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A couple of non-typical tasks and solutions

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  • A couple of non-typical tasks and solutions

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    Project 1- Compressor repair

    With a lot of time at home these days, I decided to fix my old Campbell Hausfeld Power Pal air compressor. Disassembly showed a large section of one of the polymer piston rings was missing and a lot of searching convinced me that parts for my old compressor (model 33MT0004) are no longer available.
    The rings are 2.253 in. OD, and 2.117 in. ID and only .162 in. wide. I suspect the original to possibly be Delrin, but I didn’t have any in a size that would work. I did, however, have a piece of nylon 5/8 inch thick that I could use.
    This part would be a problem. After some thought, I decided the only way I could hold the work and machine both ID and OD on such a thin piece of stock would be a faceplate.
    I counterbored the four corners of my blank for flathead screws and used a hole saw to rough out the center. This material did not saw well at all, and the hole saw was a real bear to use. The blade overheated and melted the nylon to the saw multiple times before I finally chewed out the center. You can see how rough the hole is in the photo.
    Fortunately with sharp high speed steel with a lot of relief, the nylon turned very easily and left a smooth surface. I bored out the center to the finished ID with some ease. At that time, I realized that as I approached the .068 inch thickness of the part turning the OD in this flexible material, I would have trouble regardless of how sharp my tool was. The only solution would be to provide support to the work.
    I turned a plug to a snug fit out of hardwood to hold the dimension of the ID and turned the OD to size.
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    To cut the part to length, I faced the part, reinserted the plug, and used a parting tool. This didn’t go so well. The soft material deformed as I was cutting it, progressively forcing the part away from the faceplate as it cut deeper, and the result was a ragged cut and about .020 in. of flash on my part.
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    It took some head scratching to think of how to trim such a fragile part to size, but then I remembered how an antique leatherworker’s slitting gage works where the leather is cut between the blade and a fence as it is drawn through. I freshly faced the stock still on the faceplate, reinserted the plug, and slid the part on the plug with the clean smooth side against the stock.
    I couldn’t turn the part under power because the work would just spin on the plug. The plug turned with the assist of tailstock pressure. I had good control of the thickness between the stock and a cutter. Using light passes; I squeezed the part with my fingers against the plug while I rotated the faceplate with my other hand. I was able in this way to cleanly trim the width to size.
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    The plug did further duty off the lathe acting as a mandrel in the vice while I used a jeweler saw and a fine blade to cut the angled slit in the new ring.
    The compressor is working again.

    Attached Files

  • #2
    Project 2 – Antique Gun Repair Click image for larger version

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    The lifter broke in my old Iver Johnson Top-Break revolver. You can see it in my shamelessly included drawing from Firearms Assembly 4. For my taste, this is a darn small, technically difficult part/assembly to reproduce, but certainly not available anywhere I know of. It was hardened so I elected to make the replacement part out of a block of O1 steel I had been saving. The steel proved to be workable, but very hard to cut. It cost me a band saw blade just slabbing off a blank to make a new lifter from. Using carbide, though, machining the blank to the thickness necessary on the mill was a snap.
    The broken lifter parts were glued together to enable me to get the shape right (photo above). Aside from getting the eventual shape right, there is a pivot area that is counterbored and radiused end and a tiny pin that would need to be exactly located.
    I started by drilling and carefully counter-boring the pivot hole in the plate. The counter-bore happened to be the same radius that defined the outside of the part and its depth determined the critical thickness at the pivot. I then took the plate out of the mill, and I positioned the broken/glued lifter using the drill bit through the hole in each for location. The part would not have stood up to the rigor of hand marking the external shape with blue and a scriber. Just light handling broke the glue a couple times. Instead, I masked off the protruding drill bit locating the part to the plate and spray painted it all.
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ID:	1875890 When the paint was dry, but the part lightly stuck to the plate, I put the plate back in the mill and lined up off the existing small pin to drill the hole in the plate. I removed the part, drilled the hole, and then with tiny bites and multiple clampings, I removed as much of the waste as practicable. The paint showed where the cuts could be made.


    • #3
      . Continued:
      The drill bit through the pivot hole and on top of the blocks for reference allowed machine cutting of most of the radius. The picture shows the progress.
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ID:	1875897 From here, everything had to be done by filing, measuring, filing and more measuring. Eventually it was pronounced satisfactory and hardened with a propane torch and oil. Such a small part didn’t make much smoke in the oil, but enough to cause SWAMBO to comment. The tempered part with the new pin is shown. Click image for larger version

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      • #4
        Nice work on your project #2. Gun parts can be very trying but rewarding. I still consider the old timers that perfected the various designs back 100 years ago, most of it was hand work, amazing.



        • #5
          Interesting posts and good work. Will be interesting to see how the nylon works as a piston ring. One of the worries I have with nylon is it's ability to absorb water and swell, may have to watch for this if getting moisture passing through the compressor.
          West Sussex UK


          • #6
            Great work on the compressor ring. Why did they chose plastic piston rings? Do they have to stretch over the piston to sit in the groove?


            • #7
              [QUOTE=challenger;n1876005]Great work on the compressor ring. Why did they chose plastic piston rings? Do they have to stretch over the piston to sit in the groove? [/QUOITE]

              Aluminum piston and cylinder and no oil wouldn't have likely done well with iron rings. I actually wondered about nylon being hygroscopic when I saw they were used in other make compressors, but since the compressor runs pretty warm, I figured moisture would not be a problem. Time will tell.
              As far as stretching over, no the ring is cut at a low, maybe 25 degree angle at one place. It rides in a groove in the piston that is deeper than the ring thickness and has an O-ring underneath that acts as a spring to hold the piston ring against the wall of the cylinder.


              • #8
                I see. I wasn't aware that it is an oiless compressor.