Ye Olde Slip Roll

Home shop machinist.

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Test fitting the home shop carb heat cuff on the Continental A65 muffler. The hose to the carb heat valve is 2-inch ID SCAT tubing.

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. That column showed how to make a J3 Cub-style muffler for a Continental A65 engine. If you remember, we ended up finding a new factory muffler. In addition to being lighter and more durable than our DIY muffler, it included flanges for carb heat and cabin heat cuffs.

Download this drawing in PDF format.

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.

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. 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.

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. Twelve inches refers to the maximum width material it can process.
The feed rollers (the two front rollers) are adjusted to the thickness of the material. The slip roll (the roll in the back) is initially adjusted with thumb screws (not shown) to produce a slight curve. Tighten the adjustment on the slip roll and repeat until you get the final curl you want. The part shown being formed is the 2-inch diameter outlet tube.
A feature of the machine allows swinging the top feed roll out of its socket to remove tight fitting parts (left). It took about 10 passes through the slip roller to make the cuff, which was just under 4 inches in diameter (right).

Both parts, the cuff and outlet, are cylindrical and constructed of thin (0.040-inch) 6061-0 aluminum sheet. 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.

As the name implies, this tool combines the functions of three old-school sheet metal tools in one. The model used for this article was an inexpensive 12-inch capacity benchtop machine that I would classify as hobby grade. It was purchased on Amazon for $380. A 30-inch version sells for around $840.

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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 same goes for making bends: There are other ways, but few work better than a press-type brake.

The plywood glue-block was turned into a 3.8-inch diameter dowel on the wood lathe. A roughing gouge was used from square block (left) to finished diameter (right). Power sanding on the lathe with 120-grit sandpaper smoothed out the tool marks.

For forming sheet metal into cones and cylinders, the slip roll is pretty much the only option. 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.

To make the carb heat cuff, I measured the diameter and the width between the muffler flanges. 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. The same simple math was used to size the sheet needed for the outlet connector to fit a 2-inch SCAT hose. About 1/2 inch was added to make an overlapping connection called a joggle. The joggle provided the necessary real estate to rivet the joint.

The fish mouth opening for the outlet coupling was made using a 2-inch hole saw. It would have been impossible to clamp and cut the hole without the support of the sacrificial dowel.

The outlet connector is attached to the cuff with rivets. 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 problem was that our shop wasn’t set up to weld thin aluminum sheet. Billy suggested it might be possible with some practice, but he would probably need several sets of parts. Even then, Billy said there was only a slim chance that one might come out acceptable, so rivets were the fallback choice.

The dowel also provided the support necessary to drill the air intake holes (left). A steel tube with 2-inch ID was used to accurately gauge the location of the joggle joint in the outlet tube. Two flat-head pop rivets fixed the diameter (center). A scribe line of the fishmouth opening was made on the outlet tube using a fine-line paint marker (right).
Flange mount line.

The 2-inch diameter hole in the cuff for the outlet connector was another challenge. Using a nibbler was considered, but it was decided the cleanest way to make the opening would be to fishmouth the opening with a hole saw after slip rolling the cuff to a cylinder.

Mounting flange with Delrin.
With the outlet tube clamped to the Delrin plastic arbor and mounted in the indexer, a series of stop holes were drilled (left) along the scribe line at 36° intervals. A 0.040-inch slitting saw was used to cut fingers from each stop hole (right).
Flange with Delrin tool.

To prevent the cuff from collapsing and snagging on the hole saw, I made a large dowel to provide sacrificial support for drilling the cuff in the mill. I used a similar idea to hold the outlet tube in the indexer for drilling and slitting. But instead of wood and a wood lathe, I used my metal lathe and some 2-1/2-inch diameter Delrin rod.

It will probably be a few months before we can test the effectiveness of the cuff. As any pilot knows, for carb heat to be effective it must absorb enough heat from the exhaust system to raise the air temperature high enough to prevent ice from forming in the carburetor or melt any ice that does form.

Sawing slits in the flange.

Confirmation will come during the initial run-up of the engine: With carb heat applied, we’ll watch the tach and listen for a rev drop (hot air is less dense than cold air, which results in less power for a given throttle setting). Hopefully the test will prove there are enough inlet holes for the engine to breathe and enough volume to provide sufficient heat. Only time will tell. In the meantime, get into the shop and make some chips!

Using the scribe mark as a reference to line up the outlet coupling, the fingers were carefully bent to conform to the ID of the cuff. The finger position was then transferred to the outside of the cuff to locate the position of the holes for the rivets.

The title, “Ye Olde Slip Roll,” pays homage to the story of WW-II Captain Charlie Brown and his crew of the B-17 Ye Olde Pub as detailed in the book “A Higher Call” by Adam Makos.

The wood dowel was used to hold the outlet coupling to the cuff while drilling the rivet holes. A quick wipe with some solvent cleaned up the scribe lines.

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