Cutting fluid performance in fine grinding

139

Wear, 86 (1983) 139 - 149

CUTTING

FLUID PERFORMANCE

S. CHANDRASEKAR

IN FINE GRINDING

and M. C. SHAW

Department of Mechanical and Aerospace Engineering, College of Engineering Applied Science, Arizona State University, Tempe, AZ 85287 (U.S.A.)

and

(Received November 1, 1982)

Summary

The cluster overcut fly grinding (COFG) technique in which a small patch of grains is used to grind a surface instead of using the entire wheel face was considered as a means for evaluating grinding fluids in terms of reduction in adhesion and wheel wear. The rate of wear was found to be similar for the COFG and complete wheel cases for a variety of grinding combinations provided that the undeformed chip thickness was maintained constant and an adjustment was made for the difference in the amount of material removed per grain in the two cases. At a normal wheel speed (6300 ft min- ’ (32 m-i s)) A1203 was found to give less wear than Sic when grinding AISI-SAE T15 tool steel and an undiluted grinding oil was found to be the most effective of the fluids tested. However, when titanium alloy Ti6Al-4V was ground at normal speed, Sic was found to give the best results using an extreme pressure grinding oil water solution of concentration 10 wt.?&

1. Introduction

In a recent paper, Kumar and Shaw [l] discussed metal transfer and wear of Sic and A1203 abrasives when dry grinding AISI-SAE T15 tool steel and a titanium alloy (Ti-6Al-4V). While A1203 gave less wear when grinding T15 tool steel, Sic gave less when grinding the Ti-6Al-4V alloy. Sic was found to oxidize excessively when grinding T15 tool steel, giving rise to a relatively high attritional wear rate. By contrast, A1203 tended to acquire a layer of built-up metal when grinding both T15 and the Ti-6Al-4V alloy which gave rise to a microchipping mode of wear. While Sic also tended to form a built-up layer and to microchip when grinding the Ti-GAl4V alloy, this action was not as pronounced as for the Al,O,-(Ti-6Al-4V) combination. In no case was there any evidence of a solid state reaction between work and abrasive. The present paper extends the study of ref. 1 to include several types of grinding fluids. 0043-1648/83/0000-00001$03.00

0 Elsevier Sequoia/Printed in The Netherlands

140

In the present study as well as the previous one conducted in dry air , wear rates were determined by the cluster overcut fly grinding (COFG) technique which has been fully described in refs. 2 - 4. This method employs a small cluster of grains measuring i in X $ in (3.2 mm X 3.2 mm) on the dressed wheel surface to surface grind a groove k in (3.2 mm) wide in the workpiece surface. The wheel speed V and a wheel depth d of cut are the same in the COFG test as in the corresponding complete wheel situation. The table speed u, in the COFG test is reduced from the speed vW in the full wheel case in the manner described in ref. 2 so that the undeformed chip thickness t of individual chips remains the same. This is done since practically all grinding results are functions of t and it is desired that all conditions be as identical as possible in the COFG and full wheel cases. As explained in ref. 3, wheel wear should be the same for COFG and comparable (same V, d and t) with complete wheel tests when the length L, of work ground by the complete wheel is related to the length L, of work ground by the cluster as follows: (a) when microchipping wear is dominant, Lw nD -.--=04 Lc w (b) when attritional wear is dominant, [l]

Lw _ L

VW =-..-UC

(lb)

where D is the wheel diameter, w is the circumferential length (i in (3.2 mm)) of the cluster and uc and uw respectively are the table speeds for the cluster and complete wheel tests.

2. Experimental

study

conditions

The nominal chemical compositions of the work materials used in this are given in Table 1. The T15 steel was a sintered material with a

TABLE Nominal

1 composition

Material

T15 tool steel Ti-6Al-4V

(wt.%)

of work materials

Composition C

0

Fe

W

Cr

V

co

Al

Ti

1.57 0.1

0.2

72.04 -

12.6 -

4.0 -

4.9 4.0

4.89 -

6.0

89.7

median carbide size of 0.3 pm and was heat treated to a hardness of 65 HRC. The Ti-6Al-4V alloy was an annealed material of hardness 32 HRC. The grinding wheels used were A60J8V and C6OJ8V. The wheel speed V was

141

chip thickness t = 150 pin 6300 ft min- ’ (32 m s-i) and the undeformed (3.75 pm) in all cases. This corresponded to a wheel depth cut d of 500 pin (12.5 pm) and a table speed IJ = 75 ft min-’ (0.38 m s-l) for the complete wheel. The rate of fluid flow corresponded to 0.5 U.S.gal mini (3.15 X lo-’ m3 s-l). Except as noted elsewhere, the following fluids were used: (a) a 10% concentration of a heavy duty (S + Cl) water-base cutting fluid (TRIM EP) in water; (b) undiluted grinding oil (White and Bagley number 2192); (c) 10 wt.% NaNOz in water; (d) 10 wt.% K3P04 in water buffered with an equimolar concentration of NaH2P04 (after ref. 5). Fluids (c) and (d) are inorganic salt solutions that have been found to be effective in grinding titanium alloys in the past [5 - 81. The undiluted grinding oil was not used on the Ti-GAl-4V alloy because this sometimes constitutes a fire hazard. All combinations of abrasives, work and fluid were tested by COFG and under comparable complete wheel conditions so that the corresponding rates of wear (eqns. (1)) could be compared.

3. Results 3.1. Tl5 tool steel Figures 1 and 2 show the mean cumulative wear per cut (Ad/n) plotted against the number n of cuts for A1,03 and Sic grains respectively when cutting in the COFG mode using different fluids. A1203 is seen to wear less rapidly in all cases and the best fluid of the group when grinding T15 tool steel with A1,03 was the undiluted grinding oil. The best fluid for the Sic-T15 combination was the TRIM EP water-base fluid although these results were not nearly as good as those for the best A1203 combination. Figure 3(a) shows the cumulative wear for several individual A1203 grains when grinding the T15 tool steel using the undiluted grinding oil while Fig. 3(b) shows comparable results when grinding dry in air. The grinding oil is seen to be relatively effective in reducing adhesion and buildup and hence in reducing microchipping wear. Grain 12 of Fig. 3(a) is the only one showing a slight indication toward build-up while grain 12 of Fig. 3(b) shows considerable evidence of build-up in the form of “negative wear”. Figure 4 shows plots of mean cumulative wear Ad against the length L, ground for A1203 and Sic abrasive grinding the T15 tool steel using grinding fluid (a) with complete wheels and for the COFG technique (L, converted to L,). These results are representative of a number of plots all of which show excellent agreement between full wheel and COFG tests when interpreted in terms of eqns. (1). Scanning electron microscopy (SEM) pictures of used wheel surfaces were useful in verifying the presence of metal build-up on abrasive surfaces. For example, Fig. 5(a) shows the A1203 wheel surface after grinding the T15

n

2

i 5 e P IO-E i !5

14 -

n

.I

/‘-., 0

Fig. 2. Variation in the mean cumulative fluids: curves a - e, as for Fig. 1.

wear per cut (Ad/n)

with the number

II of cuts for Sic in COFG

of T15 tool

steel for various

Fig. 1. Variation in the mean cumulative wear per cut (Ad/n) with the number n of cuts for A1203 in COFG of T15 tool steel for various fluids: curve a, air; curve b, 10 wt.% NaN02; curve c, 10 wt.% K,PO,; curve d, 10 wt.% TRIM EP; curve e, undiluted grinding oil.

16 0.4

16-04

167

‘U! 0

0

i

Ii

9 P

0 ti

d‘pv 0

ni

0 -:

9 0

7

9 N I

240-6

200-5 .a

4

6

12

16

20

24

Fig. 4. Variation in the mean cumulative wear Ad with the length Lwground for A1203 and Sic grinding T15 tool steel using 10 wt.% TRIM EP: curve a, full wheel with Sic; curve b, COFG with Sic; curve c, full wheel with Al,Os; curve d, COFG with AlzOs.

-.- _.

(a)

(b)

Fig. 5. SEM pictures of the surface of an Al203 wheel after grinding T15 tool steel using (a) 10 wt.% NaNOz solution and (b) 10 wt.% TRIM EP solution.

c

146

U! ?f ‘PV

147

tool steel using the relatively ineffective NaNOz solution (fluid (c)). Figure 5(b) shows the same grinding combination except for the fluid which was the 10% TRIM EP solution. These photomicrographs clearly show considerably greater adhesion for Fig. 5(a) than for Fig. 5(b). The wear mode in the former case (Fig. 5(a)) was predominantly microchipping while in the latter case (Fig. 5(b)) it was predominantly attritious. 3.2. Ti-6Al-4V alloy Figures 6 and 7 show the mean cumulative wear per cut (Ad/n) plotted against the number n of cuts for A1203 and Sic grains respectively when cutting in the COFG mode using different fluids. Sic is seen to wear less rapidly in all cases and the best fluid for the Sic-(Ti-GAl-4V) system was the TRIM EP solution. The best fluid for the Al,O,-(Ti-6Al-4V) combination was the NaNOz solution. Figure 8 shows the mean cumulative wear Ad for several individual Sic grains when grinding the Ti-6Al-4V alloy using the TRIM EP solution. Considerable negative wear (metal build-up (MBU)) is seen to occur on grain 15 even when the most effective fluid of the group is used. The extent of the MBU was greater for the titanium alloy than for the T15 tool steel, particularly with the less effective fluids. Figure 9 shows representative wear results for COFG and full wheel tests. This is for the system Al,O,-(Ti-6Al-4V)-TRIM EP. Other combinations gave equally good correlation between COFG and full wheel tests. When the Ti-6Al-4V alloy was ground, chips were found to adhere to the wheel when a less effective fluid was used which led to chipping and a high rate of wheel wear. Figure 10 shows SEM photographs showing chips adhering to the wheel face after grinding the Ti-6Al-4V alloy with Sic and a 10 wt.% NaNOz solution. Figure 11 shows the curves of mean cumulative wear per cut (Ad/n) versus number II of cuts for the Al,O,-(Ti-6Al-4V) system when using NaNOz solutions of various concentrations. Results for higher concentra-

(a)

(b)

Fig. 10. SEM pictures at (a) high magnification and (b) low magnification of an Sic wheel after grinding Ti-6Al-4V alloy using 10 wt.% NaNOz solution as the grinding fluid.

148 16 0.4 r

Fig. 11. Variation in the mean cumulative wear per cut (Ad/n) with the number n of cuts for Al203 grinding Ti-6A1-4V alloy in the COFG mode: curve a, 1 wt.% NaNOz; curve b, 5 wt.% NaNOz; curve c, 10 wt.% NaN02.

tions were about the same as for the 10 wt.% concentration. Since the residue left behind on the machine when the fluid evaporates becomes more troublesome as the salt content increases, the 10 wt.% concentration of inorganic salt appears to be about optimum. The same conclusion was reached when using different concentrations of NaN02 with the SiC-(Ti6Al-4V) system.

4. Conclusions (a) The COFG technique gives wear results that are similar to those obtained with a complete wheel when different grinding fluids are evaluated. The COFG technique provides a useful method of evaluating different fluids in less time, use of less material ground and with a smaller sample of the fluid being evaluated.

149

(b) The tendency for chips to adhere to the wheel face causing chipping is essentially the same for COFG and when grinding with a complete wheel. (c) When T15 tool steel is ground at a conventional wheel speed (6300 ft min- ’ (32 m s-l)), A1,03 gave better results than Sic and the undiluted oil was the most effective fluid tested. (d) When Ti-6Al-4V alloy was ground at a conventional wheel speed (6300 ft min-’ (32 m s-l)), the Sic abrasive gave better results than A1203 and the TRIM EP solution was the most effective fluid tested. (e) A 10 wt.% concentration was found to be optimum for all systems tested when NaNOz in water was the grinding fluid.

Acknowledgment The authors wish to acknowledge a grant from the Materials National Science Foundation, that has made this investigation monitor, Dr. L. Toth) possible.

Division, (project

References 1 K. V. Kumar and M. C. Shaw, Metal transfer and wear in fine grinding, Wear, 82 (1982) 257. 2 K. V. Kumar, M. Cozminca, Y. Tanaka and M. C. Shaw, A new method of studying the performance of grinding wheels, J. Eng. Znd., 102 (1980) 80. 3 K. V. Kumar and M. C. Shaw, A new method of characterizing grinding wheels, Ann. CZRP, 28 (1) (1979) 205. 4 K. V. Kumar and M. C. Shaw, The role of wheel-work deflection in grinding operations, J. Eng. Znd., 103 (1981) 73. 5 I. S. Hong, E. J. Duwell, W. J. McDonald and C. E. Mereness, Coated abrasive machining of titanium alloys with inorganic phosphate solutions, ASLE Trans., 14 (1971) 8. 6 L. P. Tarasov, Grindability of tool steels, Trans. Am. Sot. Met., 43 (1951) 1144. 7 M. C. Shaw and C. T. Yang, Inorganic grinding fluids for titanium alloys, Trans. ASME, 78 (1956) 861. 8 I. S. Hong, Use of inorganic phosphate solutions in the centerless grinding of titanium alloys with coated abrasives. In M. C. Shaw (ed.), New Deuelopments in Grinding, Carnegie Press, Carnegie-Mellon University, Pittsburgh, PA, 1972, p. 860.