Milling difficult features in titanium

The minimum web, or pocket floor, thickness in machined titanium alloy parts, such as aircraft spars, was traditionally limited to 0.100 ” unless expensive backup tooling was fabricated to enable the normal tolerance of ±0.010 ” to be held on thinner pocket floors, or chemical milling was performed instead. The limitation of 0.100 ” often blocked efforts to decrease aircraft weight because of higher fixturing costs.

The objective of one study was to develop milling methods that would allow thinner webs to be machined to the required ±0.010 ” tolerance without the need for backup tooling.

Testing was conducted in two phases. A milling process was developed to produce 0.060 “-thick webs within a ±0.010 ” tolerance. The process was further refined to produce webs as thin as 0.040 ” within the required tolerance without backup tooling. The process imparted the required surface finish of 125 μin. Ra or finer, and no tool chatter or unusual noise were noticed.

The sequence of operations began with roughing to within 0.250 ” to 0.500 ” of net thickness. If desired, the part thickness was measured after roughing. Then, finish milling to net thickness was performed, and the tool tabs were cut off during the final operation.

The cutter ramped at 4° from the outside edge of the pocket to the centerline. Work was performed from the pocket’s center to the outside walls to produce the net web thickness and fillet radius. Cutting speed was 150 sfm with a 0.004 ipt. Tool life was 45 minutes.

A test part with five pockets was machined and measured to evaluate machinability. Web thickness varied from 0.039 ” to 0.046 ” with no oil canning, or distortion, of the webs. More than 100 production spars were then produced with pockets similar to the test part, and the ±0.010 ” tolerance was met in all cases for the pocket floors.

This same process helps to minimize warpage in other thin sections, such as splice plates. Even when the part is resting on a flat plate, which amounts to backup tooling, we achieved effective distortion control.

The process allowed designers to reduce weight by about 1 lb. per 100 sq. in. of web surface without the expense of backup tooling. It applies in those instances where thinner webs are of sufficient strength.