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Need to meet surface roughness of 12.5 Ry on pinion gears

Dear Sirs,
I have a dilemma, which I hope you may have some advice. I need to meet a 12.5 Ry surface roughness spec on some pinion and side gears. The material is SCR 420 and we forge the billets at 1150° Cel and target 87 HRB, however this leaves a lot of scale on the forgings after they run through normalizing process. We currently use S170 shot for all of our gears, but for these gears the S170 still leaves to large of surface impact on tooth surface, we have done trials using S110 and it made improvement but going much smaller than that will take up to much time in machine and is not cost effective. Could using grit or some other media give me the results I am looking for? Any idea or answer will be greatly appreciated.

Thank you for taking the time to look over my question.

Best Regards,

Paul Armstrong
Production Manager - Battle Creek, Michigan
February 14, 2011

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I was going to suggest aluminum oxide or silicon carbide. The last is heavier and thus provides a higher impact energy. But it's hard to say not being there. Have you tried calling in vendors of abrasives and blasting media? I am sure they could have a better feeling of what may work without bringing in a new headache.
G. Marrufo-Monterrey, Mexico

Guillermo Marrufo
Monterrey, NL, Mexico
February 26, 2011

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It sounds to me that your main concern is the surface variation due to blasting. I'm not sure that you can improve that any more than to reduce the pressure. Another alternate process would be to look into high energy centrifugal mass finishing systems.

AF Kenton
retired business owner - Hatboro, Pennsylvania
March 1, 2011

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Thank you so much Mr. Marrufo and Mr. Kenton for your responses.

I have been working with vendors on different media and processes too, though we are still struggling to find something that can meet, I will look into the ideas you presented.

Thank you again for your time and knowledge.

Best Regards,

Paul Armstrong
- Battle Creek, Michigan, USA
March 14, 2011