GD&T Question

[DavidTunnard]

Hi all,

don't know if this is the best place to post this as it's more of a general drawing question rather than inventor specific. But thought I would ask.

I want a hole to be symmetrically placed on a plate. How do I call this out using GD&T? any help is appreciated.

[NigelHay · Accepted solution]

Like this?

[DavidTunnard]

Yeah, I'll go with that for now. Thanks! 

I'll leave it up for a while longer before accepting it as a solution in case someone knows a reason as to why this isn't right. Or do you know for definite that this is correct?

[NigelHay]

That's how I would do it unless anyone else knows better?

[hosford]

https://www.gdandtbasics.com/symmetry/

[NytCat]

David, I'm no GD & T guru but I must respectfully disagree with Nigel's solution.  You don't want to use symmetry for threaded holes unless you want to make enemies in your manufacturing and inspection/QC departments.  It's difficult enough to show that a cylinder is symmetrical between two planes but then you're going to throw threads into the equation?  I'm not even sure that's possible; others with more knowledge may know the answer to that.  Keep it simple, if you can, and change that to a position symbol.  To back me up, I'll refer you to the warning in the link provided by Mr. Hosford.  Just my two bits.

[edit] Forgot to mention, you should also drop a diameter symbol in front of your tolerance zone value.

[edit2] Added screen capture.

[DavidTunnard]

Thanks for the link. I regularly use GD&T basics for help so have already read that. Their examples show symmetry on a dimension. for example on the width of a slot. I can't see anything for where to put the feature control frame for a line going through the centre of the part.

[DavidTunnard]

Hello,

Thanks for the reply and explanation.

We do use position and I will be using it on these holes to ensure location and perpendicularity. I want to use symmetry as the holes will be going on a laser-cut plate. These are not always exactly 10mm. so if I dimension to the perfect centre, which would be 5mm in this case, the holes might not be exactly in the centre of the real-life plate. Hope that makes sense.

The use of the symmetry call out here is just to ensure that the machinist takes a measurement of the plate, halves the dimension, and uses that for his measurement. We don't have an inspection department here, so it might not be scrutinised as much as you think. But can see your point if I were to be sending this plate out to another machine shop.

[NigelHay]

NytCat is correct. Although the symmetry method is technically correct, it's use won't win you any friends as setting up & measuring can be complex. Probably not too bad for a laser cut part but still difficult to QA.

[NytCat]

David, I completely understand your design intent as well as the situation with how you believe the shop will interpret your drawing.  I cannot imagine there's anyone in this forum that hasn't had to conform to their company's internal rules/methods vs what they were taught.  In the end you have to go with what is going to work for everyone.

Just to explain the way I drew the solution I offered, which is how I believe the ASME Y14.5 standard has been set up.  The datum A indicates the width of the part regardless of what that width actually measures.  Stating the true position of the hole without a reference dimension indicates that the hole(s) must be located midpoint between two planes, one on each outer surface of the width of the part, again without regard to the width dimension.  When the datum is positioned opposite the dimension arrow, it's stating "this width" not "10".  You cannot place a datum on a centerline and you cannot place a control frame on one either.

So, as first stated, that's just my interpretation, I claim no higher education on the subject.  I have the standard here, I've read through it (mostly) but that doesn't mean I'm using it right either.  I do however feel strongly enough about it that if you were here, I'd bet you the next round that I'm right, heh-heh.