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: Concentricity help required
: Concentricity help required -- hanniffypaul	Post Reply	Top of thread	Engineering Forum General

Posted by: Kelly Bramble ® 06/29/2010, 16:32:14 Author Profile eMail author Edit

Not sure what you mean by "want to set a concentricity tolerance". Build, specify? See picture

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: Concentricity help required
: Concentricity help required -- hanniffypaul	Post Reply	Top of thread	Engineering Forum General

Posted by: Pinkerton ® 06/29/2010, 16:06:19 Author Profile eMail author Edit

Can't help with the Tolerance definition, but it sounds like it should be machined between centers with a follow steady-rest. Either that or slow down the final cut rates and depths. For it not to be concentric the roughing cuts must be bending the stock. Dave

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: : Concentricity help required
: : Concentricity help required -- Pinkerton	Post Reply	Top of thread	Engineering Forum General

Posted by: rwolfejr ® 07/09/2010, 13:57:05 Author Profile eMail author Edit

Your concentricity to the un-turned portion that is clamped in your chuck will only be as good as the precision level, or repeatability of your chuck. You can only expect to hold a tolerance of whatever your chuck is capable of repeating. The part being turned must also be true at the chucks gripping points. If you're turning this taper on a round bar... it needs to indeed be round or you can accumulate more "out" condition. Spindle bearing wear will also have an effect on concentricity, especially under heavier cuts. You could battle bearing wear with a final kiss cut... as Dave mentioned. Also remember chucks wear out. Some quicker than others. We've used a few cheaper chucks here at work on some not-so-precision large quantity parts, and change them roughly once a year. We've decided that for about 2,000.oo per chuck, per year or so, it's less expensive than a 12,000.oo chuck that might last three, maybe four years? The "throw-aways" aren't worth rebuilding though, whereas a top shelf chuck would be. Hard to say for certain which is most economical in the long run? Good luck!! Bob

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What is the difference between concentricity and runout tolerances?
: : : Concentricity help required -- rwolfejr	Post Reply	Top of thread	Engineering Forum General

Posted by: rajan84 ® 07/17/2010, 00:57:20 Author Profile eMail author Edit

Not sure if you can say if there is an equivalent between the two. Runout is typically with respect to a datum AXIS. The concentricity is with respect to a datum FEATURE. TIR or total indicated runout is how far out the shaft runs when rotated between two centers. Concentricity is how round one section is. They are two different things. _____________________ Modified by rajan84 at Sat, Jul 17, 2010, 00:58:32

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: What is the difference between concentricity and runout tolerances?
: What is the difference between concentricity and runout tolerances? -- rajan84	Post Reply	Top of thread	Engineering Forum General

Posted by: RWOLFEJR ® 07/18/2010, 12:05:41 Author Profile eMail author Edit

I'm thinking you might be a little confused? Someone pleaase correct me if I'm wrong but... Circularity is how round something is... not concentricity. Concentricity is the T.I.R. relationship of one feature or section of the shaft or what have you... to another feature. You could be turning a square bar to a round shape on the end and still be striving for concentricity of the square to the round. If you have a 2" bar chucked up and turn the end down to 1" diameter... then you remove your part... place the 2" portion on a pair of precision rolls or balancing ways... Then take an indicator reading on the 1" diameter portion... You will be reading concentricity of the 1" diameter to the 2" diameter. (If you're taking these T.I.R. measurements between centers you'd be looking for the direction of the run-out and the magnitude of the variation of the run-out between the two sections to determine concentricity. One section out by .001 at 12 o'clock vs. second section out by .001 at 6 o'clock = nonconcentric by .002) If your chuck is worn... or better yet picture a chuck with independant jaws... you chuck the part off center by a few thousandths... your turned portion will be off by a few thousandths. You will turn a crankshaft. The two features will not be concentric. They could still be round or circular but not concentric. Then say you have an oval shaped part (not circular) that you chuck up in a perfectly good precise chuck. (Tubing for example isn't round... it has some ovality allowance.) Depending on where the ovality of your 2" portion happens to hit it will / can effect the run-out or concentricity of the 2" section to the 1" section. If you are looking for circularity you have to use a tri-micrometer at minimum. You can't read circularity with a regular micrometer. Example... In centerless grinding you can sometimes encounter a situation where the part will effect a subtle three lobed cam look. (This is a wierd thing that can happen if you don't have your support blade posisitioned correctly.) This situation of non round can't be measured with a regular micrometer. Reason being at any point around the part you will read the same dimension or diameter. Say one anvil of mike is on the high of the lobe then the other will be on the low. As you turn the part slightly you will be measuring "less than peak high" to "higher than minimum low." Naturally a CMM would pick this stuff too. All that to say... If you are looking for precision circularity for whatever reason... You have to measure the correct way or you could be fooled. If you are looking for a fit on two mating components of some "x" magnitude... and they are not circular... the fit will not be what you were after. Say you are looking for a .004 shrink fit for a bearing on a 4" diameter shaft because it has been determined that that fit will keep the bearing in position with the loads in your system. So you warm up and install your bearing designed with looser than normal ball fit of "x" magnitude... that is meant for your particular application and if all was circular and correct size you will have the appropriate running clearance in your bearing and also the proper squeeze or shrink fit to your shaft. Now on the other hand say your shaft was not round but rather a cam as mentioned... then you'd have a distorted fit that would result in problems.

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