Datum Callout 1

[uptick]

A throttle shaft with two tap holes for mounting the throttle plate.
Let put the shaft horizontally (left to right), draftman draws a vertical line between & midpoint of these two tap holes and calls it a datum A. He then make a basic dimension from this datum A (this vertical line) to one tap hole, a basic dimension from this tap hole to the other tap hole. He wants to control one end of the shaft so he makes a basic dimension from datum A above to the end of the shaft.

The question is: IS he correct to callout datum A ( a line established between & midpoint of these two tap holes)?
The Inspector will use two pins (each for each holes) & locates the center point between two pins for his datum.
Please advise. Thanks

 

[drawoh]

uptick,

A datum must be a feature you can locate from. This means it either is an edge, or a feature of size.

Ask your drafter to propose a fixture that will pick up his datum.

JHG

 

[looslib]

Make the end of the shaft your datum and dimension the 2 holes from there.

"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

 

[PaulJackson]

If the throttle plate itself stops translation along the the throttle shaft via two tapped holes and two identical clearance holes in the throttle plate... then the 2X threaded hole pattern in the throttle shaft would be the functional secondary datum features... the functional primary naturally would be the cylindrical rod ends that mate with the bearings or bushings in the body.

The coordinate system reference for inspection purposes would then be central to the 2X pattern on the axis of the rod ends for any fcf's that referenced the rod-ends primary and the 2X threaded holes secondary.

It would be logical for the basics to be referenced to that origin... the datum identifiers however must be placed on the features that establish these centerlines not on the lines themselves.

Paul

 

[uptick]

Thanks drawoh.

Looslib. That is exactly what I have in mind to tell him. But I am not sure his way is correct. Thanks.

 

[uptick]

PaulJackson. Great post. Thanks.

 

[fsincox]

I agree with Paul, I do not know what a throttle plate is, but in principle the features of a hole pattern can be a valid datum.
Frank

 

[KENAT]

Per Paul, the way to do this 'properly', at least to ASME Y14.5M-1994 would be to make the 2 hole pattern a datum.

Using patterns for datums can be problematic as the topic isn't covered in much detail in the standard. However, for 2 holes it's simple enough.

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(probably not aimed specifically at you)
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