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Tool material: The tool substrate
Tool substrates of solid and brazed end mills include cemented carbide and high-speed steel. If solid carbide and brazed end mills are used with a low rigidity machine, chipping and breakage can occur and the advantage of using them can not be gained.
Cemented carbide has higher wear resistance than high-speed steel, therefore, solid carbide and brazed end mills are generally used in a machine whose main spindle can rotate at higher speeds. Compared to solid carbide end mills, high-speed steel end mills have higher toughness and are less likely to fracture. Additionally, the tool unit price is lower, but, due to lower cutting speeds, machining efficiency decreases.
The table below shows a guide to choosing a tool substrate.
Tool material: The grade
Secondly, choose a suitable grade according to the workpiece material being machined.
For ferrous workpiece materials such as carbon steel, alloy steel, pre-hardened and hardened steel, choose the cutting tool grade according to its hardness.
If the workpiece material is 40HRC or under, high-speed steel can be used, but generally, for materials of 30HRC and over, cemented carbide is recommended. When choosing cemented carbide, a general-purpose (Al,Ti)N coated type carbide for machining a material hardness of 55HRC or under is recommended. If the hardness can be limited to 40HRC or under, a highly weld-resistant coated carbide for machining low hardness material is suitable. For 55HRC and above, using coated carbide designed for machining hardened steel can lengthen tool life (see table below).
An ideal tool material for cutting stainless steel, titanium and heat-resistant alloys is a general (Al,Ti)N coated carbide. However to prevent a build up of cutting heat, a sharp tool geometry also needs to be chosen.
Tool material: The workpiece
For non-ferrous workpiece materials, choose the tool material according to the workpiece. To machine copper alloy, CrN coated carbide that has a low coefficient of friction with copper alloy is suitable. To machine graphite, GFRP and CFRP, hard and highly wear-resistant diamond coated carbide is suitable. For aluminium alloys, non-coated carbide and non-coated high-speed steels are sufficient.
The use of DLC coated carbide with high welding resistance and a low friction coefficient with aluminium alloys also allows longer tool life and higher efficiency (see table below).
After choosing a tool material, choosing suitable geometry is the next step.
The peripheral cutting edge geometry
Choose a peripheral cutting edge geometry by taking the required surface finish into consideration.
For roughing, with a surface roughness height of around 12.5μm, choose a roughing type flute geometry. In general, the use of a roughing type flute can allow machining efficiency of 3～5 times that of an ordinary flute.
For semi-finishing with a maximum surface roughness height of around 12.5μm, a semi-finishing geometry (notched flute) should be used. With semi-finishing teeth, the machining efficiency can be higher than that of an ordinary flute. However, for stainless steel and heat-treated steel, there will be a little difference between a semi-finishing type and an ordinary flute.
For finishing, an ordinary flute should be chosen.
The number of flutes
In general, solid end mills with more flutes are designed with smaller chip pockets and have higher rigidity. Chip discharge becomes poorer but machining accuracy is usually improved. On the contrary, fewer flutes improve chip discharge but machining accuracy suffers.
The table below shows a guide based on choosing the number of flutes.
The helix angle
The helix angle of solid end mills is usually 30°. As the helix angle is increased, the sharpness improves, when the helix angle is decreased, deflection from the machined surface is reduced. End mills with a large helix angle are used when machining materials with low cutting resistance such as aluminium alloy or when machining materials that require a sharp geometry such as hardened steel and difficult-to-cut materials.
Meanwhile, end mills with a low helix angle are used for high wall surface accuracy or when end mills with high cutting edge strength are required.