Modern Machine Shop

FEB 2018

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MMS FEBRUARY 2018 60 TOOLING TECHNOLOGY How do you finish the surface of a metal part to a mirror-like sheen without putting it through an abrasive process like grinding? For some appli- cations, roller burnishing could do the trick, and without actually removing any metal from the part. In basic terms, burnishing is a method for polishing a surface such as metal through sliding contact with a harder object. The burnishing tools offered by Cogsdill Tool for machining metal use highly polished tapered steel rollers, as you can see in the image above. Different Approaches to Finer Roughness Values Surface finishing, whatever the method, is about reducing the Ra, or Roughness Average, value of the surface. Machining a metal surface leaves microscopic peaks and valleys that we call rough- ness. Basically, the Ra value is a formula which measures the average distance between the tops and bottoms of these points relative to the mean line which cuts through them. The finer the surface finish, the shallower the valleys and shorter the peaks. This process enables fast and repeatable finishing of metal surfaces to mirror-like quality, but without removing any metal. BY JEDD COLE | ASSOCIATE EDITOR What Is Roller Burnishing? The first noticeable difference between abrading and burnishing a surface to accomplish this peak and valley reduction is that one removes metal from the part while the other does not. Abrasive finishing cuts or tears away the peaks in the surface, thereby bringing the average peak and valley distances closer together. But Cogsdill says that this also leaves sharp projections in the contact plane of the machined surface. Burnishing doesn't have this problem. At first glance, a burnished part looks as if the metal surface has been smeared smooth. But that would be incor- rect. The burnishing tool's polished and hard- ened rollers actually perform cold f lowing of the surface and subsurface material, which results in a controlled plastic deformation of the part. Two Types of Deformation Material deformation comes in two varieties: elastic and plastic. Elastic deformation occurs when stress is applied at a pressure below the material's yield point; bend a piece of plastic a little, and it bends back. Plastic deformation, on

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