CETME cross pin drilling help.

[vmtz]

I am in the process of building my Centerfire CETME and need a bit of help.

I have build one G3 and it came out great. However, the barrel was installed in the trunion.

On the new CETME, as I am sure you all know, the barrel is not installed. I have pressed it in and have the bolt gap set at .019. Now all that is left is to drill for the cross pin. What is the best way to do it and what methods have you all used?

Here is the URL to my first build. You'll have to cut and paste it since I don't know to make the link live on this board. http://www.falfiles.com/forums/showthread.php?s=&threadid=170747

Thanks in advance for the help.

Vince

[rustypirate]

The best way to do this is to use a 3/16 end mill to begin the cut into the barrel.
This will cut cleanly into the sloped sides of the barrel and provide a flat-ish surface for the drill bit to start in, proventing the drill bit from drifting off-center and messing up the trunion.

Once the end mill work is done, drill the remainder with a 3/16 drill bit then ream to 5mm and press in the pin.

The ideal tool for all this drilling and milling is obviously a mill, but a drill press will work. I do not recomend trying this with a hand drill. I personally use cobalt drill bits and high-speed-steel chucking reamers.

The trickiest part of the whole process is clamping the receiver down for the drilling. With a round receiver and drilling off-center, this can be a challenge.

I have never really been able to do it to my satisfaction, but it allways works out with persistance. Be carefull of denting the receiver when clamping it down.

[roland3]

ya but with the hole already in the trunion and the barrel only getting a half of hole doesnt the drill deflect over in to the hole that is already there in the trunion?

[vmtz]

The best way to do this is to use a 3/16 end mill to begin the cut into the barrel.
This will cut cleanly into the sloped sides of the barrel and provide a flat-ish surface for the drill bit to start in, proventing the drill bit from drifting off-center and messing up the trunion.

Once the end mill work is done, drill the remainder with a 3/16 drill bit then ream to 5mm and press in the pin.

The ideal tool for all this drilling and milling is obviously a mill, but a drill press will work. I do not recomend trying this with a hand drill. I personally use cobalt drill bits and high-speed-steel chucking reamers.

The trickiest part of the whole process is clamping the receiver down for the drilling. With a round receiver and drilling off-center, this can be a challenge.

I have never really been able to do it to my satisfaction, but it allways works out with persistance. Be carefull of denting the receiver when clamping it down.

Sorry, the trunion is not in the receiver yet. What method would you suggest for clamping the trunion and how would you get the set up square?

Vince

[rustypirate]

ya but with the hole already in the trunion and the barrel only getting a half of hole doesnt the drill deflect over in to the hole that is already there in the trunion?


Yes, it does, and that is exactly why you should use the end mill first, to square up the sides of the hole where the barrel is, and create a flat surface for the drill bit to bite into.

[Perro]

it still deflects - hole in the trunnion or not
the hole in the trunnion is slightly larger in diameter so the drill has the ability to start walking

milling a flat into the barrel and CENTER DRILLING it with a center drill to start it out straight is a good idea as well



Sorry, the trunion is not in the receiver yet. What method would you suggest for clamping the trunion and how would you get the set up square?

Vince
make a jig
cut two holes into a block of steel - 1 hole for the trunnion area of the receiver, and one hole for the cocking tube of the receiver
cut this block of steel in half lengthwise
drill two holes through the end of the flat plate
tap one side of the holes
install bolts into the holes and tighten them into the threaded holes

you have just basically made a clamp that clamps the receiver square to the Z axis of the tool doing the drilling

weld a couple tabs to the plate so you can bolt it down to the bed of your mill, or drill press, and whalla - you have a fixture for drilling barrel pin holes

[rustypirate]

Sorry, the trunion is not in the receiver yet. What method would you suggest for clamping the trunion and how would you get the set up square?

Vince


that may make clamping it easier. You could even use a clamp into the well of the trunion just as long as it does not push against the barrel face and ruin the bolt gap adjustment.

In Factory rifles, the barrel pin is drilled after the trunion is installed into the reveiver, but in a factory new trunion, the barrel hole is not pre-drilled, so they do not need to mess with the end mill thing, they just drill and ream it.

No matter what, it is a round object clamped to a flat surface and drilled off center, which can be tricky.

[roland3]

i think im going to just get it set up. rough it out with a 3/16 emill and then finish endmill it with a 5mm endmill I have. if it comes out big or i have to go over sized a couple thou, I will just grind up a correct size dowel pin to press in. Pressing it should be fun also. maybe i can just tap it in with brass. if your going to drill it, the trunion is hard as hell so it should act as a guid and not let a drill cut in to it.

[roland3]

also do the century cast ss recievers come with a hole in them for the barrel pin? or is that another obstacle to figure out.

[vista461]

They do not have the hole in already , at least mine did not

[rustypirate]

Jamie is correct, they do not bother to drill a hole for the barrel pin.

[roland3]

in case anyone can use this information., I took some measurments on my trunion for the hole locations. first I just dusted both ends of the trunion on the suface grinder. then put a .866" guage pin in the end of the trunion where the barrel presses in. Then held it in a v block out the side of a kurt vice on a mill. so the trunion is square in the x and y axis.next I put a 298 guage pin in the barrel pin hole and rotated the trunion until the pin would go in to a good drill chuck. checked it for up and down travel. now im within a couple thou at worst.
the center of the barrel pin hole was .312" in from the front of the trunion. and .438 out from the center of the barrel hole..I will use these numbers to pick up the reciever which will have no hole when I go to pin the barrel.

[rustypirate]

Keep in mind that the front of the trunion and the front of the receiver are different dimensions. the trunion shlould be inserted into the receiver until the rear of the trunion contacts the front of the inner guide rails. The front of the trunion will be recessed into the front of the receiver slightly.

[roland3]

depth mikes, to measure the difference.

[rustypirate]

OK, just so long as you know, I would hate for you do drill in the wrong spot.

[texlurch]

You might also look around for some of the pilot point style bits. The outside doesn't have the large taper like standard bits, so they aren't as prone to walking.

[Perro]

You might also look around for some of the pilot point style bits. The outside doesn't have the large taper like standard bits, so they aren't as prone to walking.


these are called centerdrills, and are a wise idea to start your hole after milling it flat with your end mill

i have used this set to do my 1919s in 4140 grade chrome moly hardened steel without a problem, and they also did a bunch of other peoples plates too

http://www.littlemachineshop.com/products/product_view.php?ProductID=1230&category=

these are stiff and stubby so it starts the hole out straight without wander - then you switch over to your standard jobber type drill, and go to town

[texlurch]

Those work very good, here is what I was referring to:

http://www.mytoolstore.com/dewalt/dw1956.jpg

The Dewalt brand name is Pilot Point; I see them called split point or 135 degree in other places...

They also have some that the outer edge is actually beveled more forward, and don't have as pronounced pilot in the center. Those work real good, I think they are Milwaukee brand in black cobalt.

[Perro]

ahhh
thought you were talking about center drills - my bad

[texlurch]

Be a good idea to have both... matter of fact I need a new little center bit... may have to order that exact set you posted or run down to the tool store when I get home... ayup!!

[gunner]

For those of us without access to a end mill, is it going to work using a drill press?

[texlurch]

For those of us without access to a end mill, is it going to work using a drill press?


Yup. I have a mini-mill/drill press and works fine. Just take your time and measure/line up everything 3 times....

[texlurch]

I came up with something else while trying to press out a barrel pin, and the receiver kept trying to roll in the press.

With the bolt and carrier removed, look up thru the mag well, and you can see a nice square recess in the trunnion. I can see no purpose for it, so far as locking the bolt, or guiding the mag. However, I had some scrap aluminum bar about 3/4 thick laying around, and notched the end to fit tight into the trunnion recess. With about 3 feet sticking out, it makes a perfect fulcrum to keep the trunnion/gun from rotating in the press. A shorter version should be perfect for drilling.

I'll take some pics when I get back home....