DRF Change Impact Example

[Tegguy]

I have been a design engineer for ~10 years now using GD&T almost daily. I have recently started a new position with a younger team and have noticed a lot of DRF changes to make the pierce surface the primary datum for holes. This is obviously causing a lot of useless datums to be generated as well as increasing the tolerance stack up between everything. I am working to help teach them GD&T basics but one thing I'd like to communicate it the impact of having one feature to A/B/C and another to B/C/A... I know this impact mentally but I was trying to find a visual aid/ real world example to help communicate this to the team.

Does anyone have any good things they've used in the past to help communicate this type of a topic? Right now the best thing I have is something with an exaggerated angle on B with something that mounts to it that I need parallel to A but was hoping the smart people on here might have some other ideas (PowerPoint, 3D printed, ect)

Thank You

 

[3DDave]

Draw a picture where A, B, and C aren't perfectly made perpendicular and one or two of them are relatively narrow, pointing out that the Primary makes maximum contact and the others only provide orientation that isn't constrained by the primary.

Feel free to exaggerate. The holes will end up so far off the BASIC dimensions relative to part surfaces (still in-spec from the datum feature simulator/true geometric counterpart surfaces) they might get really angry.

Edit: Sorry - blacked out with the initial rage of seeing the same crap I've been pushed with about holes having to be primarily constrained to the piercing surface and didn't initially see your plan.

I made a foam core model to show that it was a dumb thing to do - we had a large casting and doing it that way would put the clocking of the holes on opposite ends out by 1/2 inch, which would be a problem with only DIA 0.06 available clearance. Did the stupid method work? Of course, because while the crack smoking team of GDT decorators put down garbage callouts, the crack smoking CNC and QA/QC group didn't bother reading those callouts or inspecting to them. Win-win.

 

[cowski]

Did the stupid method work? Of course, because while the crack smoking team of GDT decorators put down garbage callouts, the crack smoking CNC and QA/QC group didn't bother reading those callouts or inspecting to them. Win-win.

I got a good laugh out of this, thanks.

 

[Tegguy]

Draw a picture where A, B, and C aren't perfectly made perpendicular and one or two of them are relatively narrow, pointing out that the Primary makes maximum contact and the others only provide orientation that isn't constrained by the primary.

I think I'm following but we curious if you have a picture to help communicate

 

[3DDave]

Draw a nominally rectangular block with rhomboid or trapezoidal section where one of the slanted faces is where the holes go. Evaluate the condition for each variation of slope.

 

[Tegguy]

Draw a nominally rectangular block with rhomboid or trapezoidal section where one of the slanted faces is where the holes go. Evaluate the condition for each variation of slope.

Thank you! I got it and I think I came up with a sudo real world example to help communicate this topic (where using the pierce surface would not work for the assembly)