Does GD&T Require a Diameter Symbol?

Here’s another common question that comes up in a GD&T class:  Suppose we are applying a position tolerance to a hole. It would typically have a diameter symbol in front of the number, as in the first example below.  But what if we omit the diameter symbol in front of the number, as in the second example? Would the shape of the tolerance zone still be assumed as cylindrical?

 

nodiasymbol.jpgAnswer: No, it’s not cylindrical. Without a diameter symbol, a tolerance zone defaults to two parallel planes (unless the BOUNDARY concept is invoked). So the second drawing above is ambiguous; the zone will be two parallel planes, but we don’t know the direction of those two planes.

Two solutions:  First, if the intent is to control the position in all directions, you must add a diameter symbol as in the first example above. Second, if we really intended two parallel planes, we must graphically indicate the direction of those planes:

 Now, it is clear that we are controlling position in the left/right direction. Of course, that means that there is no position control in the up/down direction. So let’s take it one more step:

nodiasymbol3

This example creates two sets of parallel planes — one in the vertical direction and one in the horizontal direction. The result is a square tolerance zone; thus, it’s very similar to using the traditional coordinate or plus/minus tolerancing method. But here we still have the advantage of clearly identifying the datum references, and we also have the MMC modifier to gain bonus tolerance, something that the coordinate method cannot do.

18 Comments

  1. In that Example Diameter Symbol is require because that hole has circular shape for to control that size of hole we should required to put Diameter Symbol in this GD&T.

    • Manoj, You are wrong for this example. Yes, a diameter symbol is required if you are calling out the size of a round hole but the size of the hole is not a part of this example. The tolerances shown are ONLY controlling the LOCATION of the hole center, not the SIZE of the hole. Therefore, a diameter symbol is not used to call out the RECTANGULAR tolerance zone of the hole center in the example GD&T.

  2. What is not clear in the text here is how far away from True Position can the centreline of the actual hole be in the left-right direction. And would that distance be any different if the diameter symbol is there or not?

  3. Thanks for the comment, Big Chris. I suppose we should have said explicitly in the text that the resulting tolerance on the hole for ALL examples given is up to 0.15 away from the true position (discounting any bonus tolerance that might be available due to the MMC modifier).

    So the tolerance amount is the same; the only difference a diameter symbol makes is that the 0.15 location error can be in any direction (radially). Having no diameter symbol makes the 0.15 location error restricted to only a single direction (up/down, or left/right). But the number itself is not affected by omitting the diameter symbol.

  4. So Figure A is Correct method if we want to define the position.

    • Yes — usually the first picture given above would be the way to control the position.

  5. i have one more question
    Concentricity without a Diameter symbol in from of value is correct way or not .
    or it is consider as diametrically if diameter symbol is not there.

    • Concentricity must have the diameter symbol in front of the number. We’d probably understand the intent even without the diameter symbol, but the rules say that it’s required.

    • I have topic related to this with one of the supplier. He is saying that if there is diameter symbol before the value it is coaxiality and if it’s not it is concentricity. In ISO 1101 there is no specification of this.

  6. left/right directio – position 0.3 A/B/C
    up/down direction is enough 0.3 A/B
    what for is second 0.3 A/B/C?

    • Although the second tolerance of 0.3 only controls up/down, it references all three datums because that allows the part to be measured while fixtured only once. By having common datums, the part will be held against A, B, and C. But it’s true that if datum C were dropped from that up/down position tolerance, it would not change anything in the meaning of the GD&T.
      (If the left/right position tolerance eliminated datum B, however, then it would significantly change things, because the important datum C would become secondary, not tertiary.)

  7. Sorry, can you explain that last bit? Why would removing Datum B from the left/right tolerance significantly change things? Is it not the same as removing C from the up/down tolerance?

    • It has to do with the order of the datums: primary, secondary, and tertiary. For a perfect part, all three surfaces of the part would be perfectly flush with the fixture/gage surfaces. Of course, an imperfect part will not be able to make full, flush contact on all three datum features.

      So, since we can’t be flush on all three surfaces, that’s where the datum sequence comes in. We want to still keep A, B, C in the left/right callout because — even though we might only be measuring the position in the left/right direction (datum C) — we want the part held more flushly with datum B.

      If datum B were removed from that callout, then the inspector would adjust the part so that it sits on datum feature A first, but then flatten it out on datum C — datum B is removed from the fixture/gage.

      I’m unable to attach a picture of this idea right now, but I hope this description helps a little. Thanks for the question, Fin!

  8. Hello gentlemen, for me it is not clear yet, for the explanation I assume that the right side image has a mistake, it should be correct to add the diameter symbol before the tolerance or add cote lines to specify the direction for apply the tolerance, but in the actual drawing is not correct according the GD&T.
    So if I saw this, am I reading wrong if I use 0.3 tolerance in up/down plus 0.3 right/left?
    Thank you for your support

    • Greetings Jose — you are correct. The picture on the right is not proper GD&T (that is one of the key points of the article).

      So it should either have a diameter symbol, or it should have extension lines (vertical or horizontal) to indicate the direction, if non-cylindrical. All other pictures in the article show the proper way to apply GD&T to that hole.

  9. So if 2*Square Root (X Actual-X True)Square + (Y Actual-Y True)Square = [True Position / Ø0.3(M) / A / B / C], what would be the correct formula for [True Position / 0.3(M) / A / B / C] with out the Ø symbol?

    • Consider the position callout above that is tagged in the left/right direction of the hole: The position calculation would simply be
      2*|X Actual-X True|. That result should be less than or equal to 0.3 (although the MMC modifier means that there may be bonus tolerance which allows for more than 0.3).

      If the same callout is applied in the vertical direction, then there would be a separate calculation of 2*|Y Actual-Y True|.

  10. I need help explaining a Profile question from my boss. Why is the highest deviation doubled? I have tried to explain and could really use some help. Diagrams would be very helpful. Thanks in advance for your generous help with this matter.

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