Bending Sheet Metal (Part 2) Sport Aviation / Experimenter "Technically Speaking" Article May 2017
In part one of this article we discussed, in depth, the theory and the process for developing a flat layout for manufacturing a sheet-metal part. In this article, we will take it to the next step: the practical process of converting the flat layout to a bent sheet-metal part. The tool that we use for bending sheet-metal is called a “brake.” There are several different types of brakes. A “press brake”, commonly found in manufacturing environments, uses a vertical positioned die bending the sheet-metal over a stationary “V” block. A “cornice” brake, which has a solid clamping bar the full width of the brake, is limited to simple straight bends. And the “box and pan” brake. The box and pan brake is sometimes referred to as a finger brake because of its individual fingers which can be configured in a nearly unlimited fashion to make some of the most complex sheet-metal parts. If you are interested in purchasing a sheet-metal brake for building experimental aircraft, this would most likely be your first choice.(Figure: 1) In our shop we use a box and pan brake manufactured by Mittler Brothers. It has a very unique combination of individual bed fingers as well as leaf fingers and a complete selection of radius fingers from 1/16 inch to 5/16 inch radius.
In part 1 of this article we talked about the necessity for bending aluminum around a radius and paying particular attention to the minimum bend radius. The amount of force required to bend sheet-metal tightly around a radius can be quite substantial. As the sheet-metal gets thicker, the amount of force required increases substantially. Normally the brake is rated for the maximum gauge material that the machine is capable of bending. This requires that you select a brake compatible with the thickness of material that you are bending. For example, a 16 gauge brake is capable of bending 16 gauge (.062”) mild steel. And a 22 gauge brake is capable of bending 22 gauge (.030) mild steel. Even though the standard for gauging brake capacity is using mild steel, we can get away with bending a little bit thicker material when bending aluminum. A 16 gauge brake could typically bend up to (.080”) aluminum and a 22 gauge break could bend up to (.040”) aluminum. You may be able to bend thicker material than the break is rated for, however, it is possible to do permanent damage to the brake if you exceed its limitations. Keep in mind that a high quality sheet-metal brake is a precision tool. Keeping it in good working order and properly adjusted will allow you to make precision bends consistently. Fortunately, most of the small experimental aircraft use sheet-metal thicknesses that are relatively small. Typically .016” to .063”. As a result, some 15 years ago we elected to purchase a 16 gauge box and pan brake that continues to serve us well on a daily basis. If you’re interested in purchasing a brake, selection of the gauge rating is only one of the criteria. You also must choose the width of the brake, which will determine how long of a piece of sheet-metal you can bend. And as you might imagine, both gauge and length significantly increase the cost. We have found that a high quality 48 inch box and pan brake is probably the biggest bang for the buck. In addition, it’s easy to move and doesn’t take up a lot of space.
Before we can begin using the brake, it’s necessary to make sure that we have it properly adjusted. Trying to make precision bends without first setting up the brake can become very frustrating. Remember, it is a precision tool. The adjusting mechanisms are located on each end of the brake. There are two adjustments that will be necessary any time that you change material thickness. Clamping pressure, and clamping finger setback. At first glance you may think that this seems a bit cumbersome just to bend a piece of sheet metal, but I assure you, once you’ve done it a few times it really is quite simple. To start with, clamping pressure should be only enough to keep the material from slipping during a bend. One of the biggest, and most common, mistakes made is the use of excessive clamping pressure. Excessive pressure can cause the clamping fingers to shift forward, leaving inadequate space for the aluminum during the bend. Clamping pressure can be adjusted by the two nuts located on the stem of the locking cam.(Figure: 2) By adjusting the nuts up or down the clamping pressure can be increased or decreased. Keep in mind that all adjustments need to be accomplished on both ends of the brake. Typically, we stick small sample pieces of the material, that were about to bend, 2 to 3 inches from each end of the brake. While engaging and disengaging the locking cam, we will make adjustments to the adjusting nuts until we have just the right amount of tension on the sheet-metal. Next, we will need to adjust the clamping finger setback. The purpose for this is to provide enough space between the bending leaf finger and the nose of the clamping finger at the point of maximum bend. If not enough space is provided at the maximum point of bending, damage to the aluminum or the radius fingers is possible. Think about this: If you are applying 50 pounds of force on the 15 inch handle, the bending leaf that is making contact with the sheet-metal, 1/4” away from the bending axis, is applying 3000 pounds of force to the sheet-metal and clamping finger. The whole purpose of the sheet-metal brake is to take advantage of this leverage. With the counterbalance weight installed, the amount of force required to bend a piece of sheet metal is really quite small. If you find yourself applying an inordinate amount of pressure during the bend, it probably means that you have it mis-adjusted. Don’t force it! Adjusting the clamping finger setback is quite simple. Move the bending leaf to the angle that you will be bending, typically 90°, and insert your sample piece of aluminum used to check clamping pressure in between the radius of the clamping finger and the bending leaf.(Figure: 3) We then simply release the locking clamp and rotate the adjusting screw knob in or out until we have approximately 1.5 times the thickness of the material between the clamping finger and bending leaf. Having excessive space will typically result in a larger radius than the radius of the bending finger. This of course, will throw off all of your bending calculations, and subsequently the final dimensions of your bent component. Once we have adjusted the finger setback on both sides of the brake, we will reengage the bracket locks and we are now ready to begin bending our piece of sheet metal.
Positioning the flat layout for bending can seem daunting at first, but once you understand the basic premise, you will be saying to yourself, “why didn’t I think of that”. In part one of this article we created a flat layout that consisted of flats and bend allowance. The bend allowance, and hopefully “only” the bend allowance, is the section of the metal that we will be bending in the finger brake. The bend will begin at precisely the end of the flat section and continue bending to precisely the beginning of the next flat section. The trick here, is how do we position the flat layout so that the beginning of the bend allowance starts at exactly the bottom of the radius finger. Keep in mind this is the section that is underneath the clamping finger. Once clamped in place, we are unable to see at least half of the bend allowance section of the sheet-metal. So how do we precisely locate the mark on the sheet-metal that denotes the beginning of our bend when we can’t see it? The trick is, move the mark to where we can see it.
We call this, the “site line”.(Figure: 4) One of the known variables is the radius of the clamping finger. The distance from the bending axis to the leading edge of the radius finger, in our case, is 1/8”. By placing a site line on the flat layout that is projected 1/8” from the edge of the bend allowance that will be under the brake, we now have a site line that we can use as a positioning reference. By looking straight down from above the brake, we can align the leading edge of the radius finger with the site line while clamping the flat layout in position.
By default, this positions the beginning of the bend allowance at exactly the beginning of the radius on the clamping finger. Although it may appear close in some circumstances, the site line is not the center of the bend allowance. As a result, if you create a site line and then inadvertently position the layout 180° from your intended position, this will completely reposition the bend allowance and your final product will be long on one flat section and short on the other. On parts containing multiple bends, you may find it more convenient to place the site line onto your layout after you decide which orientation is necessary. This will allow you to check for interference between the bent legs and the clamping fingers. In our maintenance classes the students have a contest to see how close they can bend their part to the drawing dimensions of a sheet-metal project. The students that perfect this technique can bend their parts to within .010” of the final dimensions. Working with sheet-metal has always been considered an art rather than a trade. And like any artistic endeavor, building a sheet metal airplane, a component, or even doing a repair can be both satisfying and rewarding. And like any artist, having the right tools and knowing how to use them is the secret to making the canvas come alive.