August 2009 / Volume 61 / Issue 8|
Tips for sanding titanium parts
By Edward F. Rossman, Ph.D.
Sanding, or benching, titanium alloy parts is a major cost element of the manufacturing process. Sanding removes burrs and improves surface finish to achieve drawing specifications. Sanding is also done to blend steps between cutter passes and remove material that milling missed. There are some difficult areas to access when milling and it is sometimes more efficient to sand and blend at the bench than to spend extra time milling certain features. Benching can cost less per hour because those employees are usually paid less than machine operators and because expensive capital equipment is not being tied up.
Minimizing sanding offers the potential for large savings because it typically adds 10 to 20 percent to the cost of titanium alloys parts. The key reasons for the high cost of sanding are material hardness, demanding specifications and inefficient milling operations.
Deburring titanium alloys via sanding takes a lot of effort. The titanium alloys are often quite hard (41 HRC), which makes sanding difficult.
Titanium alloys lose some desired properties if they get too hot during sanding. If the surface gets above 800° F, small cracks form and fatigue life is reduced. Therefore, aggressive sanding, especially when generating white sparks, is not allowed. The workpiece surface often turns blue when the temperature goes above 800° F. To avoid that, sanding must be done at slow speeds to maintain material integrity.
At Boeing, a contributor to the high cost of sanding is the company’s deburring/sanding specification. This specification requires a 32μin. Ra finish on titanium alloys if sanded, even if the drawing requirement is 125μin. Ra for the surface in question. This requirement does not usually apply when sanding other metals.
Sanding creates scratches, or points, in the workpiece surface. In contrast, milling generates points that project up from the surface. The machined “up” points pose less of a fatigue problem than the sanded “down” scratches. Fatigue tests performed during development of the Super Sonic Transport led to the 32μin. Ra surface finish requirement if sanding is performed. Further testing in 2003 confirmed that bad fatigue test results were generated from sanding titanium alloys.
One of our research efforts was aimed at changing the specification. If this specification could be changed to allow the surface to be rougher when sanded, such as 125μin. Ra, then the benching savings for large airframe parts would be cut in half. This cost applies to all machined titanium alloy parts.
The surface finish research included canvassing other major companies to find their requirements for sanded titanium alloys. I found that many companies are less stringent in their requirements. Some demand a 63μin. Ra when sanding and others merely require the 125μin. Ra finish called out on the milled part. In no cases did the sanding specification allow a rougher finish than that specified by the drawing.
Over the years, sanding personnel at the shop have complained about the high cost of sanding to meet the 32μin. Ra finish requirement, and NC programmers have often responded by reducing step-over distances during finish milling to produce a surface that meets the 125μin. Ra requirement without sanding. However, the added finish-milling hours from using smaller step-over distances is usually higher than the sanding savings. The additional milling hours also ties up expensive capital equipment. So we often end up spending more money than we save to quiet the sanders’ complaints.
Efforts to get specification relief will probably not succeed because sanding reduces the fatigue life of titanium alloys. Studies to quicken the sanding process will not bear much fruit because the heat from aggressive sanding also reduces the fatigue life.
There are also safety issues from the potential for titanium fines to ignite, and titanium fires are difficult to extinguish.
The research results on fatigue life of sanded titanium alloys requires that a 32μin. Ra surface finish must be met if sanding is involved. The challenge is to develop a surface finishing method that does not reduce fatigue life and that does not expand the cost of milling to astronomical levels. The most likely area for reducing benching costs is to employ some means of vibratory finishing. [Editor’s note: Vibratory finishing will be covered in Edward F. Rossman’s Shop Operations column in the October issue.] CTEAbout 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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