Cutting Tool Engineering
April 2009 / Volume 61 / Issue 4

Milling difficult features in titanium

By Edward F. Rossman, Ph.D.

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.

This process is needed because traditional milling spring passes are not effective when machining titanium alloys. Thin cleanup passes tend to push metal away from the cutter in the Z-axis direction, and they impart heat and surface stresses into the part. Leaving a heavy cut for last works because the pull of the cutter helix balances the push of the cutter. One of Boeing’s suppliers said he achieved superior dimensional results on titanium alloys by leaving plenty of material for the final milling pass on pocket floors. Cutter dullness could certainly upset the balance of forces. By not doing final spring passes, you can save time and produce a dimensionally better part.

Cutting from the pocket center first to the outside wall last leaves plenty of solid and rigid material next to the cutter at all times.

Milling thin flanges in titanium alloy parts, such as aircraft spars, is also difficult. With a minimum flange thickness of 0.060 ", the height-to-thickness ratio was limited to 20:1. To machine thinner flanges required expensive backup tooling or performing chemical milling. These limitations often blocked efforts to decrease aircraft weight.

The objective of another study was to develop methods that would allow thinner flanges to be machined to the required ±0.010 " tolerance without the need for backup tooling. Testing confirmed the original design limitations.

A new process allowed flanges as thin as 0.040 " with a ±0.010 " tolerance to be machined, convincing engineering to add a note to the drawing that would allow machine mismatches along the sides of the flanges. The process included roughing to within 0.5 " of net thickness, finish milling the top 0.5 " of the flange, stepping down in the Z-axis and milling the next 0.5 " of flange to net thickness and continuing to step down until the flange was completed. The cutter mismatch is then blended by manual sanding if needed.

The new process allows designers to reduce part weight. Fixturing costs are kept low because backup tooling is not required. CTE

About the Author: The late Edward F. Rossman, Ph.D., was an associate technical fellow in manufacturing R&D with Boeing Integrated Defense Systems, Seattle. Rossman’s Shop Operations column is adapted from information in his book, “Creating and Maintaining a World-Class Machine Shop: A Guide to General and Titanium Machine Shop Practices,” published by Industrial Press Inc., New York. The publisher can be reached by calling (212) 889-6330 or visiting www.industrialpress.com.
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