Chip breakers | Need , Purpose , Principles of chip-breaking
Need and purpose of chip-breaking
Continuous machining like turning of ductile metals, unlike brittle metals like grey cast iron, produce continuous chips, which leads to their handling and disposal problems. The problems become acute when ductile but strong metals like steels are machined at high cutting velocity for high MRR by flat rake face type carbide or ceramic inserts. The sharp edged hot continuous chip that comes out at very high speed:
- Becomes dangerous to the operator and the other people working in the vicinity.
- May impair the finished surface by entangling with the rotating job.
- Creates difficulties in chip disposal.
Therefore it is essentially needed to break such continuous chips into small regular pieces for:
- Safety of the working people.
- Prevention of damage of the product.
- Easy collection and disposal of chips.
Chip breaking is done in proper way also for the additional purpose of improving machinability by reducing the chip-tool contact area, cutting forces and crater wear of the cutting tool.
Principles of chip-breaking
In respect of convenience and safety, closed coil type chips of short length and ‘coma’ shaped broken-to-half turn chips are ideal in machining of ductile metals and alloys at high speed.
The principles and methods of chip breaking are generally classified as follows:
Self chip breaking – This is accomplished without using a separate chip-breaker either as an attachment or an additional geometrical modification of the tool.
Forced chip breaking – This is accomplished by additional tool geometrical features or devices.
a) Self breaking of chips
Ductile chips usually become curled or tend to curl (like clock spring) even in machining by tools with flat rake surface due to unequal speed of flow of the chip at its free and generated (rubbed) surfaces and unequal temperature and cooling rate at those two surfaces. With the increase in cutting velocity and rake angle (positive) the radius of curvature increases, which is more dangerous.
In case of oblique cutting due to presence of inclination angle, restricted cutting effect etc. the curled chips deviate laterally resulting helical coiling of the chips. The curled chips may self break:
- By natural fracturing of the strain hardened outgoing chip after sufficient cooling and spring back as indicated in Fig. 1.27 (a). This kind of chip breaking is generally observed under the condition close to that which favors formation of jointed or segmented chips.
- By striking against the cutting surface of the job, as shown in Fig. 1.27 (b), mostly under pure orthogonal cutting.
- By striking against the tool flank after each half to full turn as indicated in Fig. 1.27 (c).
b) Forced chip-breaking
The hot continuous chip becomes hard and brittle at a distance from its origin due to work hardening and cooling. If the running chip does not become enough curled and work hardened, it may not break. In that case the running chip is forced to bend or closely curl so that it breaks into pieces at regular intervals. Such broken chips are of regular size and shape depending upon the configuration of the chip breaker.
Chip breakers are basically of two types:
- In-built type.
- Clamped or attachment type.
In-built breakers are in the form of step or groove at the rake surface near the cutting edges of the tools. Such chip breakers are provided either:
After their manufacture – in case of HSS tools like drills, milling cutters, broaches etc and brazed type carbide inserts.
During their manufacture by powder metallurgical process – e.g., throw away type inserts of carbides, ceramics and cermets.
Overall effects of chip breaking
- Safety of the operator(s) from the hot, sharp continuous chip flowing out at high speed.
- Convenience of collection and disposal of chips.
- A chance of damage of the finished surface by entangling or rubbing with the chip is eliminated.
- More effective cutting fluid action due to shorter and varying chip tool contact length.
- Chances of harmful vibration due to frequent chip breaking and hitting at the heel or flank of the tool bit.
- More heat and stress concentration near the sharp cutting edge and hence chances of its rapid failure.
- Surface finish may deteriorate.
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