Holes on Angled Surfaces

[PRuggiero]

Guys,

I'm trying to think of the best way to dimension from holes which sit on an angled surface to my A|B|C datums. I've attached my first shot, which I think is correct up to the part where I label the edge of my angles surface 0,0. How can I cleanly ordinate dimension the holes?


Thanks,
Pete

 

[PRuggiero]

View A attached

 

[PRuggiero]

View B attached:

Sorry if there is a better way to do this...

 

[pmarc]

As long as both dimensions 1 (shown on view A) from datum axis B to the edges of angled surface are basic I do not see anything seriously misleading in that approach, although if I had to use this method, I would probably set one origin at datum axis B (since this is the origin of datum reference frame A|B|C) and dimension all the holes from that point. I would also try to apply basic 45 degrees dimension to one of the axes of the holes and not to the top surface of angled plate.

My question however is: how is datum feature C exactly defined? Does whole eliptical hole serve as datum feature C or just its width or length?

There are also other methods. The one that seems to be quite simple is to locate just one of the holes (or one row of the holes) directly from origin of DRF A|B|C and apply basic dimensions from that hole (or row of holes) to other holes in view B. Since basic dimensions are not cummulative, the location of the other holes would be clearly defined.

 

[MechNorth]

Pete,
Pls give us some idea of what the funcionality / functional relationships are between the features & the datums, and between the datums. I am leaning towards an auxiliary datum reference frame based on my perceptions.
The problem with dimensioning from the edge of a plate (even with basic dimensions) is that people WRONGLY will believe that the features can be checked from the edcge of the plate rather than from the datums. In this case, I have used an off-part 0/0/0 point with basic offsets to the datums so that people don't associate dimensions with actual feature edges.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[PRuggiero]

Thanks for the feed back guys.

Datum c is meant to be the short dimension of the slot

The box bolts down on datum a and is aligned to the mounting surface by two pins which go into the hole and slot datum b and c respectively

I was trying to stay away from adding more datums but i see your point

 

[PRuggiero]

What do you guys think about this method?

 

[PRuggiero]

View C

 

[MechNorth]

Could you pull everything into one sketch for us? I can't tell what your actual datum-B and datum-C features are.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[PRuggiero]

Good idea... See attached.

Datum B is a hole

Datum C is the width of the slot

 

[PRuggiero]

Forgot View A, added to make it a little easier to see the entire picture

 

[MechNorth]

You have the right idea. Don't forget to control your datum features wrt the higher precedent datums.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[PRuggiero]

Jim,

Thanks for the feedback, I felt kinda odd dimensioning holes from a non normal view (with respect to the holes).

 

[MechNorth]

It may seem unnatural to dimension that way, but everything needs to tie back to the datums with basic dimensions.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[PRuggiero]

What if on that same angled face (with the holes) I had a pocket which I wanted to control back to A B C but also control a little tighter with respect to the holes. What's my best route here? Do I setup another set of datums with the holes and then profile the pocket to that?

 

[MechNorth]

Then you're getting into something more interesting and challenging.
First, principle of simultaneous requirements. Second, composite tolerancing of non-similar features, depending on the amount of inter-feature control required.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[pmarc]

Jim,
What you said is absolutely correct IF "the holes" in PRuggiero's recent question are understood to be the ones currently selected as datum features B and C.
However my understanding was that he is attempting to tighten the relationship between the pocket and the pattern of holes at the angled plate. In that case simultaneous requirements and composite tolerancing may be useless.

 

[MechNorth]

Pmarc, why would it be different?

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[pmarc]

Okay, I see and agree that with some tricks this could be somehow accomplished.

 

[PRuggiero]

I understand how simultaneous requirements would keep the holes and pocket together during inspection but I don't see how a composite tolerance would allow me to tighten the relationship between the holes and pocket while keeping the relationship back to DEF somewhat "looser". Unless you guys are saying I need to setup datums based on those holes and then do a second profile for the pocket to those datums....?