Page 21 - machining_titanium_05_2019

Poor machinability of hard-to cut titanium grades is a serious obstacle for increasing productivity in
rough milling. To overcome this problem, industry uses progressive machining methods such as high
speed milling (HSM), high feed milling (HFM) and machining with high pressure coolant (HPC) supply.

Example MILLING TITANIUM
In semi finish trochoidal milling slots in blisk (bladed disc) produced from
annealed Ti-6Al-4V, a 7-flute 10 mm (0.4 in) Dia. solid carbide endmill
(SCEM) is applied with the following cutting parameters:
ap=40 mm (1.57 in), ae=0.8 mm (0.031 in).
The SCEM material is carbide grade IC902. Estimate starting Vc.

The operation can be considered as light-duty cutting.
In accordance with data in Tables 8-10:
Vo=75 m/min (246 sfm), Km=1, Ke≈1.8 (ae/d=0.08).
Ks=1. Vc=75×1×1.8×1=135 m/min (443 sfm)

Feed Per Tooth
Cutting speed Vc, feed per tooth (advance per tooth) fz is a key parameter in milling. In
the USA, it is often called “chip load” – fz reflects largely mechanical and thermal load on
the tooth of a milling cutter. Increasing fz thickens generated chips that affects the ability
of the chips to remove heat from the cutting zone, and causes a growth of cutting forces.
However, seriously reduced fz results in chips that are too thin, worsens cutting action
and dramatically reduces tool life. Therefore, selecting the feed per tooth should ensure
an appropriate chip thickness, and maintaining the same chip thickness during a cutting
process is a formula of good performance. While advanced CAD/CAM systems can program
a tool path that maintains constant chip thickness, operating via feed per tooth is easier
to understand and more practical, so metal cutting workers require this information.

In milling, the chip has complicated variable-thickness shape, while the thickness changes from
minimum to maximum. Average chip thickness hm is used for cutting characteristics, engineering
calculations and selecting fz. There is a mathematical relationship between average chip
thickness, feed per tooth, width of cut and tool diameter - hm may be considered as a function
of fz and vice versa. The average chip thickness is a computed value, but this feature does not
diminish its importance: hm characterizes mechanical load on a milling cutter and a machine
tool; in experienced hands it is one of the key parameters for estimating milling performance.

Various sources of technical information recommend the following equation for
approximate calculating hm if the cutting edge angle of a milling cutter is 90°:

hm=fz× √ae/d (2)

fz=hm× √d/ae (2a)

Equation 2 is simple and easy to use in shop floor conditions, and gives good results that are
suitable for finding hm for peripheral milling (machining by cylindrical or slab mill, 90° endmill –
Fig. 11) and one kind of 90° face milling (Fig. 12). Also, it may be accepted for milling slots
and grooves by a disc milling cutter in some cases. In other circumstances, the equations for
average chip thickness differ. Tables 11 and 12 provide summarized data for finding hm and
maximum chip thickness hmax that are suitable for rapid calculations under factory conditions.

19