Machining and Inspection

[metaldork]

Anyone with CMM experience please! Is it practical to probe a cast surface to establish Datum A. I am afraid this is throwing some numbers out of whack. I would think you would probe a truer surface and then set the part on that?

 

[Belanger]

Right you are... the datum is actually derived from the high points of the surface. In other words, set the cast part on a truer surface (such as a granite table/surface plate) and then probe that table/plate to establish the real datum.

However, cast parts often use datum targets (a circular "balloon" symbol divided in half). Do those show up on your print?

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[ajack1]

Yes it is possible, but not an especially good idea. As you say it is more common to have a fixture with feet or datum blocks on and use that.

 

[pmarc]

This is how it should be done whenever possible. Datums should be derived from datum feature simulators of sufficient geometrical quality (like granite table, surface plates or equipment simulating datum targets) and not from datum features.

 

[MechNorth]

I have "converted" a lot of CMM users over to the use of datum simulators on their CMMs. CMM "salesmen" like to tell buyers that they don't need fixturing anymore, that's an unnecessary waste of money; the truth is that it's just as important as for open-setup inspection. It's even more important for castings with datum targets because you are just using the high points, which the CMM cannot reliably find.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[dingy2]

I have some experience on CMMs and datum targets are the way to go on castings. I will assume that the casting is sand rather than die cast.

Even if the Designer only placed a datum and no datum targets, you will not easily find the "high spots". If you place the part on a surface simulating datum A, it probably would rock anyway which makes measuring pretty difficult. The 94 standard defaults to targets if the set up is not stable while the 09 standards refers to another standard.

If the casting is not cylindrical, there are 3 points on a primary datum, 2 on a secondary and 1 on a tertiary. On the primary and secondary, keep the targets as far as possible from each other and never on an edge. The tertiary target is in the center of the surface. Mark on your drawing where you placed the datum targets or try to get the targets placed on the drawing.

Good luck.

Dave D.

http://www.qmsi.ca

 

[metaldork]

Yes there are datum targets and the fixture is designed to those basic dimensions with (note) 1/2" dia. gripper buttons. I question this but I also imagined that they would mimmick this situation on the CMM table but the part is upside down and they are probing the cast surface. We are getting some unexplainable numbers and we've eliminated the fixture or movement of the part during manufacturing as the culprits.