datum question

[rhmeng]

Hi,
I have a part that I am dimensioning, and I want to make sure I am correctly using datums. Is this the correct way to use the datums A and B that are not perpendicular to each other? I can call out a basic angle between -A- and -B- and then i can dimension from the "imaginary" surface that is perpendicular to -A-? Then I can put positional tolerances on the holes wrt -A- and -B-, but in reality it is really checked to -A- and this imaginary surface?

 

[greenimi]

You did not tell us how this part works, how thin it is and what are its relationships with the mating parts

 

[rhmeng]

it mates to a part that is almost the same as this one. 4 screws go through the 4 holes in the bottom left area of the part. It is about an 1/8" thick.

 

[greenimi]

In that case I would use the flat surface as primary datum feature and the pattern of the four holes as secondary ( at MMC) and then locate ( position and /or profile ) all the other features

 

[rhmeng]

ok but then where would i dimension from if I am using the pattern of holes as the datum feature. I am unsure of how to dimension off of a pattern.

 

[rhmeng]

after doing some research, it seems like it is not good practice to create a datum from a pattern of 4 holes which do not have simple geometric relations between themselves. The 4 holes that you are saying I should use a pattern to create a datum from have odd angles and distances to eachother.... Basically i just want to know if the way I currently have it (in the picture) with the datums is an acceptable way of using feature control frames. Thanks.

 

[greenimi]

Basic dimension to the features, basic angles, profile (all around for the rectangular features) for the outside edges
Basic dimensions to the cetner of the holes and then locate them (position)
Basic dimensions should come from the center of the pattern (most preferred solution)

 

[greenimi]

So, you are centering this part on something that has "odd angles and distances to each other". Interesting concept. is it a very robust design? Works (can you assemble it) ?
What is the scrape rate?

 

[greenimi]

Also, see fig 4-4/y14.5-1994
FIG. 4-4 INCLINED DATUM FEATURES

So, in your case you can use it, but is entire non-functional approach.

 

[M4C7]

I agree with greenimi that from a functional standpoint, it sounds like it would be best to use the flat surface as a primary and the pattern of holes as the secondary datum feature. While you may not be able to easily establish a "center" for the feature pattern, that is not strictly necessary, per ASME Y14.5-2009 Para. 4.12.3:

"The origin of the datum reference frame may be established at the center of the pattern of the datum feature simulator…, or at any other location defined with basic dimensions relative to the datum feature simulator…”

I believe this has been discussed a couple of times on this forum by people with a lot more experience than I have. Here are some relevant threads you can look through

http://www.eng-tips.com/viewthread.cfm?qid=364740

http://www.eng-tips.com/viewthread.cfm?qid=283384

 

[rhmeng]

@greenimi, thanks for the info and figure 4-4 is where I originally got the idea from. It makes sense to me with my minimal machining experience that this would be easiest for the machinist to use these two surfaces as Datums (the ones i have in the picture above, and in figure 4-4) vs establishing datums off of a hole pattern. From a functional standpoint, however, once i did more research I see you are correct it should be off of the hole pattern, I was not familiar with this so I was skeptical... now I changed my drawing. Thank for all the help/lessons.
@M4C7, thanks for pointing out that the part does not need to be dimensioned off of the center of the holes, this will most likely make things much easier for me