Newb ? - CAD Part interference & mating part dims

[pabmartin]

Hi, Im a beginner Alibre design 12 cad user, making my first tool design in CAD for steel machining. I have a fundamental question regarding the conventional method to define the final mating surface of parts for CNC level machining.
If I have 2 aligned and mating surfaces such as a dovetail and slide or rod through bushing, or even gear mesh what is the conventional method to designate the allowable / or desired fitting space for precision assemblies?

For instance if I have a 3/8 drill rod in a 3/8 hole, though a 1' steel plate bushing and desire a tight tolerance, snug fit yet free enough to rotate. Using manual milling methods I would bore & ream a 3/8 hole and usually it fits perfect. If happens to be just too snug I lap either part with 600 or finer to fit. How does one specify these tolerances in a CNC specification thay I will outsource to one or several shops?. I could adjust both surfaces by a couple of thousandth's or just one part surface... or privide a -/+ allowable tolerance and hole for the best during assembly?

My specifoc situation is the manufacture of a small CR machined steel tool that has several precision aligned components that are free to slide and rotate along various axis in its dozen part assembly. My assembly in Alibre identified this problem by pointing out offending part interferences when parts of the same exact dimesion are assembled with a mated constraint. I want to correct this in my part designs the right way or at least understand the industry standard from the start. Thanks for your help.

[brunog]

That's where dimensional and geometrical tolerancing come along : all dimensions need a + or - tolerance based on a specified clearance fit.
The Machinery's Handbook's inspection section should give you most of the information you need.

Best regards

Bruno

[pabmartin]

Thanks Bruno, I thats precisely what I needed. Now in my ancient version of Machinerys Handbook 18, I have this ignored chapter on ALLOWANCES and TOLERANCES. ALl these neat tables for the types of part fittings. Not too scary. OK so now I have the data, I just need to see how to input it into Alibre Deign. Should be a breeze

[Javerh]

Usually the interference is just ignored inside CAD. The tolerances don't show in the CAD-parts. They're only written in manufacturing drawings.