Actual sizes of precision parts 2

[cjccmc]

Years ago there was the idea that most machined parts tend to be produced closer to MMC, the supporting reason that machinists worked that way to avoid rejects due to excessive material removed. I'm evaluating some fits of spherical bearings into bores that were not optimally sized and wondering if I should expect most parts to be produced, on average, considerably closer to MMC than LMC. For example, for size range of .7500-.7505 inch dia for both the bearing the housing bore, is it likely that I will have the bearings closer to the .7505 end and bores closer to .7500?
I realize this is not a GD&T issue, but there seem to be a few members here who deal with part inspections and would like to get their input. Thanks!

 

[dgallup]

You have to know something about the manufacturing process before you can know what kind of distribution to expect. I would expect that with only .0005 total tolerance this diameter is centerless ground and that the machine will be set up as close to nominal as possible.

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[MintJulep]

If your tolerance assignments are correct it doesn't matter.

If it matters than your tolerances are wrong.

The whole purpose of tolerances is to ensure that this sort of thing doesn't matter.

For example, with centerless grinding, on a large production run I would expect that the initial setup would be towards the LMC limit so that as the wheels wear more parts can be made before MMC is exceeded. If the run is short enough so that tool wear is not a concern then...

 

[drawoh]

cjccmc,

I would expect this to depend on the manufacturing process and the fabricator.

I have been told by a machinist that he makes holes and shafts close to MMC, making the fit as accurate as possible. This requires his process to have accuracy way within the tolerances you are specifying on your drawings.

If his process is marginally within your tolerances, you should expect your parts to scatter across the tolerance range, in a bell curve, that possibly extends outside your specifications.

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JHG

 

[cjccmc]

Thanks for your inputs. There doesn't seem to be a clear consensus that parts with this size and tolerance band will be made significantly closer to the MMC condition. That's a good outcome for the issue I'm dealing with, which is too mundane to bore everyone with bunch of details

 

[drawoh]

cjccmc,

One more thought in answer to your question...

I understand that ball bearing manufacturers sort their bearing parts by size. Once they do this, they can assemble the precision bearings out of parts that approach MMC, and assemble the lower precision bearings out of the remaining parts. A lot of bearing manufacturers offer you different precision levels in their bearings.

In this scenario, the fabrication process cannot reliably achieve the required tolerances. They achieve them by sorting the parts, and by having a use for the non-conforming parts.

If you go to a subcontractor who agrees to ship you balls of .7500-.7505 inches diameter, you accept everything within the range. Based on your contract, you have no right to assume the actual dimensions are skewed in any particular direction.

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JHG