Centerless Grinding for Tight Tolerances or Improved Surface Finish

All wire that we supply meets a specific ASTM standard based on the material. Within these specifications, there are callouts for diameter tolerances. These tolerances are specific to the size of the diameter. As an example, all Music Wire must meet tolerances defined in ASTM A228. Table 5 below is from ASTM A228. Looking at Table 5, you can see that wire diameter .080” to .250” must have a diameter tolerance of +/- .001”. So if you have a .240” wire diameter, then its diameter is in spec at .241” to .239”. 

But let’s say a customer says that’s not good enough; I need a tolerance of .240 +/- .0002”. In addition, the customer may say I want this surface to be really smooth. Usually the customer engineer will specify the finish but if not we would assume a 16 Ra finish.

We would achieve these requirements using a centerless grinder. Our centerless grinders can meet +/-.0002” tolerance and surface finish specifications from 5 – 16 Ra with 5 being very fine. We find that most wire has an existing surface finish of > 25 Ra which is usually acceptable for most applications. We use Profilometers to measure surface roughness (Ra) and Laser Mics to measure diameter. The Laser Mics can measure up to +/-.00001”.

Please note, we cannot centerless grind any wire in a coil or that has not been straighten – it must be straighten and cut first and to tight straighten tolerances. 

The centerless grinding cost is a function of the amount of material to be removed, the tolerance, finish and the length of the part. When we centerless grind, we use a wire diameter that is slightly larger than the finish diameter. As an example, let’s say the customer requires .224” Music Wire +/- .0002”, S&C 10 ft., 16 Ra. We might select .226” Music Wire. We would first straighten and cut the .226 to 10 ft. Then using one of our centerless grinders, we would grind material off from the diameter along the entire 10 ft. piece in order to meet the specifications. This process requires machine operator expertise and experience as there are many machine parameters that have to be set, monitored and adjusted. First piece and pieces produced during and at the end of the process would be tested for customer conformance using the Profilometers and Laser Mic.

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