Position and orientation of slot

[SDETERS]

We are having a small issue with a slot in a casting. The slot is a tertiary datum. The center of the slot is within the print. The issue is that the slot is rotated 1.5 degrees about the center of the slot. We want the tight location of the center slot which we have, we are also ok with the slot to rotate to a certain degree. What would be the best way to control the slot so we would allow it to rotate and keep the tight location of the slot?



Thanks

 

[3DDave]

Don't use a slot. If the slot rotates and the center distance to what I presume is a pin that fits into the slot, then the part will rotate due to the rotation of the slot. So you need a feature that doesn't rotate; use a hole.

But, if you insist, use a datum target that is the at the exact location and of the exact diameter and use a profile tolerance on the slot that only refers to the mating face and that datum target. It will automatically be a separate requirement and allow the slot to turn as much as you like. I would probably add an angularity tolerance for the slot width to the mating face and the compound of the secondary and the datum target to prevent the slot from turning exactly 90 degrees.

 

[donatim24]

Can you clarify what you mean when you say, "The slot is rotated 1.5 degrees about the center of the slot?" It sounds contradictory that the center is correctly located, but that the slot is also rotated.

 

[SDETERS]

I have attached an image example. This is an extremely quick sketch, I hope this helps. The dotted line is the nominal condition, and the solid line is the actual part. The slot is to size and location, it is rotated a small bit.

Once again thanks for the help and feedback.

 

[donatim24]

SDETERS, thanks for the sketch. I see two options:

1. Follow 3DDave's advice and use a hole.
2. Put a tighter position tolerance on the slot. This will constrain the rotation of the slot to the level you determine is acceptable. This will allow you theoretically looser tolerances compared to the hole, but the difference may only be academic.

Do you know for sure if the slot is really located according to requirements? I ask because the position tolerance (which you would need to constrain the slot as your tertiary feature) should control this sort of rotation.

 

[SDETERS]

The position tolerance on the print is really tight, I think this really depends on how the center plane of the slot is picked up on the CMM. We have a waiver, due to the fact that the slot does not meet the position tolerance on the print. But, when we average the center point of the two slot ends this averaged point is where it needs to be. Now that I am thinking about this a bit more, the reason we have a slot is if the center position starts to slip, then we will really have an issue. Why not a hole if this position is good I see this point now. I was not taking into account the other variable and the reason we need a slot.

Thanks again, I now have some options to think about.

 

[jassco]

Hi, SDETERS:

A slot has 6 DOFs. You will need to split this datum feature into two. The first one is a slot controlling 4 DOFs with loose tolerances. The second one is a "pin" controlling 2 DOFs (a translation and a rotation) with a tighter tolerance. You will need to use customized datum reference frame to control both the slot and the "pin".

Best regards,

Alex