Shop Math 101: 1/100″ = 1/4″

Does your fabrication shop lack confidence in your drafting/detailing department?

Have you struggled to get buy-in when trying to roll out new processes, technology or deliverables?

Did you wonder why? More importantly, do you KNOW why?

Getting to the Root of Trust Issues

My entire career spanning manufacturing to construction, fabrication shops have had trouble trusting the information they’re given. And there’s good reason. It takes time to master a domain and learn the work. And production staff are busy building and fabricating. They don’t have time to run into the office every time something is wrong. As a result, office staff take longer to train and often persist with producing lower quality work.

But there’s also a lot of reasons that are not good. Downright bad in fact because they’re simple to resolve. Things you’re NOT doing wrong but are causing problems. These trust issues are easily corrected if they’re understood properly. One such issue is fabrication tolerances that I categorize as Shop Math.

The Dynamic Between Tolerance and Rounding

I’ve had trouble explaining this verbally so I figured a more graphic (yet generic) representation would be in order. In basic terms, you need to use a rounding factor 1/2 the amount of your fabrication tolerance to achieve the desired result. As the title of this post suggests, a value of 1/100″ of an inch, can result in a deviation of 1/4″. That’s real Shop Math in practice.

My example uses both fractional inch and decimal to more clearly illustrate the point. You don’t want to get me started in why everyone should use decimal, that’s another post. But decimal also has the same dynamic, it’s just less hidden and more easily fixed. You get a lot less pushback in a shop by adding an extra decimal than by changing the denominator of a fraction to a number the shop says they don’t fabricate to.

Let’s take the following example..,

In this example, lets assume our tolerance is 1/8″ (construction field tolerances, that’s fairly common).

The TOP RED dimensions are all ROUNDED to the nearest 1/8″ to match our target Tolerance. A fairly common practice. The BOTTOM GREEN dimensions are all ROUNDED to the nearest 1/16″ which is half our target Tolerance.

Each graphic shows a line 6″ long. Half that is 3″. We’re going to make a gap and dimension from each end to that gap. Maybe it’s a weld joint in pipe or perhaps a mortar joint in concrete block. Doesn’t matter what to illustrate the problem.

For the graphics on the LEFT (Quadrants 2 & 3), the gap is just shy of 3/16″ of each side of middle to forced the dimension to round UP. For the graphics on the RIGHT (Quadrants 1 & 4), our gap just heavy of 3/16″ of middle to force the dimensions to round DOWN.

Rounding = Tolerance = Confusion = Mistrust

Let’s focus on the TOP RED in the following illustration…

If you take the two parts and add them, they vary by 1/4″. Adding 3/8″ (our rounded gap size) to either of them does not equal 6″ either.

Quadrant 1: 2 3/4″ + 2 3/4″ + 3/8″ = 5 7/8″
Quadrant 2: 2 7/8″ + 2 7/8″ + 3/8″ = 6 1/8

Between these 2 examples, a mere 1/100″ difference in our gap results in a 1/4″ difference and neither adds up to the 6″ of the total length. This is a 1/4″ TOLERANCE because we set ROUNDING to 1/8″ to match our fabrication accuracy.

If you ever wondered why your shop doesn’t trust your drafting/detailing, this is one reason. The Shop Math just doesn’t add up. They see the sum doesn’t add up to the whole and leads them to question the accuracy of your drawing and your staff.

Rounding = 1/2 Tolerance = Trust

Now lets focus on the BOTTOM GREEN portion of our illustration…

Between the left and right (Quadrants 3 & 4) we’re still making the gap just shy and just heavy of 3/16″ from the center. As you recall, we said our fabrication tolerance target was 1/8″ but here, the dimensions are ROUNDED to 1/16″. This is HALF of our target fabrication Tolerance.

Here, it doesn’t matter of the dimensions round UP or DOWN due to the slight variation in the gap, the dimension are the same. Furthermore, if you add the parts, you get 6″.

Quadrant 3: 2 13/16″ + 2 13/16″ + 3/8″ = 6″
Quadrant 4: 2 13/16″ + 2 13/16″ + 3/8″ = 6″

Same Geometry – Different Clarity

As you can see, we had different results between rounding UP and DOWN when our ROUNDING value equals the Tolerance we’re trying to achieve. When we round to HALF the Tolerance, those small variations are masked and all our numbers add up.

So if you’re dimensioning for the shop, it’s important to realize this little change can mean questioning or trusting your data and staff. Additionally, if you take the time on the shop floor to explain WHY they see these differences, they quickly realize that the information that leads them to question the data (and your people), is also the very same data that’s most likely to be wrong. What IS accurate and didn’t change, is the geometry itself. This is an extremely critical point to highlight if you’re trying to get your shop to use automation and drive machine tools from CAD/BIM geometry.

The model/geometry, is the MOST right data we have. It’s just not human readable and what we provide as human readable is prone to errors such as these. This is one reason you’re seeing terms like “Model Based Enterprise” starting to float around in the Manufacturing space. It’s also a reason you’re seeing more shop go paperless, eliminating dimensions when possible by leveraging automation.

These efforts can be challenging and often require a leap of faith. But if everybody understands dynamics like this, it can be extremely helpful in moving all of your staff to more digital workflows. Because they trust the geometry and you eliminate what’s confusing them.

Revit Fabrication Parts – Control w/Dimensions

AutoCAD was famous for it’s command line. It was easy to move items and type locations, distances or coordinates. Revit isn’t quite as intuitive for those coming from AutoCAD.

There’s a lot of reasons you need more control of Fabrication Parts in Revit. You may want to align the ends of pipe for a rack. Or perhaps you want to control the spacing between pipes in a run of parallel pipes.

At first it appears like the best you can do is drag items close. Eyeball them up so to speak. The traditional methods used in AutoCAD just won’t work. Methods like drawing construction geometry and using point filter and/or object snaps.

You can precisely control placement and location when moving to Revit from CADmep. In Revit, you simply place dimensions and edit them. Seems easy enough but there’s a couple nuances that can leave users frustrated. We’ll cover how to do this below.

Adding Dimensions in Revit

You can use the Annotate tab on the Ribbon in Revit. You’ll use the Linear, Aligned and Angular dimensions the most.

When you place a dimension between parts in Revit, the obvious thing would be to double-click the dimension to edit it. You’ve likely seen the following dialog…

If you see this dialog, you’re on the wrong path. This is not where you’d edit a dimension to control part placement. For controlling parts with dimensions in Revit, you actually select one of the parts you dimensioned.

The following image shows a Fabrication Part selected. But there’s still a problem. If the dimension text is black, you can not edit it. This is because one of the parts are over constrained. If you find a Lock icon on one of the parts, try unlocking it.

After unlocking the part, you may need to deselect and re-select the part for the dimension text to be editable. In the following image, you’ll see the dimension text is now Blue.

With the dimension text Blue, you can now click on the text to edit it as shown in the following image.

With the edit box for the dimension text activated, simply type the desired value and press <Enter> or click out of the edit box.

You’ll see the part move to the dimension you entered. The key to determining which part moves when editing a dimension is based on the part you select. If you just wanted to align the parts, you can delete the dimension afterward. On the other hand, if you want to maintain that relationship, highlight the dimension. You’ll see a unlocked Lock icon as shown in the below image.

If you click to Lock the icon, this relationship between parts will be maintained going forward.

The below video shows three pipes modeled with various end lengths. We’re using dimensions to align the ends of the pipe. We also delete the dimensions afterward. Moving one of the pipe ends later will not move the ends of the other.

Pipe spacing is set using dimensions just like before only this time, the dimensions are retained and the lock icon locked When one pipe later moves, the other moves to maintain the spacing.

ITM Dimension/Option Locking Hack

When you build content, it’s often desirable to have certain dimensions or options locked. This even applies to connectors, seams and dampers but to a lesser degree.

If you have a lot of Dimensions and/or Options to Lock or Unlock, you don’t have to individually pick each one. You can lock or unlock many very quickly provided they’re in a row.

The trick is simple….pick the button to lock/unlock the first field you want to change, and then while still holding the pick button drag your mouse up or down. This is a fast an efficient way to lock large groups of properties without picking each one.

The following recording shows this process. We’re using Pattern Number (CID) 910 as our example.