Surface Finish: Spun Metal Parts

February 24th, 2010

The final appearance of a spun metal part is determined by its contour and surface finish. By definition, the process of metal spinning moves metal. Therefore the “look” of the finished part can vary significantly depending upon the spinning technique, quality of the tool, and parent metal. To be assured that your metal spinning will have the final desired appearance, customers should include both a profile and surface finish specification on their print.

A profile specification tells us how close the contour of the spin formed part must be to the theoretically perfect shape. I think of this as the “lumpiness” factor. Is your spun shape cosmetic, requiring a very smooth profile because you are going to powder coat it in high gloss black? Does your metal spinning require a near perfect profile to be functional, for example a part used in a critical air flow application? Achieving a good contour is easier on the surface of the part touching the tool, rather than the surface of the part that is spun. If your spin formed part needs a smooth contour, incorporate a profile specification on your print. For further information, research the use of surface profile in geometric tolerancing.

A surface finish specification tells the spinner how rough the surface can be. A metal spinning may have an excellent contour, but the surface finish may be too rough. Imagine the difference in surface finish between a metal file and a shiny, chrome bumper. When you run your fingernail across each of these surfaces, they feel different. Your fingernail vibrates differently. The spinning process creates spinning lines, resulting in surface imperfections which can be mitigated by altering the spinning technique and other parameters. Typically the better the surface finish required, the slower the spinning cycle, the more expensive the spun part. RMS, or Root Mean Square, is a common specification used for surface finish. A typical spinning, without a lot of attention to roughness, will have a surface finish RMS of 125 microinches. As the RMS number decreases, the smoothness increases. Metal spinnings with an RMS of 64 microinches are quite smooth, while those with a RMS of 32 microinches may have had secondary polishing operations added to remove almost all surface imperfections.

While most engineers and designers focus on part geometry and fit requirements, don’t forget the importance of profile or surface finish if the appearance of your spun metal part is critical.


Cut Costs and Delivery Times with HMS Design Support





Send us your drawing.

(.jpg, .gif, .pdf, .dwt)

> President’s Message

> Strategic Planning