DRF for module with pins

[dthom0425]

Hi all,

I have attached a picture of a redacted model that I'm working with...may features are missing on this but it should get the point across.

This part slides in between rails (on the thickness), blindly hits some receptacle bushings for the pins you see in the part and then seats on the 2X flat surface the pins are coming out of.

I'm wondering just how you could go about setting up the appropriate datum reference frame for this part. Historically, everyone here just uses the typical 3-2-1 style which, from a functionality standpoint, doesn't make too much sense to me for this part.

I'm toying with the idea of making the 2X flat face (that seats the pins) the primary datum with the pattern of 2X pins being the secondary datum. To me, those are the alignment features for this part. My concern was that the 2X flat faces are a little small, not sure how reliable that surface will be?

Let me know your thought.

Thanks in advance.

 

[Jacob Cheverie]

Is there an issue with using the thickness/width as primary datum, pattern of pins as secondary, and 2x flat faces as tertiary? What kind of contact do you have between the width and the rails?

 

[dthom0425]

It is an atmospheric fit which is why I didn't want to make the thickness a datum.

 

[Jacob Cheverie]

Why not make the pattern of pins primary and the 2x faces secondary then? It is like you suggested, but with more repeatability of the primary datum.

 

[Burunduk]

What features have the most amount of contact and the tightest adjustment to the mating part? My bet is the flat faces. If true they should be referenced as the primary.

 

[dthom0425]

The two pins engage in a bushing (slight clearance), then the 2 pin flat surfaces bank up against the mating flat surface. 2X pin as datum feature A and 2X flat surface as secondary datum feature B seems like it could work and mimics how my part is mating.

 

[Burunduk]

If there is clearance between the pins and the mating holes and a fully stable face contact on the flat surfaces, the two flat surfaces should be the primary datum feature. Face contact is what expected to stabilize the part in the interface. Unless I didn't understand you correctly.

 

[dthom0425]

My concern was that the flat faces are very small and wouldn't create a stable surface.

 

[Burunduk]

It should be according to the function.
What type of fit is between the pins and holes? If it is transition fit or tighter and the contact between the pins and mating holes is what prevents the part from rocking, then the pins as primary make sense.

 

[3DDave]

What controls the location and orientation of the part? Is it loosely held by hand or is it firmly attached to a rigid guide rail system?

To decide, build a model where the faces are at least 2 degrees off from each other and decide how it would fit correctly if made that way. If you envision the part flipping up to allow the pins into the holes, then make the pins primary. If the top or bottom cannot be anything but horizontal then that is primary.