Dimensioning and tolerancing close fitting mating parts

[John2002]

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&quot versus (plus 0.000 minus .002&quot.

Thanks for your help. I would appreciate any feedback.

John

 

[meintsi]

A couple of thoughts....

1) Size on size fits for anything can can be able to move MUST be avoided. (even if the movement is slow ««« high torque)

2) There is no difference between your two tolerancing combinations except for its "nominal size".

3) I believe that bilateral or limit tolerances are the way to go in order to maintain design intent in this case.

4) Holding an overall tolerance of .001" on a tongue and groove will not skyrocket the cost unless the groove is very long or the realtionships of the t&g to the surronding surfaces must be kept close.

 

[Hush]

I use three methods depending on machining method and overall tolerance.

1. For manual machining I use max material unilateral tolerance. This allows the machinist to rough cut above and finish into the zone.

2. For CNC I use bilateral. If I don't know how it will be machined I use max material tolerancing.

3. Although not preferred if I have a large tolerance say +/- .06, I'll sometimes pick a spot on the high side, say +.02/-.10 on an OD to provide the machinist or operator an easy target and still make sure I have lots left over for possible reworks.

 

[SL280]

In your tolerances, there is a case where you have zero zero off from the base dimensions. That means you cannot assemble or have very difficult time to assemble, even though the parts are made to the spec.

The clearance is determined by what kind of movement you have to have with the two mating parts. If it is OK to have just move or slide, then you can give .002 or more clearance, but if there are too much, then they are too loose to slide as well. Precision parts can have 0.0005 clearance.

The tolerance also relates to the straightness or parallelism over the slide stroke.

This is an old listing, but just a little tip.

 

[John2002]

Hello,

I just wanted to thank everyone for their replies to my post. Your comments were very interesting and helpful.

Sincerely,
John

 

[toododd]

You can also use the Standard Fits section in the Machinists Handbook as a guide....

 

[Tunalover]

john2002-
If you're interested in learning how to dimension and tolerance both features so that they are guaranteed to fit 100% of the time (provided the features meet the drawing!) then I'll pass it on to you. It used GD&T per ASME Y14.5M-1994 dimensioning, tolerancing, and formulas.

Tunalover