Circular Runout minium surface length

[dtmbiz]

The company standard requires a minimum surface length (along axis) of 2.5MM (.098") in order to apply a Runout control. No one here seems to know why?

1) Does this have anything to do with inspection capability, for instance minimum diameters for dial indicator tips that are available ?

2) Is it an absolute requirement to use a dial indicator for Runout control inspections ?
a. Are there other ways with todays technology to inspect Runout without using a dial indicator ?

 

[chez311]

1) That's probably a reasonable assumption. Though a probe could touch a surface infinitely thin the concern might be that it might have a propensity to touch the edge (or come off the surface all together/touch the wrong part of the probe) instead of the surface if there is significant orientation error.

2) Though the indicator is the most common tool to inspect run out and indeed was the basis for definition of the tolerance we know as run out, run out just describes a tolerance Zone consisting of 2 coaxial circles or cylinders ( depending on if it's circular or total run-out respectively) of variable size but a fixed distance between them as specified in the FCF located and oriented to the specified datum reference frame. This can also be accomplished by scanning or CMM or any other inspection method which would measure the surface in relation to the specified DRF.

 

[dgallup]

I have certainly specified and measured circular runout on much smaller surfaces. A minimum length for the DATUM surface would seem far more useful to me.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[chez311]

dgallup,

I agree - though for a FOS used as a datum feature even more useful might be a minimum length to diameter (or length to width for a width-shaped feature) ratio.

 

[pylfrm]

dtmbiz,

That seems like a strange thing for a company standard to require. If nobody there knows the reason for the requirement, then maybe the reason has disappeared. Maybe it wasn't even a good reason in the first place.

Can you describe the company a bit? What sort of parts and processes do you deal with?

1) Indicator tips are readily available much smaller than that. See here for example.

2) Drawing interpretation standards generally do not specify inspection methods.
a. If you can get the part rotating about the proper axis, a wide variety of devices (including contact and non-contact) can be used to measure displacement. As chez311 mentioned, there are also methods that do not involve rotating the part.


chez311, dgallup,

I think such simple rules would only be useful in very limited circumstances. In general, there are many more factors involved.

The cylindrical surface of 10 mm diameter and 3 mm length would be a reasonable primary datum feature reference for a runout tolerance applied to the other cylindrical surface on McMaster-Carr 92871A069, but it would be much less reasonable on McMaster-Carr 90224A155.


pylfrm

 

[chez311]

pylfrm,

I won't disagree with you there - general rules are often a bit clunky in their application and can of course be broken if you know the reasoning behind them and when they apply. Length of the primary datum feature vs. controlled feature (or length+distance from the primary datum feature) could certainly be a consideration as well - however again of course theres extenuating circumstances where that could be mitigated for example through composite or multiple datum features.

I think you'll agree though that there are practical limitations on the length to diameter ratio of primary datum features - see

https://www.eng-tips.com/viewthread.cfm?qid=456957

 

[dtmbiz]

Thanks to all for inputs

Ref: ASME Y14.5M 1994 - Datum Features 4.3
"However a datum feature should be accessible on the part and be of sufficient size to permit its use."

It has been my experience necessary to point out the above ASME quotation over the years for datum features in a DRFs that
did not meet that requirement.

The company's product is gas-turbines and has manufactured them for decades.

The component is a torque meter housing cover.

The considered features) were 10 mm - 15 mm in diameter and from 0.63 mm - 0.86 mm length at mean dimension.

The 2.5 MM minimum length standard is in both the company's global and local design standards.
I suspected it had to due with the quoted ASME reference or the company's present inspection capability.

The reasoning for a number of the standards are unknown to the present workforce. Unfortunately the standards have been around for a long time, and the "why" has been lost.

Never had the chance to contact any one in metrology as last Friday was the last work day for a number of employees including myself.

Another curiosity that hasn't been answered.

 

[dtmbiz]

BTW
The previous post should have included:

The diameters are internal

Inspection capabilities past or present