Hello,

I have a question regarding the best way to dimension and tolerance close fitting, mating parts.

I have two parts made from 60-61-T6 aluminum that mate with a tongue and groove fit. I have a 0.250" wide tongue that must fit into a 0.250" wide groove. There cannot be any interference between the tongue and groove, but the parts should fit together with the least amount of clearance possible at the width.

Periodically, the part with the groove will slide on the part with the tongue very slowly, but the sliding is so intermittent and slow that friction and wear is not an issue.

I think the maximum amount of clearance I could get away with between the parts would be about .004" & I would feel more comfortable with .002".

1.which of the following dimension / tolerances would be best to keep clearance to a minimum (or are both the same)? They both have the same amount of total clearance possible, but from a machining standpoint, would one produce better overall results for the majority of parts?

A. Groove 0.250" wide +/- 0.001"
   Tongue 0.249" wide + 0.000" -0.002"
   Worst case = 0.004" clearance

B. Groove 0.250" wide + 0.002" -0.000"
   Tongue 0.250" wide + 0.000" -0.002"
   Worst case = 0.004" clearance

2. Are there any general guidelines when dimensioning and tolerancing mating parts like this? I want to eliminate as much clearance as possible and still keep costs reasonable. At what point would tolerances begin to really become a cost issue?

3.Provided the total variance in tolerance is the same, in general, is there any difference in machining cost between bilateral or unilateral tolerancing? For example, (plus or minus 0.001") versus (plus 0.000 minus .002").

Thanks for your help. I would appreciate any feedback.

John