Rolling a superior finish

When attempting to produce a quality surface finish on a cylindrical part, grinding is often the first method considered. Many companies are now using roller burnishing tools on lathes and mills to produce a superior finish.

Consider the normal surface finish produced by traditional machining, either turning or boring. The traditional surface finish produced is a function of the tool nose radius and the feedrate. This machined finish, when magnified, looks like a fine thread form, a series of hills and valleys.

The action of the roller burnish tool pushes the hills into the valleys, significantly improving the surface finish. This is a metal displacement process; there is no metal cutting.

Roller burnishing tools, like the Roll-A-Finish Tool by Cogsdill Tool Products Inc shown here, are designed for finishing a bore feature. The tool has numerous rollers mounted in a cage device. The rollers sit on a tapered mandrel and can be easily adjusted for size control.

These rollers are fed onto the part at a relatively high feedrate. Features with finishes of 80 to 120 µ´´ are preferred for burnishing. This fairly rough finish is beneficial to the process, assisting the metal displacement method.

Advantages of roller burnishing

• Roller burnishing is a cost effective method to achieve a good surface finish. Burnishing on a CNC lathe or mill can be less expensive than standard grinding.
• Roller burnishing can provide a surface finish between 2 to 12 µ´´ on a turning or milling machine. This removes the need for secondary surface operations like grinding.
• The rolling method also has a work hardening effect for many material types, resulting in increased surface hardness. A harder surface will resist premature wear and improves corrosion resistance.
• Size control is improved by burnishing. Use of a roller burnishing tool has the possibility of consistently attaining a 0.0005´´ tolerance. In some situations an improvement in part roundness can also be obtained.
• These tools can be used on non-CNC machines such as automatic chuckers and multispindle machines. The accuracy of the roller burnishing method is a function of the tool design, not the accuracy of the machine that is holding the tool.

When burnishing, it is important to apply a generous flow of coolant. The coolant supply must be clean and free of chips which can be detrimental to the process.

Typical applications for this process include production of components for the hydraulic industry, where a precise sealing surface is required. The seal is often produced by an O-ring or rubber seal rubbing over the surface of a hardened steel shaft. The roller burnish method meets the need for a strong part at a precise size and most importantly a quality surface finish that eliminates fluid leakage.

Whether your requirement is close tolerances or superior surface finish, roller burnishing may be the tool that satisfies your machining needs.

Steve Rose is a manufacturing consultant and president of RTSI, Solon, OH, which also offers Internet web site development. Mr Rose can be reached by phone 440.542.3066; fax 440.542.3006; e-mail
srose@cnc-training.com or on the web at http://www.cnc-training.com

Originally published in the March 2000 issue
of Tooling & Production.
Please Note: some pictures or diagrams are only available through the printed media.