Fixture build and Datum scheme- inquiry

[Manpreet Singh]

I am attaching a simplified part drawing that I have to make a checking fixture of. Following are my queries:

1. I was thinking of holding the A datums using 4-way locator pins (conical), so both holes are concentric but then I realized if I do that, how would the part touch the B-datum, what are your thoughts on it?
2. The C datum has positional tolerance to it. What would be the pin size to locate C-datum?

 

[Wuzhee]

Use pins for Datum A. You can make them coaxial.
Datum B would be the face of what makes datum A.

Datum C simulator diameter will be Hole MMC minus Pos. tol.

 

[Manpreet Singh]

So, should we just use Ø4mm pin for A-datums?
Also, does that mean we should use (5.07-.20)= 4.87 pin for C-datums?

 

[Wuzhee]

For physical gauging, yes because that's what is written on the sketch.
Or use a CMM

 

[Wuzhee]

 

[Manpreet Singh]

Sorry, since I am new to this. One of my co-workers statement is that C-datum should be constrained with its True Geometric counterpart i.e. Ø5.07 mm pin. What do you think of this statement

 

[greenimi]

One of my co-workers statement is that C-datum should be constrained with its True Geometric counterpart i.e. Ø5.07 mm pin. What do you think of this statement

I would use Ø5.07 - 0.2

 

[Burunduk]

The drawing doesn't show the feature control frame that references the 3 datum features, so any suggestions on the size of datum feature simuoator pins are only guesses. Ø5.07 - 0.2 assumes C was referenced at MMB.

 

[Manpreet Singh]

Lets say if there is another hole below C Datum hole, referencing the datum features as attached. Then what would be the pin size of C datum.

 

[drawoh]

1. I was thinking of holding the A datums using 4-way locator pins (conical), so both holes are concentric but then I realized if I do that, how would the part touch the B-datum, what are your thoughts on it?

Your holes are concentric by definition.

I agree that you cannot use cones. Your feature-of-size datums are not very accurate. Are your references to Datum[ ]A Regardless of Feature Size (RFS)?

2. The C datum has positional tolerance to it. What would be the pin size to locate C-datum?

Look up diamond pins. Your datum[ ]C feature controls rotation only. Your pin needs only to contact the sides of the hole. Otherwise, your Ø0.2 positional tolerace is more than made up for by the +0.5/0 tolerances of your datum[ ]A features.

--
JHG

 

[Wuzhee]

Lets say if there is another hole below C Datum hole, referencing the datum features as attached. Then what would be the pin size of C datum.

Still D5.07-0.2 because Datum C is referenced to A|B thus making it MMC minus position.
Also for Datum D, C is RFS, no datum shift allowed.

 

[Burunduk]

Lets say if there is another hole below C Datum hole, referencing the datum features as attached. Then what would be the pin size of C datum.

Since C is referenced at RMB,
the datum feature simulator for C would be the largest pin that fits in the actual datum feature C hole, while kept at fixed distance from simulated datum axis A and perpendicular to simulated datum plane B. An expanding pin is typically used for that.