True Position and Datum requirements
True Position and Datum requirements
(OP)
I need some justification on this one...
Our design group has released a drawing with a hole pattern on a sheet of honeycomb core. The requirement of the holes is TP of .250 LMC relative to Datum 'A' only. Datum 'A' is the planar surface of the honeycomb. My question is...
In controlling the position of these holes is it possible to control to only 1 datum feature? Since this is concerning POSITION with basic dimensions, doesn't there need to be more controlling features, ie. datum 'B' and/or 'C'?
Joe Jackson
Goodrich Corporation
Our design group has released a drawing with a hole pattern on a sheet of honeycomb core. The requirement of the holes is TP of .250 LMC relative to Datum 'A' only. Datum 'A' is the planar surface of the honeycomb. My question is...
In controlling the position of these holes is it possible to control to only 1 datum feature? Since this is concerning POSITION with basic dimensions, doesn't there need to be more controlling features, ie. datum 'B' and/or 'C'?
Joe Jackson
Goodrich Corporation





RE: True Position and Datum requirements
RE: True Position and Datum requirements
I thought so, but when addressed the design group claimed that the pattern itself is "floating", but...
doesn't that contradict the purpose of positional requirements?
Also, 2 of these locations are for an insert with a drilled hole. They did the same for these 2 holes, TP relative to only datum 'A', then labeled these 2 holes as Datum 'B' and 'C' respectively. They then use these datum features (holes) to control surface profile of the exterior features!
My point is...if the hole pattern itself is not contained as you stated, then how are these 2 features gonna control any other features of size?
BTW... Is there any justification of this principal in the ASME 14.5, I am not finding any direct explanation of this?
Joe Jackson
RE: True Position and Datum requirements
RE: True Position and Datum requirements
If you want to control the position of the feature pattern as well as the features within themselves then use a composite true positional callout. In ASME Y14.5M it spells out all the typical uses of TP. I suggest getting the spec and validate this foryourself. Is this honeycomb core being machined in its solid state or stretched out? If you're designing a panel the features for potted inserts are usually completed after the panel has its face sheets assembled.
RE: True Position and Datum requirements
The honeycomb is being fabricated as a sandwich with inserts for drilling. The drilling of the 2 "datum Holes" are for post fixturing on a fixture plate for CNC routing. The drawing is for the build of the sandwich with 2 holes drilled after cure for location purposes.
The problem is that the design is trying to control the location of the plugs and the 2 holes in relation to the plug centers with the TP callout relative to only 1 planar datum 'A'.
My problem is that I am trying to convince or justify, if you will, to the designers that this is a problem in manufacturing. I need justification from the ASME standard or likewise stating that it is theoretically impossible to control the position of the plugs and holes relative to 1 planar datum! Any help with that is greatly appreciated. Thanks.
Joe Jackson
RE: True Position and Datum requirements
The rest of the features will be referenced from these tooling holes? Unless I'm missing something this sounds okay to me.
RE: True Position and Datum requirements
I agree with Heckler and ewh, it is somewhat unusual but not incorrect. It may be incomplete, which seems to be the problem you have with the single datum reference.
If I understand your example correctly, Datum A is one planer surface of the panel. The two holes (Datums B & C) are drilled into this surface. The existing callout controls perpendicularity to the surface (A) but doesn't control their location relative to the plugs for instance.
If the other dimensions and tolerances are such that the holes are located "close enough" to the center of the plugs to work and the holes are perpendicular to Datum A, then all subsequent features can be relatively precisely located using all three Datums A, B & C.
Regards,
RedPen
Sr. Designer/Lead Checker
RE: True Position and Datum requirements
The ASME standard explicitly calls out a procedure where you have two positional tolerances for the hole pattern. One pattern is from datums A, B and C, and allows a TP tolerance of something like 1.5mm dia. The other is from datum A, and allows 0.25mm dia. The indication is that you are not concerned about the location of your pattern, but you are concerned about the location of the holes with respect to each other.
Your pattern cannot just float. There has to be a tolerance for how far it can float.
JHG
RE: True Position and Datum requirements
It would seem that the holes can be related using LMC with relation to the OD of the insert. That would of course require a datum/s relationship for that purpose. A separate detail would best serve that purpose.
At the assembly level, the two holes could then be used to establish a datum reference frame for the location of other holes or the perimeter of the honeycomb panel.
RE: True Position and Datum requirements
I hope this helps!
Tunalover
RE: True Position and Datum requirements
The last sentence of your explanation is confusing. Can you reviiew to see if error in typo or something.
Also I am a little lost on your explanation of not requiring a datum for orientation on the circuit board.
Ringman
RE: True Position and Datum requirements
Tunalover
RE: True Position and Datum requirements
Is there something that I am missing on the geometry of the hypothetical circuit board in the discussion?
RE: True Position and Datum requirements
Without clocking, the inspector can choose any angular orientation of his choosing. It's simply a matter of how he wishes to establish his coordinate system. The tolerances don't care what coordinate system is used or how it is oriented! Normally, the dimensioning method will infer the coordinate system (often with one datum running lengthwise of the board and the other at a right angle to it).
Tunalover
RE: True Position and Datum requirements
I don't believe that Y14.5 allows inference of a feature as a datum.
RE: True Position and Datum requirements
Tunalover
RE: True Position and Datum requirements
to center lines, center planes, or axes. (See Y14.5M-1996 section 4.3.2)
RE the circuit board problem... if only the two datums are established, you are saying that the rotational orientation of the part is not important. (See Y14.5M-1996 section 4.4.2) If the dimensioning scheme allows the inspector to infer the datums, why not just identify the datums and remove any ambiguity?
Ed
RE: True Position and Datum requirements
Tunalover
RE: True Position and Datum requirements
Perhaps this is the statement that is the cause for misunderstanding.
"Normally, the dimensioning method will infer the coordinate system (often with one datum running lengthwise of the board and the other at a right angle to it)."
I believe that myself and others have interpreted it to be accepting of implied datums.
RE: True Position and Datum requirements
Im not sure whether you got the justification you were looking for or not. After reading through the responses and trying to digest, I think not so.
I am of the opinion that for you to apply LMC to ANY hole, a datum, other than a plane would be required. (Or in addition to). This will effectively control the minimum edge distance. Do you agree and has that portion of your situation been resolved?
RE: True Position and Datum requirements
I should have chosen my wording more carefully. I said:
"Normally, the dimensioning method will infer the coordinate system (often with one datum running lengthwise of the board and the other at a right angle to it)."
What I should have said:
"Normally, the dimensions will infer the coordinate system (often with one of the datum reference planes running lengthwise of the board and the other at a right angle to it)."
The datum assigned to the hole axis has two mutually perpendicular datum reference planes passing through the axis. The best choice of the angular orientation of these two planes is inferred from the dimensions between the two holes.
Tunalover
RE: True Position and Datum requirements
You are describing something I have done several times, mostly on sheet metal. The extreme case is that I bent all four edges to work as gussets. It is difficult to locate holes from an edge, and easy to locate the holes from each other, since they are located on the same flat surface.
Datum_A is the bottom surface. Datum_B is a hole, and Datum_C is a second hole. The flanged edges are located by sloppy tolerances or a sloppy GD&T profile.
I can now register the part precisely, and inspect it.
The GD&T standard shows simple parts with circular shapes and hole patterns inside, where there are only two datums, the base and the OD. This makes sense to me, as long as there is only one hole pattern, and there are no rotation issues.
Once the geometry requires you to control rotation, you need three datums.
JHG
RE: True Position and Datum requirements
This practice is good for when one or more holes are created BEFORE the part outline. In PCB manufacturing, the router often uses one or two holes as a reference when forming the outline. This is amenable to "step and repeat" processes where many circuit boards are created from a large sheet of material.
You said "This makes sense to me, as long as there is only one hole pattern, and there are no rotation issues."
I've had many hole patterns on the same part dimensioned this way using 'A' as the primary and 'B' as the secondary. There is no limit to how many hole patterns can be dimensioned this way in one part.
Tunalover