Reasonable Tolerancing For Press Brake Bending

Sheet metal and plate bending may look easy to those who aren’t familiar with the procedure, but it’s actually one of the more challenging operations in many metal fabrication shops. It shouldn’t be too difficult, at least in theory. Sheet metal bending science has established formulas for bend allowances and bend deductions that take material elongation into account for specific materials. Why, then, do those who work with press brakes need so many practice pieces? How about we make bending easier?

Sure, if you tolerate in a reasonable manner.

That was the consensus reached at the end of a lively debate in November of last year at a Chicago steakhouse. Steve Benson, president of ASMA LLC in Salem, Oregon, was seated at one end of the conference table. Benson is a well-respected authority on press brakes and a regular contributor to this publication. Larry Boden, a press brake tooling product training specialist from Mate Precision Tooling in Anoka, Minnesota, was our counterpart.

Benson noted that they had a lengthy conversation about how tolerancing is commonly misused in the sheet metal manufacturing industry, and how realistic tolerancing is still one of the simplest and most practical ways to achieve efficient bending.

It Depends on the Materials Used

Bending is now more quicker and more predictable thanks to the accuracy and reproducibility of current press brakes and precision-ground tools. However, the sheet metal itself is still a major unknown.

For example, as Boden put it, “when designing parts for a sheet metal application, you must take a realistic view of the material characteristics, consistency of makeup and temper, yield strength, tensile strength, and grain direction, to name just a few variables.”

“But most important,” Benson said, “is the variation in material thickness.” Tolerance callouts are particularly affected by this issue. There can be noticeable angular differences between objects that differ by only a few thousandths of an inch.

The root of the issue is found in the process of material specification. Fabricators typically consider of 10-ga. mild steel as being 60 KSI and 0.135 inches thick. However, these are not absolute standards; rather, they represent averages, and there is some variance from one sheet or batch to the next. Gauge-zone tolerances are typically 0.006 in. for standard 10-ga. A36 material. As a result, the range for 10-ga. is between 0.129 to 0.141 inches.

The 0.012-inch discrepancy “is more than enough to cause problems when it comes to forming on the press brake,” Benson added.

The emphasis is Boden’s. “Especially,” he continued, “if the applied tolerances are unreasonably tight.”

Rotational Shifts

The die opening determines the degree to which the angles vary. The higher the angular variation, the smaller the die opening in relation to the material thickness. So what is the reason? It’s a fundamental exercise in air bending. The bend angle in air bending is not dependent on the die angle. In air bending, the punch must descend far enough into the die space to overbend for springback, hence the primary purpose of the varied die angles is not to bend different angles.

The relationship between the aperture width and the penetration depth into the die space is what determines the bend angle of an air bend. Unfortunately, the thickness of the material is not as consistent as the position of the punch tip at the bottom of its stroke, which determines the depth of penetration.

It is especially important to accurately measure the depth of penetration when working with small die apertures. If you adjust the depth of penetration on a wide die, the resultant bend angle will only shift by a small amount, but if you do the same on a narrow die, the resultant bend angle will shift by a large amount. For this reason, the bend angle is sensitive to variations in material thickness, especially for more narrow dies.

“A mere plus or minus 0.006 in. variation in material thickness can represent as much as 4 degrees of angular variation,” Benson stated. Changing the direction of the bend from perpendicular to the grain (across) on one part to parallel with the grain on another can produce even more diversity.

Changes in Dimensions

Angular dispersion is only the beginning of the issues. The elongation behavior of a material in a bending motion can be affected by variations in its thickness.

Because of this, the requirements for the initial blank size have shifted. Take that 10-ga. A36 material into account. The bend deduction is 0.221 in. at the thin end of the gauge range (0.129 in. ), and 0.237 in. at the thick end (0.141 in. ).

Benson remarked, “That means the bend deduction can vary by as much as 0.016 in.”

When working with parts that have several bends, this makes it nearly hard for many press brake operators to maintain a tolerance of 0.005 in. A minor misstep at the first turn might easily balloon into a catastrophic one around the last.

Consistency in Content has many Advantages.

This is not to suggest, however, that such precise bending never occurs, so long as the fabricator maintains adequate quality standards, particularly with regard to the raw material. Whether they’re using an antique press brake or one of the state-of-the-art bending devices available today, press brake operators can benefit from high-quality materials.

• If at all possible, stick with one central service provider and minimize switching between suppliers.
• Shape components so that their arcs maintain a constant angle relative to the grain.
• Check and arrange components in whatever order you like. This includes variations in thickness (for which an operator must measure each individual piece), color or tint from batch to batch, surface condition, and other signs of a change in material qualities.

Is This Level of Accuracy Necessary?

Precision fabrication is all about showing off the skills that come from using cutting-edge tools and the most efficient methods. Let’s pretend a company is proud of the fact that it can bend parts with a 0.0050 in. dimensional tolerance. Surely that gives you an edge over the competition.

Possibly, but only if maintaining such a tolerance is beneficial to the final result. But suppose it is worthless. A fabricator might not always need to hold an exact 0.005 inches. If the tolerances were loosened to 0.010 in. or 0.020 in., what would happen? What if we increased the angular limits from half a degree to one or two degrees?

Benson argued that having a reference for comparing called tolerances would have a major impact on manufacturing operations.

When Benson refers to a “handy guide,” he means a chart like the one in Figure 1 that lays out acceptable ranges of variation for a produced part’s linear and angular dimensions. You’ve decided to put an end to all unreasonable requests and excessive expectations.

As was demonstrated, while it is possible to achieve a high degree of precision with a single feature or dimension, the “reasonable” tolerance tends to relax when multiple features (whether angular or dimensional) are measured together. Dimensional and angular differences will be more pronounced with thicker material.

Both the angular and dimensional tolerances can be raised as the number of turns grows. Technicians who specialize in bending know this, and they often take it for granted. It all comes back to differences in physical properties.

Take the dimension variance into account. The thickness difference that arises from bending materials of different thicknesses, each of which has different elongation properties during bending, must be taken into account by the “edge to the outside surface of the bend” dimension. The technician faces an additional set of variables that expand upon the first one if the part needs to be bent again. Since the initial bend stretches the material to a variable extent, the second bend will be dimensionally inaccurate even before the punch tip makes contact with the sheet because of variations in material thickness.

Modest Alteration, Significant Effect

Tolerances for bending don’t have to be lax by any means. As Benson and Boden described, a significant amount of time and energy is spent fixing what may seem like insignificant errors. Small adjustments can have a big impact when bending.

As examples, Boden used Figures 2 and 3. Dimensionally and in terms of material thickness (0.0105 inches), the two drawings are similar. Figure 2 has a tolerance of 0.030 in. for its overall dimension of 6.110 in., while Figure 3 has a tolerance of 0.020 in. Unlike Figure 3, which does not depict reasonable tolerances, Figure 2 does.

That seemingly insignificant shift in tolerance levels is crucial. That’s because there should be three groups of variables included in that overall dimension. Since there are three flange dimensions and four bends to reach the target diameter of 6.110 inches, Benson noted, “these would be the possible errors over these areas.”

Benson and Boden both stressed the importance of the time, money, and effort required to obtain the precise tolerances that can be attained by a precision bending process. Achieving such precision can be worthwhile if doing so is required for the product to perform as expected.

If that’s the case, why not just “engineer out” the issue by basing component designs on tolerances that are actually feasible? The importance of following proper bending design requirements cannot be overstated. According to Boden’s analysis, “When you apply such guidelines to your drawings, your scrap rate will decrease, and it follows that your production rates will increase.”