Correct formula for symmetry?

[RalliartQA]

i have been looking thru a couple of books and checked a couple of other sources and as i said im new to this so i need a little help if anyone can. im just looking for the basic formula for symmetry. thanks

 

[dingy2]

Symmetry applies to non-cylindrical products making sure that a keyway, as an example, is centred relative to the outside width.

Let's say we have a symmetry call out of 0.010. This means the centre of the keyway must not move any more than 0.005 above or below the centre plane created by the width.

This is ugly to check and the method that I will describe has about 90 -95% confidence as long as the outside width surfaces are parallel to each other.

Here is a quick and dirty way - take the part and place it on a granite table. Take an indicator on a height gauge and zero off on one side of the keyway in the centre. Take the part out and flip it upside down. Take a reading on the opposite side of the keyway with the indicator. It must not read more than .010 in our example.

Real Way - Measure the actual width of the part and the width of the keyway. Now place the part on a granite table and zero off on the top of the part. Come down 1/2 the actual width of the part to create a centre plane. Now come down an additional 1/2 actual width of the keyway and touch the surface of the keyway. We have a tolerance of +/- .005. Check at the edge of the keyway and then the bottom of the keyway making sure the size does not change.

Hope this helps.

A CMM will be able to copy this method.

Dave D.

http://www.qmsi.ca