On the Influence of the Medium on the Cutting Process
Abstract Generated abstract
This paper examines whether gaseous media influence chip formation during metal cutting at speeds typical for carbide tools. The authors describe a lathe apparatus that permits cutting tests in an argon atmosphere and compare results with cutting in air for Armco iron and copper, using chip shrinkage as the main indicator. The experiments show no practical effect of argon versus air at high cutting speeds when built-up edge is absent, apart from the absence of temper colors on chips in argon. The authors argue that short contact times and dense chip-tool contact limit gas interaction, and suggest that earlier contrary findings may reflect conditions involving intensive built-up-edge formation.
Full Text
On the Influence of the Environment on the Cutting Process
M. F. Poletika, V. N. Osipov
The question of the mechanism by which the environment influences the process of chip formation is of great theoretical and practical interest. In particular, it is important to know in what the effect of atmospheric and other gases on the cutting process manifests itself and how substantial it is.
Most of the known studies in this field pertain to comparatively low cutting speeds, when the temperature of contact between the chip and the tool is low or equal to room temperature (microspeeds). It is noted that, with an increase in speed and, consequently, in cutting temperature, the effect of the medium on chip formation diminishes and, at relatively low temperatures, disappears altogether [1].
Recent studies carried out in England [2] lead, it would seem, to opposite conclusions. Experiments in vacuum \((10^{-5}\ \text{mm Hg})\), in neutral (nitrogen), and in active (oxygen) media, carried out on medium-hard steel at high speeds, showed that the cutting force both in vacuum and in nitrogen is approximately 50% higher than in air, while the quality of the machined surface is considerably worse. Oxygen has the opposite effect.
In order to determine how substantially a gaseous medium can affect chip formation at high cutting speeds, we designed a special apparatus that makes it possible to carry out experiments on a lathe in a neutral argon medium at speeds normal for the operation of a carbide tool (Fig. 1).
Argon from cylinder 1 is supplied to chamber 2, which is mounted on bearings on the workpiece 3 installed on the lathe. Holder 4 with the cutter can move in a special slot in the wall of the chamber. The appropriate seals ensure in the chamber, after the cover is installed, an excess pressure of about \(0.5\ \text{ata}\).
Armco iron and copper were used as the work materials. The former was machined with a cutter made of T15K6 alloy with the following parameters: \(\gamma = 20^\circ\), \(\alpha = 12^\circ\), \(\varphi = 70^\circ\), \(\lambda = 0^\circ\). In cutting copper, two cutters were used: a carbide one (T15K6) and a high-speed-steel one (R9), with the same geometry: \(\gamma = 10^\circ\), \(\alpha = 12^\circ\), \(\varphi = 70^\circ\), \(\lambda = 0^\circ\).
For the experiments on iron, a range of variation of cutting speed was selected such that, as the latter increased, chip shrinkage decreased and built-up edge was absent. The feed in all experiments remained unchanged and equal to \(S = 0.1\ \text{mm/rev}\); the depth of cut was \(3\ \text{mm}\).
Fig. 1
Legend:
- ● — results of experiments in an air medium
- ○ — results of experiments in an argon medium
Labels in the graph:
- Copper
- Ductile cast iron
Axes:
- \(\xi\)
- \(v\), m/min
Fig. 2
From Fig. 2, on which the results of measuring chip shrinkage are plotted, we see that the effect of the medium is practically absent. The scatter of the experimental points at low cutting speeds, indicating instability of the chip-formation process, is approximately the same both in air and in argon. As the speed is increased, when the cutting process stabilizes, the points corresponding to different media fall exactly on a single curve.
The only difference between the cutting process in a neutral medium and cutting under ordinary conditions that we were able to observe is the absence of temper colors on the chip at all cutting speeds.
Thus, it may be asserted that at high cutting speeds, corresponding to the descending branch of the chip-shrinkage curve, the composition of the gaseous medium has no effect on the cutting process. One reason for this is the excessively short interval of time during which a particle of chip metal remains in the contact zone, insufficient for interaction of this particle with the oxygen of the air. A second and, in our view, more essential reason in this case should be considered the higher density of contact between the chip and the tool than at low cutting speeds.
An analysis of work [2] shows that its authors carried out experiments under conditions in which intensive built-up-edge formation was observed, which explains the different results they obtained. Apparently, during built-up-edge formation, owing to the continuous destruction and renewal of the built-up edge, the “average” density of contact between the chip and the tool decreases, thereby creating more favorable conditions for the manifestation of the effect of the medium.
REFERENCES
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Yu. M. Vinogradov. Influence of technical gases and active cooling-lubricating fluids on friction and surface cleanliness in metal cutting. In: Cooling-Lubricating Fluids. Mashgiz, 1954.
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Influence of the medium on the cutting process. Express Information “Cutting and Cutting Tools,” No. 9, VINITI, Moscow, 1964.