Material removed from a workpiece with the help of a tool are known as chips. Chips are formed by shearing. Machining of material is highly dependent on chips.
Chips are formed during the machining process. As the cutting tool engages the workpiece, the material ahead of tool is sheared and deformed under tremendous pressure. The deformed material then seeks to relieve its stressed conditon by fracturing and flowing above the tool in the form of chip and hence the chips are formed.
Machining can be a finishing or semi-finishing process done by removal of material from the workpiece. The excess material is removed from the workpiece in the form of chips.
Some chips are also made for buiseness purpose. Coiled SS chips are sold for scrubbing and cleaning household vessels.
Types of chip removed during a machining process depends on:
1) Material used as workpiece.
2) Material used in cutting tool.
3) Dimension of Tool.
4) Speed of cutting.
5) Feed Rate.
6) Cutting environment like temperature.
7) Friction between tool and workpiece.
8) Forces involved in metal cutting.
The knowlwedge of chip type and when they are produced are very essential because it tells a lot about the condition of machining.
Mainly chips are of three types :-
1) Discontinuous chips.
2) Continuous chips.
3) Continuous Chips with built up edges (or BUE chips)
These three types are elaborated below:
1) Discontinuous Chips:-
If the chips during machining process is not continuous i.e. formed with breakage are called discontinuous chips.
Discontinuous chips are formed when brittle or hard metals like brass, bronze and cast iron are used as workpiece in the machining process.
Discontinuous chips are also formed in ductile material when the friction between tool and workpiece is high. Discontinuous chips is not a good sign for machining of ductile material as it gives poor surface finish and machining process becomes slow.
Discontinuous chips are formed in the following conditions:-
i) Material of workpiece: When the workpiece is made of brittle material such as cast iron and bronze.
ii) Bevels: Small bevels of cut tools.
iii) Cutting Speed: Cutting speed is low.
iv) Feed: When the feed is large and chip is thick.
v) Rake Angle: Small rake angle.
vi) Friction:High friction forces at the chip tool interface.
vii) Depth of cut: Large value of depth of cut.
2) Continuous Chips:-
Continuous chips are the chips formed during machining without breakage or without segments.
These chips are formed by the continuous plastic deformation of metal without fracture in front of the cutting edge.
Continous Chips are mainly formed during cutting of ductile material like mild steel, aluminium and copper.
Factors responsible for formation of continuous chips:-
i) Material of Workpiece: When ductile material is used as the workpiece.
ii) Cutting Speed: Cutting speed is high.
iii) Chip thickness: Thickness of chip is less.
iv) Rake Angle: Rake angle is large.
v) Depth of cut: Depth of cut should be small.
vi) Lubricants and Coolants: Lubricants and coolants should be used efficiently.
vii) Coefficient of friction: Coefficient of material of tool should be low.
vii) Friction: Friction of chip on tool face should be minimum. This condition can be achieved by:
a) Polishing tool face.
b) Use of efficient cutting lubricants.
c) Using tool made of low coefficient of friction.
3) Contnuous Chip With Built Up Edge :-
This type of chip is similar to continuous chip and it has a built up edge adjacent to the tool face and also is not as smooth as continuous edge.
Actually, Built Up Edge (BUE) is an accumulation of material against the rake face, that siezes to the tool tip, seperating it from the chip.
BUE is form during machining of ductile metal when excessive friction exists between tool and workpiece.
Formation of Built Up Edge:
Built Up Edges are formed mainly due to friction between the tool and interface of chip. Because of friction between the tool and the chip, very intense heat is genrated near the nose of the tool. The compressed metal near the nose of the tool get welded due to high temperature near the nose. This compressed metal near welded to the nose is called built up edge.
The chip flows through this built up edge and gets broken and get carried away by the chip and are called as built up edge chip.
Rest of the built up edge stick to the surface of the workpiece and make it rough.
As built up edge is formed nearr nose of tool, rake angle of the tool changes and so the cutting force also changes.
Bulit Up Edge are formed in the following conditions:-
i) Wokpiece Material: Metal used as workpiece is ductile.
ii) Friction: High friction exists near the face of the tool.
iii) Coolant: Insufficient coolant and lubricant.
iv) Temperature: High temperature near the tool face while cutting.
v) Feed Rate: High value of feed rate.
vi) Cutting Speed: Low cutting speed.
vii) Rake Angle: Very small rake angle.
Different factors that causes Discontinuous Chips, Continuous Chips and Continous Chips with Built Up Edge are sum up below:
1) Material Types:
|Discontinuous Chips||Brittle and ductile but hard materials used as workpiece.|
|Continuous Chips||Ductile materials used as workpiece.|
|Continuous Chips with BUE||Ductile materials used as workpiece|
2) Cutting Speed:
|Discontinuous Chips||Cutting Speed is high or medium.|
|Continuous Chips||Cutting Speed is high.|
|Continuous Chips with BUE||Cutting Speed is low or medium.|
3) Depth of Cut:
|Discontinuous Chips||Depth of cut is high.|
|Continuous Chips||Depth of cut is small.|
|Continuous Chips with BUE||Depth of cut is Medium.|
4) Rake Angle:
|Discontinuous Chips||Rake angle is small.|
|Continuous Chips||Rake angle is Large.|
|Continuous Chips with BUE||Rake angle is Small.|
5) Feed Rate:
|Disontinuous Chips||Low Feed rate.|
|Continuous Chips||Medium Feed Rate.|
|Continuous Chips with BUE||Excessive Feed rate.|
6) Friction near Tool Face:
|Disontinuous Chips||Very high friction between tool and workpiece.|
|Continuous Chips||High friction between tool and workpiece.|
|Continuous Chips with BUE||Low friction between tool and workpiece.|