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Sheet Metal Holes as Datum Features

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If I specify a hole in a practically thin feature, such as a piece of sheet metal, is this still considered a cylindrical datum? Is this a poor choice of datum? In this video, we look at an example to help us answer these questions.
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Опубликовано:

1 авг 2024

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Комментарии : 10

@scottevans9553 Год назад

This is excellent advice. Problem is if you are NOT responsible for the design, it is tough to get it changed when parts are waiting for inspection. In my career as a mechanical inspector I saw so many poorly-chosen datum structures. As an inspector and student of mechanics and physics, you know that if you use the axis of a 3mm-long sheet metal hole as the primary datum that the results will be skewed, possibly by a long way from reality... So, what do you do? If you measure and report to the drawing you are (not intentionally but effectively) misleading your requestor as to the true nature of the piece(s). Yet you are bound to report to the drawing, such as it is. What I did, especially late in my career when I worked with a bunch of fresh, clean-behind-the-ears engineers, was report it both ways. In other words, the way it was written and the way it should have been written. When the engineer sees the comparative results he is often quick to correct an errant datum scheme. It can be a slippery slope. Occasionally, they actually do know what they are doing and it represents the actual usage. In that case, you just do your best. 🙂

@shankar554 Год назад

you could have mentioned CAN MAY and MUST rule to differentiate between primary and secondary datums differences

@luisquiroz9355 Год назад

Hi Shankar, that rule is new to me, could you please help with a short description? Thanks a lot!

@shankar554 Год назад

@@luisquiroz9355 If a datum feature CAN stop a DOF and if it MAY(has permission) stop that DOF then it MUST stop that DOF. So primary datum has an obligation to stop all possible DOF that it MAY stop. Hence mentioned it was a good idea to bring up this rule to explain things better.

@scottevans9553 Год назад

I like this rule though I have never seen it written this way either.

@shankar554 Год назад

@@scottevans9553 In control frame of the datum column you can write any of the 6 DOF(x,y,z,u,v,w)(write only ones which that particular datum can control). In other words if datum A can control u,v and x and u write next to the datum only u,v then it means that even though datum A can control 3 DOF it will "only" control 2 DOF as mentioned. Hope I explained clearly.

@scottevans9553 Год назад

@@shankar554 Yeah, for sure. But if they have specified (x,y,z,u,v and w) then it would be my estimation that they know what they are doing, or cynically, they messed up the output defaults in their CAD program. I was more talking about the good ol' days when much was implied and you had to know how to interpret the DRF properly. Is this Can May Must Rule new after Y14.5-2009?

@XiaoBuding-ol8og 8 месяцев назад

How about using 3 holes for the primary datum？ or 2 holes and 1 surface(datum target :10x10mm) for the primary datum?

@Gdandtbasics 6 месяцев назад

We can certainly use a pattern of 3 holes as a primary datum feature, however that would only like down 5 degrees of freedom, so our second datum feature would have to be something that could control that final degree of translation.