The problem was that our shop wasn’t set up to weld thin aluminum sheet. TIG welding was initially considered to attach the outlet, but a few test welds on some scraps of 0.040-inch-thick aluminum by my ace welder Billy Griggs resulted in burn through. The outlet connector is attached to the cuff with rivets. It would have been impossible to clamp and cut the hole without the support of the sacrificial dowel. The fish mouth opening for the outlet coupling was made using a 2-inch hole saw. The joggle provided the necessary real estate to rivet the joint. About 1/2 inch was added to make an overlapping connection called a joggle. The same simple math was used to size the sheet needed for the outlet connector to fit a 2-inch SCAT hose. Adding the material thickness (0.040 inch x 2) and some basic math (C=πd) gave me the circumference and the length of the sheet needed for a perfect cylinder. To make the carb heat cuff, I measured the diameter and the width between the muffler flanges. Yes, it’s possible to get close by stretching sheet around a form, but there will always be some springback, whereas a cylinder (or cone) formed with a slip roll will retain its shape. Power sanding on the lathe with 120-grit sandpaper smoothed out the tool marks.įor forming sheet metal into cones and cylinders, the slip roll is pretty much the only option. A roughing gouge was used from square block (left) to finished diameter (right). The plywood glue-block was turned into a 3.8-inch diameter dowel on the wood lathe. The same goes for making bends: There are other ways, but few work better than a press-type brake. There are a lot of ways to cut sheet metal, but few tools do a nicer job than a shear when it comes to making straight, distortion-free cuts. The model used for this article was an inexpensive 12-inch capacity benchtop machine that I would classify as hobby grade. If there was ever an ideal job for an all-in-one benchtop shear, press brake and slip roll machine, this has to be it! The sacrificial dowel was made by gluing up six layers of ¾-inch plywood.Īs the name implies, this tool combines the functions of three old-school sheet metal tools in one. It took about 10 passes through the slip roller to make the cuff, which was just under 4 inches in diameter (right).īoth parts, the cuff and outlet, are cylindrical and constructed of thin (0.040-inch) 6061-0 aluminum sheet. A feature of the machine allows swinging the top feed roll out of its socket to remove tight fitting parts (left). The part shown being formed is the 2-inch diameter outlet tube. Tighten the adjustment on the slip roll and repeat until you get the final curl you want. The slip roll (the roll in the back) is initially adjusted with thumb screws (not shown) to produce a slight curve. The feed rollers (the two front rollers) are adjusted to the thickness of the material. Twelve inches refers to the maximum width material it can process. This 12-inch benchtop machine combines the functions of a sheet metal shear (left), a bending brake and slip roll (right) into a relatively compact space. Openings at one end allow air to be drawn in, and an outlet tube at the other end provides a way (usually via SCAT-type hose) for hot air to be ducted to the carb. The carb heat system on the A65 is as simple as it gets: A sheet metal cuff wraps around the flanges to form a heat chamber. While there’s not much point to cabin heat on an open-cockpit tandem like the Pietenpol, a hot air/carb heat system is absolutely required on almost any engine with a carburetor.
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In addition to being lighter and more durable than our DIY muffler, it included flanges for carb heat and cabin heat cuffs. If you remember, we ended up finding a new factory muffler.
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That column showed how to make a J3 Cub-style muffler for a Continental A65 engine. This month’s project is a follow-on to the Pietenpol muffler that was the subject of the Home Shop Machinist column in the May 2019 issue. The hose to the carb heat valve is 2-inch ID SCAT tubing. Test fitting the home shop carb heat cuff on the Continental A65 muffler.