• The advantages of using GD&T (Geometric Dimensioning & Tolerancing) in your drawings – My Story

    Here’s the job I was working on a long time ago – a massive granite beam held in place with M20 tie-rods in an anchor plate. The clearance holes in the beam were Ø30mm, so lots of room.


    So I did the drawing using classical tolerancing, which should have been fine. A ±0.01mm tolerance should be more than needed to clear in case the holes weren’t perfectly straight.

    This is how the tie-rods looked in the manufactured plate – not straight and there’s no way this will fit in the granite beam. The shop apologized and remade the plates with straighter tapped holes and it worked. They only had to remake 2 plates. Of couse the schedule took a hit.


    Here’s the thing in this case, and many others – there are some serious disadvantages with classical dimensioning.


    • There’s no way to say how straight the holes should be
    • It was only two plates – what if it was 1,000? The vendor could rightly have decided not to remake them, a financial loss to the company.


    So I redid the drawings using GD&T, as below. This is a 3-D system that ensures there only one way to measure and verify the part. In this case, it allowed me the following benefits:


    • I could loosen up the unnecessary tolerance to 0.030 from 0.010, saving manufacturing cost
    • I could tighten up the tolerance where needed, by specifying a PTZ (Projected Tolerance Zone ‘P’) on the axis of the tapped hole.
    • This saves money and time, as the requirement is unambiguous and the part will be delivered correctly the first time.



    See the red axis boundary indicators below that the PTZ represents. This ensures that the cap plate will fit over the rods.

    Comments 18 Comments
    1. GarethW's Avatar
      GarethW -
      Nice article and example of the advantages of GD&T. Thanks for sharing.
    1. sam341's Avatar
      sam341 -
      GD&T is also commonly called Geometric Dimensioning and Tolerancing.
      As you point out, GD&T allows for many more details to be represented in the drawing, including surface finish.
    1. CADJockey's Avatar
      CADJockey -
      Geometric Dimensioning and Tolerancing. Here's a good link for mostly US based, but also others to learn more. I have had Don Day teach classes at a few of the places I have worked. I highly recommendhim and Tec-Ease to anyone in need of learning or refreshing their GD&T.
      http://www.tec-ease.com/
    1. camadden458's Avatar
      camadden458 -
      Thanks for the article! It is well laid out and illustrates the advantages of GD&T. Note: GD&T stands for Geometric Dimensioning and Tolerancing.
    1. GarethW's Avatar
      GarethW -
      Quote Originally Posted by CADJockey View Post
      Geometric Dimensioning and Tolerancing. Here's a good link for mostly US based, but also others to learn more. I have had Don Day teach classes at a few of the places I have worked. I highly recommendhim and Tec-Ease to anyone in need of learning or refreshing their GD&T.
      http://www.tec-ease.com/
      GD&T is a useful knowledge to have. In the UK I had some training recently with Iain Macleod Associates who I would certainly recommend.
    1. GarethW's Avatar
      GarethW -
      Also, well spotted you two. I have corrected the article title
    1. nikhil3586's Avatar
      nikhil3586 -
      Its good to hear from you. But, if the two plates are matched, it will of course follow the axis and tend the rods to be straight.
    1. broby's Avatar
      broby -
      Here is also a classic example of over specification on a manufacturing drawing!
      It is stated that an M20 rod is to go through a Ø30mm hole, that is 5mm clearance around the tie rod and the hole has to be in the correct position by ±0.01mm? Are you crazy or what? Did the manufacturers use a hand drill to so badly stuff the manufacture of these plates?
      I do concede that the point of the article is to allow the inclusion of a tolerancing scheme that will allow the end user (manufacturer) to understand what is important, but come on guys, don't forget that as tolerances tighten, manufacturing costs go up!
    1. BallPein's Avatar
      BallPein -
      Quote Originally Posted by broby View Post
      Here is also a classic example of over specification on a manufacturing drawing!
      It is stated that an M20 rod is to go through a Ø30mm hole, that is 5mm clearance around the tie rod and the hole has to be in the correct position by ±0.01mm? Are you crazy or what? Did the manufacturers use a hand drill to so badly stuff the manufacture of these plates?
      I do concede that the point of the article is to allow the inclusion of a tolerancing scheme that will allow the end user (manufacturer) to understand what is important, but come on guys, don't forget that as tolerances tighten, manufacturing costs go up!
      I have to agree that the intent of the article is correct but the example is a wee bit hard to take serious.
      Im an Engineer and also a trained toolmaker and I think this example is a bit Childish to say the least, and from a practical point of view the 20mm bar over that length will be relatively easily manipulated to fit through the Granite and then the top plate will fit anyway.
    1. ivnicholas's Avatar
      ivnicholas -
      It is a good article. I am currently taking a course through ASME. It is doing a lot to change my perspective on design drawings. There are a lot of little considerations that make a world of difference in design.
    1. lockbits's Avatar
      lockbits -
      Hi There, I love GD&T. It is a wonderful thing, if the guy you give the drawing to can understand what you've written. I use GD&T in my drawings where I can, because it make sense. However, it is not easy to get good training in GD&T everywhere in the world (even here in Australia) and that can lead to mistakes being made.
      We have a lot of stuff made in China, and some of our suppliers will send parts back with dimension inspection sheets that quite clearly indicate that they have not understood the drawing (GD&T). This has cost us time and even delayed a product launch.
      The moral of the story is that GD&T is a language, and both parties need to be able to understand it, otherwise you can end up with a costly mess on your hands. Please bear in mind who will be interpreting the drawing.
    1. marcasmith's Avatar
      marcasmith -
      There are many reasons to use GD&T, however one must realize that no matter what it is extra work and expense - up front for you and downstream for (some) vendors. There has to be a good reason to justify using it. The example I've given here is to point out one really good justification - protecting your professional butt against having to pay for a large order of parts that are rejects.
    1. marcasmith's Avatar
      marcasmith -
      Look closely you'll see those are imperial dimensions except for the metric holes. And it turns out after the fact those holes were drilled by hand.

      Quote Originally Posted by broby View Post
      Here is also a classic example of over specification on a manufacturing drawing!
      It is stated that an M20 rod is to go through a Ø30mm hole, that is 5mm clearance around the tie rod and the hole has to be in the correct position by ±0.01mm? Are you crazy or what? Did the manufacturers use a hand drill to so badly stuff the manufacture of these plates?
      I do concede that the point of the article is to allow the inclusion of a tolerancing scheme that will allow the end user (manufacturer) to understand what is important, but come on guys, don't forget that as tolerances tighten, manufacturing costs go up!
    1. GarethW's Avatar
      GarethW -
      Quote Originally Posted by lockbits View Post
      It is a wonderful thing, if the guy you give the drawing to can understand what you've written.
      Quote Originally Posted by lockbits View Post
      We have a lot of stuff made in China, and some of our suppliers will send parts back with dimension inspection sheets that quite clearly indicate that they have not understood the drawing (GD&T). This has cost us time and even delayed a product launch.
      Yep - sounds familiar. I try to use GD&T sparingly and only when necessary. I usually back up the drawing with emails and phone conversations to make sure people absolutely understand what I want I if put on a GD&T dimension.
    1. Douglas J. Carr's Avatar
      Douglas J. Carr -
      Quote Originally Posted by BallPein View Post
      I have to agree that the intent of the article is correct but the example is a wee bit hard to take serious.
      Im an Engineer and also a trained toolmaker and I think this example is a bit Childish to say the least, and from a practical point of view the 20mm bar over that length will be relatively easily manipulated to fit through the Granite and then the top plate will fit anyway.
      Excellent.
    1. npanchal_007's Avatar
      npanchal_007 -
      I want to Creo-1.0 sheet metal and simulate,structure,thermal,model chek tutorial.
    1. wbrokow1's Avatar
      wbrokow1 -
      I agree with lockbits. Many shops and I mean really good shops don't have a clue when it comes to GDT. Most will compensate by doing everything with a CNC and call it a day. You pay more but they get you a good part every time. I use GDT on super-critical features to be sure they get it right. You have to know the capabilities of the shop and their equipment you are using before you contract with them.
    1. Vishal Pharande's Avatar
      Vishal Pharande -
      nice article but its basic things
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