Prudent selection of milling cutters (cutter size and geometry for a given workpiece size and material, the fixture and machine used) results in removal of given volume of metal in the shortest possible time. With poor selection, one ends up with lost production, poor insert life, and excessive costs.
General Considerations:
To enable the inserts to enter a cut at a favourable angle, the cutter diameter should be selected around 1.5 times the workpiece width. The specific cutter geometry is dictated by the workpiece material, its hardness, cross section, tolerances and finish.
Climb milling is recommended for milling with indexable inserts but there should be no backlash in the table feed mechanism as it produces feed surges that can chip the inserts. Proper workpiece support is ensured by solid fixturing which also absorbs clamping and cutting forces. Feeds and speeds should be based on spindle horse power.
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Cutter Geometry:
Inserts can be positioned in cutters relative to both radial and axial planes and further with negative, neutral or positive rake (Refer Fig. 16.49). Horse Power consumption is dependent more on radial rake than axial rake.
Chip formation, direction of chip flow, and forces into or away from the spindle are controlled by axial rake. The combination of radial and axial rakes determines the shear or true rake angle.
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‘A large lead angle produces a thinner chip, reduces cutting pressure and removes metal at faster rate’.
Negative Rake Cutters:
These are ideally suited if machine horsepower is adequate, heavier feeds are required and point of contact at the workpiece requires a strong cutting edge. However workpiece should have sufficient cross section and strength to resist increased cutting forces.
Negative rake cutters are suited for all steels and cast irons but not used for aluminum and plastic. These impose increased cutting forces and entry shock. Their relatively high cutting temperatures may work-harden the surfaces being machined.
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Coarse pitch with maximum chip clearance is used for heavy feed per tooth and maximum depth of cut. Medium pitch is recommended when more inserts are needed to reduce entry shock and cutting pressures while maintaining feed rate. Pine pitch is preferred for high production milling where economy is desired and depth of cut does not exceed 6 mm.
Positive Rake Cutters:
These provide the most efficient cutting action, consume less power, reduce entry shock, and are less likely to deflect thin workpieces. These are used for nonferrous materials, free-machining steels, stainless steels and cast iron.
They perform well with work-hardened materials and are recommended where there is a considerable spindle extension. It is more suited for old machines and where parts are difficult to support. Surface finish is better due to lower pressure and deflection.
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It has weaker cutting edges and these tend to lift the work pieces. Number of cutting edges per insert is limited to four.
Coarse pitch with larger chip gashes is used for high depth of cut and when horsepower is limited. Medium pitch is used for moderate feed per insert and entry shock must be reduced, and more than one insert is necessary to cut. Fine pitch is used when depth of cut is less than 6 mm and edge break out has to be controlled during thin-wall machining.
Shear Angle Cutters:
These cutters have a negative radial rake and positive axial rake. These are best suited for both job shops and high production shops for all steel and cast iron jobs. They have strong cutting edges and their shearing action deflects chips away from surfaces being machined.
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Maximum feed per tooth is possible. Parts can be roughed and finished in a single pass with good finish. Larger lead angles can be used resulting in lower cutting pressure and thin chips.
Coarse pitch is preferred for general purpose milling for deeper cuts if adequate horsepower is available. Medium pitch enables more inserts and thus reduces entry shock and still maintain high feed rate. Fine pitch is used for high production milling of thin section pieces with good finish.