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Dimple Dies Make Stronger Panels

Posted in How To: Body Chassis on September 23, 2016
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Sheetmetal is used in a variety of ways on our rigs. A thin, flat sheet of steel can be flimsy and weak toward bending. Use of thicker sheet can add strength and rigidity, but also adds cost and something we try to avoid, extra weight. Production factories add rigidity to panels such as a pickup bed by corrugating it with ridges. In the aftermarket fabrication world one might use a bead roller to add raised ridges, but flaring holes with dimple dies is possible. Cutting holes sheds more weight and adding flared holes restores panel stiffness. Lighter means faster.

With a set of dimple dies and a hydraulic press, you can do your own flared holes in your home shop. We visited with fabrication maestro Rob Bonney of Rob Bonney Fabrication in Peoria, Arizona to garner some dimpling tips we could share with readers doing their own work.

To begin with, Bonney suggests making your holes slightly oversized to make the flare process smoother. A hole size about 0.030 inches larger than the nominal hole size of the die seems to work well. If cutting holes with a hole saw, you’ll usually get a slightly oversized hole anyway, so this usually works out well for this method. It’s also a good idea to deburr your holes with a die grinder or similar tool before you flare with the dimple dies. Small imperfections in the drilled hole may result in a crack or tear as the metal is stretched during the flaring process.

Bonney shared with us that there are two basic methods of metal bending: air bending and coining. Air bending is a process where a die pushes on a workpiece to form or bend it, but the die(s) never fully contact the piece. Coining occurs when the metal is stamped under extreme pressure between two mating dies that fully bottom against the formed metal, resulting in greater bend precision. When dimple dies are used they are coining the sheetmetal, so the harder the applied pressure the more accurate the flare will be with less metal spring-back.

Aluminum sheet is the easiest common metal to flare and typically the least likely to crack. Mild steel is a bit harder and requires greater effort to dimple. Chromoly sheet tends to be the most resistant to flaring and can be prone to tearing the metal when flared, especially very thin sheet. A bit more oversizing of the hole can help reduce the tendency for the sheet to tear.

If working at home, you’re likely not using a high-dollar press, but a simple 12-ton or 20-ton hydraulic press to do your dimple die work. Bonney explained that the final quality of flare and consistency is dependent on the die pressure used. A high-capacity press will coin the sheet more completely, resulting in less curl in the sheet material, but you can still get fine results in a home press. When placing your die in your press, always do your best to center the die under your press ram so the dimple die resists tilting which produces an uneven flare in the metal. Typically lube is not used when flaring holes in sheet.
When doing a series of flares, always work from one end of the piece to the other. While the piece may tend to curl across its length, it’s easier to deal with straightening a piece that was worked from one end to the other. Sometimes flipping the inside die to the other side of the sheet and pushing back on the flare with the press or a hammer on the die can help take some of the curl out of the sheet.
When planning to do flared holes, the order in which you work the piece can make a difference. In the case of this support strap and this dual-battery tray, the bends were put into the sheet before the hole flares were made. Since the dimple dies make the sheet want to curl, doing the bends first made the piece more rigid before the flaring process. Of course, this assumes you can work your dies into the piece properly.
There are times when you’d like to pack flared holes fairly close together. You’ll need to allow spacing for the adjacent flare to clear the dimple die. One trick to buy yourself a little tighter spacing interval is to grind a spot on the outer diameter of the die as shown here.

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