Using True Position to Control Symmetry

[kcmomde]

We stamp connector terminals, tabs and pins they use true position callouts on the drawings to control symmetry, but there are no basic dimensions called out. And we are not dealing with holes or diameters on the drawings. How do you inspect, measure, calculate the true position. The datum called out is usually the width of the terminal, so I have been told to measure the centerline of the width to the centerline of the feature, but then what?
Does anyone have experience using true position in this manner?

 

[David DeLong]

We stamp connector terminals, tabs and pins they use true position callouts on the drawings to control symmetry, but there are no basic dimensions called out. And we are not dealing with holes or diameters on the drawings. How do you inspect, measure, calculate the true position. The datum called out is usually the width of the terminal, so I have been told to measure the centerline of the width to the centerline of the feature, but then what?
Does anyone have experience using true position in this manner?

Positional tolerance for symmetry is used quite a bit since symmetry can get complicated. Is the positional tolerance in Maximum Material Condition or is it shown in Regardless of Feature Size?

If the feature is shown in RFS, then it is pretty easy. If the tolerance shown in the feature control frame is .010 ", it means that it can be off center no more than .005.

If the feature is shown at MMC, then one would have to calculate the tolerance. Let us use a tab width shown on the drawing as .500 +/- .005 with the .010 positional tolerance . Here is what you would do to calculate the tolerance. Measure the tab width. I am using a value of .496". Subtract that value from the MMC value of the tab (largest value) or .505 - .496 giving us .009. Add that value to the positional tolerance of .010 and we get a total of .019. The actual tolerance at that particular tab width is .019/2 or +/- .0095". The tab cannot be off center more than .0095".

Hope this helps.

 

[Paul F. Jackson]

Kcmomde,

If their drawing has true position callouts where there are no datum features specified in the feature control frames and there are no basic dimensions then the drawing is incomplete (technically un-inspectable).

That said... there are some basics that need not be stated but are implied i.e. right angles 90 degrees and features of size drawn symetrically centered upon one another in the same view having a basic displacement to one another of zero.

Check with your customer to shed some light on what features are to be used to establish the datum reference frame for measurement and what those "un-implied" basics are... and get it in writing to protect your interests.

If they have given you a CAD model sometimes they will have you refer to the model as basic but that still doesn't address the DRF definition.

Functionally the DRF should mimic how the connector is constrained for its six degrees-of-freedom in its assembly with its mating parts... what stops its three rotations and its three translations. The feature's plane, width's center-plane, or axis that constrains the most degrees-of-freedom and the most rotational degrees-of-freedom (if tied with another) is the functional primary datum feature... with that progression following in the secondary and tertiary.

If you do not get reliable feedback from the customer about the DRF I would examine what orients and locates the part in the assembly functionally and choose those features to create the DRF.

Paul

 

[optomist1]

Paul,

Good post that's my read as well, how do you get past True Position without any Basic Dimensions? Excuse the pun but it seems basic...like you stated you need more info.

md