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Highly Efficient Grinding of Ceramics and Hard Metals on Grinding Center
Highly Efficient Grinding of Ceramics and Hard Metals on Grinding Center T. Nakagawa (1). K. Suzuki; Inst. of Ind. Sci. Univ. of Tokyo, and T. Uematsu, Toyama Pref. Coll. of TechJJapan
SUMMARY A machining center provided with necessary counterplans a s a grinding center was used with a newly developed cast iron bonded diamond wheel in grinding o f ceramics and hard metals. Highly efficient three dimensional form grinding was found to be possible by utilizing the rigidity and numerical control function of the machining center and the high toughness and grinding ratio of the grinding wheel. The maximum stock removal rate of 42000mm3/min in deep grinding of alumina and that of 2000mm3/mm/min in high feedrate grinding of silicon nitride were obtained. Decrease of grinding force and increase o f stock removal rate were attained by inprocess dressing in creep feed grinding. Specially devised, numerically controlled tool paths for improving the grinding ability were proposed and the helical scan grinding among them was found to improve surface roughness.
1.
INTRODUCTION
In the final stage of manufacturing ceramic mechanical parts, grinding by diamond wheels is usually adopted in order t o provide configurational and dimensional accuracy t o the parts. But ceramic materials are, as well known, extremely hard and the grinding efficiency is very low. Thus it is said that the great majority of the cost of ceramic components is occupied by the machining cost. However, mechanical parts of complex shape made of ceramic materials are being required more and more in recent years. Same things can be said for hard metals a s cemented carbide. It is, therefore, a very important and urgent problem t o develop a more efficient method of stock removal for ceramic materials and hard metals. One of the solutions for the problem is considered t o utilize a rigid machine with three dimensional grinding function and tough grinding wheels. The cast-iron-bonded (CIB hereinafter) diamond wheel, which was developed by one of the authors et al. several years agolll, is expected to be one of such wheels. A machining center(MC in short) is one of those machine tools and is widely used in the field of metal cutting. If an M C is used in grinding, many advantages The versatile are expected as the Table 1 shows. numerical control(NC in short) function, automatic tool and pallet changing (ATC and APC) functions will realize the generation of desired, complicated three dimensional configurations, easy truing and dressing of wheels and automatic grinding operation. The high rigidity of an MC will make heavy, deep or creep feed grinding of ceramics and hard metals possible. A new type of M C provided with functions and counterplans necessary for grinding operation can be called a grinding centerrGC in short). The main objects of the present research work are to pursue the possibility of grinding of ceramics and hard metals by an MC combined with CIB diamond wheels, and to investigate the proper grinding conditions and grinding ability. Further object is to discuss additional functions t o an M C and the solution of the problems in realizing an ideal grinding center. 2.
[Relative advantages]
[Functions of machining center]
Economical feasibility
Substitute of specialized grinding center
Contour grinding
YC function for controlling m?ving axes
Heavy grinding
High rigidity of machine
Creep feed grinding
Variable table speed
Grinding of multi sides of work
Selection of deslred tool by automatic tool ChangerlATC)
Easiness of truing
NC function + Dressers
~~~
and dressing Zssy work change
Automatic work changing with pallet changerlAPC)
*Insufficient chip removal
Countermeasure only for coarse chips
*Disadvantage
Table 2
I I
Machine
Grinding wheel
Cast iron bonded diamond wheel Mounted u h e e l l D i a m e t e r ~ ~ 3 0 , W l d t h : ~ = 3 O m m ~ Straight w h c c l ( ~ ~ l 5 L0 ~ z 1 0 / 1 3 . 5 m m l Cup wheelG75, b-51 Mesh size=#100 - 4 0 0 Concentration: CC-75-125 Dressed before use by WA stick
Grind'g fluid Water solubla oillJohnson'a JC707. x50) Measurement
Tool dynarnorncter~Kiatler.9257A)
Wxkpieces
Silicon nitride ceramics Alumina ceramics Aard rnetalslCemented carbide:12wtlCo,H~AE7.
.rl 0
U 9
i 54
0
w.
Annals of the ClRP Vol. 35/1/1986
Revised MC (Horizontal type, 7.5kW lEN4DB-G, Niigata Enqq. Co. Ltd) ~~
EXPERIMENTAL EQUIPMENTS AND CIB DIAMOND WHEEL
Table 2 lists the experimental epuipments mainly used-MC is rebuilt in the form of a grinding center provided with a dust f r s e cype, highly rigid and accurate spindle and a specially designed dust cover for the slideway. A large quantity of grinding fluid for efficient cooling is supplied by a high power pump through a paper filter and a centrifugal micro separator. Wheels with small diameter a s mounted wheels are required in three dimensional form grinding. Though the machine used here possesses a spindle speed of 6000rpm, it may not be sufficient for effective grinding with existing mounted wheels. Thus it is necessary that wheels should have enough grinding ability and keep a high grinding ratio even at a low grinding speed. Otherwise, the dimensional accuracy o f the product will be lowered and frequent wheel changing, truinq and diameter compenSation will be necessary. Further, wheels must be enough tough in order to utilize the high rigidity and power of an PIC. The CIB diamond wheel is such a wheel with high toughness and abrasive holding force, and the grinding ratio is higher than that of resinoid wheel a s shown in Fiq.l[21. And the CIB wheel keeps rather high grinding ratio even at a low grinding speed as shown in The CIB wheel is manufactured by mixing diamond abrasive, cast iron powder[3] or fibers and a small amount of carbonyl iron powder, by compacting it to a
I
Experimental equipments
Yesh size of diamond purticle (and depth of cut a urn)
Pig.1
Comparison of grinding ratio between cast iron bonded(CIB1 diamond wheel and resinoid wheel
205
-
-
0 .A
-
U
4
-
D
<
500
E C
. . I
bl
-
0
-
Cast iron bond wheel
y- o
175)
A
(' 2 5 ) \Bronze
/ /
(75)A4
m
HA
bond,
L-
,Concentration
y-1725)
I
0 350
700
Grinding speed % I m l m i n Fig.2
Grinding ratio of CIB diamond wheel at low grinding speed Fig.4
desired form under the pressure of 6-8tonlcm' and then by sintering it in the atmosphere of ammonia cracking gas at the temperature of 1373-1413K for about 30-60 minutes. The carbonyl iron powder is mixed for improvement of sinterability and abrasive holding force.
3. THREE DIMENSIONAL FORM GRINDING
BY
Typical HP-Silicon nitride sample of three dimensional shape by NC form grinding. (Mounted wheel with corner radius rc=5mm, %=lPBm/min, b=lmm,a=O. 1-1 Omml
NC FUNCTION 4. GRINDING WITH DIFFERENT WHEELS IN TOOL MAGAZINE
The NC function of an H C makes it possible to generate two or three dimensional configuration easily[4]. Form grinding was tried by using a mounted wheel (&~30mm,b=30mm,#120/140). shows a sample formed trom a n alumina block (NGK, 6Ox60x15mm) by a kind of two dimensional plunge grinding. The wheel was fed by 5mm along the Z-axis after several cycles of NC plane. The grindlng plunge grinding in one X-Y conditions were %=470rn/min, vf=lOOmm/rnin and a =0.4mm. The grinding time required was about 5 hours. illustrates another product ground from a hot pressed silicon nitride block (NGK, 6Ox60x15mm) using a mounted CIB wheel 4 = 2 0 m m , b = 2 O m m , #120/140). Corner radius of 5mm was given to the wheel for three dimensional form grinding. The workpiece was ground by one p a s s movement of the wheel according to a 2x112 axes N C program prepared with an automatic programming tool. The pick feed along the Z-axis was lmm and the grinding speed was 188mlmin. The feedrate of 10-30 mmlmin was selected according to the change of depth of cut(a=O.l-lOmmI by means of the override function. The total finishing time including the coarse grinding took around eight hours. It is found afterwards that the total grinding time for the part could be reduced remarkably by selecting proper grinding conditions.
Blade like alumina ceramic sample formed by NC plunge grinding with a mounted wheel (Work size:60x60x15mm, W h e e l : ~ - 3 O m m , ~ 3 0 m r n ,Grinding conditions:gt470m/min,uf--130mmlmin,b=5mm,a=O.4mm)
A variety of shapes, dimensions and mesh sizes of wheels can be selected freely using the ATC function. This will lead to a great improvement of the grinding efficiency for complicated configurations. illustrates an alumina component (Kyocera A484, Hv=1400) ground from original block dimensions of 82xlOOxl41mm. Flat surfaces, straight or arc steps, straight, circular or elliptical grooves and concave or convex contours were formed by one pass grinding with automatically changed cup wheels l$=75mm,#140-300), straight wheels ($=130-150mm,#140) and mounted wheels ($=20mm,#140). The grinding conditions were a =1-lOmm, b=10-70mm, y=5-50mm/min and 5=60-500m/min. Fig.6 illustrates a state of NC grinding o f an elliptical groove with a mounted wheel ~ ~ = 2 O m m , # 1 2 0 1 1 4 0 , r , = 5 m m ) . The net grinding time o f the product was about ten hours.
Fig.3
Fiq.5
Alumina ceramic block formed by different types of grinding wheels. (Alumina, Hv1400, 82xlOOx141mm)
. = . E E
140
oMateria1: Alumina (A392) OWheel :$150.~10,#170/200. Conc.100
120
Og.I500m/rnin a-3.0 mn ti-10
L -
-.
lhvn cut('F,1
nrn
100
h.c
80 0
2
60
W
40 .r(
U
2
e
0
c :
e
a
20
bl
8
0
.
A
.
Down
-
cut(
4)
0
-20 0
I
I
I
I
200
400
600
800
(12000)
(0)
(24000)
Feedrate y Fig.8
Fig.6
State of NC contour grinding of elliptical groove bv a mounted wheel o n a n alumina ceramics block. ($=20mm mounted wheel with 5mm corner radius)
The roughness of the flat portion ground by a cup wheel(&=75mm, W300) attained to Rmax=0.8-l.Oum inspite of rather coarse mesh size of the wheel. Thus any suitable grinding wheel can be selected according to the requirements such as smooth surface or high stock removal rate. 5.
DEEP GRINDING UTILIZING HIGH RIGIDITY
The high rigidity in the spindle assembly and in the table driving mechanism of an MC derives the potentiality of the tough CIB wheel in deep grinding or creep feed grinding or high stock removal grinding of ceramics and hard metals. illustrates products formed from alumina blocks (Narumi China, A392, 5Ox68x125mm) by deep grinding with straight wheels (Ql50mm, b=10613.5mm 1170/200). Every circular or elliptical contour on the products was formed by two dimensional one pass grinding in the original rectangular cross section plane IX-Y plane). The depth of cut was especially large at the corner part and the maximum value of as much as 22mm was attained under the conditions of b = 1 Omm, y =40-1 OOmmImin and vs=940m/min. The table driving mechanism of an MC has features of rigid construction and backlashless mechanism by means of a ball screw drive. Thus it can be expected
1
1000
I
1200
1401 (36Mx)) (42000)
I mm/min , I z'Imm'/min)
Relation between grinding force( %, I$) and stock removal rate in grinding or straight groove on an alumina block.(Wheel was dressed for each feedrate)
that down-cut grinding is conducted as safely as up-cut grinding adopted in traditional grinding machines, and this will lead to increasing the possibility and flexibility of NC form grinding. Fig.B illustrates grinding force when deep straight grooves were formed on a n alumina block(Narumi China,A392) by up and down cut grinding. It is observed that vertical grinding force P, is nearly equal and horizontal force is rather small and in the opposite direction in down-cut grinding comparing those in up-cut grinding. Vertical force in down-cut grinding was often observed to be smaller in some of the other experiments. It is also worth while noticing in that remarkably large amount of stock removal rate as much as 42000 mm3/min was attained under the conditions of YE=1400mm/min, a = 3 mm, b-lOmm and ~ = l 5 O O m / m i n for an alumina ceramics. This value of stock removal rate is sufficiently comparable with that in metal cutting process. 6. SELECTION
OF FOR CIB WHEEL
OPTIMUM
GRINDING
CONDITION
Relatively superior surface roughness can be obtained in the present method as described in the former section. Here the grinding conditions for higher stock removal rate are investigated. An MC permits to select the grinding speed and the feedrate continuously in a wide range so as to realize the optimum grinding condition for a given wheel and workpiece material. 6.1 Rapid Feed Grinding High feedrate is one of the solutions for high productivity, though the feedrate is limited by the depth and width of cut and the strength of wheel and which is workpiece. Here the depth of cut of 0.5mm, far larger than that in conventional grinding, and the The workpiece width of cut of 1.0 mm were selected. was silicon nitride. illustrates the vertical grinding force
. E
L!
[ y = 2 0 0 0 mmlmin,a=0.5 mm, b = l mm,Down cut]
300
-
%=SO0
200
; - 100 C m c
4
2 Fig.7
Alumina ceramic samples showing deep contour grinding with a straight wheel(+150mm) (a)Sample with elliptical post and several grooves (Width of Cut b-13.5mm and 5.5mm)
0. 0
...
%=2000 m/min
o.o..D. OODDOD.oO..Do .OD.
I
I
2500
5000
Grinding distance L Fig.9
(b)Samples composed of different configurations, i.e., square, circle and ellipse(b=lOmm)
\5=1500
0
0
E
%-1000
1 7500
I
mm
Change of the grinding force with grinding distance for different grinding speed. (Silicon nitride, Hv=1700)
207
loot I
z
%=5OOm/min, a-lmm, b = 4 m m , Down cut, ~i~N~:~vi700 300
y = 3 0 0 0 mmlmin
o
u
0
.
0
0
40
t
Y=4000
pt 0
2000
1000
4000
3000
5000
Grinding distance L / mm
"
~~
0
2500
5000
7500
Grinding distance L I mm Fig.10
Fig.11
Effect of grinding speed on the change of grinding force in creep feed grinding. (Silicon nitride, Hv=1700)
Duration of constant grinding force in grinding with medium depth of cut and rapid feed. (Silicon nitride, Hv=1700)
for several values of grinding speed vs where the feedrate is 2000mm/min. Under the condition of %>=1500m/min, the force b is less than lOONlmm and rather stable. In order to obtain higher stock removal rate under a low grinding force, the feedrate was increased up to Vf=4000mm/min with the speed %=2000 mlmin as the Fiq.10 shows. The stock removal rate 2' attained to 2000mm31mmlmin and that the grinding ability was completely hold after grinding of rather The grinding long distance (about 8000 mm in Fig.10). ratio was about G = 4 6 0 f o r y = 3 0 0 0 mm/min and G -370 for Y . 4 0 0 0 mmlmin. 6.2 Creep Feed Grinding with Inprocess Dressing Creep feed grinding is expected to be one of efficient and qualified grinding. Fia.11 illustrates an experimental result of creep feed grinding for silicon nitride workpiece with a straight type CIB wheel(ds~l50mm,~10mm,Y170/200). The vertical force FA is found to be saturated after the initial wear of diamond particle and a steady state of grinding continues after that. But creep feed grinding may sometimes cause a sudden increase of grinding force and/or loading of the grinding wheel. This is due to the lack of chip pockets on the CIB wheel. Inprocess dressing(1D in short) is considered to be effective to avoid such accidents. The effect of ID by means of electro discharge machining or lwse alumina abrasive contained in grinding fluid(powder dressing) has already been confirmed in fundamental experiments. Here the effect of ID was investigated with a dressing stick as shown in Fig.12. In this case grinding wheel is dressed at every stroke. ( a ) Effect of inDrocess dressin effect of ID on Region [ A 1 in Fiq.13 shows 'the grinding force when silicon nitride was ground under the conditions of %=8OOm/min, y=lOOmm/min. The force in grinding with ID reaches a steady state of far smaller vaiue at the earlier period 0 % grinding than that without ID. In the region [ e l and [Cl in Fiq.13, grinding with ID under the other conditions were subsequently conducted with no initial dressing and a was steady state of rather small grinding force maintained in both regions. The adoption of ID was found to bring the increase of stock removal rate. Maximum feedrate was y.100 mmlmin in grinding without ID under the conditions of However, the feedrate %=500m/min, b - l m m and a = l m m . could be increased up to 2OOmmlmin in case of ID method. The stock removal rate in this case is Z=800m3/min which means the accomplishment of considerably efficient grinding.
rnding wheel nt nozzle
ressing stick IGC 880)
Fig.12
Schematic inprocess dressing method with dressing stick.
la:lmm. b=4mm. Doun c u t , Si3Nq(Hv=1700)]
.
400
000
O0
300
OD
208
O
U I4 W 0
m
dressing]
dressing] 0 0.00
, ~ o o . ~ ~ ~.,'ao o 8
4
Q c
,[With
200 181
100
..I
I4 c)
Y-1OOmm/min
,
I
Y=100
Y.150
0
0
2500
5000
7500
Grinding distance L / mm Fig.13
(b) Durability of arindinq wheel It was apprehended that the wear of grinding wheel might increase by adoption of ID method. But in fact, with ID at the grinding ratio((; ) in grinding Y=lOOmmlrnin was about 240 and 870 at the initial stage (L10-2800mm) and steady stage(L=2800-5300mm), respectively, and the reduction of G by the ID method was less than 201. The value of G may not be so small for silicon nitride ceramics. Since the amount of protrusion of the abrasive particle from the matrix cast iron is maintained rather constant by the ID method, it is preferable that the bonding strength of the abrasive should be as high as possible. Though the ID method with dressing stick is effective to keep grinding force constant, it is not preferable for three dimensional grinding. Powder dressing is recommended in that case.
\\[Without
K€
Attainment of constant grinding force by inprocess dressing. (Silicon nitride, Hv=17001
7. NEW GRINDING METHODS BY TOOL PATH CONTROL The excellent performance will be given to a grinding center by positive utilization of NC function. This means to prepare suitable NC programs or software for giving special relative movements between a wheel and a workpiece, in addition to normal NC programs for straight or contour grinding. Such devised NC programs are expected to have effects on cooling of wheels, removing of ground chips, inprocess powder dressing, increasing the force acting on the cutting edge of each
Uheel
<
Step backwards
Feed
I 1 )Stepatop
(2)Step-back
(3ISlou in-slow out
(5)Reciprocating rolling
(6)Comb type
Work (4)
Rollrng
B
(7)Zig-rag
181Helical scan
19)Cros.q hatch
Fiq.15
Clearance
Principle of step-back grinding which moves backwards intermittently.
. ‘ 0.04
d
fa
Il0)Surtace grindxng
(11)Hclical
1121Chop grinding
5 I
0.03
”
0.02
r(
0
U m r(
Fig.14
Schematic tool trajectories for realizing highly efficient grinding
0.01
2
2 D
o
0
abrasive particle, eliminating residual stock removal and improving surface roughness. Fiq.14 illustrates representative schematic tool trajectories developed for the above purpose. In the step-stop movement (l), the tool is stopped intermittently for sufficient cooling. The step-back movement (21, shown in Fiq.15, includes intermittent, short backward movement during the forward movement and is expected to have effects on cooling, removing ground chips, inprocess powder dressing and eliminating residual stock removal. In the slow in-slow out movement (3). the feedrate is lowered at the both ends of a workpiece so as to avoid breakage or chipping of the workpiece. Trajectories (4),(5);(6) and ( 7 ) are two dimensional movements designed for cooling, chip removing, inprocess powder dressing, and for increasing the force acting on each abrasive particle. Helical scan grinding ( 8 ) described later in detail and cross-hatch grinding ( 9 ) are intended for the improvement of surface roughness. The surface grinding program (10) is convenient when a grinding center is used as a surface grinding machine. The three dimensional helical and chop grinding program (111 and (12) are used in effective grinding of inner or outer peripheral surface of cylindrical workpieces. The programs for all these trajectories are stored in the custom macro area of the NC device as subprograms. The operator has only to input some numerical data. 7.1
Step-back Grinding As an example of effects of above tool path control, the elimination of residual stock removal by A deep groove the step-back movement is described. (a.5.0mm. b=5.5mm) was formed on an alumina block by a straight wheel ($=148mm, b=5.5mm, #120/1401. When the wheel was fed alternately forward with a feedrate of ~ = 5 0 m m / m i nand backward with y=200mm/min, the thickness of residual stock removal was zero as illustrated in Fiq.16, though it was about 20 urn thick in ordinary forward grinding with y-50mm/min. Of course the grinding time increased because of the backward movement and the apparent feedrate was 33mm/min in the above case. But the residual stock removal did not decrease in ordinary forward grinding with V=33mm/min. This fact means that the step-back movement eliminates residual stock removal without decreasing grinding efficiency. 7.2 Helical Scan Grinding Surface roughness in grinding can be improved by wheels with fine abrasives, low feedrate, or by small depth of cut. But stock removal rate will also be reduced in those methods. The helical scan grinding method proposed here was devised so as to improve surface roughness with keeping stock removal rate high. In the helical scan grinding method, the wheel axis is inclined relatively to the feed direction as illustrated in Fiq.l7(al. The actual distance between
20
40
60
80
Grinding distance L / mm Fig.16
Elimination of residual stock removal (Step-back grinding, Alumina ceramics).
Grindinq wheel
y /
c Feed direction
(a)Inclined spindle method Fig.17
Fig.18
(b)Helical scan grinding method Schematic helical sca. grinding realized by controlling a tool path.
Effect of helical scan grinding on improvement of surface roughness, and photographs showing appearance of surface. (Work: Kyocera Cermet, Ceratip N-5)
209
Table 3
Problems to be solved and their counterplans for realizing an ideal grinding center.
[Counterplansl
[Problems a causes1
Dust cover.
Fine filter for coolant p u m p
Abnormal wear a t slideways of machine hg ground chips insufficient grinding speed for small size wheel
r
1
Truing on a machine s i n g NC movemect
insufficient grinding quality due to swing of grind'g wheel
Dulling of wheel or not so long dressing interval
I
I
I
Hard metal sample formed by NC creep feed
abrasive particles will become smaller and the surface roughness is expected t o be improved. Though there is no grinding machine suitable for such an operation, the N C function can easily produce such a relative movement a s illustrated in Fiq.l7(b). Fig.18 illustrates surface roughness of workpieces (cermet, Kyocera Ceratip N5) ground by helical scan grinding with a mounted wheelf$=30mm, h-20mm. #lOO/l20). Though rather rough surface may be observed from the photographs, the measured roughness is found to be steeply improved with the inclination angle. When the angle is 45 deg., the roughness Rmax is about 0.6um, and this value is about one seventh of the roughness in ordinary grinding ( 0 deg., Rmax=4.2um). 8. APPLICATION OF THE TECHNIQUE T O HARD METALS
The new grinding technique proposed here was also applied t o hard metals and satisfying results were obtained. Fiq.19 shows a product formed by NC creep feed grinding from a cemented carbide block (12wt%Co, H Ad7.5, 6Ox60x20mm). All the ground parts were f%rmed by a single pass of different wheels. Fiq.20 shows the relation between the feedrate and the surface roughness of a hard metal block ground by a CIB cup wheelf&=75mm,#140) at !+,=165m/min and a=O.lmm. The surface roughness gets better with the reduction of feedrate and a superior mirror finished surface under 0.1 m Rmax can be obtained when feedrate is smaller than about 2Ommlmin. The reason for getting such a smooth surface is due to the flattened surface of the diamond particles gripped firmly in cast iron matrix.
a 6 \
X
2 YI YI
al
c m J
a
o-2i
Cup wheel [$=75m, X140) n
I
Inprocess dressing by ED machining/?order dressing Specialized NC tool path for GC
I Fig.19
I I
with a larger power I Pump internal cooling grinding
in grind'g efficiency
Need for more complicated configuration
Increase of spindle speed up to 6000rpm
Establishment of the adaptive control
I
Addition of more control axes
9. REALIZATION OF IDEAL GRINDING CENTER An ideal grinding center will be realized based on an M C by solving the problems listed in Table 3 and by providing an MC with their counterplans. Some of the problems have already been solved in the course of the present research work and the counterplans such as dust cover for slideways, dust-free structure of the spindle, spindle speed and specialized NC tool paths were provided. The other problems have also been solved basically. Inprocess dressing of wheels is important as described in above sections and the effectiveness of EDM or powder dressing has been confirmed fundamentally. Cooling of the engaging point of a wheel and a workpiece is also important. Supplying a large quantity of pressurized grinding fluid is one of the solutions, but more reliable method is to adopt a spindle-through or tool-thourgh fluid supplying system. Grooves provided on the surface of grinding wheels are also effective for cooling and chip removing, and applying of some patterns of grooves has already been tried. 1 0. CONCLUSIONS
Highly efficient three dimensional form grinding of ceramics and hard metals has been accomplished by the combination of a grinding center based on a machining center and newly developed cast iron bonded diamond wheels. The rigidity of the machine, NC functions and the toughness of the wheels are found to be essential keys of this new grinding technology. The fundamental experiments showed that a n ideal grinding center will be completed by additionally adopting inprocess dressing , specially devised, numerically This new technology controlled tool paths and so on.. is expected to open a new ceramic.age. [ACKNOWLEDGEMENTS]
15
The authors wish to thank MIS Niigata Engineering Co. Ltd. and Makino Milling Machine Co.Ltd. for providing machining centers, MIS Fuji Die Co. Ltd., Sintobrator Ltd. for providing grinding wheels, MI6 Narumi China Corp. Xyocera Corp., Yoshida Xogyo X.X., NGX Spark Plug Co. Ltd. for providing workpiece materials. Further they extend sincere thanks to Prof. of Nippon Inst. of Tech. and Mr. A.Yanagisawa M.Kimura for their assistance.
0.1
[REFERENCES) 0 0
10
20
30
40
50
Feedrate vf I mmlmin
Fig.20
210
Realation between feedrate and surface roughness obtained with a cup wheel. (Work: J I S V3, 12wt%Co, HRA=87.5,ds=75mm,#140)
[11 Y.Hagiuda, K.Karikomi, T.Nakagawa: CIRP annals 19E1 Manufacturing Technology, 30,1,(1981)277-281 (21 T.Nakagawa, Y.Hagiuda, K.Xarikomi: Proc. of 5th ICPE,(1984)369-374 [ 3 1 T.Nakagawa, C.S.Sharma: Modern developments in Powder Metallurav. - - . vo1.9.(1977)347-388 [41 T.Nakagawa, K-Suzuki, T.Uematsu: Proc. of Winter ~~~~~l sin ti^^ of ASME, vo1.17,(1g85)1-8
SUMMARY A machining center provided with necessary counterplans a s a grinding center was used with a newly developed cast iron bonded diamond wheel in grinding o f ceramics and hard metals. Highly efficient three dimensional form grinding was found to be possible by utilizing the rigidity and numerical control function of the machining center and the high toughness and grinding ratio of the grinding wheel. The maximum stock removal rate of 42000mm3/min in deep grinding of alumina and that of 2000mm3/mm/min in high feedrate grinding of silicon nitride were obtained. Decrease of grinding force and increase o f stock removal rate were attained by inprocess dressing in creep feed grinding. Specially devised, numerically controlled tool paths for improving the grinding ability were proposed and the helical scan grinding among them was found to improve surface roughness.
1.
INTRODUCTION
In the final stage of manufacturing ceramic mechanical parts, grinding by diamond wheels is usually adopted in order t o provide configurational and dimensional accuracy t o the parts. But ceramic materials are, as well known, extremely hard and the grinding efficiency is very low. Thus it is said that the great majority of the cost of ceramic components is occupied by the machining cost. However, mechanical parts of complex shape made of ceramic materials are being required more and more in recent years. Same things can be said for hard metals a s cemented carbide. It is, therefore, a very important and urgent problem t o develop a more efficient method of stock removal for ceramic materials and hard metals. One of the solutions for the problem is considered t o utilize a rigid machine with three dimensional grinding function and tough grinding wheels. The cast-iron-bonded (CIB hereinafter) diamond wheel, which was developed by one of the authors et al. several years agolll, is expected to be one of such wheels. A machining center(MC in short) is one of those machine tools and is widely used in the field of metal cutting. If an M C is used in grinding, many advantages The versatile are expected as the Table 1 shows. numerical control(NC in short) function, automatic tool and pallet changing (ATC and APC) functions will realize the generation of desired, complicated three dimensional configurations, easy truing and dressing of wheels and automatic grinding operation. The high rigidity of an MC will make heavy, deep or creep feed grinding of ceramics and hard metals possible. A new type of M C provided with functions and counterplans necessary for grinding operation can be called a grinding centerrGC in short). The main objects of the present research work are to pursue the possibility of grinding of ceramics and hard metals by an MC combined with CIB diamond wheels, and to investigate the proper grinding conditions and grinding ability. Further object is to discuss additional functions t o an M C and the solution of the problems in realizing an ideal grinding center. 2.
[Relative advantages]
[Functions of machining center]
Economical feasibility
Substitute of specialized grinding center
Contour grinding
YC function for controlling m?ving axes
Heavy grinding
High rigidity of machine
Creep feed grinding
Variable table speed
Grinding of multi sides of work
Selection of deslred tool by automatic tool ChangerlATC)
Easiness of truing
NC function + Dressers
~~~
and dressing Zssy work change
Automatic work changing with pallet changerlAPC)
*Insufficient chip removal
Countermeasure only for coarse chips
*Disadvantage
Table 2
I I
Machine
Grinding wheel
Cast iron bonded diamond wheel Mounted u h e e l l D i a m e t e r ~ ~ 3 0 , W l d t h : ~ = 3 O m m ~ Straight w h c c l ( ~ ~ l 5 L0 ~ z 1 0 / 1 3 . 5 m m l Cup wheelG75, b-51 Mesh size=#100 - 4 0 0 Concentration: CC-75-125 Dressed before use by WA stick
Grind'g fluid Water solubla oillJohnson'a JC707. x50) Measurement
Tool dynarnorncter~Kiatler.9257A)
Wxkpieces
Silicon nitride ceramics Alumina ceramics Aard rnetalslCemented carbide:12wtlCo,H~AE7.
.rl 0
U 9
i 54
0
w.
Annals of the ClRP Vol. 35/1/1986
Revised MC (Horizontal type, 7.5kW lEN4DB-G, Niigata Enqq. Co. Ltd) ~~
EXPERIMENTAL EQUIPMENTS AND CIB DIAMOND WHEEL
Table 2 lists the experimental epuipments mainly used-MC is rebuilt in the form of a grinding center provided with a dust f r s e cype, highly rigid and accurate spindle and a specially designed dust cover for the slideway. A large quantity of grinding fluid for efficient cooling is supplied by a high power pump through a paper filter and a centrifugal micro separator. Wheels with small diameter a s mounted wheels are required in three dimensional form grinding. Though the machine used here possesses a spindle speed of 6000rpm, it may not be sufficient for effective grinding with existing mounted wheels. Thus it is necessary that wheels should have enough grinding ability and keep a high grinding ratio even at a low grinding speed. Otherwise, the dimensional accuracy o f the product will be lowered and frequent wheel changing, truinq and diameter compenSation will be necessary. Further, wheels must be enough tough in order to utilize the high rigidity and power of an PIC. The CIB diamond wheel is such a wheel with high toughness and abrasive holding force, and the grinding ratio is higher than that of resinoid wheel a s shown in Fiq.l[21. And the CIB wheel keeps rather high grinding ratio even at a low grinding speed as shown in The CIB wheel is manufactured by mixing diamond abrasive, cast iron powder[3] or fibers and a small amount of carbonyl iron powder, by compacting it to a
I
Experimental equipments
Yesh size of diamond purticle (and depth of cut a urn)
Pig.1
Comparison of grinding ratio between cast iron bonded(CIB1 diamond wheel and resinoid wheel
205
-
-
0 .A
-
U
4
-
D
<
500
E C
. . I
bl
-
0
-
Cast iron bond wheel
y- o
175)
A
(' 2 5 ) \Bronze
/ /
(75)A4
m
HA
bond,
L-
,Concentration
y-1725)
I
0 350
700
Grinding speed % I m l m i n Fig.2
Grinding ratio of CIB diamond wheel at low grinding speed Fig.4
desired form under the pressure of 6-8tonlcm' and then by sintering it in the atmosphere of ammonia cracking gas at the temperature of 1373-1413K for about 30-60 minutes. The carbonyl iron powder is mixed for improvement of sinterability and abrasive holding force.
3. THREE DIMENSIONAL FORM GRINDING
BY
Typical HP-Silicon nitride sample of three dimensional shape by NC form grinding. (Mounted wheel with corner radius rc=5mm, %=lPBm/min, b=lmm,a=O. 1-1 Omml
NC FUNCTION 4. GRINDING WITH DIFFERENT WHEELS IN TOOL MAGAZINE
The NC function of an H C makes it possible to generate two or three dimensional configuration easily[4]. Form grinding was tried by using a mounted wheel (&~30mm,b=30mm,#120/140). shows a sample formed trom a n alumina block (NGK, 6Ox60x15mm) by a kind of two dimensional plunge grinding. The wheel was fed by 5mm along the Z-axis after several cycles of NC plane. The grindlng plunge grinding in one X-Y conditions were %=470rn/min, vf=lOOmm/rnin and a =0.4mm. The grinding time required was about 5 hours. illustrates another product ground from a hot pressed silicon nitride block (NGK, 6Ox60x15mm) using a mounted CIB wheel 4 = 2 0 m m , b = 2 O m m , #120/140). Corner radius of 5mm was given to the wheel for three dimensional form grinding. The workpiece was ground by one p a s s movement of the wheel according to a 2x112 axes N C program prepared with an automatic programming tool. The pick feed along the Z-axis was lmm and the grinding speed was 188mlmin. The feedrate of 10-30 mmlmin was selected according to the change of depth of cut(a=O.l-lOmmI by means of the override function. The total finishing time including the coarse grinding took around eight hours. It is found afterwards that the total grinding time for the part could be reduced remarkably by selecting proper grinding conditions.
Blade like alumina ceramic sample formed by NC plunge grinding with a mounted wheel (Work size:60x60x15mm, W h e e l : ~ - 3 O m m , ~ 3 0 m r n ,Grinding conditions:gt470m/min,uf--130mmlmin,b=5mm,a=O.4mm)
A variety of shapes, dimensions and mesh sizes of wheels can be selected freely using the ATC function. This will lead to a great improvement of the grinding efficiency for complicated configurations. illustrates an alumina component (Kyocera A484, Hv=1400) ground from original block dimensions of 82xlOOxl41mm. Flat surfaces, straight or arc steps, straight, circular or elliptical grooves and concave or convex contours were formed by one pass grinding with automatically changed cup wheels l$=75mm,#140-300), straight wheels ($=130-150mm,#140) and mounted wheels ($=20mm,#140). The grinding conditions were a =1-lOmm, b=10-70mm, y=5-50mm/min and 5=60-500m/min. Fig.6 illustrates a state of NC grinding o f an elliptical groove with a mounted wheel ~ ~ = 2 O m m , # 1 2 0 1 1 4 0 , r , = 5 m m ) . The net grinding time o f the product was about ten hours.
Fig.3
Fiq.5
Alumina ceramic block formed by different types of grinding wheels. (Alumina, Hv1400, 82xlOOx141mm)
. = . E E
140
oMateria1: Alumina (A392) OWheel :$150.~10,#170/200. Conc.100
120
Og.I500m/rnin a-3.0 mn ti-10
L -
-.
lhvn cut('F,1
nrn
100
h.c
80 0
2
60
W
40 .r(
U
2
e
0
c :
e
a
20
bl
8
0
.
A
.
Down
-
cut(
4)
0
-20 0
I
I
I
I
200
400
600
800
(12000)
(0)
(24000)
Feedrate y Fig.8
Fig.6
State of NC contour grinding of elliptical groove bv a mounted wheel o n a n alumina ceramics block. ($=20mm mounted wheel with 5mm corner radius)
The roughness of the flat portion ground by a cup wheel(&=75mm, W300) attained to Rmax=0.8-l.Oum inspite of rather coarse mesh size of the wheel. Thus any suitable grinding wheel can be selected according to the requirements such as smooth surface or high stock removal rate. 5.
DEEP GRINDING UTILIZING HIGH RIGIDITY
The high rigidity in the spindle assembly and in the table driving mechanism of an MC derives the potentiality of the tough CIB wheel in deep grinding or creep feed grinding or high stock removal grinding of ceramics and hard metals. illustrates products formed from alumina blocks (Narumi China, A392, 5Ox68x125mm) by deep grinding with straight wheels (Ql50mm, b=10613.5mm 1170/200). Every circular or elliptical contour on the products was formed by two dimensional one pass grinding in the original rectangular cross section plane IX-Y plane). The depth of cut was especially large at the corner part and the maximum value of as much as 22mm was attained under the conditions of b = 1 Omm, y =40-1 OOmmImin and vs=940m/min. The table driving mechanism of an MC has features of rigid construction and backlashless mechanism by means of a ball screw drive. Thus it can be expected
1
1000
I
1200
1401 (36Mx)) (42000)
I mm/min , I z'Imm'/min)
Relation between grinding force( %, I$) and stock removal rate in grinding or straight groove on an alumina block.(Wheel was dressed for each feedrate)
that down-cut grinding is conducted as safely as up-cut grinding adopted in traditional grinding machines, and this will lead to increasing the possibility and flexibility of NC form grinding. Fig.B illustrates grinding force when deep straight grooves were formed on a n alumina block(Narumi China,A392) by up and down cut grinding. It is observed that vertical grinding force P, is nearly equal and horizontal force is rather small and in the opposite direction in down-cut grinding comparing those in up-cut grinding. Vertical force in down-cut grinding was often observed to be smaller in some of the other experiments. It is also worth while noticing in that remarkably large amount of stock removal rate as much as 42000 mm3/min was attained under the conditions of YE=1400mm/min, a = 3 mm, b-lOmm and ~ = l 5 O O m / m i n for an alumina ceramics. This value of stock removal rate is sufficiently comparable with that in metal cutting process. 6. SELECTION
OF FOR CIB WHEEL
OPTIMUM
GRINDING
CONDITION
Relatively superior surface roughness can be obtained in the present method as described in the former section. Here the grinding conditions for higher stock removal rate are investigated. An MC permits to select the grinding speed and the feedrate continuously in a wide range so as to realize the optimum grinding condition for a given wheel and workpiece material. 6.1 Rapid Feed Grinding High feedrate is one of the solutions for high productivity, though the feedrate is limited by the depth and width of cut and the strength of wheel and which is workpiece. Here the depth of cut of 0.5mm, far larger than that in conventional grinding, and the The workpiece width of cut of 1.0 mm were selected. was silicon nitride. illustrates the vertical grinding force
. E
L!
[ y = 2 0 0 0 mmlmin,a=0.5 mm, b = l mm,Down cut]
300
-
%=SO0
200
; - 100 C m c
4
2 Fig.7
Alumina ceramic samples showing deep contour grinding with a straight wheel(+150mm) (a)Sample with elliptical post and several grooves (Width of Cut b-13.5mm and 5.5mm)
0. 0
...
%=2000 m/min
o.o..D. OODDOD.oO..Do .OD.
I
I
2500
5000
Grinding distance L Fig.9
(b)Samples composed of different configurations, i.e., square, circle and ellipse(b=lOmm)
\5=1500
0
0
E
%-1000
1 7500
I
mm
Change of the grinding force with grinding distance for different grinding speed. (Silicon nitride, Hv=1700)
207
loot I
z
%=5OOm/min, a-lmm, b = 4 m m , Down cut, ~i~N~:~vi700 300
y = 3 0 0 0 mmlmin
o
u
0
.
0
0
40
t
Y=4000
pt 0
2000
1000
4000
3000
5000
Grinding distance L / mm
"
~~
0
2500
5000
7500
Grinding distance L I mm Fig.10
Fig.11
Effect of grinding speed on the change of grinding force in creep feed grinding. (Silicon nitride, Hv=1700)
Duration of constant grinding force in grinding with medium depth of cut and rapid feed. (Silicon nitride, Hv=1700)
for several values of grinding speed vs where the feedrate is 2000mm/min. Under the condition of %>=1500m/min, the force b is less than lOONlmm and rather stable. In order to obtain higher stock removal rate under a low grinding force, the feedrate was increased up to Vf=4000mm/min with the speed %=2000 mlmin as the Fiq.10 shows. The stock removal rate 2' attained to 2000mm31mmlmin and that the grinding ability was completely hold after grinding of rather The grinding long distance (about 8000 mm in Fig.10). ratio was about G = 4 6 0 f o r y = 3 0 0 0 mm/min and G -370 for Y . 4 0 0 0 mmlmin. 6.2 Creep Feed Grinding with Inprocess Dressing Creep feed grinding is expected to be one of efficient and qualified grinding. Fia.11 illustrates an experimental result of creep feed grinding for silicon nitride workpiece with a straight type CIB wheel(ds~l50mm,~10mm,Y170/200). The vertical force FA is found to be saturated after the initial wear of diamond particle and a steady state of grinding continues after that. But creep feed grinding may sometimes cause a sudden increase of grinding force and/or loading of the grinding wheel. This is due to the lack of chip pockets on the CIB wheel. Inprocess dressing(1D in short) is considered to be effective to avoid such accidents. The effect of ID by means of electro discharge machining or lwse alumina abrasive contained in grinding fluid(powder dressing) has already been confirmed in fundamental experiments. Here the effect of ID was investigated with a dressing stick as shown in Fig.12. In this case grinding wheel is dressed at every stroke. ( a ) Effect of inDrocess dressin effect of ID on Region [ A 1 in Fiq.13 shows 'the grinding force when silicon nitride was ground under the conditions of %=8OOm/min, y=lOOmm/min. The force in grinding with ID reaches a steady state of far smaller vaiue at the earlier period 0 % grinding than that without ID. In the region [ e l and [Cl in Fiq.13, grinding with ID under the other conditions were subsequently conducted with no initial dressing and a was steady state of rather small grinding force maintained in both regions. The adoption of ID was found to bring the increase of stock removal rate. Maximum feedrate was y.100 mmlmin in grinding without ID under the conditions of However, the feedrate %=500m/min, b - l m m and a = l m m . could be increased up to 2OOmmlmin in case of ID method. The stock removal rate in this case is Z=800m3/min which means the accomplishment of considerably efficient grinding.
rnding wheel nt nozzle
ressing stick IGC 880)
Fig.12
Schematic inprocess dressing method with dressing stick.
la:lmm. b=4mm. Doun c u t , Si3Nq(Hv=1700)]
.
400
000
O0
300
OD
208
O
U I4 W 0
m
dressing]
dressing] 0 0.00
, ~ o o . ~ ~ ~.,'ao o 8
4
Q c
,[With
200 181
100
..I
I4 c)
Y-1OOmm/min
,
I
Y=100
Y.150
0
0
2500
5000
7500
Grinding distance L / mm Fig.13
(b) Durability of arindinq wheel It was apprehended that the wear of grinding wheel might increase by adoption of ID method. But in fact, with ID at the grinding ratio((; ) in grinding Y=lOOmmlrnin was about 240 and 870 at the initial stage (L10-2800mm) and steady stage(L=2800-5300mm), respectively, and the reduction of G by the ID method was less than 201. The value of G may not be so small for silicon nitride ceramics. Since the amount of protrusion of the abrasive particle from the matrix cast iron is maintained rather constant by the ID method, it is preferable that the bonding strength of the abrasive should be as high as possible. Though the ID method with dressing stick is effective to keep grinding force constant, it is not preferable for three dimensional grinding. Powder dressing is recommended in that case.
\\[Without
K€
Attainment of constant grinding force by inprocess dressing. (Silicon nitride, Hv=17001
7. NEW GRINDING METHODS BY TOOL PATH CONTROL The excellent performance will be given to a grinding center by positive utilization of NC function. This means to prepare suitable NC programs or software for giving special relative movements between a wheel and a workpiece, in addition to normal NC programs for straight or contour grinding. Such devised NC programs are expected to have effects on cooling of wheels, removing of ground chips, inprocess powder dressing, increasing the force acting on the cutting edge of each
Uheel
<
Step backwards
Feed
I 1 )Stepatop
(2)Step-back
(3ISlou in-slow out
(5)Reciprocating rolling
(6)Comb type
Work (4)
Rollrng
B
(7)Zig-rag
181Helical scan
19)Cros.q hatch
Fiq.15
Clearance
Principle of step-back grinding which moves backwards intermittently.
. ‘ 0.04
d
fa
Il0)Surtace grindxng
(11)Hclical
1121Chop grinding
5 I
0.03
”
0.02
r(
0
U m r(
Fig.14
Schematic tool trajectories for realizing highly efficient grinding
0.01
2
2 D
o
0
abrasive particle, eliminating residual stock removal and improving surface roughness. Fiq.14 illustrates representative schematic tool trajectories developed for the above purpose. In the step-stop movement (l), the tool is stopped intermittently for sufficient cooling. The step-back movement (21, shown in Fiq.15, includes intermittent, short backward movement during the forward movement and is expected to have effects on cooling, removing ground chips, inprocess powder dressing and eliminating residual stock removal. In the slow in-slow out movement (3). the feedrate is lowered at the both ends of a workpiece so as to avoid breakage or chipping of the workpiece. Trajectories (4),(5);(6) and ( 7 ) are two dimensional movements designed for cooling, chip removing, inprocess powder dressing, and for increasing the force acting on each abrasive particle. Helical scan grinding ( 8 ) described later in detail and cross-hatch grinding ( 9 ) are intended for the improvement of surface roughness. The surface grinding program (10) is convenient when a grinding center is used as a surface grinding machine. The three dimensional helical and chop grinding program (111 and (12) are used in effective grinding of inner or outer peripheral surface of cylindrical workpieces. The programs for all these trajectories are stored in the custom macro area of the NC device as subprograms. The operator has only to input some numerical data. 7.1
Step-back Grinding As an example of effects of above tool path control, the elimination of residual stock removal by A deep groove the step-back movement is described. (a.5.0mm. b=5.5mm) was formed on an alumina block by a straight wheel ($=148mm, b=5.5mm, #120/1401. When the wheel was fed alternately forward with a feedrate of ~ = 5 0 m m / m i nand backward with y=200mm/min, the thickness of residual stock removal was zero as illustrated in Fiq.16, though it was about 20 urn thick in ordinary forward grinding with y-50mm/min. Of course the grinding time increased because of the backward movement and the apparent feedrate was 33mm/min in the above case. But the residual stock removal did not decrease in ordinary forward grinding with V=33mm/min. This fact means that the step-back movement eliminates residual stock removal without decreasing grinding efficiency. 7.2 Helical Scan Grinding Surface roughness in grinding can be improved by wheels with fine abrasives, low feedrate, or by small depth of cut. But stock removal rate will also be reduced in those methods. The helical scan grinding method proposed here was devised so as to improve surface roughness with keeping stock removal rate high. In the helical scan grinding method, the wheel axis is inclined relatively to the feed direction as illustrated in Fiq.l7(al. The actual distance between
20
40
60
80
Grinding distance L / mm Fig.16
Elimination of residual stock removal (Step-back grinding, Alumina ceramics).
Grindinq wheel
y /
c Feed direction
(a)Inclined spindle method Fig.17
Fig.18
(b)Helical scan grinding method Schematic helical sca. grinding realized by controlling a tool path.
Effect of helical scan grinding on improvement of surface roughness, and photographs showing appearance of surface. (Work: Kyocera Cermet, Ceratip N-5)
209
Table 3
Problems to be solved and their counterplans for realizing an ideal grinding center.
[Counterplansl
[Problems a causes1
Dust cover.
Fine filter for coolant p u m p
Abnormal wear a t slideways of machine hg ground chips insufficient grinding speed for small size wheel
r
1
Truing on a machine s i n g NC movemect
insufficient grinding quality due to swing of grind'g wheel
Dulling of wheel or not so long dressing interval
I
I
I
Hard metal sample formed by NC creep feed
abrasive particles will become smaller and the surface roughness is expected t o be improved. Though there is no grinding machine suitable for such an operation, the N C function can easily produce such a relative movement a s illustrated in Fiq.l7(b). Fig.18 illustrates surface roughness of workpieces (cermet, Kyocera Ceratip N5) ground by helical scan grinding with a mounted wheelf$=30mm, h-20mm. #lOO/l20). Though rather rough surface may be observed from the photographs, the measured roughness is found to be steeply improved with the inclination angle. When the angle is 45 deg., the roughness Rmax is about 0.6um, and this value is about one seventh of the roughness in ordinary grinding ( 0 deg., Rmax=4.2um). 8. APPLICATION OF THE TECHNIQUE T O HARD METALS
The new grinding technique proposed here was also applied t o hard metals and satisfying results were obtained. Fiq.19 shows a product formed by NC creep feed grinding from a cemented carbide block (12wt%Co, H Ad7.5, 6Ox60x20mm). All the ground parts were f%rmed by a single pass of different wheels. Fiq.20 shows the relation between the feedrate and the surface roughness of a hard metal block ground by a CIB cup wheelf&=75mm,#140) at !+,=165m/min and a=O.lmm. The surface roughness gets better with the reduction of feedrate and a superior mirror finished surface under 0.1 m Rmax can be obtained when feedrate is smaller than about 2Ommlmin. The reason for getting such a smooth surface is due to the flattened surface of the diamond particles gripped firmly in cast iron matrix.
a 6 \
X
2 YI YI
al
c m J
a
o-2i
Cup wheel [$=75m, X140) n
I
Inprocess dressing by ED machining/?order dressing Specialized NC tool path for GC
I Fig.19
I I
with a larger power I Pump internal cooling grinding
in grind'g efficiency
Need for more complicated configuration
Increase of spindle speed up to 6000rpm
Establishment of the adaptive control
I
Addition of more control axes
9. REALIZATION OF IDEAL GRINDING CENTER An ideal grinding center will be realized based on an M C by solving the problems listed in Table 3 and by providing an MC with their counterplans. Some of the problems have already been solved in the course of the present research work and the counterplans such as dust cover for slideways, dust-free structure of the spindle, spindle speed and specialized NC tool paths were provided. The other problems have also been solved basically. Inprocess dressing of wheels is important as described in above sections and the effectiveness of EDM or powder dressing has been confirmed fundamentally. Cooling of the engaging point of a wheel and a workpiece is also important. Supplying a large quantity of pressurized grinding fluid is one of the solutions, but more reliable method is to adopt a spindle-through or tool-thourgh fluid supplying system. Grooves provided on the surface of grinding wheels are also effective for cooling and chip removing, and applying of some patterns of grooves has already been tried. 1 0. CONCLUSIONS
Highly efficient three dimensional form grinding of ceramics and hard metals has been accomplished by the combination of a grinding center based on a machining center and newly developed cast iron bonded diamond wheels. The rigidity of the machine, NC functions and the toughness of the wheels are found to be essential keys of this new grinding technology. The fundamental experiments showed that a n ideal grinding center will be completed by additionally adopting inprocess dressing , specially devised, numerically This new technology controlled tool paths and so on.. is expected to open a new ceramic.age. [ACKNOWLEDGEMENTS]
15
The authors wish to thank MIS Niigata Engineering Co. Ltd. and Makino Milling Machine Co.Ltd. for providing machining centers, MIS Fuji Die Co. Ltd., Sintobrator Ltd. for providing grinding wheels, MI6 Narumi China Corp. Xyocera Corp., Yoshida Xogyo X.X., NGX Spark Plug Co. Ltd. for providing workpiece materials. Further they extend sincere thanks to Prof. of Nippon Inst. of Tech. and Mr. A.Yanagisawa M.Kimura for their assistance.
0.1
[REFERENCES) 0 0
10
20
30
40
50
Feedrate vf I mmlmin
Fig.20
210
Realation between feedrate and surface roughness obtained with a cup wheel. (Work: J I S V3, 12wt%Co, HRA=87.5,ds=75mm,#140)
[11 Y.Hagiuda, K.Karikomi, T.Nakagawa: CIRP annals 19E1 Manufacturing Technology, 30,1,(1981)277-281 (21 T.Nakagawa, Y.Hagiuda, K.Xarikomi: Proc. of 5th ICPE,(1984)369-374 [ 3 1 T.Nakagawa, C.S.Sharma: Modern developments in Powder Metallurav. - - . vo1.9.(1977)347-388 [41 T.Nakagawa, K-Suzuki, T.Uematsu: Proc. of Winter ~~~~~l sin ti^^ of ASME, vo1.17,(1g85)1-8