Profile Tolerance - 2 Different Coord Systems

[ewh]

-Lofted part with datums defined using 6 datum points on surfaces.

-Mold for part, with convenient surfaces for datums.

Since both part and mold are defined relative to three primary datums, is it a matter of concern that the datums are not defined exactly the same? Does it matter that the mold has datums defined from flat orthogonal surfaces and the part from six datum points?
I think I know the answer, but need to explain my reasoning to the project manager, and don't want to seem too ignorant on the issue (especially if I overlooked something). Thanks!

 

[drawoh]

ewh,

How easy is it to design a molded part with datum targets such that you can use the datum targets on the female mold? The really serious GD&T types would probably insist on you doing this, but at some point, it is not possible.

I believe the crude assumption with tooling is that you need to be ten times as accurate as your part. If your mold is fabricated with that accuracy, then your part datums can be located from your mold datums.

JHG

 

[ewh]

Thanks for your response, JHG.
My reasoning is that the part and the mold are each tied down to three primary datums, regardless of where those datums are.
As you state, mold tolerances are much tighter than the part tolerances, and it only seems reasonable to use the most efficient method to inspect the items. However, to please the customer, we have to use their part datums when inspecting the part. There are situations where the part inspection is "bought off" the mold.
Since the mold tolerance is always tighter than the part tolerance, wouldn't the part always be within it's own tolerance, even though the mold uses 3 different datums?
What is the worse case situation using two different datum systems?

 

[drawoh]

ewh,

When you translate from one coordinate system to another, you lose accuracy. If you lose enough accuracy, your part goes out of specification. It is desirable to use the same datum system throughout your manufacturing process. Then again, if your losses are tiny enough, they are irrelevant.

Some people have trouble with terminology like desirable and preferred. They insist on absolutes.

Is your customer inspecting your tooling or auditing your drawings? If your parts pass inspection, they should not care how you do it. This sounds like the equivalent of somebody specifying the drill size for tapped holes.

JHG

 

[PaulJackson]

ewh,

When datum target points are used to constrain all six degrees-of-freedom for a component’s DRF the dimensions describing their location have to be basic. Typically those basics are relative only within the system of targets themselves so that the “nest” definition can be transported simply and identically to other details such as machine registry fixtures, gages, or even to iterative probing CNC or CMM programs that hunt for the actual surface condition that only an actual physical registry would provide.

Using the same targets in two or more different systems will (theoretically) never deliver equitable registry with the one system! That said “what’s the marginal difference”…it depends on the surface conditions “the margins between subsequent constraints and expected registry surface qualities…specifically roughness, form, and orientation.”

I once asked a respected GD&T guy (a member of the Y14.5 committee for the 1994 rev) to help me by convincing a team to describe cast datum targets on a new transmission project. I wanted him to support my assertion that we needed targets described in such a system described above. Well he not only did that but insisted that there be zones around each target area that constrained both the form and orientation of the target areas. I thought this is “overkill” but the group bought it along with the datum target registry scheme that I was trying to sell so I wasn’t too disappointed.

Since that time I have seen numerous designs that use the full complement of target points solely for the first operation and then they use the secondary and tertiary points or just the tertiary point with the new machined datum surfaces for the DRF. You can’t do that and expect to repeat precisely the same target spots because the original primary and/or secondary is gone!!!

I am a purist at heart which makes me cringe when the rules (I perceive) are violated, so to answer your question I would say NO! You cannot use equivalent datum target designations with differing datum basic datum target locations for all six-degrees-of-freedom….but …I have found solace in marginal victories of GD&T understanding in the past so I won’t discount that for you.

Paul

 

[drawoh]

PaulJackson,

I think you are talking about casting a part with datum targets, machining the datum targets off and specifying new datums. This is a different issue from ewh's question. I strongly agree that once you have the part, the datums should not change through the manufacturing process.

In a very narrowly defined sense, a casting die is a separate part from the casting itself. Making good casting drawings is very much more difficult than making good machining drawings. I would make sure I selected the right datums on the part from casting through to machining. If these are not convenient datums for the die, I would not worry about it.

JHG

 

[ewh]

Thanks for the input, guys!
It turns out that the actual concern was with HOW the mold datums are controlled, not whether they were the same as the part. It was agreed that any discrepancy between the final part relative to the mold datums would fall well within our tolerance.
So, different issue, problem solved.

 

[PaulJackson]

Drawoh,

In the first two paragraphs I was trying to explain that the datum references established solely by target points |A|B|C| and |A|D|E| will have two distinctly different |A|'s for each DRF because the secondary and tertiary are integral in defining the targets for |A|.

The third paragraph was just an experience I had that demonstrated the diversity in opinions about datum targets.

The fourth paragraph relates to your response and the fifth paragraph tried to tie both situations to a conclusion and offer some "how much difference does it really make" retraction to the problem.

Sorry my writing isn't more succinct with my thoughts.

Paul

 

[PaulJackson]

As I look back on the original question it appears that I was answering a question that was not asked! Sorry for the detour.

Paul