On the Cutting Behavior of High Polymers

I. Some Observations in Polytetrafluoroethylene Cutting

Abstract

The mechanism of chip formation is studied in order to find the optimum cutting condition for high polymers. The orthogonal machining operation was adopted to observe the phenomena of chip formation, and to measure the cutting force and the deformation of work material during cutting process for PTFE under machining conditions; cutting speed: 0.02-762m/min, depth of cut: 0.023-1.0mm, rake angle of tool: -30°-40° having straight cutting edge. We choose as work materials Teflon No. 1 and No. 5 which are du Pont's products.
We found the peculiarities fall within cutting phenomena:
(1) Types of chip: Two types of chip, continuous and discontinuous, appeared in whole range of our experiments (Photo. 1 & 2), and they were influenced by the cutting conditions. Chips formed at high cutting speed were varied from continuous to discontinuous due to decrease of rake angle (Fig. 2), and chips at low speed were always continuous.
(2) Deformation of work materials: The patterns of deformation of work material around the cutting edge were influenced by the rake angle as shown in Photo. 3 and Fig. 3. The amount of deformation were very large as compared with that in the case of metal cutting.
(3) Cutting force required in machining: The larger the rake angle the smaller the cutting force and the more it turns from downward to upward (Fig. 5 & 6). In the lower range of cutting speed, the cutting force decreases with decreasing cutting speed, but in the higher range, it decreases with increasing cutting speed (Fig. 7). These phenomena may be explained by the time and thermal effects on the strength of work material in machining process.
(4) Critical rake angle: The authors named the rake angle which makes the thrust force F_t=0 as “Critical rake angle”, it depends upon the depth of cut as shown in Fig. 8. Using these relations, we may machine under the optimum condition in which the residual strain will be the smallest.
(5) Friction on tool rake face: Friction force between chip and rake face was very small due to the low shear strength of work material, and coefficient of frictions less than 0.2 (Fig. 9). The so-called “shear plane” or “shear zone” is not recognized in these cases.
It should be re-emphasized that the following matters were observed in PTFE cutting.
(1) When the cutting speed is high (few hundred meter per minute), high polymers behaves in the same way as metals, so the work material to be cut can be assumed as a perfectly plastic material.
(2) When the cutting speed is very low (below one meter per minute), the elastic deformation being large, friction force on the rake face is small, and shearing in the so-called “shear plane” can not be observed. Therefore, the mechanism of chip formation in these cases will be considered to be rather tearing and large compressive strain, than shearing.