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08-19-2020, 08:29 PM | #1 |
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Join Date: Feb 2008
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Open track build questions
I’ve got a couple gun blanks glued up, and I would like to have graphite open tracks. My friends and I are making guns at the same time (we’re all unemployed here on Maui so what better time!) The guns are varied in size and shaft diameter. So my question is what is the best way to go about making an open graphite track? Seems like my euro pipe guns have wide shallow tracks that allow for a variety of shaft diameters. Would I need a specific router but that matches each shaft size (7mm, 7.5mm, 8mm)? Could I just use a 1/4” round bit and very the depth of cut? I seem to remember years ago spearq8 describing a method for pour the track and using the shaft to mold it, but I couldn’t find it in my searches ( this method interests me a lot). I’m having a hard time deciding on a method, and would prefer to avoid buying lots of bits if possible.
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08-20-2020, 12:35 AM | #2 |
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Re: Open track build questions
The idea of the "mold" is to reduce track friction the absolute minimum and thus maximize shaft velocity without having to add band power. Basically you want to have 3 points of contact with track to shaft .... one at mech ... on in middle and one at the very end where line over shaft goes. I guess if you have precise instruments you can do that with a controlled router cut, but the way I do it is I just use a slightly over sized shaft say 8mm if I want to use 7 to 7.5mm shafts and then I use one layer say red and the other layers another color like yellow. The idea is to then remove one inch length in 3 locations mentioned. Then you tape out the sides of your routed track and put a layer of mixed epoxy and then just lay the shaft over. Once cured you should have a perfect track with only 3 inches of contact. This has zero impact on accuracy but a tremendous improvement in shaft velocity. I have shared this with quite a few gun builders and now they all use it as standard. You might actually buy a gun that has this but you can't really see it as the shaft to track gap will only be about .5mm to 1mm. I use this system on every gun I modify and although I used to think it did not make much of a difference and not worth mentioning, everyone else who has done it says it make a huge difference.
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08-20-2020, 01:06 AM | #3 | |
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Re: Open track build questions
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08-20-2020, 02:03 PM | #4 | |
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Re: Open track build questions
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I’m thinking I route out a dovetail groove to pour my graphite epoxy into. Then make a tape dam at the muzzle end and mechanism end of the track. Make sure the blank is level. Then fill the track with my epoxy mix. Insert the shaft into the mechanism and lay it on the fresh poured epoxy and check that the shaft is level. Are the tape dams enough to hold the shaft level and straight? Have you ever got a shaft stuck in the epoxy? Or had epoxy overflow to the mechanism? I like the idea of using the shaft to mold a track, but I’m worried there are more risks than using a router table to cut groove. |
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08-20-2020, 11:53 PM | #5 | |
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Re: Open track build questions
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08-21-2020, 01:04 PM | #6 |
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Re: Open track build questions
In the Ol Days we used to call these Speed bumps. We would make a Shallow Track Lower than the Mech Height and then put in a few short lengths of thin Stainless rod which looked like segments of a Railroad Track. You need to bend the ends of the sections so you can bury those in drill holes. Put in two or three shots along the track. The resulting surface was so slippery a shaft would slide out of the track at almost any angle. The other characteristic of the Speed Bumps was to impart a "Zing" sound when the gun was fired..
One interesting partial track idea was Terry Maas and Malibu John's PArtial Capture Track Guns. These have sections of Enclosed Track holding the shaft and open space between.. they looked funny, but they apparently worked well. In any case. You can make an Open Track using a variety of tools including just a Table Saw.. A Square track is arguably a better track than a Round Track anyways.. Use what you have. If it's a 1/4" Bit, make a center pass, and a few passes on either side and then take a Flat Head Wood Screw and sink it into a dowel and then sand/file it until it's the right diameter and sharp edged and scrape the track flush to the center cut.
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08-23-2020, 01:34 PM | #7 |
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Re: Open track build questions
I’ll let you guys know what method I go with once I have trigger mechs in hand for alignment. That screw head idea is brilliant! I love DIY tools like that.
The idea of reducing surface area friction to increase shaft speed is a very interesting one to me. If a segmented track reduces friction, would a 1/8” groove down the center of the track do the same thing? Or the square track? I plan on shooting a 7.5mm shaft out of this build, and I’ve got an 8mm shaft I can mold from so I will try to get some scrap wood together and make some test tracks of different methods |
09-01-2020, 06:49 AM | #8 |
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Re: Open track build questions
My understanding is that if the band pull is perfectly aligned with the shaft sharkfins, the pull would be (theorically) perfectly straight with no vertical component.
However this is not feasible since even with rollers the band diameter increases while the band contracts, therefore throwing out the window any intent of having the pull direction perfectly aligned with the shaft. That said, there is always going to be a vertical component of the pull when using circular bands (through holes in the muzzle area). The shaft track, is meant to support the shaft and negate this vertical pull from affecting the shaft tail area and ruining the flight of the shaft. Besides, when sliding forward, the thin layer of water between shaft and track actually prevents any real friction between the two materials. I have checked well used guns (wooden as well as aluminium) with full shallow tracks and the finishing coats are not worn out, at all, in the track area. The shaft just floats over the track. I don't think there is any perceptible benefit from using partial tracks, speed bumbs (besides the zing sound, which seems cool as heck!) or low friction tracks (graphite or teflon or whatever). Mikel |
09-01-2020, 10:55 PM | #9 |
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Re: Open track build questions
Like Jon said, it does not have to be made with fancy tools. First two I made, the track was cut with my table saw. I used a trim router to round over the edges. Some of the nicest guns I've seen were made with very crude tools, while some CNC guns were nothing spectacular.
If you have access to a router, you can pick up a 5/16" or 3/8" box core bit (see photo) and route the track that way. 3/8" may be kind of big even with an epoxy coating. IIRC I used the 5/16" centered on the blank, and ran it in both directions which made the track just a hair over 5/16" A simple method would also be to make a router plane and use a section of old shaft cut at an angle and sharpened. Basically you just need two pieces of wood about 8-12 inches long 3 inches wide 1 inch thick. Glue them along the long edges to make an "L" drill a tight fit hole at an angle for the shaft and run it along the blank, as you go, tap the rod to increase the depth of the cut. See ideas here: https://www.bing.com/images/search?q...mageBasicHover I've always drilled and chiseled the mech pocket, it's easier to slowly take away wood. It's hard to center and plunge a router bit and work it back and forth without taking away too much wood. Use 2X4 practice blanks, the same dimensions as your finished gun to set up routers, drill press holes, cuts etc. Once you got the machine set up, do the same to the good blank. |
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