How to handle draft in a tolerance analysis 1

[C98hris]

Can someone give me some help about handing draft angles in a tolerance analysis. Attached is an image of three parts. The two blue parts are mounted to the purple part on faces which are drafted. I know all the green dimensions and associated tolerances, but I would like to establish the tolerance stack up for the red dimension. Is it normal to consider the distances between the dashed lines caused by the draft angles or is this usually ignored. If it is included the analysis becomes quite complex, having to consider tolerances of the draft angles as well as horizontal positions of the blue parts. I would just like to understand if it is standard practice to include this level of detail.

Thanks

 

[djhurayt]

great question C98, I will be looking forward to responses, I have to deal with castings and drafts pretty regularly.

 

[3DDave]

Only worry about it if it matters. Otherwise you'll need to create a calculation to evaluate it. If it's really important, don't depend on as-cast features without understanding the specifics of the process, including any variability from heat treating and straightening.

 

[drawoh]

C98hris,

It looks like high school trigonometry to me. Alternately, you can sketch this up on your favourite CAD software.

Draft angles to me mean you are using castings, which are not very accurate. If accuracy matters, you need a different process.

--
JHG

 

[KENAT]

Simplistically, yes you need to allow for the draft. Essentially evaluate where the parts actually 'touch' to give the resultant stack.

Now in the real world parts are not infinitely stiff, sure typical metal castings are near enough that for tolerance calculations they can be treated so in most situations. However, plastic parts with draft are often quite flexible & soft which will impact eventual stack in many cases.

Remember that the tolerance on teh 2 draft angles could be going in opposite directions e.g. for nominal 2° draft with +-.05° variation one surface could be at 1.5 ° and the other at 2.5° - depending on size of feature this can end up with quite a big impact on linear tolerance.

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