Fly Cutting a Cove With a Boring Head

Home shop machinist.

Shop-made piston pin press tool. Note the cove in the Delrin cradle.

Fly cutting is a great way to put a “finish” on a flat and true face. The sweeping tool marks are not only instantly recognizable, they are, to my eye, infinitely more attractive than the swirly patterns made by an end mill. I covered the basics of fly cutting on plane surfaces in the February 2015 issue (“Fun with Fly Cutters”). But as this month’s project will demonstrate, fly cutting can also be used for coving. Because I was after a precision radius, I used a boring head, which allows you to precisely dial in the radius of the cutter path. For more on boring heads, see the October 2015 issue (“Boring on the Vertical”).

The concept behind a pulling tool is simple: Tighten the nut to extract the pin toward the socket.

The task at hand was to make a tool to gently press a piston pin onto a connecting rod. I made this tool for my friends who are not yet 100% sure they’ll need it, but since removing the piston pin required a makeshift drawbar to pull it out, there’s logic in the idea that some gentle persuading may be necessary to reinstall it.

A ratcheting box wrench was used to speed up pin removal. Note the proximity of the adjacent cylinder.

The engine in question was down because of a cracked exhaust port on one of the cylinders near the firewall. Not a common issue, but it happens. The typical fix is a replacement cylinder assembly, which includes the cylinder with valves, the piston and rings. You reuse the rockers and piston pin at your discretion. Lately, anybody who’s needed a replacement cylinder assembly for any of the popular legacy engines knows that the wait times have been excruciating. There was nothing to do but bide our time. So when the word came down that delivery was imminent, the teardown could finally begin!

Exploded view of the press tool, left to right: the tension nut, aluminum bushing, 7/16-14 all-thread, piston pin, piston, Delrin cradle and fixed nut.

It was a Saturday afternoon when I got the text: “Do you have, or could you make, a piston pin removal tool?”

“No and maybe” was the answer. I grabbed my calipers and some graph paper and headed to the hangar to see what was what.

A boring bar positioned in the horizontal tool holder (left). The radius was rough-set using a caliper. With the mill turned off, the workpiece was centered by eyeballing the sweep of the tool path over the target radius (red line). The machining stock was a 1x4x5-inch piece of Delrin (right). Note the left side had a preexisting beveled edge. The opposite side was beveled later to give the look of the part the proper symmetry.

There are two types of piston pin removal tools: push and pull. For aviation, the predominant type seems to be the ACS push tool sold by Aircraft Spruce and Aircraft Tool Supply. No one seems to offer a pull-type tool for aircraft engines. The ACS tool is nice, but it is for rebuilding or refurbishing complete engines. In our situation, where we were removing and replacing one cylinder assembly, there’s simply no way to make it work if an adjacent cylinder is in the way.

Fly cutting is an “interrupted cut” process, so start out by taking a shallow cut with slow feed. Delrin machines super easy, so as long as the machine doesn’t start shaking, the spindle rpm can be relatively fast. On my benchtop mill the combination started to shake above 600 rpm, so 600 rpm was the setting used (left). The cove was made 2¼ inches wide and deep enough to allow a finished part height of 1-3/4 inches (right).
The cradle was set up (left) and drilled dead-center for a 7/16-14 threaded hole (right).

A few chin rubs and head scratches later, we concluded that we could put together a shop-made puller with some all-thread and tubing. While we had to be careful not to damage the pin or connecting rod, the piston was going to be replaced, so there was no need to worry about it getting dinged during removal. As luck would have it, I had a foot-long section of 7/16-14 all-thread with a couple of nuts and washers, but no suitable tube to provide clearance to make a drawbar sleeve. Then the light went off: a socket! Sockets come in handy all the time when you need a right-size sleeve or bushing on the arbor (or hydraulic) press. All we needed was a 1/2-inch drive (deep) socket big enough for the pin to be extracted. The OD on the pin was 1.125 inch, so any socket 1-1/4 inch or larger would work!

A Siding T-bevel was used to gauge the angle of the beveled side of the stock (left) and then to scribe a line on the side to be beveled (right).

Once the pin was removed our attention turned to putting it back. The fit is not a tight press fit, but definitely tighter than you could possibly remove by hand. Reinstalling the pin carried the burden of doing absolutely no damage to the new piston.

Two parallels (left) were stacked to eyeball the scribe line level for milling (right). A metal part sticking out this far from the vise jaws would be particularly sketchy to mill, but Delrin is particularly easy to machine. Nonetheless, the table was fed front-to-back, so the primary cutting forces were more or less perpendicular to the vise jaws.

Which bring us to the main feature of the press: a Delrin pad machined with a cove to ever-so-gently cradle the piston while the pin is pressed back in place.

The aluminum bushing provides a “soft” surface to impart pressure on the piston pin. After turning the shoulder to fit the ID of the pin (0.490 inches), it was drilled to fit the 7/16 all-thread (left). After parting off and facing square, the last touch was to deburr all the edges (right) and check the assembly.

The deadline to turn in this column was before we had a chance to see how it works. The hope is, everything should slide together without much, if any, force. But, just in case, we’ll have our shop-made press ready to go. I’ll post an update on the KITPLANES® website to let everyone know how it turned out.

That’s it for now. Time to get back in the shop and make some chips.


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