plug gage tolerance

[lfw618]

I have not been able to find a clear answer on this. Does the tolerance of a plug gage apply just to size, or does it apply to the form of the gage along its length, ie cylindricity.

I know that a size tolerance should imply from, but I thought at least some plug gages were centerless ground, and I'm not sure how the form tolerance along the entire length would be achieved with that process consistently?

 

[ornerynorsk]

First of all, centerless grinding is incredibly accurate and consistent, and produces very round and straight forms when done right. Very much within the realm of gauge quality work. Decades ago when I was in microhydraulics I would routinely receive batches of pistons and poppets that were consistent to each other within 50 millionths, and this was a normal day in the shop for this grinding company. If you wanted real cheap and sloppy work it was +/-0.0001. In fact, it is the only practical method when the limitations of between-centers grinding has been reached. Can you imagine trying to grind a 4 inch long by 0.020" diameter pin between centers. It's not going to happen.

Secondly, you would have to check the ANSI/ASME B89xxxxx standards for methodology of measurement of gauges. What I've seen from some gauge makers is multiple measurements along an axis, and then repeating those measurements along the axis 90 degrees from the first set. So technically, you may not be detecting out-of-roundness or straightness. Someone more versed in geometric tolerancing and more familiar with the standards will no doubt give you a better answer.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[IRstuff]

A plug gage is designed for a specific purpose, checking the diameter of a "hole." From what I understand, "holes" are distinct from "bores," the latter requires a bore gage. Therefore, a plug gage is likely to be locally cylindrical within its tolerance, but is global cylindrical properties are likely looser.

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[lfw618]

Wow, I guess I had the wrong impression of centerless grinding, I had thought it would be locally very repeatable, but maybe not as much along the entire length.

Thank you both for the insight, sounds like I might expect decent cylindricity along the entire length, but it may not be the same as the class of tolerance of the gage. Especially as they are not necessarily measuring straightness when certifying the gages.

 

[ornerynorsk]

To clarify, centerless grinding IS very repeatable, PARTICULARLY in high ratio length/diameter work pieces. It's an excellent method for finishing plug and pin gauges. As opposed to between centers grinding, which does very poorly with high L/D ratio work pieces.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.