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Piloted Chamber Reamer Tolerances?

Discussion in 'GUNSMITHING & FIREARMS' started by TTD, Apr 15, 2017.

  1. TTD

    TTD Canada Active Member Active Member

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    Hi guys,
    I realize this is mostly a powder-burning, firearm section of the forum & my question pertains to an airgun build I’m currently doing, but the parameters are basically the same (other than AG barrel is softer steel than a PB barrel)

    Anyways, after a lot of research on the topic I’m now in the process of making my very first chamber reamer (piloted) for a .177 pellet-shooter project I’m currently working on & was wondering if any of you fellas “in-the-know” could clear up a few details for me that I either missed in my search or info wasn’t provided?

    1) Do you turn your reamers to the exact final, finished size needed, or do you make it .0005” (+/-) smaller to allow for bore lapping/polishing after cutting with reamer? BTW, barrel will be mounted in 4-jaw chuck, appropriate pin gauge in bore & indicated with DTI to be perfectly centered so chuck runout *shouldn’t* be a factor…

    2) Should (solid) pilot be sized to just kiss top of lands (i.e-exact fit with bore minor diameter) or would I also make it slightly smaller (.0005”- .001“) to prevent galling of lands? I ‘may’ be able to make it a live pilot w/brass bushing replacing solid pilot if need be, but .177 reamer is already pretty tiny without drilling/tapping end for pilot sleeve.

    3) Now this might be a silly question, but I’m new at this & need/want to learn sometime anyways, soooo - how many flutes/cutting edges? Have read that you generally don’t want all your cutting edges to coincide with rifling lands to prevent “train-tracking” (“bump, bump, bump” while reaming). My barrel has 12 lands/grooves, so should I make the reamer with 5 flutes (1 flute always in contact with land), or could I get away with 3 or 6 flutes? I don’t have a spin index so will be using a hex collet block in vice to index flutes, hence the 3 or 6 flute reference. 5 flutes @ 72* apart might be difficult for me to accomplish accurately without a spin index…

    My apologies for all the noob questions, just want to make sure I’m going at this the right way & not fubar a $100 barrel!

    Thank you very much for your time & any insight you can offer…


    Todd
     
  2. ghostdncr

    ghostdncr United States Active Member Active Member

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    I'll start off with some random thoughts. First, do you have the square collet block that so often accompanies the hex? If so, I'd make my reamer an eight flute. I haven't done the math, but it seems eight flutes divided between twelve lands would give good, consistent contact throughout a 360 degree revolution.

    I probably wouldn't bother lapping the chamber. I say this because instead of creating a chamber to precisely locate a cartridge casing, you are instead creating a freebore "chamber" to orient the pellet prior to firing. A good fit will certainly help accuracy, but consider the manufacturing tolerance on even high-grade match pellets. You may be chasing your own tail here.

    I would aim for a fixed pilot diameter of .0003-.0005" under land diameter. If using a live pilot, I'd probably shoot for .0002" under.

    In the case of a reamer this small and short, I'd also make an external support for the reamer shank. Something similar to a false muzzle that closely fit both the reamer shank and OD of the barrel's chamber area, just to keep everything working in a straight line.
     
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  3. TTD

    TTD Canada Active Member Active Member

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    Thank you so much for the advice/tips, ghostdncr....it is greatly appreciated!:encourage:

    No, I don't have a square collet block yet, just the hex block (used at that!)....and the only reason I even have that is I just happened to be at the local tool supply store that buy/sell/trade looking for something else when I spied it & thought, "Ya, that might come in handy". Will get a square one soon though.
     
  4. ghostdncr

    ghostdncr United States Active Member Active Member

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    Not sure if this would be helpful or not, but a quick search of eBay using the term "5C collet block set" returned several individual square blocks and sets of the hex and square blocks for under $50 shipped. Certainly not best quality, but a little patience and messing about with them would probably get you to where you need to be with this project. Most of these cheap blocks (shop owned) I've checked are ground within .0002-.0003" and the real errors seem to arise from combining them with cheap collets. Get yourself a good quality collet (Lyndex, Hardinge, etc.) in whatever size needed. You should find them to be somewhere within .0005" runout or less when new.
     
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  5. Msalm

    Msalm United States Iron Registered Member

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    Couldn't you use an extremely sharp, short boring bar with the compound set to 2 degrees or so and bore out the 'seat' followed by the compound travel for the throat? Be dialed in as close as you can get in the lathe, easily .0002".
     
  6. TTD

    TTD Canada Active Member Active Member

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    I actually did think of that approach at first Msalm, but with a minor bore diameter (ie- land diameter) of only .01755” x .725” chamber depth I would need an extremely small boring bar (1/8”+/-)….and knowing my luck coupled with lack of skill/experience, I would end up snapping it off or something stupid like that. If it was a .25 or .30cal I might give the boring bar idea a shot, but I just don’t possess the skill/talent yet at this time to try that on such a small calibre. Besides….I wanted to learn how to make a reamer anyways, so if nothing else it was a good excuse for some “hands-on” training...;)

    You will have to excuse the poor drawings below (just used SketchUp), but this was how I modeled the required profile of reamer. I removed all dimensions just to “de-clutter” pic, but the rest is drawn to scale & fairly self-explanatory - reamer in yellow, bolt in green, black where o-ring sits. Also, drew it as a D-bit just to simplify things, but in reality it’s actually a tapered cylinder.
    [​IMG]
    Reamer, barrel & flo-thru bolt configuration:
    [​IMG]

    As drawn, the flo-thru bolt probe I designed for this build will seat the skirt of pellet .010” past Transfer Port, so unlike the original factory bolt probe that NEVER pushes ANY pellet far enough to clear the port, the pulse of air that pushes pellet on its way once trigger is pulled will no longer blast into the side of the skirt deforming the soft lead in the process…which normally results in terrible accuracy. At the front end, headband of pellet will be fully engaged with rifling.

    I made the reamer out of ¼” O1 drill rod. After talking to some knowledgeable & reputable fellas on a few airgun forums I frequent that have successfully made numerous small reamers like this, I opted for a simple 3-flute design with (strangely enough I thought) NO secondary relief cut needed leading up to the cutting edge. Apparently the softer steel of an airgun barrel cuts pretty easy & the guys “in-the-know” have not encountered any problems by deleting this secondary relief. They have been doing this for a long time, so who am I to argue?

    Not too bad I suppose for a first try, but still not completely satisfied with my attempt. I plunged my end mill .010” past centerline to provide a bit of rake to cutting edges of reamer, but don’t think I made the cut quite long enough into rear relief section next to main shaft. Should have made it so center of end mill stopped cutting once it was centered with rear shoulder of “fat” section instead of front shoulder. Now I’m too leery of setting everything back up in my milling attachment & buggering up the cutting edges for the sake of continuing the cut by another .050” or so. I may just clamp reamer in the vice & carefully use the Dremel w/grinding wheel to touch it up a bit.

    Still going to harden/temper this one & give it a whirl on an old .177 barrel I have here for a trial run first. If it cuts fine then I will proceed with my good barrel, but if something goes awry I’ll just start over & make a new one…fingers X’d
    [​IMG]
     
  7. Msalm

    Msalm United States Iron Registered Member

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    Ah, makes more sense....I was thinking spring piston break open type that would have a much shorter chamber area.
     

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