Turn-Milling of Hardened Steel - an Alternative to Turning

Turn-Milling of Hardened Steel

- an Alternative to Turning

H. Schulz (l),T. Kneisel, Technical University of Darrnstadt, Germany Received on January 5,1994

Abstract: Turn-milling with parallel axes is a relatively new method for precision machining of rotationally symmetrical workpieces. Here the kinematic conditions and their influence on the design of tools and the choice of technological parameters are shown. Using CBN while cutting hardened steel (> 62 HRC) a tool life is achievable which clearly exceeds that of the hard-turning process. Surface qualities beneath RZ<2 pm can be achieved. Keywords: Turn-Milling, Hard Cutting, CBN Aims of High-Precision Cutting In high-precision cutting surface qualities down to RZ= 1pm as well as dimensional and shape tolerances of IT4 are often required. Normally these aims can only be achieved by grinding. Recently efforts have been made to substitute the grinding process with all its problematic strains by methods with geometrically defined cutting edges. When rotationally symmetrical workpieces are to be produced, turning and most recently turn-milling can be real alternatives to grinding. Special Characteristics of Turn-Milling with Parallel Axes The combination of a fast rotating milling cutter and a slowly rotating workpiece results in several advantages [I, 21: Short chips will be produced because of the process kinematic. Even when a material is machined which normally creates long chips, chips will be appropriate for automation. Machining with optimized cutting speed is possible, both for large and unbalanced workpieces as well as for workpieces with small diameters. Workpieces with non rotationally symmetric geometries are easily produced. Contrary to turning there are only small oscillation frequencies in the linear axes because the rotational speed of the workpiece is relatively small. This opens new fields of applications such as shaft-hub connections in polygonalform. Machining of hardened steel without any coolant is possible. Basic Kinematic Factors Influence of Radial Deviation on Surface Quality Usually the single edges of a milling cutter rotate on different radii. Therefore irregularities in the structure of the surface occur. The exact, quantitative effects on the degeneration of surface quality can be described by a non-linear equation system (eq. 1 and 2). It can be numerically solved by means of the multidimensional newtonian-method. Fig. I demonstrates this for a two-edged cutter.

q .COScp1- r2 .coscp2 = 0

-(TI f

2x

+ (p2 + ACP)- q * sincpl - r2. sincpp = 0

(2)

with: q: Acp: f: cpi:

rotation radii of the single cutting edges angular pitch between edges 1 and 2 feed per rotation unkown angles o: angular velocity vf: feed rate The surface uneveness, expressed as the averaged peak-tovalley height Rz, can be calculated using the angle c p l :

R, = max(q,r2)-q.coscpl (3) Thus, a quantitative evaluation of the allowed radial deviation is possible. Even a slight eccentricity undermines an improvement of surface quality normally obtainable using multiple cutting edges (fig. 2). The cutting edge with the largest radius determines the surface quality and a milling cutter for high surface qualities has either only one edge or has to be used at infeeds f similar to a single edged cutter.

10

kinematic roughness Rz [ pm ]

*010 w mm

one cuttind edge .-....... I--.-

0 Fig. 2:

2

4

two cutting edges

6 8 1 0 radial deviation [ pm ]

Influence of the radial deviation on surface quality

(1) Achievable Feed Rates The restrictions mentioned above concerning the number of cutting edges as well as the realizable infeeds result in reactions to the efficiency of the process. Under the premise that goals have been set for a technologically optimized cutting speed as well as a pre-determined surface quality (e.9. Rz = 1pm), infeeds and rotational speed of the cutter have to be adjusted so that these values are maintained. As can be seen in fig. 3, the infeed travel per tool rotation along the contour is dependent upon the diameters of the tool and those of the workpiece. It can be calculated by means of the angle cp: 2 2 - d ~ ~ + ( d w + 2 . R , ) +(dw+dT) coscp = (4) 2.(dw+2.R2).(dw+d~)

Fig. 1:

Formation of kinematic uneveness of a dual-edged cutter with radial deviation

Annals of the ClRP Vol. 43/1/1994

93

with d w being the workpiece diameter and dT the tool diameter. Rate of feed is calculated by implementing cutting speed combined with the speed of the tool nT:

In fig. 5 the correlation for different numbers of cutting edges is graphically depicted. Given the diameters of workpiece and tool the number of cutting edges should be chosen in a way that the feed per revolution is optimal. tool diameter [ mm 1

T

i

0

20 40 60 80 1000 20 40 60 80 100 workpiece diameter [ mm ]

..

boundaw conditms kinematic roughness Rz = 1 pm * surface only generated by the edge with the largest radius fig. 3:

Meshing conditions during peripheral milling of curved workpiece contours

Dependent on the given values, fig. 4 shows the achievable feed rates vf at the machining diameter. Referring to eq. 6 y is expected to be reciprocally proportional to the tool diameter, because the number of revolutions per minute of the cutter decreases with its increasing diameter. This effect can only moderately be compensated. When the flight path of the edges shows a lower curvature larger feeds at the contour are allowed. Angle 2(p, travelled per cutting action, cannot be increased at will because of the workpiece's curve. There is an upper limit of 2qmax which results from the tangents on the workpiece's contour and characterizes the meshing conditions by an infinitly large tool diameter.

feed rate [ mm/min ]

3 000

fig. 5:

optimal tool life

a

Influence of the number of cutting edges on the choice of tool's diameter

Chip Formation During Turn-milling of Tempered Materials Turn-milling hardened steel produces a saw-tooth like chip as already described by several authors [3 - 71 (fig. 6). The chips mainly consist of the non-deformed basic material with a tempered, martensitic structure and the single lamellas connected with each other by a zone of tetragonal, quench hardened martensite which is newly developed during chip formation. The material piles up in front of the cutting edge until the increasing stresses result in a crack which spreads from the workpiece surface in the direction of the cutting edge. Nearer to the cutting edge the increasing pressure tensions reach a level which prevents the material from further cracking. The material volume which lies between the cutting plane and the crack is pushed over the cutting plane as well as the crack plane. The material directly in front of the cutting edge is plastified and transformed by the high pressure tensions and the high temperatures.

2 000

1 000

0 0

20

40

60

80

100

tool diameter [ mm ] fig. 4:

Achievable feed rates at a prescribed surface quality

Number of Cutting Edges and Tool Diameter In order to achieve the feed rates which are shown in fig. 4, tools with multiple cutting edges must be utilized. Here the cutting work is done by all cutting edges while the surface geometry is generated only by the cutting edge with the largest radius. A separation of tasks is achieved: Feed per revolution f which is responsible for the surface roughness and feed per tooth fz for tool wear. The matching tool diameter can be calculated by using eq. 4 and h-values which are optimal to wear:

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f i g 6:

Chip formation during turn-milling of lOOCr6 (62HRC) wifh CBN (vc = 400dmin)

Fig. 7 demonstrates the dependence of the lamella size on the thickness of the chip. The thickness is at maximum in the middle of the chip and chip segments are very coarse. At the ends of the chip the thickness as well as the size of the lamellas decreases. With decreasing thickness no change in chip formation occures as described in literature [5, 61. Even thin turn-milling chips with maximum thickness of 10 15 pm show no flowing chip formation, they remain segmented.

-

)ol life travel [ m ] m

2 HRC

klQl

sin le edged c d e rA1203 + TIC a = 10". Y = -10" rE = 0.8'mm chamfer O,2x2O0

iae%=%&.E 0.1 mm

ap= 5mm fz= 0.1 mm upcut milling no coolant Fig. 7:

Chip lamellas and depth of cut

cutting speed vc [ mlmin ] Influence of the cuffing speed on tool life quantity while cuffing 1OOCr6 with mixed ceramics

Fig 9:

Influence of the Feed per Tooth on Tool Life Travel Low feeds per tooth cause an increased friction and result in longer cutting paths at the same feed path. Higher feeds lead to higher mechanical stresses on the cutting edge due to increasing chip sections (fig. 10). So a clear maximum of tool life travel between 0.1 and 0.15 mm is observed for the feed per tooth. Fig. 8:

Chip formation during machining lOOC'r6 with mixed ceramics (fz = 0,05mm) .a

tool life travel If [ m ]

The high friction during chip formation heats up the chips. At small thicknesses they melt and form small balls which have diameters of some tenths of a millimeter (fig. 8). An enlargement of the depth of cut e.g. by increasing the feed per tooth causes this kind of chip to disappear. Furthermore this melting of the chips is influenced by the cutting edge geometry as well as the cutting material. The ball-like chip shape results more often with cutting edges of mixed ceramics with a protective chamfer than with sharp-edged tools made of CBN. But even then a dull edge leads to a melting of the chips.

100Cr6,62 HRC

chamfer 0.2 x 20'

Influence of the Cutting Speed According to the method implemented the workpiece is milled along a spiral where the width of contact is equal to the lead ap. The feed path is equivalent to the length of this path curve and calculates as:

i

+VBO.IO~~ VB =width of VB 0.15mm wear land

+

1

I

0

0.05

I

1

0.1

0.15

0.2

feed per tooth ti [ mm ] with lax being the length of the workpiece. If economic tool liie travels are to be achieved the cutting speed has to be chosen in a relatively tight range. When cutting 100C16 with mixed ceramics the range is between 350 and 400 mlmin (fig. 9). Similar dependencies between tool wear and cutting speed can be observed for most of the other cutting materials. This dependency of tool life travel on the cutting speed is typical to the hard-cutting process and can be explained by the operative mechanism of self-induced hot-cutting. When using higher speeds there is less time available to transport energy created by the chip formation via heat transport over the tool's cutting edges. The cutting zone is more heated intensely and with increasing cutting speed there is a softening of the material with positive effects on tool life travel.

Fig. 10: Influence of the feed per tooth on tool life travel Influence of the Cutting Process on the Workpiece The strong temperature development in the transformation zone can heat the chips up to their melting temperature. Workpiece and tool however remain relatively untouched, i.e. both remain cold and warm up only a little. Therefore the thermal strain at the workpiece's boundary layer remains relatively low which is a positive effect for their further application especially where tempered components are concerned [5. 7, 81. The relatively small oversize present during high-precision machining results in small nominal pressure angles of the tool. The short heating up of tool and workpiece during the actual cutting process is followed by a long phase of cooling down and the maximum temperature is cooler than the

95

maximum temperatures for continuous cutting [9]. Hence the cutting speeds used for turn-milling necessarily have to be higher than those for the hard-turning process.

Rz [ c l m l averaged peak-to-valley-heighl

l!mb!e=

100Cr6,62 HRC

tsel

single-edged cutter CBN Q = 10". Y = 0" ra = 1.2'mm a Darameters ae = 0.1 mm ap= 2.5 mm fz = 0.1 mm upcut milling no coolant

fig. I I : Material structure at different cutting speeds while machining 16MnCr5 with CBN

.

0

I

,

200 400 600 800 1 0 0 0 feed path If [ mm ]

fig. 14: Development of surface roughness while cutting IOOCr6 (62 HRC) with CBN

fig. 12: Surface with burnt grain boundaries resulting from machining 16 MnCr5 with excessive cutting speeds (CBN, vc = 450 m/min)

Fig. 11 shows the boundary layer of a workpiece out of 16MnCr5 which was hardened to 62HRC after case hardening. The boundary layer shows no disadvantageous influence by the cutting process. The martensitic structure spreads to the edge and new hardening zones cannot be seen. If cutting speeds are excessive, there is the danger of burning the grain boundaries in the boundary layer and the development of hardening cracks (fig. 12). Machining with Optimized Parameters Best machining results can be achieved only by using polycristalline cubic' boron nitride. Fig. 13 shows the characteristic progress of wear. After a slow increase at the beginning and a long phase of relative constant width of wear land a rapid growth of wear occurs at the end of tool life travel. The course of the averaged peak-to-valley-height RZ mainly follows the ascent of the width of wear land (fig. 14).

Bibliography A New [ I ] Schulz H. (1990) High Speed Turn-Milling Precision Manufacturing Technology for the Machining of Rotationally Symmetrical Workpieces, Aruds of ClRP 107-109 [2] Daniel A. (1991) Koaxiales Drehfrasen 6, rotationssymmetrischer Prazisionsbauteile, ' Darmstaskr . . Fertiaunastechnisches Svmposuro Hohe Geschwodgkeit und Prazision beim Zerspanen" [3] Berktold A. (1992) Drehraumen geharteter Stahlwerkstoffe, Dissertation RWTH Aachm [4] Shaw, M.C. and Vyas A. (1993) Chip Formation in the Machining of Hardened Steel, of CIRP. 4211, 2933 [5] Kdnig W., Klinger M. and R. Link (1990) Machining Hard Materials with Geometrically Defined Cutting Edges Field of Applications and Limitations, Annals of CIRP, 61-64 [6] Ackerschott G. (1989) Grundlagen der Zerspanung einsatzgeharteter Stahle mit geometrisch bestimmter Schneide, Dissertation RWTH A a h [7] Konig W., Berktold A. and Koch K.-F. (1993) Turning versus Grinding - A Comparison of Surface Integrity Aspects and Attainable Accuracies, Banals of CIRP. 4211, 39-43 mit [8] Kallabis M. (1991) Raumen geharteter . Werkstoffe .. kristallinen Hartstoffen, P [9] Palmai Z. (1987) Cutting Temperature in Intermittent Cutting, I n t . c h . Tools Wet. 2712,261-274

-

m,.

-

width of wear land [ mm ]

m,

100Cr6,62 HRC

-0 200 400 600 800 1 0 0 0 feed path if [ m ] fig. 13: Wear progress while machining 1OOCr6 (62 HRC) with CBN

96

Summary Turn-milling with parallel axes is an economic alternative to precision-machining of hard materials by turning, especially for the production of workpieces where certain limitations for turning exist. The main advantage of turn-milling over grinding is that coolant is not necessary. Surface quality of the workpieces can be compared with those achieved by grinding. Extreme qualities of Rz< 1,5 pm cmnot be achieved due to the structure of the cutting materials. At Rz values below 2 pm tool life travel of approx 500 m can be achieved if conditions are optimized. This is equivalent to a cutting time of approx. 10 h. The attainable time-cuttingvolumes are in the range of the turning of hard materials.