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Q. I’ve done a few mild restorations in my time, but recently I’ve gone overboard (for me, anyway) on a ’55 Chevy shortbed, second series. As you can see from the picture, I’m trying to recess some ’92-96 Corvette taillights into the rolled pan. My question is, what is the best way to get a consistent radius on the outside edge of the pocket on all four pockets? I’ve been using a 2-inch diameter 40-grit disc, and I’ve made a template for the radius corner by drilling a 3/32 inch hole in a piece of sheetmetal, then making cuts tangent to the edge, but there has to be a better way.
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A. Yes, making a consistent radius on a welded edge can be quite challenging! Here are some approaches you can consider. First, after the recess is welded to the rolled pan, sand the weld until all the bumps are gone, on both the face and the recessed surface going in toward your taillight mounting flange. You can probably tolerate some tiny low spots in the weld at this stage, as long as you think your radius edge will smooth them in the next steps. If the welded edge needs any touch-up, however, this is the time to do it!
Next, use Dykem (machinist’s ink) to coat the edges you want to radius. This will make the next steps much easier!
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The first step is to create a 45-degree chamfer on the edge. It’s much easier to get an even chamfer than a radius edge, particularly if you have used Dykem on the metal, since the high contrast between the dark ink and the shiny metal of the chamfer makes it easy to see any inconsistencies in the width of your chamfer.
There are several ways to make this chamfer. Probably the most accurate way is to use a sharp, half-round file. Hold the file at an angle to the edge, and take long, sweeping strokes. Each stroke will make the chamfer a little wider, and it’s easy to judge when you have a uniform width around the entire taillight recess.
While the file is a great way to go, it’s a pretty slow process, so (being somewhat impatient) I usually do this portion of the job a different way. A drum sander can do almost as good a job (if you take great care), and it’s MUCH faster. One of the keys to this is to use the largest diameter sanding drum that you can. If you use a small diameter drum, you are likely to get a scalloped surface. If you do get any scalloping from a sanding drum, the old trusty file is the best way to fix it!
Once you have a perfect chamfer around each taillight opening, it’s pretty easy to turn it into a uniformly-radiused edge. I’d put some more Dykem around the edge, and take off the corners of your chamfer. If the chamfer is at 45-degrees, these next cuts would be at 22-degrees in both directions. These chamfers will reduce the width of the first chamfer, and your goal is to end up with three chamfers the same width.
Once you’ve achieved this stage, it’s easy to get a fully-rounded edge, by doing some light filing, or hand sanding. Some people use a flap wheel at this stage, and it’s great tool, but it takes very good hand control to keep the radius uniform. If you do use a flap wheel, the larger diameters work best, and make sure you use the type that cuts on the edge (similar to the drum sanders) rather than the ones that cut on their face, like a sanding disk. Be sure to hold the flap wheel so it’s cutting ALONG the edge, not ACROSS it! The flap wheel will form a groove where it touches the edge you’re rounding, but this is actually an advantage, since the rounded groove in the flap wheel will help to make your rounded edges more consistent.
Again, if anything goes wrong, and you start losing the consistency of the radius on the edge, careful work with a sharp, half-round file is the best way to correct any hiccups and get back on track. CT