Titanium machining data

Required forces and power

The forces that are generated during machining are three: tangential or cutting force, thrust or separation force and axial force caused by the tool advance; multiplied by the relative working speeds, they define the necessary power to the machine.

Machine power can be calculated, with a fair degree of accuracy by measuring the power required during cutting operations, minus the power required by the unloaded machine.

ln general, machining titanium does not require high levels of power, especially with CP Ti. Although these levels of power are a bit superior than those of stainless steel, values are lower than those of structural steels with medium hardness and strength.

Any machine tool that is powerful enough to work structural steels can also be used with titanium, provided that it is fitted with the necessary tools.

Choice of tools and tool life

The main factors that determine the performance of a tool are:

- material

- geometry (corners, shape, accuracy)

- conditioning (heat treatment cycles, surface coatings).

People often (wrongly) choose extremely sophisticated tools, including sintered tips and tips with high cobalt content or tips steel with high hardness (70 HRc).

Material is important, but correct geometry is much more important.

lt is much easier to obtain good results from tools with correct geometry and of common material, than from sophisticated materials with inappropriate geometry. Tips with radius 1.2 are recommended for the first cuts and tips with radius 0.4 for the final cuts.

These materials are more subject to splintering because of the great depth of pass used with titanium, and do not bring the expected advantages.

The most frequently used materials are high-speed steels known as M2-M7-M33 M42 (according to AISI-ASTM standards). We also recommend Sandvik H13A.

Good results can also be obtained by using sintered carbide tools, which increase productivity, especially in turning and boring operations; extent is lesser in drilling, tapping, broaching and similar operations.

The recommended grades are C-2 according to ASTM standards and grades K1O, M1O, K20, and M20 according to lSO standards. (Sandvik H13A)

Coatings are widely used for tools (TiN, TiCN) but in the past they were not looked on favourably, as there was thought to be chemical-tribological incompatibility between titanium of the work piece and titanium of coating.

This fear is unfounded and some advantages can be obtained using tools in C-2 coated in TiN, especially for machining of Ti-alloys with high degree of hardness.

However in many cases such advantages does not exist, or they are so limited that they do not justify the high cost of tool.

Cutting fluids

The cutting fluids used in titanium machining must be chosen carefully. The right refrigerant will reduce the cutting temperature and rubbing and will thus greatly increase working life of tool.

In certain cases in the past, use of fluids with halogens (chlorine, fluorine, bromine and iodine) after heat treatment with not correctly cleaned piece probably caused stress corrosion cracks in operation.

These findings have not always been confirmed by specific laboratory tests, so the use of these fluids is not even forbidden in Aeronautical field except for special cases. The work pieces should be thoroughly washed and cleaned after processing, especially if they are subjected to heat treatment or to welding or to high temperature service.