Analysis of Methods for Obtaining a Fixed Chip Formation Zone
Abstract Generated abstract
This paper analyzes existing methods for obtaining a fixed chip-formation zone in metal cutting experiments, distinguishing approaches based on withdrawal or destruction of the tool-holder supporting element. It identifies practical and methodological limitations of prior spring-actuated and explosive devices, including insufficient interruption speed, manual control requirements, limited rigidity, damage to the preserved zone, and constraints at high cutting speeds. The authors propose a stop-actuated mechanism in which impact removes the support, continued machine motion separates the tool from the chip, and a latch protects the fixed zone. Experiments on difficult-to-machine materials, including high-manganese steel, showed close correspondence between tool sections and fixed chip-formation-zone sections, with applicability claimed for larger cut sections and high cutting speeds.
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ANALYSIS OF METHODS FOR OBTAINING A FIXED CHIP-FORMATION ZONE
G. L. Kufarev, V. I. Livshits
(Presented by the joint scientific seminar of the departments of machine tools and metal cutting and of mechanical-engineering technology)
In the practice of research on the process of metal cutting, a number of methods are known for obtaining a fixed chip-formation zone, which can be divided into two groups according to the difference in their effect on the supporting element of the tool-holder structure:
a) withdrawal of the supporting element;
b) destruction of the supporting element.
As an example of a method of group “a,” one may cite the method described by T. N. Loladze [1], which consists in the fact that, in turning, termination of the cutting process is produced under the action of the compressive force of a spring, as well as the cutting force, which causes the rotation of a special device carrying the cutter (the cradle) about a horizontal axis downward.
During the cutting process, this rotation is prevented by a locking pin inserted into a special hole in the cradle. In order to actuate the spring, the locking pin is knocked out of the hole in the cradle by a hammer blow.
Experience in the use of devices based on this method has shown that such devices are in fact unsuitable for operation because the edge of the locking pin is crushed very rapidly. In addition, the method under consideration for obtaining a fixed chip-formation zone has, in principle, a number of disadvantages:
1) Insufficiently rapid termination of the cutting process.
2) The impossibility of regulating the rate of the external action that terminates the cutting process as a function of the cutting speed.
3) Immediate separation of the chip-formation zone and the cutting tool after termination of the process is not ensured, which causes additional deformations of the zone and distorts the picture of the process.
4) It is difficult to ensure the preservation of the chip-formation zone obtained, that is, to avoid damaging it through rotation of the workpiece or impacts of the chip against the carriage or the tool.
5) It is difficult to obtain a fixed chip-formation zone at a specified location of the workpiece.
6) In non-free cutting, in order to obtain the result the researcher has to perform the following three control actions in immediate succession: deliver a blow to interrupt the process; stop the longitudinal motion of the carriage; and retract the transverse ...
the carriage. At a cutting speed above 80 m/min, it is impossible to carry out these actions manually without damaging the fixed chip-formation zone.
7) Owing to the specific features of the method under consideration, the devices for carrying it out are insufficiently rigid, which does not allow them to be used in studying the cutting process of difficult-to-machine materials; and in experiments on relatively easily machined materials, their range of application is limited to cut sections of no more than \(1.5 \text{ mm}^2\).
As an example of a method of group “b,” one may cite the method described by E. D. Salomonovich [2]. Here the supporting element of the tool-holder structure, which is destroyed at the moment the cutting process is stopped, is a cast-iron key, and the source of energy for its destruction is the explosion of a powder charge. The time during which the normally occurring cutting process is disturbed before its cessation is determined by the time required to destroy the key. This method has a higher speed of response than the one described above; however, it also has the shortcomings listed earlier. In particular, there remains the need for rapid manual control of the carriage; obtaining a fixed zone at any point of the workpiece is possible only with additional complication of the method; the devices for carrying out this method are difficult to manufacture and operate. It should be noted that the maximum charge magnitude allowable for safety reasons limits the greatest cutting speed at which methods using an explosion can be applied.
The desire to eliminate these shortcomings led to the creation of a method for fixing the chip-formation zone that ensures the obtaining and preservation of an undistorted chip-formation zone on any materials, including difficult-to-machine ones, at any cutting speeds and any cut sections.
The essence of the method is that cessation of the cutting process is carried out by impact of a stationary stop (Fig. 1) and the moving machine member \(a\), traveling at the cutting speed and carrying the body of tool holder 1; anticipatory separation of the tool surface and the chip is ensured by the trajectory of motion of the cutter, formed as a result of continuation of the motion of the machine member and rotation of cradle 2; and preservation of the chip-formation zone from damage by the moving machine member is ensured by locking the cradle after impact in the horizontal position.
Fig. 1.
In the proposed method, disturbances of the normal course of the cutting process before its cessation are reduced to a minimum, since they are determined only by the elastic deformation of the stationary stop of a very rigid structure.
When the moving machine member meets the stationary stop it does not stop, but, after transmitting to the supporting element of cradle 5 (the key) the action that removes the obstacle to motion, continues...
movement in the same direction. Owing to this movement, the surfaces of the tool and the chip are separated, which precludes additional deformation of the cutting zone and distortion of the picture of the process.
The fixed stop, as it were, protects the fixed cutting zone together with the chip from additional action and from damage by the moving member of the machine tool.
By the proposed method, fixed zones of chip formation were obtained when cutting, on a planing machine, very difficult-to-machine high-manganese steels with a carbide insert of grade VK8. The sequence of operations is as follows: on the table of the machine tool a fixed stop \(d\) and a specimen of the material under investigation \(b\) are fastened. On the hinged board \(a\) of the slide support there is fastened a body 1 carrying a cradle (tool holder) 2.
The cradle is set in the raised (vertical) position by means of a supporting element—a key. Then the cutting insert is fastened in the tool holder. After the slide support has been set to the required thickness of cut, the working stroke of the slide is switched on. This is the only control action in the proposed method that is carried out by the experimenter. All the remaining operations take place automatically. As the machine slide moves forward, the striker pin 6 runs into the stop \(d\), as a result of which the key is cut off and the cutting process is terminated. Under the action of the impact, as well as of the continuing motion of the slide and the tension of spring 4, cradle 2 turns together with the axle in the bearings, separating the chip and the tool; and it is fixed by latch 7 in such a way that it passes over the stop without touching either it or the obtained zone together with the chip. Turning the holder about the axis of rotation above the tool tip makes it possible to carry out rapid separation of the chip and the tool and to protect the cutting edge from breakage.
It should be noted that, in the proposed method, the speed of interruption of the cutting process changes automatically when the speed of the member performing the main motion is changed. The proposed method can be used in planing and milling devices, and also in turning devices with a tool rotating on an arbor and with the specimen fastened in the tool holder.
Experimental verification of the proposed method was carried out on specimens of various difficult-to-machine materials. The carbide cutting insert had a positive rake angle and a chamfer on the cutting edge with dimensions measured in advance.
Quality control of the result of fixing the chip-formation zone was carried out by comparing sections of the tool and of the obtained zone perpendicular to the cutting edge of the tool. Experimental verification showed the high quality of the result, i.e., complete coincidence of the sections of the tool and of the fixed chip-formation zone in experiments on specimens of various difficult-to-machine materials, including high-manganese steel G13L.
In this case the cross section of the cut reaches \(4\ \mathrm{mm}^2\). The proposed method makes it possible to obtain a fixed chip-formation zone at arbitrarily high cutting speeds, since here there are no restrictions that would lead to limitation of the upper limit of the cutting speed.
REFERENCES
- T. N. Loladze. Chip formation in metal cutting. Mashgiz, 1952.
- E. D. Salomonovich. Device for instantaneous interruption of the cutting process at high speeds. Vestnik mashinostroeniya, No. 12, 1954.