Best Fit

[[sc...]]

When should I use best fit? Can this option be used when there are three datums shown in the feature control frame?

[[To...]]

If some of the 3 datum features were features of size and had an MMB designation, you could. But if it's 3 planes, no.

[[Me...]]

Tom, when is it required or best practice to use Best Fit?

[[Ra...]]

Jumping in for Tom and hoping I get the words right...

All features referenced to the same datum reference frame must meet their requirements simultaneously. I believe that you could use best fit if your features are called out to 3 datums, but as Tom stated, with no available degrees of freedom - what's the point? Adding in the modifiers Tom suggested, you may have cause to use best-fit.

[[De...]]

One example I see a lot where you would use best fit is if one of your datum features is one of a pattern of features such as one of the holes in a bolt circle, in this case best fit allows you to find the 'best fit' of all the holes in the pattern to use as your datum feature, thus allowing you to do one check that gives repeatable results rather than getting different results each time you use a different hole from the pattern as the datum feature.

Just imagine a shaft with one cylinder on the main axis called out as A and a face perpendicular to the main axis called out as B, then pattern of holes around A called out as C.

At that point 'all' the holes in the pattern must represent C in order to assure 'simultaneous requirement' is met. In this case best fit is used to find the rotational location around A where the 'best' rotational condition of all the holes in the pattern at the same time is met, this position can then be used as C since it is the location that will give the best result for all of the holes.

If you chose only one hole as Datum C, you would essentially be saying that one hole is perfectly oriented and all the other holes have the rotational error, using best fit essentially is saying - no hole is perfectly oriented, so find the rotational orientation that gives the best result on all holes then report the actual error of each of the holes in relation to that best rotational orientation.

Hope that helps some - this is a tough subject to explain.

[[sc...]]

Thanks everyone, that make sense.
Best Regards,

Scott