Runout for a cylinder with a flat face

[handleman]

I'm designing a part that's basically a large-ish (over 2" diameter) disk/cylinder with a small-ish (less than 1/8") concentric hole. Then the disk is cut such that it has a flat, sort of like a wrench flat. So the cylindrical surface is no longer complete. I want the hole to be concentric to the outside diameter of the truncated disk.

I may be splitting hairs here... I know that I could specify the hole as the datum and then control the runout of the outside diameter, but it seems to me that it would be difficult to fixture the part to truly inspect it that way due to the hole being so small relative to the large part. OTOH, if I specify the large OD as the datum and control runout of the hole, does that mean that the hole surface is sort of "undefined" in the area where there's no corresponding outside diameter datum?

Hope that makes sense...

 

[CheckerHater]

If your "large" cylinder still has at least 2/3 of the original cylindrical surface, you may be able to specify datum targets to show exactly how to grab it with chuck jaws (I was thinking 3-jaw but your case could be more complicated)
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[mkcski]

Just a thought. But you might consider making the hole a datum and using profile and BSC dims to control the location of "flats" relative to the hole.

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[chez311]

Keep in mind as well that if you are specifying total runout it also controls form - now that you are switching the datum reference that form control will now apply to the ID hole. As such you may need additional form control on the OD surface.

 

[AndrewTT]

Runout is not the only way to control coaxiality. Can we know what the design intent of the part is?

 

[pmarc]

handleman,
First question to answer would be: Which of the two diameters is more important from functional standpoint? If it is the hole, then I would not hesitate too much and choose it as datum feature.

Second question: How long is the part? If its diameter-to-length ratio is greater than 5* (this number is just an example), I would consider choosing a flat face (axial stop) as primary datum feature in a ID-to-OD coaxiality control in the first place. This in most cases better reflects how this kind of parts work in reality and additionally eliminates the problem of orientational instability of the part held for inspection using only small ID.

Next thing: There is no need to worry about the hole surface in the "undefined" area because the hole itself is full cylinder that can be easily verified against size and Rule #1 requirements (assuming you are using ASME). However, more problem can be with size control (LMC limit) of the truncated OD - simply because in the area lying on the opposite side of the flat there is no chance to find opposed point for size limits verification. If you choose OD as datum feature this may be a bigger problem. If you decide to control OD relative to the ID, the problem may be of less significance depending on the type of coaxiality control you will choose (runout vs. position).

Another option would be to control the OD and the flat with profile of a surface.

*EDIT: I just noticed that I originally used incorrect value for diameter-to-length ratio.