Hole Pattern Datum - Limited Constraint

[somervil09]

Hello All,

This may be a simple question, but I haven't seen a clear illustration that directly addresses my situation. I need to establish a secondary datum using a pair of holes, the datum being the midpoint between the two centers (point P). See the attached sketch for illustration. The rotation of the part should be defined by the dia.12 hole at the center of the part. Essentially I want to define point P as the X,Y origin and fix rotation using the third hole. Calling out the 2-hole pattern as datum B would fully constrain the part without regard to the third hole (correct??), so what would be the best way to establish the DRF as I would like it to be?

 

[jassco]

Hi,

Attached please find Fig. 11-38(a) from James Meadows' book on ASME Y14.5-2009.

What do you think of this approach?

Best regards,

Alex

 

[chez311]

jassco,

As I said I was unsure whether your first scheme with the width dimension was acceptable per the standard, it seems that according to Meadows it is. That being said, I'm not familiar with all the ins and outs in how to apply these datums with irregular FOS, however it seems to me that Meadows' example is applied to pins so the boundary exists OUTSIDE the part. In your example the boundary is INSIDE the part - troublesome to say the least when trying to create datum feature simulators (ie: why LMC cannot be functionally gauged like MMC because the controlled boundary of LMC is inside the part). I'm still also not convinced that this would be the best way to specify the datums.

 

[somervil09]

Apologies for the delayed response, I was frying bigger fish so to speak. As for application details, this part is a plastic plate that two other components are mounted to. The mounted components need to locate within .010" to each other (and have their own associated tolerances). The 2x 5mm holes are actually 2 molded posts that mate to a corresponding hole and slot in the mating component. The center thru-hole is centered with a tapered collet to a shaft and fixed. See the attached sketch for an example of the existing callouts (sorry for the unit change). As for "considerable shrink relative to the tolerances allowed" I have typically seen ~50% of the tolerance zone taken up by shrink, which I would say is significant.

chez311, I think you nailed the issue with the current datum scheme here: "2x holes too close together as well as far from the features they control". This was the main reason the holes (posts) were not giving good results, as far as I could tell.

My initial question was applicable to this situation, but mostly theoretical re: how one would call out the system I proposed, or if it was legal or possible. After discussing internally I think we've settled on moving the system to the center hole as datum B and 2x post pattern as datum C.

 

[chez311]

somervil09,

Ah - so the central hole is indeed critical in assembly and therefore a viable datum feature. I would say that this feature should have been specified as the secondary datum from the beginning as it controls translation of the shown component. Is this part fixed with the tapered collet BEFORE mounting the mating component to the 2x molded posts? Also is there any other feature which controls orientation around this central bore/tapered collet at this same point BEFORE mounting the mating component to the 2x molded posts - perhaps one or both of the 2x .156 holes or the width of that tab?

Forgive me for being ignorant as I am no expert in molded plastic parts, but could you expand on the issue with shrinkage a little? The reason I ask is that I was initially thinking that this would be a consistent and predictable phenomenon that would be accounted for in the initial part design (ie: basic dimensions and sizes) and tolerances - is this something that is instead wholly unpredictable and causes inconsistent variation?

 

[somervil09]

Yes, both features are important in this application. The 2x posts are mounted first, and the clocking of the shaft is not controlled (i.e. contact feature with 3rd component is round). As I said above, the question is partly theoretical and not necessarily tied to this specific application. Maybe there isn't ever a situation where it would be required, but it seemed logical here. Several other folks had the same proposal internally.

Shrink in molded parts (in my limited experience) is typically accounted for in the tooling design, i.e. the pin for creating an internal bore will be slightly over-sized from nominal to account for the movement of plastic as it cools. There can be additional variation on top of that movement due to changes in tool setup (batch to batch) or form error for small features, as well as warp, tool wear, etc.

 

[jassco]

Hi, somervil09:

Did you intentionally create the first sketch differently from the last sketch you just posted? On the first sketch, you showed two small posts with different distances from 12mm hole, i.e. those two holes are not leveled. On the last sketch, they are.

As I mentioned, you can add a dimension (FOS) between those two posts. Then you can attach your datum C to this dimension. Datum A, B, and C will immobilize your parts. The two small posts create a derived medium plane that you can use as datum C. greenimi probably disagrees with this proposal. There is a DMP between two holes or posts. It is perfectly legal to use it.

You can also create a circle tangent to the posts. That also create a quality datum. It is just like create a circle from 2p.

Best regards,

Alex

 

[powerhound]

jassco,

Regarding your last statement, you need 3 points to make a circle. Because of this, a circle would not be appropriate in your scenario because there are only two points. James Meadows shows a width. This would be appropriate but it assumes it's those tangent edges that are the functional points.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[jassco]

Hi, John:

A circle can be defined by either 2 or 3 points. If you establish two cylinders (two posts), you can create a cylinder inline and in contact with the two posts. This virtual cylinder can be a very good datum for clocking the part. I'll talk to our CMM programmer to see his opinion.

Best regards,

Alex

 

[powerhound]

If you establish two cylinders (two posts), you can create a cylinder inline and in contact with the two posts.

You see that highlighted part? That's the third point.

A circle cannot be defined by two points only, unless you don't care where the points land on that circle. I can make a circle of any diameter, any center, and any relation to those two points that I want. Just try to make a circle with two points on any CAD package. You won't be able to. It will always require a third input. If you enter two points and a diameter, you just told it where the center point has to be and then you will still have two options.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech