countersink orientations

[cjccmc]

I have a large contoured part made of uniform thk material which will have 70 countersunk holes. (picture a 1960's era car fender made of thick material) All three datums (A,B,C) will be established by target points and few if any holes will be normal to any datum but all holes need the countersink (cone) to be normal +/- 0.5 degrees to the surface at the point where it is drilled. These are oversize holes and I can allow .015 dia tol on position for each. There is no hole-to-hole tolerance requirement. If the .5 deg angle error for a given material thickness equates to .003 dia tol zone can this be done with composite tol something like:

TP .015 A/B/C
--- .003 A/B/C
70X INDIVIDUALLY

These parts use Model Based Definition where you query the model to find the location of each hole (no dimensions given). Does above callout do what I want it to, orient each hole/countersink normal to the (theoretically perfect)surface within a .003 tolerance zone? And does the "INDIVIDUALLY" break the hole-to-hole tolerance of .003?

 

[MikeHalloran]

I think the tolerance as given should please the CNC programmer who has to program the 5axis mill to make the cuts. (... since there is no reference to the part surface at the csk location.)

... but if the part is at all flexible, or inflexible but not perfectly shaped, the diameters of the csks at the part surface may be other than what you intended.

I.e. if the part is rigid and the shape is good, you can do the csks complete in the mill. But if the part is imperfect or can't be fixtured so it's rigid at the csk locatins, you will only be able to use the mill to spot the holes, and you'll have to execute the csks manually.

At least that's how it smells from here...


Mike Halloran
Pembroke Pines, FL, USA

 

[pmarc]

Some loose thougths:
- If you want to break the hole-to-hole relationship of .003 I would recommend applying orientation tolerance (most like angularity) in addition to TP .015 A/B/C callout.
- Will position/orientation tolerance apply to countersunk portion of the holes at all? And if yes, which portion of the countersink exactly? I asked about it in following thread:

http://www.eng-tips.com/viewthread.cfm?qid=351650

(reply from 12 Sep 13 10:01), but no answer was given.

 

[cjccmc]

I had in mind that the composite tol would apply to both hole and countersink, like this:

DIA .219
CSINK DIA .395 X 100 DEG
TP .015 A/B/C
--- .003 A/B/C
70X INDIVIDUALLY

 

[pmarc]

I figured out what you had in mind, but with the notation you are proposing the questions from the other thread still apply.

As for 70X INDIVIDUALLY note, it is not clear for me which segment of the composite callout the note applies to - to both or only to the lower one?
If to both, this would mean to me that the composite positional tolerance is applied to each hole individually, and that each hole is treated as a separate feature. In that case composite callout would not really make sense, because the purpose of composite positional tolerance is to deal with patterns, not with individual features.
If to the lower segment only, then, like I said in my first post, orientation tolerances are usually used to untie the hole-to-hole spacing relationship. Per Y14.5 standard INDIVIDUALLY concept is dedicated to slightly different applications.

 

[cjccmc]

pmarc, thanks for your replies. Can you post the GD&T callout you would recommend for the end result (described in my OP) I am trying to get? I care about orientation of the countersink more than the hole because that will orient the screw. I am working to 1994 std for this project.

 

[pmarc]

Here you are (additional comments are in the attachment):

http://files.engineering.com/getfil...9e1-4c5d-b7c1-5a4e16a2e2b8&file=csink_tol.png

Hope this helps and does not make things more complicated.

 

[cjccmc]

thanks for the nice graphic!

What you wrote in the 2nd bullet would apply, since the csink is related to hole D, I would need D to be normal to the surface. The only downside I see is that to end up with the Csink +/- .5 deg, I would have to split that tol between D and Csink, D being +/- .25 deg and then Csink to D +/- .25 deg. In actual use the oversize hole will not contact the screw, it just provides pass thru clearance and the Csink will eat thru 70% of the matl thk.

Is there a legal way to call out just angularity of the Csink to A/B/C, like:

DIA .219
TP DIA .015 A/B/C

CSINK DIA .395 X 100 DEG
TP DIA .015 A/B/C
ANGULARITY DIA .003 A/B/C

70X