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Here you can see the steps used to give rectangular stock a uniform, consistently-curved p
Q. I want to make new body trim and have it chrome plated. I plan to use brass for the trim, but what is your suggestion for material? Do you have a video that shows how to bend or curve the straight stock once I have created the shape? I have a machinist friend who can make a custom bit, which is expensive, but once it's done we can put on a CNC machine to crank out perfectly-profiled straight pieces. Then my dilemma is getting the trim to curve the way I want. My friends suggest that I start with a wider piece of brass plate and cut out a curved piece, then profile the edge with the custom bit. Or could I just hammer and dolly the straight pieces, possibly using some heat, to bend the straight stock into the desired contours? Any help would be greatly appreciated.
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A. Brass is a very nice material to work with for making custom trim, since it bends and chrome plates beautifully. While it is certainly possible to cut curved pieces from a wide piece of flat stock, and then machine the profile, this is not an easy task to do, even with a CNC-machining center. The problem here is that you will almost certainly want bends of a constantly changing radius, and while any curve can be expressed mathematically, you'll find this is not a trivial task to accomplish with CNC programming. Making straight or constant radius curves is easy, but it takes pretty high-end software to program 'spline' (changing radius) curves.
Therefore, machining straight pieces with the proper profile, and then hand-bending to your desired contours may be the most feasible approach. For this task, you'll be taking a piece of brass stock and bending it the 'hard' way (on edge). This is certainly possible, but it does take some careful setup. One of the challenges with bending any material the hard way is that it will have a tendency to twist, since the material is always trying to rotate so it can bend the easy way. I usually counteract the tendency of the material to twist by clamping one end of the stock to a large, flat surface, and as I'm bending, I make sure that the back side of the material always maintains contact with the backing surface.
If your bends are gradual, you may be able to do them freehand. If you accidentally over-bend an area, you can generally un-bend it without a problem. For tighter-radius bends, you will probably need to bend the stock over a fixture of some sort. It's likely you can make a fixture out of plywood for this. The material being bent will spring back a lot, so your fixture will need to have a tighter radius curve than the finished part. There is a bit of guesswork here, but you can 'sneak up' on the shape you want by re-cutting your fixture to a little bit tighter radius with each attempt. Again, if you over-bend something, it's usually not too hard to fix it, as long as you don't kink the brass stock.
This technique should work fairly well for brass stock up to about 1-inch tall, if it needs only gradual bends. For tighter bends, or for stock that is taller, it will probably be beneficial to anneal the brass before bending. This can be easily done by heating it to a dull red color. You must use care here, since if you get it too hot, it may melt, and if you heat it unevenly, you may wind up with some areas that are softer than others. This will make it very difficult to get smooth bends, since the softer areas will bend more than those that weren't heated as much. A good trick to help heat the brass evenly is to use a rosebud heating tip on your torch, and to work in a darkened room, so you can more easily see when the brass material just starts to turn a dull red color. Be very careful not to bend the brass bar when it's red hot, since it will be very likely to fracture!
If your trim will be more than 1 1/2-inches tall, then it might be better to cut the curves out of a large sheet of brass, and then contour the edges. While using a milling machine or a CNC-machining center is an elegant way to profile rectangular stock, it certainly can be done with careful hand sanding and filing! The photo at the head of this column shows how I've done this, with a good degree of control.
The only DVD I have that shows bending stock over a form with a non-constant radius is "Working with Tubing." In this DVD, I make a plywood form, cover the edge with a strip of sheetmetal, then heat steel tubing to red hot, and bend it tightly against the form. Unlike brass, steel can be easily bent while red hot.