Simulating a datum center plane 2

[semiond]

I need help in understanding this issue regarding simulating a center plane from a datum feature related to an external width dimension: let's say a width dimention is defined as a primary datum feature. When using a vise-like physical datum feature simulator with two almost parallel faces that close on the part, unless the tangent planes on both sides of the datum feature are perfectly parallel (and in the real world they're not), one of the vise faces will act similar to a primary datum plane - touching on 3 high points, and the opposite face will touch on only one point, similary to a tetriary datum plane. Now, depending on which side of the datum feature will make the more stable contact with the simulator, we might get a different separation width between the vise faces, and therefore the simulated datum plane will also be different. For example, if the measurement set up has the vise faces oriented horizontally, the side of the datum feature facing down will orient the part in the fixture, and if you flip the part upside down for a repeated measurement you might get different results on whatever control called out that datum. Now, I understand that there is only one "actual mating envelope" to the datum feature per ASME and only one of the sides facing down will produce the "minimum separation" condition per fig. 4-13, But that means that you have to mount the part twice in the fixtute and re-check your results, and I somehow doubt that this is the recommended practice... on the other hand, if the vise is oriented vertically, we will have no control over which side we stabilize better in the simulator - which is even worse. Everyone's insight will be much appreciated... Thank you!

 

[pylfrm]

Curiosity got the better of me, so I did some quick measurements on the hub of a milling cutter. Cutting diameter was about 120 mm. Hub thickness was about 12 mm. The hub faces extended from about 32 mm to 48 mm diameter.

I will identify datum features as follows:

[pre]
Datum feature A: hub face 1
Datum feature B: hub face 2
Datum feature C: hub thickness
Datum feature D: central bore
[/pre]
Measurements were as follows:

[pre]
Circular runout of datum feature A with respect to |B|D|: 0.002 mm
Total runout of datum feature A with respect to |B|D|: 0.024 mm
Circular runout of datum feature B with respect to |A|D|: 0.002 mm
Total runout of datum feature B with respect to |A|D|: 0.024 mm
[/pre]

semiond,

My experience with milling cutters is mainly from the point of view of a user rather than a designer or manufacturer. I can think of various reasons why it would be important to control the axial location of features relative to individual hub faces, but I'm not sure why location relative to the center plane of the hub thickness would matter. The only case I can think of is if I wanted to flip the cutter around without having to measure the tool offset again, but that seems rather unlikely. Are there other reasons I'm missing?


pylfrm

 

[semiond]

pylfrm,
Your figures definitely make sense to me
Symmetrical location of the body width to the hub width is obviously affecting the final location of the cutting edge to the hub, and as you assumed correctly, has to do with cutting edge(s) location repeatability between installations. Flipping the cutter to change its function from RH to LH or vice versa is brobably a rare practice and i don't know why would a customer need to do it (but maybe i'm wrong). What does happen commonly is that a cutter reaches the end of its tool life and the user replaces it with a new one. If the user does precision work he usually takes an offset and basically eliminates the need for accurate repeatability. But nevertheless, repeatability is considered to be an important parameter and most tool manufacturers try to be strict about it. In case of symmetrical tools that have two possible mounting orientations, cutting geometry location is achieved by symmetrical position, in order not to make one option more accurate and the other take the tol. stack up.

 

[pmarc]

Similar to the composite tolerance concept, only with both segments' tolerance zones independed of any external datums - interesting.

Yes. In a traditional composite position callout there is no way to have two datum-less segments because the one with smaller tolerance value will always override the other one.

The datum-less parallelism as a refinement of position could make sense because it would only control orientation of feature's axes without taking axes spacing into account. And actually the position would not have to be datum-less to make this combination work.

 

[semiond]

I agree, the pattern locating tolerance zone framework would still need to reference datums.
The datumless parallelism tolerance zones would be able to float within the PLTZF, while keeping parallelism among themselfs only, and unlike datumless postion used in last segments, would be able to dislocate in different directions without fixed distances.