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Chip Breaker

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Published in: IIT JEE Mains
8,541 Views

Use of chip breaker in a lathe 

Sandeep K / Kolkata

3 years of teaching experience

Qualification: M.Tech. (Production Engineering)

Teaches: Chemistry, English, Hindi, Physics, Drawing, Mechanical

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  1. USE OF CHIP BREAKER Use of chip breaker in machining (i) 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. (ii) 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 1
  2. USE OF CHIP BREAKER 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. 7.1 (a). 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, as shown in Fig. 7.1 (b), mostly under pure orthogonal cutting By striking against the tool flank after each half to full tum as indicated in Fig. 7.1 (c). (a) natural (b) striking on job (c) striking at tool flank Fig. Principles of self breaking of chips. The possibility and pattem of self chip-breaking depend upon the work material, tool material and tool geometry (Y, X, (P and r), levels of the process parameters (Vc and so) 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. 2
  3. USE OF CHIP BREAI,GR 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 A after their manufacture — in case of HSS tools like drills, milling cutters, broaches etc and brazed type carbide inserts A 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. W width, H = height, ß = shear angle Fig. Principle of forced chip breaking. Fig. 7.3 schematically shows 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 as schematically shown in Fig. 3
  4. USE OF CHIP BREAKER (a) (c) (b) (d) Fig. Step type in-built chip breaker (a) parallel step (b) parallel and radiused (c) positive angular (d) negative angular (a) circular groove Fig. (b) tilted Vee-groove Groove type in-built chip breaker 4
  5. USE OF CHIP BREAI
  6. USE OF CHIP BREAKER (i) Failure of cutting tools Smooth, safe and economic machining necessitate prevention of premature and catastrophic failure of the cutting tools reduction of rate of wear of tool to prolong its life To accomplish the aforesaid objectives one should first know why and how the cutting tools fail. Cutting tools generally fail by . i) Mechanical breakage due to excessive forces and shocks. Such kind of tool failure is random and catastrophic in nature and hence are extremely detrimental. ii) Quick dulling by plastic deformation due to intensive stresses and temperature. This type of failure also occurs rapidly and are quite detrimental and unwanted. iii) Gradual wear of the cutting tool at its flanks and rake surface. The first two modes of tool failure are very harmful not only for the tool but also for the job and the machine tool. Hence these kinds of tool failure need to be prevented by using suitable tool materials and geometry depending upon the work material and cutting condition. But failure by gradual wear, which is inevitable, cannot be prevented but can be slowed down only to enhance the service life of the tool. The cutting tool is withdrawn immediately after it fails or, if possible, just before it totally fails. For that one must understand that the tool has failed or is going to fail shortly. It is understood or considered that the tool has failed or about to fail by one or more of the following conditions . (a) In R&D laboratories total breakage of the tool or tool tip(s) • massive fracture at the cutting edge(s) • excessive increase in cutting forces and/or vibration • average wear (flank or crater) reaches its specified limit(s) (b) In machining industries excessive (beyond limit) current or power consumption excessive vibration and/or abnormal sound (chatter) total breakage of the tool dimensional deviation beyond tolerance rapid worsening of surface finish adverse chip formation. 6
  7. USE OF CHIP BREAKER (ii) Mechanisms and pattern (geometry) of cutting tool wear For the purpose of controlling tool wear one must understand the mechanisms of wear, that the cutting tool undergoes under different conditions. The common mechanisms of cutting tool wear are . i) Mechanical wear thermally insensitive type; like abrasion, chipping and delamination thermally sensitive type; like adhesion, fracturing, flaking etc. ii) Thermochemical wear • macro-diffusion by mass dissolution • micro-diffusion by atomic migration iii) Chemical wear iv) Galvanic wear various In diffusion wear the material from the tool at its rubbing surfaces, particularly at the rake surface gradually diffuses into the flowing chips either in bulk or atom by atom when the tool material has chemical affinity or solid solubility towards the work material. The rate of such tool wear increases with the increase in temperature at the cutting zone. Diffusion wear becomes predominant when the cutting temperature becomes very high due to high cutting velocity and high strength of the work material. Chemical wear, leading to damages like grooving wear may occur if the tool material is not enough chemically stable against the work material and/or the atmospheric gases. Galvanic wear, based on electrochemical dissolution, seldom occurs when both the work tool materials are electrically conductive, cutting zone temperature is high and the cutting fluid acts as an electrolyte. The usual pattem or geometry of wear of turning and face milling inserts are typically shown in Fig. respectively. sectionA:A flank wear notch principalflank o tch auxiliary flank crater wear rooving wea grooving wear rake surface Fig. Geometry and major features of wear of turning tools 7

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