dimension drafted surface best practice

[hellomoto]

Almost all molded plastic parts have drafts. Any comments on what the best practice is for dimensioning features with drafts?

Let's use a simplified example for the discussion:

A machined circular boss (O.D. +/-.003) on a plate has hard edges. Drawing show dimension from diameter edge of circle to one of the four edges of plate (+/-.010). Inspector can use calipers or video inspection or etc. to inspect the dimension: Easy.

A molded plate with a circular boss requires the boss to be drafted 1 deg and a R.01" fillet at the base and a R.005" rounded edge at the peak. Inspection not so easy. (e.g. Inspector with video inspection system may not be able to find hard edges, use of calipers would also be challenging due to the draft on the boss)

What would be the best way to dimension the boss location to the edge such that it can be inspected? (and what would the inspection method be?)

 

[dr_gallup]

Using video (optical comparator). you should be able to measure at the top to a Theoretical Sharp Corner at the top. In that case I would add the text TSC next to the diameter dimension. You might also want to add PLUS DRAFT to indicate that the diameter can increase as it goes down within the limits of the draft allowance. If that is not sufficiently accurate, the standard method for measuring a tapered shaft is to use a basic diameter with a toleranced height dimension. To measure it you would put a ring gage over the boss and measure the height.

 

[hellomoto]

"To measure it you would put a ring gage over the boss and measure the height."

Measure the height of the ring gage while it is on the boss?

Thanks for your reply. I just realized that, what I really want to know is how to treat drafts during inspection (short of building go-no-go gages for all features). Knowing that would enable me to dimension the part accurately.

So, how would a curved slot with draft on the inner surfaces be inspected? To simplify, let's change that to a straight rectangular slot with full radius ends, fillet at the bottom and radius edges.

 

[cgosnell]

In the area of forging design, we are moving away from individual feature measurements to an overall and/or targeted GD&T profile tolerance with the CAD model as the nominal reference.

There were two way to measure manually:

1) try to find the theo. corner
2) pick a point in the surface down from the theo. corner(in Z, of example), and then check the distance to that point in X, or Y.
3) OD and ID's were checked with radius gages.

Tooling tolerances were typically 10% of part tolerance (tool tol approx +/-.005)