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Draft/round dimensioning


New member
How do you guys normally handle dimensioning parts with draft and rounds? The main problems I'm having are that a) the draft creates two very close edges and makes the dimension ambiguous, and b) the copious amounts of rounds I put on my plastic parts are taking away the dimensioned edges entirely. I could solve the first issue by dimensioning cross-sections, but I'd still be faced with the fact that rounds throw my whole dimensioning scheme out of whack. I'm reluctant to add a dimensions apply to virtual edge-type note, because that's an incoming inspection nightmare, and seems like a major kluge. Got any ideas?


New member
Why are you dimensioning plastic parts its almost impossible. Your vendor would probably rather work directly from the model and you will only need a few critical dimensions.


New member
Sure, they probably would. I'm not willing to relinquish that kind of control to them, though, since I (and our QA department) are tired of vendors making dimension changes wherever they deem it necessary without documentation on our end to defend our position. Besides, in my mind, there is no such thing as a non-critical dimension, if everything is toleranced properly. How was this done in the days before 3D modeling?

laser guy

New member
I'm with wrladner! We don't dimension our stuff either except for critical hole to hole dimensions to keep shrink and plastic molding process in check. Draft and rounds too darn hard unless it's a tupperware mold. In our notes we call out part model is master. We also have to put the shrink rate for the material in the notes too. If you have a good tool maker they will have all non-critical dims within .005 and critical stuff within 0.002 on reasonably sized parts (5x5 or smaller).

We do c

olor different surfaces red, yellow, and green to let the toolmaker know what they have room to fudge a little on. They really like that.

Good luck



When linear dimensions apply to a corner that has been rounded we add TSC to the dimension (Theoretical Sharp Corner). This is usually easy to measure if your part can be checked on an optical comparator. The easiest way to specify draft is with a note (imbed the draft dimension with &d#). You can also show the dimension at one end of the draft with either PLUS DRAFT or MINUS DRAFT added.

You can also have a second instance of your model with all the rounds suppressed which makes the visualization of the model much better. I have requested PTC to add imaginary lines to the display option several times. I even end up drafting them in manually sometimes. No drafting standard calls for tangent lines but they all use imaginary lines. I don't know how PTC can truly claim to meet ANSI & ISO drafting standards when they can't draw an imaginary line!

If a few more people request imaginary line display maybe PTC will finally add it.