No [ - C - ] datum on engine bracket?

[JoeBlock]

Every hole or slot is toleranced to [-A-],[-B-]. [-B-] is a slot. [-A-] is a plane perp to the slot. How do I align the part for layout purposes? The part is 8 times longer then the [-B-] slot.

 

[dingy2]

Well, you should have a teriary datum. All dimensions should come from the centre of B datum with a tertiary datum included for orientation. In your present situation, you should align the sides of the slot for orientation but there is a BIG chance for error here especially since the part is quite long.

It would be better if the hole was in a positional tolerances in MMC and the slot was also in MMC. Now you could use a checking fixture (functional gauge) where there would be a pin the shape of the datum B slot but in a MMC size and theoretically oriented to the hole. Place the part over the datum B pin and one will see a certain slop. The positional checking pin for will be placed in the hole to confirm its location.

Dave D.

http://www.qmsi.ca

 

[JoeBlock]

Thanks Dave,

Appreciate the function gage info. I do have MMC. For capability studies of machined hole positions I plan to set it up on the CMM RFS for the variable readings. Maybe if the [-B-] slot locater had 2 spheres with a spring in between forcing them apart, but I just don't think I'll pass an under 10% R&R with that.

Thanks Again,

Gary

 

[dingy2]

Love to see how you do capability studies on positional tolerances since I know it can't be done. It is all hocus-pocus and make believe. I hope to have something online from a presentation I make to an ASQ group on why it can't be done properly.

As far as the 10% R & R value on critical characteristis, one must remember it is a percentage of the process and not the tolerance per MSA 3rd edition. The way it is set up on the B slot, I am pretty sure that you will not make it. You probably will be lucky to get under 30% and most likely 30 - 50%.

Good luck on that one!

Dave D.

http://www.qmsi.ca

 

[dingy2]

JoeBlock:

I finally figured out how to do this. I have enclosed a web site reflecting capability studies on positional tolerances and how it just doesn't work out. This is a part of a presentation that I made to an ASQ group in one of their dinner meetings.

See if it makes any sense to you. I hope that this will help you.

http://www.qmsi.ca/capabilityindex.html

Dave D.

http://www.qmsi.ca