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# ISO GD&T question

## ISO GD&T question

(OP)
Hi, i have a question regarding concentricity i hope some of you can help me with, i use ISO GD&T

I want 2 concentric circles to have be be toleranced concentric to eachother, to make sure they have the same center according to eachother. How shall this be illustrated? I have atteched a drawing of my idea of it, but the lead engineer disagrees. He says that using this method,there is no established centerpoint, so only one diameter can be toleranced, since the other establishes the centerpoint. But then one of the diameters doesnt have a tolerance, right?

Also shall some of the dimensions then be set as Theoretical exact dimension (TED) if i use general dimensions overall?.

### RE: ISO GD&T question

What you have is completely wrong. Concentricity only establishes the relationship of the center of two circles (or axis of two cylinders) to each other. It does not control size or form. One circle (or cylinder) is the datum and the other is toleranced to it. Also, concentricity ALWAYS has a diameter symbol in front of the tolerance value.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

### RE: ISO GD&T question

(OP)
Okey, but if i then use a circularity tolerance on the outer diameter. Then use this as a datum for concentrisity on the inner diameter will that be more correct? Then i am sure that both ID and OD are toleranced? See attached picture

Also using the OD as the datum, this references the centerpoint right?

Thank you for replies.

### RE: ISO GD&T question

Datums are the origin of dimensional measurement. So datum feature A - the outside diameter - is used to establish the theoretical axis line (which you called a point). You then "find" axis of the feature under GD&T control - the inside diameter - and compare it to the datum axis. The center of the cylindrical concentricity tolerance zone sits at the datum axis and the axis of the feature under control must lie on or inside the cylinder.

Certified Sr. GD&T Professional

### RE: ISO GD&T question

(OP)
Thank you for reply, but that doesnt really answer my questions. It would be great if someone could just show me the correct way to use the symbols on my drawing.

### RE: ISO GD&T question

JFP;

The symbols on your last sketch look fine to me. My only comment is the circularity form control on the outside is not required to define the concentric (location) relationship. It only controls the form of the outside diameter, which technically has nothing to do with finding the datum axis from it.

Certified Sr. GD&T Professional

### RE: ISO GD&T question

(OP)
Thank you very much for your reply. Using the circularity form control was only to control the OD.

But for my next question, shall the OD and ID dimensions now be Theoretically exact dimensions (TED)?

### RE: ISO GD&T question

JFP:

Definitely NOT. They are features-of-size and must be controlled with "+/-" tolerances. TED's would only be appropriate f you applied profile to control the outside feature.

Certified Sr. GD&T Professional

### RE: ISO GD&T question

(OP)
Okay, thank you so much for your help. It is apparant it is many things i dont understand about geometrical tolerancing yet. I actually have some questions regarding paralellity of multiple features too. I will make a new thread about it soon, hopefully you can help me there too.

### RE: ISO GD&T question

Your are most welcome. I am not an ISO GPS expert like many others who post to and follow this forum. But I am sure they will chime-in when the time comes.

Certified Sr. GD&T Professional

### RE: ISO GD&T question

Jorgen - You really should take a class in GD&T. Far to much to learn just asking questions on a forum. A good book can help but a class is better.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

### RE: ISO GD&T question

Jorgen Fone Pedersen,

The ASME Y14.5 standard is depreciating concentricity. Concentricity is not really meaningful unless you have an accurate cylinder to use as a datum feature. What you have is a circle. You can define one circle as your datum feature, and use the positional tolerance to locate the other circle.

As yourself how you are going to inspect anything you define by GD&T.

--
JHG

### RE: ISO GD&T question

JFP:

I totally agree with dgallup's and drawoh's posts. But be aware, this is a subject as deep as there are parts to design and dimension. Stay with it and the rewards will be great.

Certified Sr. GD&T Professional

### RE: ISO GD&T question

drawoh,

May I ask why you are mixing ASME Y14.5 into this discussion. The thread title clearly states it is ISO question, and it was explained many many many times before on this forum that concentricity in ASME is totally different animal than concentricity/coaxiality in ISO.

### RE: ISO GD&T question

pmarc,

I know there are differences, and I actually do not know what the differences are. I understand ASME Y14.5. Also, we cannot tell if the OP is visualizing a plate or a shaft. A shaft would provide a cylindrical datum feature.

Having said that, a positional tolerance should work for this?

--
JHG

### RE: ISO GD&T question

In ISO a position tolerance is no different in meaning to a concentricity tolerance applied to a 3D feature (the control is then called Coaxiality). And because in ISO concentricity/coaxiality tolerance is suggested/preferred choice over position in cases where location of features of size nominally coaxial with datum feature needs to be controlled, I see no point in using position tolerance here.

### RE: ISO GD&T question

5. Coaxiality tolerance:
- ASME - coaxiality is nothing but position tolerance applied to nominally coaxial features of size. There is no extra symbol for that characteristic.
- ISO - coaxiality is also nothing but position tolerance applied to nominally coaxial features of size, but it has a separate symbol (the same as concentricity symbol in ASME).

6. Concentricity tolerance:
- ASME - this geometric characteristic has very special definition (control of feature of size's median points relative to datum axis), so it is never a special case of position.
- ISO - concentricity has the same symbol as coaxiality, but in addition the ACS modifier (Any Cross Section) is associated with the tolerance frame. This characteristic controls relationship between the toleranced feature and the datum feature in each cross section individually.

The strike through text is still valid, but regards to ASME and I do not want to get in trouble......with the original text posted by pmarc.

### RE: ISO GD&T question

I sincerely apologize for having added only confusion to this ISO question. In the future I will hold my tongue and defer all ISO questions and waiting for pmarc to chime-in. He is the ISO vs ASME man! I will "silently" follow the posts and learn along with the OP.

Certified Sr. GD&T Professional

### RE: ISO GD&T question

mkcski,

If I could suggest something, I would strongly encourage you and others NOT to wait for my replies in ISO-related discussions. I do not have monopoly for knowledge (not to mention that I do not even consider myself expert in ISO GPS) and I am pretty sure many folks on this forum, including you, have a lot of meaningful and valuable things to say.

I jumped into this discussion only because I wanted to keep the discussion and OP focused on ISO stuff and not get distracted by some statements on how bad and meaningless concentricity in ASME Y14.5 is (sorry, drawoh ). As you most likely noticed, up to that point I did not even offer any help to OP. That is because almost everything had been already said by you, CH and dgallup.

### RE: ISO GD&T question

pmarc:

I really appreciate your response. My confidence with ISO improves as I follow this forum. I will be more reserved when I respond to ISO questions so I don't mislead the OP with false information. I also realize you must try to measure the knowledge of the OP from the content of the questions so you don't talk past them and add more confusion.

Certified Sr. GD&T Professional

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