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Thread: Chamber fluting in the home shop

  1. #1
    holescreek's Avatar
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    Chamber fluting in the home shop

    I don't know if this topic has much relevance but I figure I'll put out some info on chamber fluting anyway. Back when I was thinking about making a cetme in 7.62x39 the big hangup was needing to flute the chamber for the rifle to function. After a week or more of head scratching I finally decided that the best way to control the depth of the flutes was to use my (home made) CNC mill. My intention was to make a cutter that could be inserted into the chamber, drop into the back of the rifling and then pull it out using the X and Z axis programmed for the shape of the chamber.

    I spent a bit of time in CAD drawing the chamber cross section and finally came up with a tool that was stout enough to withstand the cutting pressure while still small enough in diameter to fit inside the bore.


    This is a close up of the actual cutter:


    Cutter inserted into the chamber:


    My first fluting experiment was done using a cheap VZ58 barrel. I figured the $25 barrel was a great learning experiment. I ground the tiny carbide insert cutter by hand, and didn't know what to expect. The mill was programmed to the chamber contour and the spindle motor was unplugged so i didn't forget and accidently flip the switch.

    Here is the finished VZ chamber:


    I took a mold of the chamber and discovered that the flutes were too shallow. Fluting a chamber is a lot like driving a submarine. You gotta trust your numbers because you can't see what is happening. I set the barrel back up in the indexer and cut them again.

    Here's a photo of the shallow first mold on top and the more pronounced fluting on the bottom mold:


    Once I had the programs finished for the 7.62x39 I bought an RPK barrel, faced it off and rechambered it for use in the cetme and fluted it for the build. I used the RPK barrel because of it's large OD which I could turn down to fit into the cetme trunnion.

    This weekend I started fluting in 7.62x51, more on that in a bit.

  2. #2
    Senior Veteran HKILLER's Avatar
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    nice work holescreek how long did i take you to figure that out?
    IS THERE A SECTION FOR MY DEATH STAR ON MILITARYFIREARM.COM?

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    Veteran spectre's Avatar
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    sweet now i need to buy stuff to make my own cnc mill.

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    holescreek's Avatar
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    It was a week of head scratching before I got the idea to use a rigid tool in the mill. Originally I had thoughts of making some sort of hand shaper with a spring tensioned doohicky and tracer type followers and whatever else ran through my head. Then I got to thinking about how nice it would be to CNC the flutes but using a bal nosed cutter was out of the question due to the OAL needed. Then one day in the shower it came to me, mix the shaper cutter idea with the CNC repeatability.

    Here is the finished RPK chamber:


    I've been reading on some forums that there are a bunch of people that bought some PTR barrels with shallow flutes. I have talked to a couple of people that have them, or think they do, and don't want to chance building with the barrels until they are either fixed or proven good. On top of that, I have been wanting to find a real heavy barrel to play with on a future build and haven't seen anything out there that I would buy either because it isn't heavy enough or just too darn expensive.

    I stumbled on some Green mountain round "gunsmith special" barrel blanks rifled for 7.62x39 for $30 each. I figured they would give me somthing to experiment with for programming/tooling up for 7.62x51. I bought an x51nato reamer and started in on one of the blanks. I turned the OD of the blank while centered on the bore ends to keep it true. I made the blank so that it would be a press fit into a cetme trunnion, then relieved it back from the far end about .010" to clear the trunnion. I ended up with a 20" barrel that measures about .853" at the crowned end. I put an 11 degree crown on the far end and reamed the chamber for x51nato.

    Top is an 18" B5 Designated Marksman heavy barrel:




    After some hours laying out the chamber dimensions and writing the new programs I set the new heavy barrel in the mill. I have 3 different programs for roughing and finishing to take it easy on the little cutter. Grinding those tiny cutters is like trying to brush a fly's hair.

    I purposely made the flutes on the new x51 chamber shallow to simulate the defects I have been reading about. I took it out of the indexer and made a mold of the chamber. In making the mold I discovered that the tip of my tool holder was hitting on the rifling as the cutter was lowering into the shoulder angle where it meets the side wall and left the flutes disconnected from the shoulder to the side of the case. This morning I modified the tool holder and re-ran one of the two finish programs on all 12 flutes and got the second program ran on 6 of the flutes before I had to stop to go to work.

    Being able to set the barrel back up and align the existing flutes is a key component to repairing bad barrels so this little excercise was good for me.

    Here is a photo of the shallow x51 flutes:


    This is the mess inside the chamber from running the second finishing program on the six flutes this morning:


    Hopefully I'll have time to run the other 6 flutes in the morning and take a mold to compare the before and afters side by side.

  5. #5
    holescreek's Avatar
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    Quote Originally Posted by spectre View Post
    sweet now i need to buy stuff to make my own cnc mill.
    I have 4 CNC axis' on mine, but the rotary A axis through hole is too small to fit a barrel through. If it wasn't, I'd set a barrel up and walk away for a few hours.

    Old men have the best toys!

  6. #6
    Otis61's Avatar
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    Nice work. I can build (put together) guns in my garage, but this is the stuff I can only dream of.
    "Without gunpowder there can be no freedom!"
    -German-American proverb during the War of Independence

    COG# 1961

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    Senior Veteran turbothis's Avatar
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    you dont have to be old to have cool toys in the garage.
    have cnc mills, will work for food.
    http://www.gunsnorthwest.com/

  8. #8
    holescreek's Avatar
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    Oh, I forgot to tell you guys that this might be a "trade secret". I read some threads by a current barrel maker where he said he was shown the "proper way" for his run of barrels but was sworn to secrecy. On the off chance that this method is close to the "real way" you guys will have to watch your back or the barrel making boogey men in black will be out to get us all.

  9. #9
    Senior Veteran turbothis's Avatar
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    lol
    let them come
    have cnc mills, will work for food.
    http://www.gunsnorthwest.com/

  10. #10
    Senior Veteran cfish's Avatar
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    Ya let em we have your back!!! With turbo in the lead!!! Seriously thats some cool stuff HC wish I had those skills!!!


    COG#1963

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