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Member

Join Date: Oct 2010
Posts: 9

# Measuring Concentricity

02/09/2011 1:19 AM

Hi. I am producing Graphite rings. O.D and ID concentricity has to be within 0.05 mm. Could any one suggest how to measure concentricity? O.D of the ring is 13.7mm+0.05/-0.00 and ID of the ring is 6.8mm+0.05/-0.1. Is there a simpler method to measure concentricity?

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Power-User

Join Date: Nov 2006
Location: San Francisco Bay area
Posts: 212
#1

### Re: Measuring Concentricity

02/09/2011 2:55 AM

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.

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Power-User

Join Date: Mar 2009
Location: United Kingdom
Posts: 308
#2

### Re: Measuring Concentricity

02/09/2011 3:03 AM

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.

Guru

Join Date: Oct 2006
Location: India, 200 Km. North of Delhi.
Posts: 1391
#4

### Re: Measuring Concentricity

02/09/2011 6:30 AM

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.

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Anonymous Poster
#3

### Re: Measuring Concentricity

02/09/2011 4:01 AM

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.

Guru

Join Date: Aug 2010
Posts: 687
#5

### Re: Measuring Concentricity

02/09/2011 6:49 AM

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.

Power-User

Join Date: Mar 2009
Location: United Kingdom
Posts: 308
#6

### Re: Measuring Concentricity

02/09/2011 6:50 AM
Guru

Join Date: Feb 2008
Location: Van Nuys, CA
Posts: 563
#7

### Re: Measuring Concentricity

02/09/2011 7:19 AM

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)

Member

Join Date: Oct 2010
Posts: 9
#8

### Re: Measuring Concentricity

02/10/2011 3:49 AM

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

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