Deep Bore Dimensioning

[dezignr]

I am looking at a drawing that requires a couple of pretty deep bores (.375 dia x 20" deep). There is a conical positional tolerance zone locating the axis, presumably to recognize the possibility of drill wander, and there is also an axis straightness RFS callout associated with the FOS. I'm struggling with how to interpret the axis straightness requirement in conjunction with the conical shaped locating tolerance. I have no experience with deep bores such as this and I am inclined to suggest a simple perpendicularity tolerance at MMC. The 2009 standard has an example of using different controls at the ends of a FOS in para 7.4.3.

Your thoughts are appreciated.

dezignr

 

[Tunalover]

How are you establishing the conical tolerance zone?


Tunalover

 

[dezignr]

The conical tolerance zone is established with basic locating dimensions and a positional tolerance of .010" at the hole start surface and .040" at the hole end.

 

[Tunalover]

Good. I would think that the positional tolerance zone alone would be sufficient but who am I to question the drawing? Is the straightness tolerance less than the smallest positional tolerance?


Tunalover

 

[dezignr]

Tuna, yes they seem to be trying to refine the position with straightness. I too believe the pos tol is sufficient but I was looking for another view point in case I'm missing something.

dezignr

 

[Tunalover]

I have nothing else to add then. Anybody else want to step up to bat?



Tunalover

 

[jassco]

Hi, dezignr:

Could you post an image to show what you have?

Best regards,

Alex

 

[dezignr]

Sorry, but I can't add a pic due to proprietary contraints.

dezignr

 

[3DDave]

It seems to me the application doesn't care what happens between the start and finish. I'd give a position tolerance for a small distance on the start, a small amount on the end, and indicate a minimum clear diameter and straightness between them.

Trying to combine straightness and position in this doesn't seem useful. One might look at it as the same problem as welding a thin rod to a plate. It may matter where it starts and where the end is located, but in between those two places the position of the center of each section is unimportant. It could be very straight, kink, or corkscrew, but as long as it is straight enough and the diameter is OK the rod would still function.

 

[dezignr]

The designer has decided to remove the straightness tolerance and rely solely on position and size limits. Functional analysis and stack-up shows acceptable axis deviation without trying to impose axis straightness.

Thanks, dezignr

 

[Tunalover]

3DDave-
Don't forget that the applied tolerances apply to the entire length of the feature. In this case the pt is a straight line function of depth into the hole. The two pts don't just apply to the entrance and exit.

Tunalover

 

[3DDave]

That's why I suggested zones along the feature. By default tolerances apply over the entire feature, but there's no rule preventing their application to a portion of the feature.