Pistol smithing / truing frame trunnion.

[felis concolorParticipant @felisconcolor37847]

Gents,

I am working on a blowback, fixed-barrel pistol, with the objective of replacing the fixed barrel, with one that I will fabricate.
To date, the gun is stripped of its parts, including the original barrel, leaving a bare frame.

Removing the press fit barrel entailed tapping out the through-frame, barrel retaining pin, adequately cooling the frame /barrel in a deep freezer, and then applying torch heat to the frame trunnion just before tapping out the barrel from the frame with hammer and wood block. YES, it was a press fit, but the barrel was eventually removed with persuasive tapping.

Now, I find that neither the frame trunnion or barrel at the chamber-end are / were round. Both are out of round by about .003-.004″.
When turning a replacement barrel, it will be round / true.

That leaves truing the frame trunnion.

My plan is to use the 4 jaw on the ML7, and with the use of a boring bar, true the trunnion to round.

Herein lies the problem….. because the frame needs to be oriented in the 4 jaw at the grip-end, the frame’s trunnion hole (about 12mm diameter) has a smaller diameter inner band that any boring bar must pass through, to machine the majority of the trunnions’ lumen. This inner band is about 9.5mm in diameter, and works as a “step” to keep the barrel from moving forward, in the frame trunnion.

My question are;

Is this a “doable” machining operation?

The trunnion lumen is not a blind hole, its just having to pass a boring bar through that narrowed band, for access to the main trunnion lumen.

What design of boring bar do I need, to machine from left to right, up to the stepped inner, narrow band….. which is of course, at the tail-stock end of the trunnion hole?

Thanks for your thoughts.

 

[Kiwi BlokeParticipant @kiwibloke62605]

You need a ‘back boring bar’. Easy enough to make one, to take a small HSS bit in a cross-hole (see description in books), or convert a right-angle-cranked HSS boring bar. Or forge and file/grind one from silver steel/drill rod: it will be fine for the limited amount of work it’ll have to do. Here’s a pic of inserted tipped ones – no idea who the company is, no doubt cost a small fortune. As you can see, it’s just like a conventional boring bar, but with the tip reversed.

Strange to have such distortion. Presumably the chamber was of round cross-section, thanks to the use of a chambering reamer, used after assembly. Good luck with rifling. Perhaps you’ll tell us how you go about it.

[Michael GilliganParticipant @michaelgilligan61133]

Classy-looking tooling, Kiwi

… and proudly made in India

https://www.renukatools.in/include/downloads/ECCENTRIC%20FINE%20BORING%20TOOLS%20V24.1.pdf

… That’s trampled upon some of my preconceived notions !

MichaelG.

[JasonBModerator @jasonb]

Would it be possible to mount the part on the cross slide and with the larger hole facing the chuck and then have the cutting tool rotate?

Could be done with a boring head or even a simple hole made bar with cross drilled hole for a HSS bit.

 

[Kiwi BlokeParticipant @kiwibloke62605]

Oh, interesting idea, Jason. One might even consider a between-centres bar. It would be an easier set-up, and avoid a frame whirling around, disconcertingly… However, I assume the OP is working on a semi-auto pistol (Webley made a single shot blow-back target pistol, but that’s an evolutionary dead-end, as far as I know). Unfortunately, all the semi-auto’s I have experience of have their frame obstructing access to the barrel line from the rear, which also makes cleaning them a fiddle… I guess in this case the barrel is fitted by offering it to the frame, whilst its axis points down a bit, to clear the back of the frame, then aligning it and pressing it home. Common revolvers are easier – they screw together.

Are we allowed to discuss gunsmithing here? I ask, because I was really looking forward to the pistol construction article, promised in MEW (or was it ME?), some years ago. It got pulled… Bah!

[Michael Gilligan]

On Michael Gilligan Said:

… That’s trampled upon some of my preconceived notions !

MichaelG.

Yeah, I know what you mean. It’s one of the side-effects of getting old: the world changes, but we don’t realise…

[FulmenParticipant @fulmen]

There is no law requiring the barrel axis to be obstructed on a fully disassembled pistol. Since the barrel is mounted from the rear it’s a good chance that it has free access to the trunnion from the rear as well.

Cutting a 12mm recess through a 9.5mm hole shouldn’t be too hard to pull off, something like a grooving tool from round HSS should do the trick. But there will be a fair amount of tool overhang, so a line boring setup should be considered as well.

[Kiwi BlokeParticipant @kiwibloke62605]

Oh dear! What was I thinking? Of course, Fulmen is quite right: I should not have said ‘all’. Many blow-back pistols, of the type with their barrels mounted to the frame, do indeed allow in-line access to the barrel seating region from the rear, albeit at a distance, when dismantled. However, not all. I guess my frustration, working on my Unique DES 69, which is a rather odd design, coloured my memory. Also, many target pistols’ barrels aren’t fixed into the frame, again possibly making access to the barrel seat easier – but not necessarily. Apologies for any confusion caused.

[felis concolorParticipant @felisconcolor37847]

Thank you gents, for sharing your insight.

To answer a few of your questions…..

Yes, the frame trunnion is accessible from both ends. Boring it from the chamber end certainly would be easier, with being able to avoid working through the narrowed, trunnion end. But putting the frame into the 4 jaw would be challenging, as there is just not much frame steel to clamp onto, in that orientation. Further…. the boring bar would need to be long, with 4″ stick-out….. not ideal.

Yes, I may be able to ?clamp? the frame to the ML7 cross slide, and put the boring bar in a collet. Since I am new / learning to using the lathe, the set up for this feels more daunting.  Setting the correct height of the frame to within a thou or two, I have zero experience with.
Pop can shims?
I do not have a myford milling attachment.

And yes, .003-.004″ does seem like a lot of out-of-round, within the trunnion bore. I am probably responsible for some of that, trying to measure the lumen with telescoping bore gauge and a mic.

I will search for a “back boring” boring bar.

 

 

[Michael GilliganParticipant @michaelgilligan61133]

This probably has too many complexities of its own, but I just offer it for discussion:

You might try embedding the frame in a block of low-melting-point alloy, to make mounting it more convenient.

MichaelG.

.

Ref. __ one example of such material

https://www.flexbar.com/products/fixturing-alloy-1-lb

[JasonBModerator @jasonb]

Lining things up can be done by putting  a rod (skimmed down barrel) into the hole and holding by that in a collet. You can then arrange your clamps and packing against the frame and when all is firm remove the rod.

[PeteParticipant @pete41194]

Is this an air gun, rim fire or center fire pistol? Without seeing both parts and knowing who manufactured your pistol, or exactly what it is. Then it’s extremely hard to be 100% sure of anything. But a 4 jaw independent chuck isn’t always the best method to hold everything that comes up that isn’t exactly round or eccentric. Something known as a Keats angle plate attached to your lathes face plate might be far easier and provide a much more secure grip. I find it a bit surprising how many have never even heard of them today. https://www.hemingwaykits.com/HK2430

It also seems more than odd that neither the hole or the barrel tenon is round. Even more so when using a press fit to assemble the parts. In fact that would likely produce a less than round chamber which would be highly undesirable in every gun I can think of. Depending on the amount of access, the chamber may well have been reamed after that press fit of course. I won’t say it’s impossible, but it would be tough for the manufacturer to machine anything like that on purpose or even by accident. I’d google your specific gun and see if anyone else has found the same. Depending on where that out of round happens to be, they may have used a press to squeeze the frame onto or against the barrel instead of doing a more conventional longitudinal press fit. If that’s how it was done? Then that out of round should show up in two or more evenly spaced areas on both the frame and that barrel tenon. And if the pistol frame is lightly built, I’d be real leery about trying to do a more conventional type of temperature differential or press fit when your reassembling it. A cracked frame might easily be the end result.

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