***Non-CMM Inspected Roundness (The last time I promise)

[[Me...]]

I know I've asked this question a billion times but everywhere I go people seem to have their opinion about it and not the correct one, in my opinion.

EDIT:

Is it acceptable to use a .0001 indicator for a bore/shaft, offset to zero and then sweep around it again and call that roundness?

[[Te...]]

The way Calypso filter a roundness the center of the circle really is not that important. It uses Minimum Feature what is does is creates the smallest and largest circle that share the same center from the data set. The mean circle of those 2 circles is the nominal and the variation of the 2 circle is the roundness, hence why the high and low are always the same. I have attach a document that might do a better job explaining it.

Filters and Outliers.pdf

[[Da...]]

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The answer is a definite No.

When you measure any form, then you always expect as a result a form error, a deviation from the perfect form. For roundness, that would be a perfect circle. Now, any measuring device has to have information about how a perfect circle looks like. For a CMM, it’s fairly easy, because the perfect form can be derived from the measuring scales and the fact that they’re perpendicular to each other.

For a hand-held measuring gauge, it’s different. It measures only distances, but not form. It does not have an inherent „knowledge“ of a perfect circle. If you sweep it around, there is no defined center point that you will turn it around and therefore you can’t determine the form error of a hole with it.

In fact, if you used a two-point distance measuring device on a three-lobed hole, you would always measure the same distance, giving you the false impression that this hole was round, when it really isn’t.

[[Me...]]

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Thank you. I remember having to buy the standard that tackles roundness to help our shop understand it better but that was at another company. I've tried explaining it to our shop and the best I could do is a broken down version of what you have explained, it is size-based.

[[Da...]]

Prior to CMM this is how they checked roundness. The bore or shaft must be set between 2 centers aligned to each other, then setting a .0001 indicator on it and rotating it checked roundness. I had to use this method back in the 80s. It is effective although not as accurate ( in my opinion.) as the CMM.

[[Ma...]]

If by "indicator" you mean a snap gage or bore gage then the answer is no.
If you have a fixed indicator and the part can be rotated on its axis then the answer is yes.
There are many roundness measuring instruments on the market, most with rotating tables some with rotating arms. In almost all cases these instruments have less systematic form error than a CMM.

[[Me...]]

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Yes we have a "Jig Bore" setup that you can adjust and set zero and then sweep the part. I've had issue with it though because I get significantly different readings between it and the our CMM.

[[Gr...]]

We use a Brown and Sharpe .00005 indicator mounted to the head of a MOORE Jig Grinder for roundness. Also, if the hole is large enough, it allows us to check straightness an perpendicularity to confirm the CMM results

[[Ad...]]

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Exactly...a form machine uses a transducer...same as an indicator. I would disagree that it can't be inspected using an indicator.

[[Ow...]]

Although it's often difficult to exactly match the CMM and checking with an indicator, I'll 2nd that there is nothing wrong with using an indicator if done properly.

The biggest difference between the CMM and using an indicator is usually human error caused by improper alignment of the piece perpendicular to spatial alignment where, the CMM makes it easy. If using a spindle and indicator to check, any runout in the spindle itself will induce error. If using v-blocks, any form error of the cylinder will negate perfect readings of roundness to a truly derived center. If using centers, the center form and alignment can effect results by inducing runout or misalignment.

Generally, most of the differences between the two can be identified as indicator procedure error or CMM filtering/evaluation methods.

The important thing here, in my opinion, is proving which one is right or wrong to the CNC machinist. Giving the machinist or engineer any reason to doubt the CMM is going to spread like wildfire and induce unneeded CMM programmer/ operator stress.

[[Me...]]

Owen Long says its good
my opinion:
for tolerances 0.1 mm or higher it may be done, but only if your results are lover than your measuring error + method error.
The thing is we are never know how the part really looks and for example if its bend shaft the results will always contain errors - its always little band on parts.

So i if i have for example 0.1mm tolerance,also lets say i have some experience in this method, my result is 0,09 then i can accept this part.But if the results are higher than 0,09 i will put it on CMM to make sure.
I other case 0,09 may be even 0,05 it depends from parts you measure, technology .... make some practice, experience will learn 🙂
It need time, cannot make good method in 5 minutes, if doing it for first need time and testing