Measuring Concentricity

Hello there,

If you are turning the rings from rod stock then a 'Tenth' indicator can be used to indicate the rings and the mandrel you will turn.

Method 1.

Turn the I.D.to a finished size on all rings. Cut the rings off in the lathe. Leave 0.2mm on the internal face of the cut-off slot. To finish the thickness accurately it is easy on a surface grinder but if not available use double sided tape on a flat disc in the lathe. Use kerosene or other means to remove the tape.

Rough the O.D. to +0.2mm on all rings.

Turn a mandrel from 12.5mm aluminium or drill-rod bar stock, leaving a perfect shoulder to seat and support the graphite discs. The diameter of the rod should be 6.8mm +0.0/-0.1mm. Do not take the mandrel out of the lathe.

Tap a 4mm screw thread and use this thread to tighten the rings with a collar against the shoulder.

Load the mandrel with the unfinished rings and snug up a collar to tighten the rings against the shoulder. Finish the O.D.

Method 2.

Producing Rings from flat graphite sheet. Sandwich the graphite between 2 sheets of acrylic, aluminium or such material. Using a milling machine, drill and ream or drill and bore the I.D. to the finished dimension. Stand the graphite sheet on a surface table. Using a height gage with a carbide tip, carefully score the sheet on both sides to create 14.5 squares approx. Snap the graphite sheet against a sharp edge. (This works on graphite up to 3mm thick.) Otherwise slit the graphite on a band saw. Leave a little extra stock on the squares for this method. Then turn a mandrel on the lathe and follow the directions in method 1 above.

Hope this helps.

These graphite rings have been turned on a lathe and the o.d and i.d machined at the same time?
If so they have to be concentric and thus in this instance no measurement needs to be made.

Bit of disagreements-

Machining on lathe is not always going to produce concentric parts even if they are made in one go. For example using drill for making centre hole don't produce concentric parts as drill tend to deflect and also tail stocks are not always aligned with headstock.

Yes if it is done with a boring tool there is very low chance of having concentricity error.But still I would say even do it once a year but but measurements is essential for validation of process.

If I am wrong here means my boss was wrong 20 years back.

Many thanks for the help/suggestions. Actually the process of manufacturing is different. These rings are made from Graphite tape (0.4 mm thick). The tape is wound and put in a mould and pressed by hydraulic press with a center punch. The O.D, I.D are controlled with precise tolerance on punch and mould. The pressed ring is then extracted. Due to pressure, the ring is dense(1.6 gm/cm3). The ring has to be concentric within 0.05 mm. While OD and ID and run out can be measured, how do we measure Concentricity? Hope I have been able to explain the problem and process somewhat better.

Given the sizes stated, I would have a tool & die shop make a jig (fixture) to hold the units with indicators set at locations need to be verified. Should be an easy measurement for operators to complete.

A mandrel and dial indicator.



http://www.mqs.co.uk/shopping_admin/product_details/product.cgi?product=100G1&cat=Concentricity Gauges&sub=Concentricity Gauges--Concentricity Gauges

I get the feeling you (and a few others here) might be thinking of TIR, and not concentricity. (See http://www.eng-tips.com/viewthread.cfm?qid=107051&page=17)

Perhaps not. Concentricity refers to when inner circle and outer circle has different centers.

T.I.R measures the ovality. I hope I am right.

Regards