# Question 4 The Machinists--What Is Concentricity??



## Randy_ (Jun 3, 2008)

As the result of some posts in another thread on making mandrels, I was looking at some sources of drill rod and noted a spec for concentricity.  The quoted figure was 0.005".

I think I know what concentricity means, generally; but am not sure I understand how it relates to drill rod and the spec mentioned above.

If anyone can offer an explanation, I would be most appreciative.

Thanks.


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## Modelmaker (Jun 3, 2008)

I left my machinist handbook at work, when I get in there I'll look it up and post again later today for you


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## Monty (Jun 3, 2008)

My WAG would be that it is within +/-0.005" of being concentric.


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## ctwxlvr (Jun 3, 2008)

&lt;a href="http://www.thefreedictionary.com/concentricity"&gt;Concentricity&lt;/a&gt;

from this it is how close to round it is. it seems that for our "tolerances" .005" seems to me to be a lot.


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## jwalt191 (Jun 3, 2008)

I hope this helps explain the difference between concitricity and roundness in regards to tolerancing.


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## Mudder (Jun 3, 2008)

It must be a misprint as concentricity is the comparison of one feature to another such as a 3/8" journal to the 1/2" outside diameter of a piece as shown by jwalt191's picture above. A common callout for drill rod is circularity (sometimes called Roundness) but .005" seems very excessive. More commonly it is called out to be .0005"


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## dfurlano (Jun 3, 2008)

I think it means that when you put the drill rod in a holder they are saying it will be within .005" using an indicator as in the first graphic.  I think they use such a large number due to  either the fact that the holder can cause a lot of your problems and the length of the rod can effect the concentricity.

Also I would be more interested in a straightness and dimensional tolerance.  McMaster Carr specs these tolerances for their rods.

Dan


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## Randy_ (Jun 4, 2008)

> _Originally posted by ctwxlvr_http://www.thefreedictionary.com/concentricity
> 
> from this it is how close to round it is. it seems that for our "tolerances" .005" seems to me to be a lot.



That's what I was thinking too!!


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## Randy_ (Jun 4, 2008)

> _Originally posted by jwalt191_
> 
> I hope this helps explain the difference between concitricity and roundness in regards to tolerancing.


Jim:  Your drawing is exactly what I was thinking in terms of the meaning of concentricity.  I'm still puzzled as to how there can be a spec for concentricity for what is effectively a simple cylinder.


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## Randy_ (Jun 4, 2008)

> _Originally posted by Mudder_
> 
> It must be a misprint as concentricity is the comparison of one feature to another such as a 3/8" journal to the 1/2" outside diameter of a piece as shown by jwalt191's picture above. A common callout for drill rod is circularity (sometimes called Roundness) but .005" seems very excessive. More commonly it is called out to be .0005"



You could very well be correct, Scott.  I found this information on the Enco web site and they did have some drill rod products for which the oncentricity was 0.0005"  I'll post a link to the page a little later.


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## Randy_ (Jun 4, 2008)

> _Originally posted by dfurlano_
> 
> .....Also I would be more interested in a straightness and dimensional tolerance.  McMaster Carr specs these tolerances for their rods.



They did quote another spec for "section" which I took to be "diameter" and it was Â±0.001".  They did not spec a tolerance for straightness which I agree is an important characteristic for material to be used as a pen mandrel.


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## Randy_ (Jun 4, 2008)

http://www.use-enco.com/CGI/INPDFF?PMPAGE=789&PARTPG=INLMK32


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## mkcaskey (Jun 5, 2008)

Hi all. I am new here but think I can help on this one. 
I believe that some of you are correct about this being a misprint, but in the enco callout. I am thinking that they wanted to callout either cylindricity, roundness or straightness. I would say the first as shown below.


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## great12b4ever (Jun 5, 2008)

I have to agree with Mudder on this one IMHO.  It is a relationship between to cylindrical surfaces on the same part, such as a truned down drill rod like 3/8" shaft to go into chuck and 5/8" for the drill bit itself. Sometimes called a stepdown drill bit.  I also agree with Mudder that I think it is a misprint, and they really mean .0005".  If it is truly .005" then this would be excessive.


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## Randy_ (Jun 5, 2008)

I still don't know what's going on.  Maybe I'll email the sellers?  If it is a misprint, it shows up on two "DIFFERENT" web sites.  Maybe it is a misprint on the spec sheets from the manufacturer and both Enco and MSCDirect are buying from the same source or maybe concentricity means something diffenent to Enco and MSCDirect than it does to us??

Thanks to all for your responses.  Even though the question is unresolved, I have learned a lot!!


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## Mudder (Jun 5, 2008)

> _Originally posted by Randy__
> 
> ...or maybe concentricity means something diffenent to Enco and MSCDirect than it does to us??



It would also appear that they mean something other than ANSI/ASME Y14.5.1M-1994 (R1999) which is the accepted standard.


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