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 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: 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  . This kind of chip breaking is generally observed under the condition close to that which favours formation of jointed or segmented chips.By striking against the cutting surface of the job, mostly under pure orthogonal cutting.By striking against the tool flank after each half to full turn.

The possibility and pattern of self chip-breaking depend upon the work material, tool material and tool geometry, levels of the process parameters and the machining environment (cutting fluid application) which are generally selected keeping in view the overall machinability.

(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.

The basic principle of forced chip breaking is schematically shown in Fig. 7.2 when the strain hardened and brittle running chip strikes the heel, the cantilever chip gets
forcibly bent and then breaks.

some commonly used step type chip breakers:

• Parallel step
• Angular step; positive and negative type
• Parallel step with nose radius – for heavy cuts.

Groove type in-built chip breaker may be of

• Circular groove or

• Tilted Vee groove

 The unique characteristics of in-built chip breakers are:

·         The outer end of the step or groove acts as the heels that forcibly bend and fracture the running chip.

·         Simple in configuration, easy manufacture and inexpensive

·         The geometry of the chip-breaking features are fixed once made (i.e. cannot be controlled)

·         Effective only for fixed range of speed and feed for any given tool-work combination.

(c) clamped type chip-breaker

Clamped type chip breakers work basically in the principle of stepped type chip- breaker but have the provision of varying the width of the step and / or the angle of the heel. Figure 7.5 schematically shows three such chip breakers of common use :

·         With fixed distance and angle of the additional strip – effective only for a limited domain of parametric combination

·         With variable width (W) only – little versatile

·         With variable width (W), height (H) and angle (β) – quite versatile but less rugged and more expensive.