Zero setting on datum

[Sa-Ro]

Refer figure 7.7 datum plane establishment from ASME Y14.5:2018

Doubt 1:
Should I take measurements (dial/probe zero setting) from datum plane A (true geometry counter part of datum feature A) or Simulated datum plane A (plane derived from datum feature simulator such as surface table, chuck, vice, angle plate etc).

Because, the datum surface of the part may have imperfections which will effect in our measurement result. Surface of the datum feature simulator shall have less imperfection than part.

Doubt 2:
If we declare a surface as a datum, the entire surface must be located with datum feature simulator. Then how to touch and dial zero setting at part surface?

Doubt 3:
If we declare a outer dia as datum (Ex: Dia 50 X 100). Should I locate the entire length 100 to achieve the datum axis?

Thank you.

 

[3DDave]

"Question;" "Doubt" means "disbelief" as in "I doubt the answer."

I know the substitution of one for the other happens in some language base; I wonder how it came about. But in conventional English the two words are distinct.

Anyway:

Measurements are from datum feature simulators except for determining local size.

You can zero on the simulator before the part is installed.

 

[Sa-Ro]

Thank you.

Consider a flat surface as a datum.

Can I dial that face and create a imaginary plane (may be called as datum plane) and take measurements from that plane or should I measure only from datum feature simulator (ex. surface plate)

 

[Sa-Ro]

For measurement from datum feature simulator....

This is the reply I received from my Quality dept.

"If we check the part at CMM, the actual surface / axis should be taken as datum since we are generating datum as theoretically (Imaginary plane, axis)"

Is this correct?

 

[chez311]

Sa-Ro,

The below is an excellent discussion on some of the merits of either method.

https://www.eng-tips.com/viewthread.cfm?qid=468228

 

[3DDave]

If a surface plate is used then the part has millions of points examined simultaneously in the millisecond it takes to settle in position to generate that plane from the part surface.

If a CMM is used to create a software calculated version of the datum feature simulator then a few points are taken over a period of 30 seconds or so for each part that is inspected, points which may or may not reflect the overall part surface.

So, yes, the CMM can also be used for this, but it is less comprehensive and more time consuming. Maybe that is what the Quality department wants.

Anyway, it seems like they will just do whatever they want to do. Let them. If the parts don't fit because they were incorrectly inspected and defective parts were sent on their way, let the upper management decide what to do about that situation. If good parts are rejected, start looking for a new job because this is just a small part of a wasteful operation.

 

[Sa-Ro]

@ Chez and 3DDave

Thank you

impressed with

If a surface plate is used then the part has millions of points examined simultaneously in the millisecond it takes to settle in position to generate that plane

and

If a CMM is used to create a software calculated version of the datum feature simulator then a few points are taken over a period of 30 seconds or so for each part that is inspected, points which may or may not reflect the overall part surface

 

[Burunduk]

I'm not saying that any inspection method is superior or inferior to the other, but it is worth to keep in mind that depending on how much form error there is on a nominally planar surface, the "millions of points" mentioned above can be reduced to as little as three points of contact - and that is for a primary datum feature. For secondary and tertiary you get 2 and 1 respectively. These points are the only connection between the datum feature and the simulated datum. Since there is form variation of the surfaces on the mating part, there is a very high chance that the 3/2/1 points involved with datum simulation are not the points that will contact the mating part at assembly. Datums have to be simulated in accordance with the product definition standard referenced in the drawing and should have exact geometrical relationship between them according to the basic geometry of the part, but the decision whether to establish them from a calculation based on sampled points or use physical datum feature simulators should be the inspection department's decision. Both methods can follow the same design intent and are equally legitimate.