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Press brake bending basics: Proper measurement of formed parts

It’s easy, if you have the right tools and know how to use them

Figure 1
How should this 90-degree bend and 30-degree, acute bend be measured? It depends on the tolerances and dimensions called on the print.

This month I would like to address the proper measurement of formed parts as I have received several emails from around the globe asking for this kind of information. I have selected two to work from, one from Egypt and one from the U.S.

Questions

From Egypt. Hope this email finds you well. This is Diaa from Egypt. First, I would like to thank you for interesting and useful articles on thefabricator.com.

Could you advise me with regard to the correct measuring practice for a few specific air bending patterns to get the correct bend dimensions and angles. One is a 90-degree bend and another is an acute-angle, 30-degree bend. For both, I need to measure from the edge to the bend (see Figure 1).

I also have an offset 90-degree bend with two bending steps, and I need to measure from bend to bend; a closed and open hem (measuring from the edge to the bend); and finally a U bend (measuring from bend to bend) with two bending steps (see Figure 2).

From the U.S. Is there an industry standard for measuring parts after a bend in a brake press? For example: if I have 1⁄8-in.-thick steel, the drawing states that the bend line is 1 in. from the edge, and the notes on the drawing say to bend down 90 degrees, what is the proper way to check the 1 in. after it is bent—inside the bend or outside, which includes the thickness of the material?

It Depends

The basic answer to all of these questions is the same: It all depends on the applied tolerance and how the dimension is called on the blueprint, CAD drawing, or sketch. For example, sometimes the information is provided on CAD prints. The 1.000-in. line would be assumed to be the bend line and the outside dimension 1.125 in., shown on the print as “bend (direction) 1.000 (1.125).”

While this information is of great importance to both the operator and quality control, it still might be missing. Providing this data is usually at the discretion of the engineer or programmer. Then again, sometimes in smaller operations, it’s all about knowing what your co-workers’ needs are.

Inside or Outside Dimensions

Unless otherwise notated, it is common practice in precision work for bends to be called between the outside surfaces of the part. Why? This simply is the most accurate and easiest to measure, and it’s the way parts are called on Diaa’s items in Figures 1 and 2.

But then in some operations the tolerance is rather liberal—say, within 1⁄16 in.—like when forming dump truck boxes or locomotive frame and cab parts. Many times these jobs are called by the inside dimensions. Because of their tolerance, they have to be “ugly bad” not to work. In these cases, part dimensions are checked using a standard tape measure, not the most precise tool but functional for the work at hand.

But in the precision world, the dimension is usually called to the inside surface of the workpiece. As you can see in Figure 3, the bend line has shifted away from the inside surface of the bend; the distance the bend line shifts occurs as a percentage of the radius and the bend angle.

Figure 2
The offset bend (top), hem (center), and U bend must be measured per the print, but operators need to know what exactly to measure and how to use measurement tools properly.

Acute Bend Angle Measurements

Now let’s look at the acute bend in Figure 1. This can be measured correctly two different ways: to the apex of the bend or to the surface of the radius. Again, this depends on what the engineer or draftsperson specifies. But one measurement is much easier to perform than the other.

Measurement to the apex of the bend is somewhat subjective without specialized tools such as an optical comparator. Without such tools, you can measure to the apex by setting an adjustable square to the desired apex dimension, and then sliding a straight edge up to meet the bottom edge of the adjustable square blade. You can also place a protractor over the bend and eyeball a measurement using a pair of calipers. Either way is a good estimate at best.

You can take a far more precise reading for acute bends by measuring from the edge to the outside radius, rather than the apex of the bend. Still, in 40-plus years in the trade, I have found that calling a dimension to the outside radius is somewhat uncommon. Even though the outside radius dimension is rarely given on prints, you can calculate it by using the formula for the outside offset, or OSOS (see Figure 4).

OSOS = {(Rp + Mt) /

[Tangent (Included bend angle / 2)]} – (Rp + Mt)

Rp = Inside radius, whether it is

the punch radius or the floated radius

Mt = Material thickness

This formula gives both the press brake operator and quality control technician a hard and precise number to measure—no more estimates. Subtract the OSOS from the edge-to-apex dimension called on the print, and you’ll find exactly what the edge-to-outside-radius dimension should be.

Hem Measurement

Forming hems is one of those aspects of sheet metal that is very operator-dependent. Hit the hem just right and the numbers work; hit it softer or harder than necessary and the flange dimension changes. Fortunately, hems are rarely specified on tight-tolerance flanges. They are usually stiffeners or placed on the part for safety reasons, such as to remove a sharp edge. But they may be specified within a tight tolerance when mating parts are involved.

Figure 3
Because the bend line shifts after forming, the edge-to-inside-surface dimension will be greater than the edge-to-bend-line dimension.

The best way to measure a hem, from the outside radius to the hem edge, is with a surface plate, angle block, and a height gauge. Place the hem’s outside radius on the surface plate, hold the part against the angle block, and take the reading from the surface plate to the edge of the hem with a height gauge.

This of course may not be an option if you don’t have these measuring devices. In this case, you can use your calipers to measure from the hem’s outside radius to the edge. This leads us to the correct use of the calipers themselves.

Caliper Calibration and Use

Many believe that a 1.000-in. gauge block can be placed between the jaws of the caliper, and if the reading is 1.000 in., all is well. To look at calibration this way is a mistake.

Instead, take the calipers and use a standard quality control gauge pin with a known length—3.000 in., for example. Check the calibration with a pin at both the top and the bottom of the caliper jaws. If there is more than a 0.005-in. error between the top and the bottom of the jaws, adjust them using the two set screws on the top of the caliper head. You need to adjust them until the error is 0.005 in. or less, while ensuring that the head still can move freely along the caliper body.

A good way to start is to lightly tighten both set screws and then back off the set screws an eighth to a quarter turn. This may work on the first try, or it may take several attempts to remove the error.

The caliper body does not move, so the error in the caliper jaws is in the head of the tool. For that reason alone, you should hold your part against the body of the caliper and slide the head up to the part to take the measurement. So in the case of Diaa’s acute bend-to-edge measurement in Figure 1, the technician should place the caliper body against the outside radius, and then slide the head up to the edge.

Because the caliper body is fixed, you can use it as a square; and if you have adjusted the error out of the caliper head, you can use it as a set of parallels. Why should you care? You can quickly check to make sure a part feature—a hat section, for instance—is square or parallel while also checking its dimension (see Figure 5). True, you would not qualify a part this way, but as a quick check for consistency during a run, its value cannot be understated.

Nonparallel Flange Dimensions

These are no harder to check than any other measurement, if you have the correct tools. Consider a 3-degree open bend, as shown in Figure 6.

To measure this, you need a surface plate, an angle block, and a height gauge. Of equal importance to the tools is how the part is held. Holding the part against the angle block incorrectly—that is, with the 90-degree bend against it—the 3-degree open bend will be tilted and cause the height gauge to take a measurement from the inside edge of the material, resulting in a bad reading. Holding the 3-degree bend perpendicular to the angle block, as shown in Figure 7, will allow the height gauge to be flat against the material edge and, hence, give you a correct reading.

More Information Is Better

There are many ways of measuring to ensure a correct measurement and good quality, but is there an industry standard? Not really. We just have some general rules and practices that relate to whichever area of the trade you are working in.

Nonetheless, even under the loosest of tolerancing and measuring, the more information presented to those who actually do the manufacturing, the faster and more precise your parts will be. There is absolutely no reason that, with a little care and a lot of knowledge, perfect parts can’t be produced.

About the Author
ASMA LLC

Steve Benson

2952 Doaks Ferry Road N.W.

Salem, OR 97301-4468

503-399-7514

Steve Benson is a member and former chair of the Precision Sheet Metal Technology Council of the Fabricators & Manufacturers Association. He is the president of ASMA LLC and conducts FMA’s Precision Press Brake Certificate Program, which is held at locations across the country.