Page 17 - machining_titanium_05_2019

Milling Tool

Selecting a suitable tool for milling titanium is a very important issue. The following three
features: cutting geometry, tool material, and design configuration, should be examined.
1. It is considered that tools for milling titanium feature positive-rake geometry with appropriate MILLING TITANIUM
edge preparation. This is quite right with respect to commercially pure and easy-to-machine
titanium grades. However, milling the difficult-to-machine grades requires a reinforced
cutting edge that is ensured by neutral or negative T-land. In many cases of roughing
applications (such as heavy-duty milling by indexable extended flute milling cutters or HFM),
tools with negative-rake geometry, which provide higher feed, are the best choice.
2. Milling titanium dictates increasing relief angle.
3. When indexable milling tools are examined, cutters with one-sided inserts are preferable.
Generally, they demonstrate better performance when compared to tools carrying double-sided
inserts. However, in rough machining wide edges of workpieces (edging), the indexable extended
flute cutters with tangentially clamped double-sided inserts can be a first-choice solution.
4. The tool material and design should ensure a sharp edge during reasonable tool
life. In conditions of intensive heat generation, a dull (“bad”) edge increases build-
up edge formation and tool wear, and leads to serious tool damage.
5. Machining titanium features considerable heat build-up and mechanical load,
particularly for difficult to machine grades, and the tool material must maintain a
correct balance between high-temperature strength and impact resistance.
6. Coated cemented carbides are the main tool materials for titanium milling. PVD coating
retains a sharp cutting edge better than CVD. The majority of indexable carbide inserts
intended for milling titanium are PVD-coated. However, CVD-coated inserts, which are usually
less sharp than PVD-coated inserts, often perform better in heavy-duty rough milling.
7. Cutting ceramics and cubic boron nitride (CBN) are not so suitable for machining titanium,
although polycrystalline diamond (PCD) has proved itself in finish milling in several cases.
8. Most machine tools today are usually designed to supply coolant through a spindle. Therefore,
applying cutting tools with internal coolant supply through the tool body is preferable.
9. Vibration strength in end milling can be improved drastically by using solid
carbide endmills (SCEM) with chatter-free cutting geometry.
10. Milling cutter accuracy is an important factor not only for ensuring the required precision
parameters in finishing operations. Low accuracy, especially in case of small-diameter
cutters, affects performance and reduces tool life. For example, excessive run out leads to
increased loading of a tooth and its early wear, which causes overloading the other teeth
and even cutter failure. Poor cylindricity of the tool shank diminishes the effective area of
contact between the shank and a toolholder that significantly decreases clamping force.

15