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Thread: Cetme L build planning

  1. #151
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    I do have a reamer. And I have a reamer print to match.

    I have a 25 inch barrel blank, of which 18 to 20 inches needs to become a barrel. I think I have a scrap of 4140 and the right size drill, too.

    The bore is only 0.270, can your tooling fit?

  2. #152
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    Quote Originally Posted by Xnke View Post
    I do have a reamer. And I have a reamer print to match.

    I have a 25 inch barrel blank, of which 18 to 20 inches needs to become a barrel. I think I have a scrap of 4140 and the right size drill, too.

    The bore is only 0.270, can your tooling fit?
    Nope, I'd just need to make a slightly smaller tool holder and since the chamber is based on the shorter .223 cartridge it doesn't need to be as long as what I've been using either. You could even use cold rolled steel for the practice piece, it's just to prove out the tooling and program. It has to be chambered to the same depth you are going to chamber your barrel blank and have the same OD.

    I'd need a copy of the reamer print and also need to know how much cartridge stick out you're going to have beyond the barrel face. Typically the stick-out equals the depth of the bolt face counterbore +.009".

    I thought I was done chamber fluting forever until I got stuck with the POS RCM barrel and had to make my own replacement. Besides, I love one-of-a-kind builds and this one looks like it's going to be pretty cool. I can at least try to help a little (and hope it works).

  3. #153
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    This also brings up an interesting question as to the width and depth of the chamber flutes. I can't remember if the .223 flutes were the same as .308 flutes or smaller. I have a piece of an HK53 chamber in the shop and if memory serves the flutes were different.

  4. #154
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    There is an L chamber photo in this thread, I think the .223 L used 12 flutes.

    I have been aiming for 9, but that was just a WAG. I'll get the reamer print and post it tonight when I get home from job hunting.

    EDIT: Here's the reamer print that my reamer was supposed to be made to match.

    Last edited by Xnke; 12-08-2018 at 12:38 AM.

  5. #155
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    More work on this today.

    Ruined the scope rail the other night-Not sure what happened, but somehow I let the part get loose in the vise and shattered a 13/64" end mill cutting the cross slots-this chucked the part to the floor, hitting the table and the broken end mill on the way down. Rail could be saved, but the bolt hold-open section is mangled. I have some stock that I can start over with when I can afford to buy another odd sized end mill to do the cross slotting.

    But today, I turned a dummy section of barrel, drilled 0.272" with a letter I drill bit. it's sloppy but the reamer pilot wasn't too rough in the bore, and was able to make two chambers. One is a drop-in fit in the trunnion, with the lip turned to 0.170" thick, so I can use some shim stock to work out exactly what thickness that lip has to be to set the bolt gap properly in my rifle.

    As far as Barrel Dimensions go:

    The lip diameter (largest OD of the barrel is 0.775"-any bigger and you can't get it into the trunnion from the rear.
    The lip thickness seems to vary-on my rifle, I need one almost 0.200" thick-but some others have needed 0.170".

    The Trunnion ID for my rifle is 0.6685", so 0.669-0.670" should be the OD of the barrel through the trunnion.
    The Length of the press fit is 0.800", from there forward to the front sight base the diameter is not critical.
    At the front sight base, the barrel diameter needs to be 0.631", my sight base is 0.6305" ID.

    I found the best fit for the chambering depth was to get the chamber cut deep, then before cutting the lip thickness, slowly face the barrel off until I had 0.120" of case protruding. This leaves me with 0.009" of gap between the back of the barrel and the bolt face.

    Next up tonight will be to weld the trunnion extension in place. I've figured out how I am going to work with the cocking tube issues, I'll be re-drilling the pin removal hole forward of the trunnion extension and just weld up the top side hole back near the receiver. Hopefully by tomorrow morning, I have an extended trunnion, reamed through 0.750 and ready to fit a barrel to.

  6. #156
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    Trunnion welded up nicely, and it seems to be something similar to 4130 steel. Welded to the mild steel extension very nicely, and even though I burned through into the (pre-reamed) bore in a few tiny spots, it didn't harden up appreciably harder. To weld it, I set it on a block of wood in a plastic dish of water, up to right behind the barrel pin hole, and put it in the freezer until completely frozen. Once frozen, I welded fast, hot, and in a hurry, and the by the time the last dime was layed the ice had only melted back just barely enough to slide it off the back of the trunnion. A quick check with the hardness file set and it's still between RC40 and RC45 until right on the weld, which is RC50 to RC55. A good US made file will JUST cut it where the weld is, but carbide tooling (Even in the old Atlas) made quick, but ugly work of it.

    Since my 3/4" reamer is carbide tipped, and it's an old reground one at that, AND one of the carbide tips already has a tiny nick in it, I ran the reamer through using a boring bar holder and the power feed, and it went through with no trouble at all. You could definitely tell when I was cutting trunnion, weld, and the mild steel extension though-the reamer cut 0.7495" through the trunnion and weld and the lathe loaded up enough to slip the (admittedly 50 year old) belt, and cut 0.7505 through the mild steel section like it wasn't doing any work at all. I'll have to run a brake hone through a few swipes and even that out a bit, or it could make pressing the barrel in a tough job.

    The trunnion extension brings the support for the barrel forward another 60mm, so the total supported length of the barrel is now 80mm. I'm contemplating trimming the extension back a bit, maybe 25mm or so, because the cartridge case isn't that long, and the barrel only really needs that extra support to about an inch beyond the chamber.

    Up next on the extension is milling a little off the flat surface on the top, so I can get the cocking tube to drop down low enough. I am going to split the difference though, and remove most of the metal from the cocking tube with a belt sander and the cocking tube will weld to the top of the trunnion extension as well as the receiver-this should improve the stability of the trunnion as much as adding the T-rails on the side will. The rear of the handguard will have a screw that threads up into the trunnion extension, and the front of the handguard will have a cross pin that pins through the front of the cocking tube as per a normal CETME-L, but instead of a front sight base, a plane jane bit of steel tube will be welded on and capped off. This thing has enough metal hanging out the front already!

    I have decided that a 20" barrel may be a bit too much-my blank will do a 24" barrel but I think 18" plus 2.5" of flash hider will be plenty, and 20" plus 2.5" of flash hider might be getting a bit long, since the barrel will be limited to 0.750". I don't want it to get TOOO whippy out there....

  7. #157
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    Trunnion is now cleaned up and honed straight, 0.7505" all the way through. Here's a test fit shot in the receiver:



    I set up and milled out the first operation on the receiver support rails last night. Since the CETME-L is flat-sided instead of cylindrical where the rails mount on, it's a bit easier to make these-but since they're flat, they are also much thicker and heavier than the HK PSG-1/MSG90/HK21e reinforcing rails. I'll have to do some fancy profiling on them to make this thing less suitable for anchoring the USS Iowa.



    I can shave them down quite a bit here, the extra stock is for work holding and because it's what I have available. After cutting in the 1.5" radius to slip over the OD of the trunnion end of the receiver, I'll work on how I can slim down the rails so that they're still supportive but weigh less...The original rifle weighed 7.5lbs, I would like this one to come in around 9.5lbs complete.

    After milling in the profile, I had to set the plate up on unequal length parallels to cut the taper on the rails. After that, a few quick swipes with a file to radius off the corners, and the rails fit into the grooves nice and tight.



    I'll cut the rails free of the base stock after facing off the far side, and probably after I get the radius cut into the forward end of the rail. They are much more stable and less likely to bend right now, but we'll see how it goes.

  8. #158
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    Looking good, nice work!
    14EH AIT Instructor-PATRIOT Fire Control Enhanced Operator/Maintainer

  9. #159
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    indeed you made a nice looking receiver out of that flat.
    say what you mean & mean what you say!
    TEC Tactical=SOT/07 i work there.

  10. #160
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    Well, I was doing SO well this evening, I spent the last two days cutting 0.010" per pass off the barrel blank to get it from 1.250" down to 0.751". Kept getting chatter problems, so had to just back off until it stopped, and 0.010" was all I was gonna get. Got down to 0.775", did the layout for the muzzle threads, main straight section, and then the press fit section that presses into the trunnion. Muzzle thread section turned down to 0.625", main straight 0.730", and then came the press fit shank, that HAS GOT TO BE 0.751". Got into the cut-and-check, shallow passes, getting closer, OK! 0.755" and one more pass to bring it home. And on that last pass, the tiny little flakes coming off, not even enough of a chip to get a curl going, and everything looks great. Except that now the press fit section is 0.750" and is a strong finger push into the trunnion, instead of requiring several tons of press.

    Crap.

    So, setting it back up in the lathe I get out the old knurling tool and lay a fine diamond knurl onto the barrel. It's now 0.755", and isn't scrap, but I was feelin' so good about getting it all just right, when it all went to crap. Oh well, that's part of why I extended the trunnion. Originally the trunnion was only press fit for 0.8" of barrel length-that's it. Less than an inch of barrel press fit into the receiver, and retained with a pin. Now, it's 3.5" of press fit and still uses the press-fit pin, so barrel support is MUCH more solid now.

    I've got another section of barrel that has to be turned down to 0.750" tomorrow and three chambers to cut-two practice chambers, and the real deal. Hopefully tomorrow evening or saturday I'll have both practice chambers done and the real barrel will have the chamber cut, and the muzzle threads finished. Once that's done, the barrel, two practice chambers, and a few other things are off to Ohio to see The Wizard saturday afternoon.

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