Intolerance of Tolerances

In a recent LinkedIn post, the topic of Tolerance Stacking was brought up. I’m not a machine designer, but I’ve spent a lot of my past life in Manufacturing. In that world, the term was used frequently. If the term was used in Construction, it certainly wasn’t when I was listening.

Tolerance Stacking can be described (in my mind) as the accumulation of allowable tolerances to a point where the design is no longer suitable for it’s intended purpose. Errors resulting from Tolerance Stacking are caused by a few things…

  • Lack of tolerance awareness
  • Poor annotation and documentation of tolerances
  • Both of the above

Tolerance Stacking Explained

The best way to understand Tolerance Stacking is from a few examples. In our first example, we see a part 10 Units long with 9 holes, equally spaced 1 Unit apart. Take note of the RED dimension on the right.

10 Unit Long Part with 9 Holes Spaced 1 Unit apart

You may have seen parts dimensioned like this. Looks pretty normal. Now lets consider this same part and assume the dimensions have a tolerance of +/- 0.0625 (1/16 Inch). Now lets also assume that all the dimensions are in the negative -0.0625. The following graphic illustrates this condition. Again, notice the RED dimension on the right.

Tolerance Stacking using the an allowed -0.0625 on each dimension.

Is the overall length really have enough tolerance to compensate for the accumulation of those tolerances?

Now lets look at the same part, same tolerances but annotated/documented differently. It’s not as “pretty” and takes up a lot more real estate on your drawing.

Same part as before but dimensioned differently.

But lets look at that same -0.0625 extreme case tolerance in this scenario. Once again, keep an eye on that RED dimension to the right.

Using an alternate annotation approach solves the Tolerance Stacking problem.

This latest example solves the Tolerance Stacking issue by clearly outlining where the tolerances are allowed. In fact, in construction, we’re already doing this. We just don’t call it Tolerance Stacking.

In construction, one of the ways we eliminate Tolerance Stacking is by dimensioning to gridlines and columns. Dimensions in relation to known fixed points minimized Tolerance Stacking.

Are you old enough to remember when rafters were layed out by hand on the job site and cut individually? You would cut one and use it as a template and use that to mark the others. You never installed the template and used the next cut as your template for another. This minimized Tolerance Stacking as well.

Geometric Dimensioning and Tolerancing – GDT

What’s less familiar, is another concept used heavily in automotive and other precision manufacturing. It’s called Geometric Dimensioning & Tolerancing or “GDT” for short.

Traditional linear tolerances have flaws. GDT on the other hand more accurately describes “features” and allowable deviation from the desired location using a more complex form of graphics and symbols.

Once again, the best way to explain this is with some illustrations. The following example shows a square part with a hole in the middle. Pay close attention to the RED dimensions.

Top Left – Perfect Part (not real world)
Top Right – Hole moved 0.0625 to the right
Btm Left – Hole moved 0.0625 up
Btm Right – Hole moved 0.0625 in both directions

In this example, you see when the hole is moved to the maximum tolerance in both directions, it’s actually further away from it’s desired position than 0.0625.

This is where GDT comes in. In this last example, GDT is used to “Describe” the allowable deviation from it’s ideal position.

GDT can more accurately describe tolerances.

There’s actually an ASME Standard for GDT (Y14.5.2) and a full explination of GDT is not only beyond the scope of this blog but my knowledge, There’s a lot of courses out there specifically for this but a good “101” description can be found here.

Given trends in Prefab, Modularization, and Construction becoming more like Manufacturing….Makes you wonder….should there be a “GDT” style of documentation for construction?

Tag Opposite End of Fitting

In CADmep, using the Size command, you can tag the size of a fitting. But on a fitting like a transition, what if you wanted to tag the size of the opposite end?

This can be easily done but the sequence is a little nuanced. Type “SIZE” from AutoCAD’s command prompt or select the “Size” tool button on the CADmep toolbar.  

When prompted to Select Objects, select the fitting. Once the fitting is selected, instead of pressing <Enter> to end the selection like you normally would, press and hold the <TAB> key while you press the <Enter> on the keyboard at the same time. Your tag will display the size of the opposite end.

Depending on your AutoCAD and Mouse settings, right-clicking to end the select objects prompt may not work and instead bring up a right-click menu. For this reason, it’s recommended you use the <Enter> key on your keyboard while pressing <Tab>

The following video shows to transitions of the same size side by side. The left transition has it’s size tagged like you normally would. The fitting on the right, the <Tab> key his being held down when the <Enter> key is pressed which results in the tag displaying the size of the opposite end of the fitting.