Tolerancing problem

[geoffthehammer]

I am drawing a pair of dovetail slideways (60 degree angles). This can be measured over (or between for the female half) balls or rollers. These will be made in batches of 100 pairs.
The female half is considerably larger than the male because a bearing material will be 'cast in' during assembly. The cast-in bearing will adhere to the female part and slide along the male part.
So, I am not too concerned about the actual size of any individual dovetail (within say +/- 0.5mm)as the bearing material will take up size variations. What is therefore important is each male component with regard to its straightness and uniformity.
To reduce manufacturing costs I want to put a fairly open tolerance on the size but a tight tolerance on the dovetail form of each male component.
Any suggestions

 

[MechNorth]

I would suggest setting the base as the Primary Datum, and use the width of the dovetail as the Secondary Datum Feature-Of-Size (FOS). Alternatively, if the base fits into a snug groove itself, its width can be used as the Secondary Datum FOS. What I prefer to use for the secondary datum is a pair of fixture holes, which set up a secondary datum plane.

From that point, it's actually fairly simple to control the angled surfaces of the male dovetail; just put a surface profile with an adequate tolerance zone. Keep in mind that a surface profile tolerance zone is two parallel planes offset from the BASIC (theoretically perfect) surface depicted on the drawing.



Jim Sykes, P.Eng, GDTP-S
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