Making a Shaping Block
Content provided by: Chris Fleming
For those of us car and motorcycle builders who have been building and restoring for years, there isn’t much in the world of wrenching that is out of reach. One thing that many seasoned gearheads can find daunting, however, is sheet metal shaping. At first glance, it’s a craft that requires not only skill and patience, but a lot of expensive specialty tools as well. We’ll let you into a deep, dark metal shaping secret: with only a few basic tools, you can make up for a lack of fancy equipment with large amounts of patience and time – plus a little guidance from us.
Usually we like to start our students off with basics such as dent removal and panel repair, but for our purposes, we’re going to jump right into some fun stuff to pique your interest. Incidentally, this lesson in metal shaping begins with some instruction in woodworking. This may seem counterintuitive, but the shaping blocks you need can be built on a budget out of scrap wood that may already be lying around your shop. Now, let’s start with the tools and equipment you’ll need to build the blocks and start down the road to shaping.
In figure 1, you’ll see everything needed to start building your bench shaping blocks (already finished in the photo) as well as what you need to do the metal shaping. We used scrap lumber (a standard 2×6 cut into squares), but if you have an old tree trunk or log sitting around, that’s even better. In addition to that, the photos show (clockwise from the blocks) a circular saw, a welding gas tank cap, a small pile of used drywall screws, an impact driver (standard drill with a driver bit works as well), a dual-action pneumatic sander, a 4.5” angle grinder with a standard metal cut-off wheel, a wooden mallet, two different body hammers, and some measuring and marking implements. You’ll also want a clamp to hold the block down while working. Figure 2 shows just a couple options for cutting your steel: standard green or red handled shears, or an inexpensive electric “nibbler” type shear purchased from our local discount tool store. The previously mentioned grinder with metal cut-off wheel can work double-duty for cutting your sheetmetal as well; just be sure to not over heat and warp the steel.
As you follow along step by step with us, keep in mind that our metal guru has years of professional training. Try not to get bogged down or frustrated if your piece doesn’t turn out how you want it the first, second, or even third time. Our expert wasted a lot of material in his first years. Remember, it’s only metal, you can cut, weld, and shape some more. At the very least, you’ll have a handy mechanism for venting frustration, and 20ga steel is a lot cheaper than therapy. Now, go through the photos and captions a few times to get your footing, then head out to the shop and get shaping.
Some of the basic tools needed as well as the finished shaping blocks.
Red or Green handled hand shears and the optional electric shear.
Two squares of 2×6 screwed together from the bottom and clamped to the table. Make sure to use shorter screws toward the center to keep from interfering with the bowl shape being carved into the top. A circle is drawn on the top as a guide for the bowl. Stay at least a ½” from the edge to minimize breaking during use of the block.
Using the angle grinder with standard metal cut-off wheel (Note: Using power tools for anything other than their intended purpose or without their guards attached is not recommended), cut slices about 1/8” apart down the entire bowl area, making sure to stay within the circle. Continue to cut more grooves perpendicular to the original. Note: Again, the goal is to show that everything can be done with whatever tools you might have laying around. Most of you probably have an angle grinder. Because this is not a wood-cutting disc, this causes MASSIVE amounts of smoke and should be completed outside or in an EXTREMELY well ventilated space. Better yet, use a cordless mini circular saw if you have one.
Knock out all the scrap from the bowl and, using your favorite grinder with a fresh 36 or 40 grit disc, sand the bowl smooth.
Multiple 8 inch pieces of 2×6 were used to build the large block. Scribe a line on each block as deep as you want your bowl. This will remind you to not place screws in that area. Now screw each block to the next, one at a time, until you have all of them screwed together. We used a 6” sanding disc to trace around for our bowl diameter. Now use the same method as the smaller bowl to cut out and sand the bowl to your desired depth.
The finished shaping blocks. The transition from bowl to flat is highlighted with black ink to help guide us a bit.
A few options for shaping mallets. On the left are two homemade wooden mallets; one from an old broken baseball bat, and another from a lathe-turned branch joined to a surplus hammer handle. On the right are two manufactured nylon shaping mallets. Our tech prefers the red mallet due to its feel and weight. But choose whatever fits your budget and personal tastes.
Cut your steel into a 4 inch wide strip about 20 inches long. To start shaping, lay the edge of the metal in-line with your marker line on the block (not pictured). Strike your mallet about an inch just inside the bowl with enough force to dent in the steel and ripple the edge. Do this down the entire length of the piece, and then repeat for the other side. The ripples, also known as “puckers” or “tucks,” should form down both edges of the entire piece (Figure 10). Continuing to work in the bowl, hammer (flatten) each tuck on the entire length of both sides. This gathers the steel and “shrinks” it.
After only one pass of hammering the tucks, there’s already a considerable amount of shape on the edges.
Continue to hammer with the mallet down the center of the piece: stretching the center rather than shrinking, like the edges did.
The combination of shrinking the edges and stretching the center produces a nicely rounded piece.
It may be a little hard to tell, but some of you may have guessed it already, our practice piece utilizes all the necessary skills and techniques to build a motorcycle fender. Here it is after one round of shrinking and two rounds stretching.
At this point our tech prefers to switch to a metal hammer with a high-crown face to continue shaping the metal; still using the shaping blocks. Another option is to clamp something into your bench vise like a welding tank cap or one of your mallets (figure 16). If you go the route of the tank cap, remember to use a wooden or nylon mallet instead of the metal hammer.
Still more shaping from only two rounds of hammering into the bowl with a high-crowned metal hammer.
After one round of planishing (lots of fast and light hammering to help smooth the steel rather than stretch it) over the tank cap (clamped in the vice and using a flat faced body hammer) a quick pass with 80 grit paper on a dual-action (orbital) sander reveals our low spots front (figure 19) and back (figure 20). No planishing was done on the back half.
A step out into the daylight shows off the difference in smoothness between the front and back.
A final round of planishing and sanding on the front, as well as a quick scrub with a scuff pad, and it’s almost done.
Another step outside really shows off the difference.
Again, using what you have sitting around (our tech used his electrical tape), scribe radius lines for the corners of your fender. Then cut or grind off the excess and touch it up with the scuff pad.
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