Evaluation of Processing Factors on Turning of Thermoplastics

P. K. ROY Armument Research and Development Establishment

Poona, Zndia and

S . K. BASU

Central Mechanical Engineering Research Institute Durgapur, Zndia

Experiments were carried out to study the effects of cutting speed, feed, depth-of-cut and tool-nose-radius on the cutting force and surface roughness produced during turning of the thermoplastics Nylon-6 and Teflon. Generalized equations for the cutting force and surface roughness in terms of processing factors have been established using the statistical technique of multiple regression analysis. The equations have been tested for acceptability by the F-test and the degree of significance of various factors has been assessed by the t-test.

INTRODUCTION hermoplastics have attained recognition as remarka-

T b l y versatile engineering materials. The properties of thermoplastics make them suited for rapid produc- tion. processes such as injection molding and extrusion. The machining of thermoplastics (such as turning, drill- ing and millir g) has also aroused much interest and gained imporf mce because of certain advantages over the molding 1 rocess. For example, we may resort to machining wl en production volume is insdflcient to justify an inve ,tment in molds and molding machine or, where design changes are frequent as in the develop- ment stage 01 when parts are to be made with tighter tolerance tha.i that which can be economically met by molding. Moreover, machining operations may be used to supplement molding operations. But the work done in the field of evaluation of optimum machining charac- teristics is not adequate to make definite inferences. Some research has been carried out by Kobayashi (1) and others mainly to find out the characteristic patterns for dependence of the cutting force on parameters such as speed, depth of cut and tool-rake angle. Narayan et al. (2) studied the change in components of cutting force with rake angle in order to find the optimum or critical rake angle for machining of plastics. Degree of sig- nificance and ranking of the machining variables affect- ing the surface finish for machining thermoplastics was assessed by factorial regression analysis by Roy and Basu (3).

This paper deals with the experiments carried out to study the effects of processing factors in turning of thermoplastics. Using the statistical technique of multi- ple regression, generalized equations have been estab- lished for the main cutting force and the surface rough-

ness in terms of various processing variables such as cutting speed, feed, depth of cut and tool-nose-radius in turning of Nylon-6 and Teflon (PTFE). An empirical relationship was also evolved to determine optimum nose-radius that could produce the best quality finish. All derivations were tested for validity by statistical tests and analysis.

EXPERIMENTAL DESIGN The objective is to establish the generalized equations

for evaluating the major component of cutting force and the surface roughness dependent on factors based on the multiple Linear Regression method (4, 5) . It was first assumed that both the main cutting force (or tangential cutting force) and the surface roughness are related to the four factors---cutting speed, feed, depth-of-cut and tool-nose-radius by the following functional form:

y = Kvbl f" d b @ (1)

where y is the yield of any particular combination of the factors, v, f, d and r , i.e., the main cutting force or the surface roughness.

Equation 1 can be written in the linear, first order form :

Y = lo%$ = a + bl XI + b2 X2 + b3 X3 + b4 X4 ( 2 )

where (I = log& and X1, Xz, Xsr and X4 represent the logarithmic transformed factors v , f, d, and r respec- tively. In this changed form, Y is the yieldandx,, X2, X3, and X4 are difTeqent factors.

Since all the factors represent quantititive variables, the yield or response may be expressed as function of the levels of variables and may be written as:

POLYMER ENGINEERING AND SCIENCE, OCTOBER, 1977, Vol. 77, No. 70 75 1

P . K . Roy and S . K . Basu

Table 1. Propertfes of Thermoplastics

Generic name Nylon4 (polyamlde) PTFE (potytetrafluoraethylene)

Trade name Grade description Sample fabrication Specific gravity Melting point Tensile strength Percentage elongation Thermal conductivity Thermal expansion

Garflon Unmodified, extrusion Extruded rods

250°C. 7.7 kglrnm2

5.3 x l o 4 cal/scc.crn"C 12 x per%

1.12-1.14

250-350

Teflon Virgin, unfilled, molding Compression molded rods 2.2 327°C. 3.5 kg/mrn2 350-400 6 x lo4 cal/scc.cm"C 10 x per"C

Y = F(X,j, x,. . . .X,) (3) wherej = 1,2, ------ n represents the nth observations in the experiment andXti represents the ith factor in thejth observation (where i = 1, 2, 3, 4). The function F is called the response function, the knowledge of which gives a complete summary of the results and also enables one to predict the response or yield for values X , that were not tested. Therefore, the relationship between Yj and X t i is of the form (when i = 1, 2, 3, 4):-

Yj = K + b, Xu + b, X , + b3 XQj + b4 X, (4)

EXPERIMENTAL SETUP Standard round bars of the thermoplastics were held

by a three jaw chuck at one end and with a revolving center at the other end on a HMT-LB.20 center lathe. The single-point turning tool was held in the tool clamp of a single component turning dynamometer which was fitted to the tool post. The magnitudes of the tangential cutting force were measured by the dynamometer dur- ing turning under various cutting conditions. At the end of turning, the C.L. A. value (in microns) representing surface roughness of each turned sample was measured. Table 1 shows the average properties and specifications of two thermoplastic materials, namely, Nylon6 and PTFE on which the experiment was conducted.

18-4-1 HSS tool of hardness 760-770 VPN was used in the experiment, tool shape used being 5-5-10-8-12-8-r.

One dimensional mechanical type turning-dyna- mometer capable of measuring cutting force with an accuracy of 21 percent was used. The surface roughness was measured by Rank Taylor Hobson Surtronic in- strument.

The experiments were conducted under the following range of parameters:-

Cutting speed, v(m/min) = 50-150 Feed, f(mm/rev.) = 0.05-0.25 Depth of cut, d(mm) = 0.5-2.5 Tool-nose-radius, r(mm) = 0.1-1.0

Figure 1 gives the general view of the experimental setup.

EXPERIMENTAL OBSERVATIONS AND RESULTS

The values of the main cutting force and the surface roughness respectively under 25 and 31 different com- binations of various levels of the factors (cutting speed, feed, depth of cut and tool-nose-radius) during turning

of Nylon8 and PTFE were observed and recorded. The tool-nose-radius in surface roughness tests was kept at zero, the reason for which will be subsequently ex- plained. A program for multiple regression analysis was made on a Honeywell-400 Computer and these data were used to obtain the set of linear equations and solved by the method of least squares (4). The summary of computer output for determining the generalized equations for the cutting force and the surface roughness are given respectively in Tables 2 and 3.

GENERALIZED EQUATIONS Referring to Table 2, the equations for main cutting

force can be written: 66 .9 dO.9 ,-0.3

P, = f" vo.ol for Nylon-6

10 .78 dO.9 r0.4 P, = v0.03 for PTFE

Similarly, with reference to Table 3, the surface roughness equations can be written:

vo,06 for Nylon-6

H C L A =

Fig. 1 . View of the experimental setup.

POLYMER ENGINEERING AND SCIENCE, OCTOBER, 1977, Vol. 17, No. 10 752

Evaluation of Processing Factors on Turning of Thermoplastics

Table 2. Computer Output for Determining Force Equations Multiple Regression Analysis

Model: Y = a + b, x , + bn x2 + b:, x:, + b, x,

Statistics for Regression Coefficient

Regression For For For For coefficient Nylon PTFE Nylon PTFE

a = bg,K 4.1 824 2.3645 - -

Value of reg. Ca Icu fated coefficient t-va lue

- - K 66 10 bi -0.01 -0.03 -2.36 -2.26 b2 0.9 0.78 81.39 37.97 b:: 0.92 0.91 90.89 43.07 b, 0.31 0.40 54.60 46.59

Analysis of Variance Due to

regression Due to error Total Source of For For For For For For variation Nylon PTFE Nylon PTFE Nylon PTFE

S.S. 9.5113 8.9070 0.0075 0.0195 9.5188 8.9265 d.f. 4 4 20 20 24 24 m.s. 2.3778 2.2267 0.0003 0.0009 - -

F-value (calculated) 6288 2277 - - - - S.S. = sum of sources, d.f. = degree of freedom, ms. = mean square = S.S./d.f. F-value (calculated) = mS' due to regression'

m.s. due to error

TESTIXG FOR ACCEPTABILITY OF EQUATIONS

Referring to analysis of variance for cutting force ex- periment as shown in Table 2 , d.f. due to regression and due to error are 4 and 20 respectively for both N!.lon-6 and PTFE. For 20 & 4 d.f., the tabulated F-value at 1 percent level of significance (iis o1)tainecl from a standard table of F ) is 4.43 which is niuch lower than the cillcu- lilted F-value in both the cases a s obtained from Tuble 2 . Therefore, overall the regression is good and the de- rived equations for cutting force during turning of Sylon-6 and PTFE are valid at 1 percent significance.

BY F-TEST (5)

Similarly, referring to Table 3 , it can be seen that calculated F-values in the case of both the thermoplas- tics are much higher than the tabulated F-value (oh- tained froin a standard tal)le of F ) . Hence, the generalized equations derived for surface roughness in turning of Nylon-6 and PTFE are acceptalde within the range of the experiment.

TESTING FOR SIGNIFICANCE OF THE FACTORS BY t-TEST (5)

From the results of the cutting force test, it is seen that d.f. due to error is 20 for which the t-value ut 0.01 level of significance is 2.845 (:is obtained from standard table of t ) . The calculated t-value for regressioii coefficients b,, b:$, and b, in the cases of both Sylon-6 and PTFE a s seen from Tuble 2 are much higher than the tabulated t-value of 2.845, but the calculated t-value of coefficient 12 , is lower. Therefore, the coefficient b I is iiot significant whereas the Coefficients b,, h:{, 12, ai-e highl!. significant at 1 percent level. Hence we can infer that the effects of the independent variables 0 1 - factors X2, S:! and X4 are significant on the dependent variulde 1'. In other words, tangential or main cutting force ( P , ) is little affected IIJV cutting speed ( c ) , hut it is significantly de- pendent on feed w), depth of cut (d) and tool-nose- radius ( r ) .

Similarly, from the results of the surface roughness experiment, it is seen that the t value for error d.f. at 0.01 level of significance is 2.771 (obtained froin the standard table oft) which does not exceed the calculated t-value of the coefficient b, but does exceed the calcu- lated t-value for b , and b, with reference to Tuble 3 . Hence, we infer that only the effect ofX, is significant. In other words, only the feed factor has a significant effect on surface roughness and the other two iiiachining variables, cutting speed and depth-of-cut have no sig- nificant effect on the surface roughness.

Table 3. Computer Output for Determining Surface Roughness Equations Multiple Regression Analysis

Model: Y = a + b, x, + by xp + b:, x:: ~~~~~ ~

Statistics for Regression Coefficient

Regression For For For For coefficient Nylon PTFE Nylon PTFE

Value of reg. coefficient Calculated t-value

a = log,,K 2.7789 3.9353 - - - - K 16 94

bi -0.06 -0.139 0.95 -2.97 b2 0.66 0.97 14.208 22.771 b:: 0.02 0.04 0.739 1.620

Analysis of Variance Due to regression Due to error Total

For For For For For For Source of Variation Nylon PTFE Nylon PTFE Nylon PTFE

~ ~~ ~ ~~

S.S. 3.3763 5.9590 0.4082 0.2757 3.784 6.2347 d.f. 3 3 27 27 30 30 ms. 1.1 254 1.9863 0.0151 0.01 02

F-Value (Calculated) 74.441 194.49 - - - - - -

S.S. - Sum of sources, d.f. = degree 01 freedom, m.s. - mean square = S.S./d.f. F-value . m.8. due to regression

m.8. due to error '

POLYMER ENGINEERING AND SCIENCE, OCTOBER, 1977, Vol. 17, No. 10 753

P . K . Roy and S. K . Basu

Figures 2(u) to 2(d) and Figs. 3 (a) to 3(c) show the individual effects of the various factors on the cutting force and the surface roughness respectively.

EFFECT OF TOOL-NOSE-RADIUS ON SURFACE ROUGHNESS

Since the feed proves to be the iiiost significant factor affecting the surface roughness, investigatioiis were made to observe its influence at various feed rates. Figures 4(u) and 4 ( h ) show the effect of tool-nose-radius on the roughness of the turned surfaces of Nylon-6 and

I I I I I I I 1 M 40 M 80 100 Ru W J

CUTTING SPEEDtV<m/min.)-

Fig. 2a. Main cutting force as a function of cutting speed during turning of dry h'ylon-6 and PTFE with H.S.S. tool, tool shape being 5-5-1 0-8-12-8-0.

0 NYLON-6 X P B

f I 1 I I

005 0 . 1 PI5 02 025

FEED, f (mm/rrv.l-

Fig. 2b. Relationship betaeen main cutting force and feed dur- ing turning of Nylon-6 and PTFE in dry condition using H.S.S. tool hocing a shupe of 5-5-10-8-12-8-0, Cutting speed used was 140 mlniin while depth of cut remuined at 1 mm.

1 Q -

6 -

4 -

1 1 I I 0.5 HJ n M t'5 39

Q@'TH OF CUT. d hm)-

Fig. 2c. Relationship between main cutting force and depth of cut f o r Nylon-6 and PTFE turned at a cutting speed of 145 mlmin and feed of 0.1 13 mmlrea. with a H.S.S. tool of the shape 5-5-10-8-12-8-0 without coolant.

1 I I I I

02 04 06 cm Kl TOOL NOSE FtADIUS,rcmm)-

Fig. 2d. Dependence of cutting force on tool-nose-radius during turning of Nylon-6 and PTFE at a cutting speed of 125 mlmin.

754 POLYMER ENGlNEERiNG AND SCIENCE, OCTOBER, 1977, Vol. 17, No. 10

Evaluation of Processing Factors on Turning of Thermoplastics

4t

I 6

0 1 I I I I I

40 60 I20 I 6 0 Z W 240

CUTTING SPEED, V (m/min)----t

Fig. 3a. Effect of cutting speed on surface roughness f o r turning Nylon-6 and PTFE at a feed of 0.15 mmlreo. and depth of cut of 1 mm. A n H.S.S. tool of the shape 5-510-8-12-8-0.

Fig. 3b. Effect of feed on .surface roughness on turning Nylon-6 und PTFE ut a speed of 140 mlmin and depth o f c u t of 1 m m using H.S .S . tool of5-5-10-8-12-8-0 shape.

PTFE ut different feed rates from which the optimum tool-nose-radius at dflerent feeds are ol)tained. From the surface roughness criterion, the nose radius of the tool for which the roughness 1)ecomes minimum is the optimum value of the tool-nose-radius. The optimum tool-nose-radius (r,),,,-) as a function of feed c f ) for differ- ent thermoplastics is shown in Fig. 4(c), which gives a

‘t c *t ‘t

L 1 I 1 I I 0 0 5 I.0 1.5 20 *5

DEPTH OF CUT, d Cmm)-

Fig. 3c. Relationship between surface roughness and depth of cut in turning Nylon-6 and PTFE at 135 mimin speed and 0.15 mmlreo. Feed with H.S.S. tool of 5-510-8-12-8-0 shape.

1 I 1 I I

OL 020 040 060 o a 1.0

TOOL NOSE RADIVS,r(mmn)-

Fig. 4a. lnjluence of tool-nose-radius und feed on surface roughness when Nylon-6 is turned keeping cutting speed and depth of cut constant at 150 mlmin and 1 m m respecticel!/. H.S.S. tools with oarying nose-radius.

linear relationship on doul,le log coortliiiates a s fi)llows:

r,,),t = 0.8 f”.” for Nylon-6 r,,),, = 0.8 f’.””5 fbr PTFE

CONCLUSIONS Our euperiments with machining of thermoplastics

show that the t1iermopl;istics such as S!.loii-6 and PTFE exhihit good macliiiial)ilit!. ~ i n d a reasoii;hl!. good sur- f k e finish over ;I wide range of cutting conditions.

POLYMER ENGINEERING AND SCIENCE, OCTOBER, 1977, Vol. 17, No. 10 755

P . K . Roy and S. K . Basu

W

I 0 .- E

< 0

U

I

w W

!!

follows:

Fig. 4b. ln juence of tool-nose-radius and feed on surface roughness when PTFE turned at speed of 150 mlmin at a depth of cut of 1 mm. Tool material was H . S . S .

*2t

ONYUIN. SLOF€=QOBS WTFE, SLOPE 012

.I I I I I I 1 I I . 1 I 1 I 1

001 a02 -03 -05 -10 45 .P 30 f Cmm /rev.> ___c

Fig. 4c. Optimum nose-radius of tool as u function of feed on turning Nylon-6 and PTFE.

From the assessment of the degree of significance of the various fktors affecting machining behavior, the following inferences are made:

Of the three principal machining variables, the feed is the most significant in affecting the surface finish of the turned thermoplastic. The effects of the other two variables, namely, the cutting speed and the depth of cut are insignificant so far its surface finish is concerned.

The cutting force developed during turning of therinoplastics is influenced primarily Ly the depth of cut. Next in order of importance, coiiie feed, nose- radius of the tool and cutting speed. The cutting speed has no significant influence.

In view of the above, the insignificant factors iii the equations for surface roughness and cuttiiig force m a y be neglected and the modified equations may lie written as

P, = C,f" db rc HCL.4 = K f X Materials C, a 1) C K x

Nylon 66 0.9 0.9 0.3 16 0.66 PTFE 10 0.7 0.9 0.4 49 0.9i

I t is evident froin the above equations that within the experimental range, the cutting force requirement (i.e., power requirement) is more in turning Nylon-6 than in PTFE, but the roughness produced is less on the turned Nylon-6 surface than that on the turned PTFE surface. This may be due to the hardness, ultimate tensile and shear strength of Nylon-6 being inore than that of PTFE.

I t has been observed that the surface finish improves with the increase in tool-nose-radius up to a limit be- yond which the surface finish deteriorates with further increase in the nose-radius. This is because, with the increase in tool-nose-radius from zero, the traces left by the cutting edges of the tool on the workpiece become smoother and smoother leaving less and less ridges or microirregularities, but beyond a limit for the radius at the tool-nose, a greater degree of deformation of the material and higher tool forces are required. This results in chatter and vibration which increase roughness. Thus there exists an optimum value for the tool-nose-radius which changes with the change in cutting conditions. The empirical relationship connecting the optimum nose radius of the tool with the feed rate for producing best quality surface finish has been established to be:

The value ofC and exponent 8 vary from 0.80 to 0.85 and 0.085 to 0.12 respectively and under any cutting condi- tion within the specified range of the experiment the values of optimum tool-nose-radius lie betwewi 0.5nim and 0.71nni for producing the best surface finish for turning Nylon-6 and PTFE.

In general, turning of therinoplastics does not differ greatly froin the turning of metals. Thus standard metal working tools with soiiie modifications in tool geometry can be used. The techniques, however, are soiiiewhat different from those employed for metals. Once the cutting force and surface roughness are evaluated under various conditions of machining, optimum processing parameters for economic machining can easily be found to attain close tolerance niachined parts of good surface finish.

ACKNOWLEDGMENT

\t7e express our sincere gratitude to Dr. Patwardhan, Director, Armament Research and Development Estah- lishment, Poona for his continuing help and encourage- ment in carrying out this work. We are also thankful to Shri S. W. Brosekar, Shri B. S . Jadhav aiid Shri S. S . hlupid of Arde, Poona aiid Shri P. K. Slukherjee of Merado, Poona for their assistance aiid cooperation.

756 POLYMER ENGINEERING AND SCIENCE, OCTOBER, 1977, Vol. 17, No. 10

Evaluation of Processing Factors on Turning of Thermoplastics

NOMENCLATURE REFERENCES 0 = cutting speed (idmin)

d r = tool-nose-radius (inm) H C L A

p , Toid =optimum nose radius (inm) K = constant b,, bz, b3, b, =exponents

= feed (mm/rev) = depth of cut (mm)

= surface roughness (CLA microns) = tangential or main cutting force (kg)

f

POLYMER ENGINEERING AND SCIENCE, OCTOBER, 1977, Vol. 17, No. 10

1. A. Kobayashi, “Machining of Plastics,” McGraw-Hill Book Co., New York (1967).

2. N. Riarayan, S. K. Basu, and D. K. Pal, “Role ofcritical Rake Angle in Machining of Plastics,” Third AIMTDR Confer- ence, IIT, Madras (1970).

3. P. K. Roy and S. K. Basu, Mech. Eng. Bull., 5, No. 3 (1974). 4. W. G. Cochran and G. M. Cox, “Experimental Design,” Asia

5. C. W. Lowe, “Industrial Statistics,” Vol. 1, Business Books Publishing House, Indian Edition (1962).

Ltd., London (1968).

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