S & W N-frame Project

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mtngun
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Re: S & W N-frame Project

Postby mtngun » Sat Nov 04, 2017 11:59 am

Bob wrote:I think that the loose bolt issue is deliberate by some gun manufacturers for production purposes. If there's a little wiggle in the cylinder at full lockup, the cylinder machining & fit are a bit less critical. The bullets passage from the cyl throats into the forcing cone (especially jacketed bullets) will serve to align the barrel & cylinder a bit.


That's possible, but I suspect the cylinder's inertia will prevent it from rotating fast enough to self-align. Also the hand is pushing the cylinder against the bolt, taking up most of the slop.

When you get to making your own cylinder, consider making it a bit longer so it fits the cylinder window better. Smith really dropped the ball on that little detail, IMHO.


I'll probably do that.

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Re: S & W N-frame Project

Postby mtngun » Sun Nov 12, 2017 1:53 pm

Fitting the oversize bolt: the cylinder slots are about 0.105" wide, and the bolt window in the frame was originally about 0.1055" wide. The logical thing would be to stone the oversize 0.107" bolt down to about 0.105" for a snug fit.

But .... I had a clever idea. 8-) Since the cylinder timing was off by a good 0.003", why not shift the timing by a combination of filing the window 0.001" or 0.002" wider on one side so that the 0.107" oversize bolt could fit, and then file the top of the bolt on one side to 0.105"? In theory that would shift the timing over about 0.003" while providing a snug fit at all points.

So I tried it, but when I checked the alignment, it was worse, not better. I figured I must have screwed something up so I started over, making a homemade bolt from barstock. Let me tell you it was a son of a gun to make that homemade bolt and in the end it would not cycle properly because its hole was located a few thousandths off. :cry: Nonetheless this photo of the homemade bolt illustrates how I was attempting to shift the timing by making the top of the bolt offset relative to the bottom of the bolt.
Image

Since the homemade bolt wouldn't cycle, I tried again with the Power Custom bolt, welding it up oversize, then grinding it down. But no joy , alignment was still worse. All I can figure is that my original alignment measurements with the factory bolt were optimistic because there was enough wiggle in the cylinder timing to allow the gage pin to self-align its way into the throat.

Here is the alignment with the oversize bolt
#1 -- 0.354" pin
#2 -- 0.352" pin
#3 -- 0.353" - 0.354" pin
#4 -- 0.353" pin
#5 -- 0.352" - 0.353" pin
#6 -- 0.353" pin
average -- 0.353" pin, or about 0.0045" misalignment.

Even thought the oversize bolt eliminates the wiggle in the cylinder timing, I could still wiggle the undersize gage pins in the 0.358" stub, so the actual misalignment is probably more than the measured 0.0045". :cry: I suspect the actual misalignment is closer to 0.006". :cry: That sucks, but it's not unusual for a mass produced revolver (perhaps CNC technology has improved tolerances on recent production revolvers, but CNC was not being used when this Dirty Harry era frame was manufactured).

The fitted oversize bolt will be absolutely necessary when I get around to making a line-indexed cylinder, but in the meantime, would it be better if I went back to the sloppy factory bolt? Perhaps, bearing in mind that the hand will take up most of the slack in the cylinder timing, and it so happens that the hand will be pushing the cylinder in the direction that it needs to go.

Retesting with the factory bolt:
#1 -- 0.353" pin
#2 -- 0.353"
#3 -- 0.3535"
#4 -- 0.354"
#5 -- 0.353"
#6 -- 0.3525"
average -- 0.3532", or about 0.0043" misalignment.

That's worse than my original measurement on Oct 21. The difference is due to how much I wiggle the pins. If I wiggle the pins aggressively enough, I can get a 0.354" or even 0.355" pin to go, but that's probably "cheating." Wiggling the pin effectively pries the cylinder into alignment.

Does wiggle in the cylinder timing allow a bullet to self-align in the forcing cone? There are several problems with that theory. 1) the front band of the bullet will still slam into the forcing cone off center. The best that can be hoped for is that the cylinder will then self-align for the remainder of the bullet 2) inertia may prevent the cylinder from moving quickly enough to self-align and 3) the hand pushing on the cylinder ratchet may take up most of the slack in the cylinder timing, anyway.

Well, my clever idea to shift the cylinder timing with the stepped bolt did not work. :oops: I'll set the oversize bolt aside for the time being and use the factory bolt with the factory cylinder. I will have to cut the forcing cone to allow for at least 0.006" mis-alignment -- in other words, the entrance to the forcing cone will have to be at least 0.370". :cry: Accuracy will probably be disappointing due to the misalignment and the oversize forcing cone, but that's a fact of life with mass produced revolvers. Buying a mass-produced revolver is a roll of the dice -- sometimes you get lucky, but I've been unlucky with revolvers more often than not. That's why people pay big bucks for FA's or for custom line-bored cylinders.

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Re: S & W N-frame Project

Postby mtngun » Fri Nov 17, 2017 2:01 pm

I wondered if I could compensate for the factory cylinder's timing error by cutting the barrel tenon offset from the barrel's bore? To test that theory, I made a dummy tenon from a short piece of scrap metal, with a 0.358" ID, except the ID was offset from the OD such that a test indicator showed 0.005" runout (that's 0.0025" offset since the offset is half as much as the runout). I marked the offset point and screwed it into the frame with the thin side at 9 o'clock as viewed from the muzzle. With the factory bolt most holes would take a 0.355" pin so call it 0.0025" misalignment, compared to 0.0045" previously.

The interference was at the 4 o'clock position as viewed from the muzzle.
Image
OK, so this gimmick does seem to improve the alignment, but I haven't found perfection yet.

UPDATE: I made another dummy tenon with 0.007" runout (0.0035" offset) to try to nail down the optimal amount of offset. Some trial and error confirmed that results were best with the thin side of the tenon set at 10 o'clock as viewed from the muzzle. Most holes still took a 0.355" pin and interference was still rubbing the cylinder at 3 - 4 o'clock. I can't explain why the results were exactly the same with both 0.005" runout and 0.007" runout, other than the usual measurement error and/or machining error in the dummy tenons.

At any rate, I still have not optimized the amount of tenon offset so I may have to make yet another dummy tenon.

UPDATE #2: I made yet another dummy tenon, this time with 0.010" runout (0.005" offset). A 0.356" pin would go in all cylinders, for about 0.0015" mis-alignment. The interference was still rubbing the cylinder at about 3:30 o'clock.

Let's summarize the results:
No tenon offset -- 0.0045" mis-alignment, but guessed actual mis-alignment was closer to 0.006" due to pins & cylinder wobbling.
0.0025" offset -- 0.0025" mis-alignment
0.0035" offset -- 0.0025" mis-alignment
0.0050" offset -- 0.0015" mis-alignment

Perfection has not been reached yet but there's enough data to suggest that 0.006" - 0.0065" offset would be ideal, so I'll plan on cutting the barrel tenon with 0.006" - 0.0065" offset. The offset tenon will complicate installing the barrel and setting the barrel-to-cylinder gap, but I think I can deal with those complications. Yeah, it's a ton of effort for a small improvement, but the factory mis-alignment really bugs me. :evil:

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Re: S & W N-frame Project

Postby mtngun » Sat Nov 18, 2017 3:57 pm

Tenon cut with 0.006" offset, which should put barrel/cylinder alignment within 0.0015".

The tentative plan is to end up with an 8" barrel, but in this photo it was 9 1/4" in case I wanted to set the barrel back and re-cut the tenon. I doubt if I ever will set it back, though, so I'll probably go ahead and trim the muzzle end to 8".

Anyway, I set it up for 0.001" - 0.0015" barrel/cylinder gap when the tenon is clocked correctly. That may turn out to be too tight, but if the need arises I can always shave a thousandth off the barrel to increase the gap. You can't adjust the gap simply by screwing the barrel in or out because the offset tenon has to be clocked.
Image

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Re: S & W N-frame Project

Postby mtngun » Sun Nov 19, 2017 9:40 am

I cut the forcing cone with a boring bar, and finally got around to slugging the barrel.

This no-name ebay barrel has an eclectic 10 grooves, almost like a microgroove, except the grooves are normal depth. A 0.347"+ pin is a very snug fit in the bore. It's not lapped and you can see some light machining marks on the lands.
Image

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Re: S & W N-frame Project

Postby mtngun » Sun Nov 19, 2017 1:27 pm

A chunk of aluminum from the scrap pile was sawed to rough shape and then lathe-bored. I didn't have the desired drill or reamer on hand so I made a D-Bit reamer, which worked well enough on the aluminum.

The aluminum was not labeled but there's a good chance it's 7075 or Fortal (same thing).
Image

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Re: S & W N-frame Project

Postby mtngun » Sun Nov 19, 2017 6:37 pm

I needed a way to position the shroud parallel to the bore. I had a ground rod that is used in combination with bushings to true various rifle actions, so I made a couple of bushings to fit the 0.680" bore of the shroud.
Image

Set the ground rod on blocks and mill the top of the shroud. Now the top is parallel to the bore. Repeat for all four sides.
Image

Next ... contouring the shroud .....

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Re: S & W N-frame Project

Postby mtngun » Mon Nov 20, 2017 2:35 pm

I had contemplated contouring the shroud to nearly exactly match the frame, but I didn't have some of the tooling needed to do fancy contouring, so I did the best I could with the tools I had and called it good. ;)

Aluminum shrouds are used on 38 special competition revolvers, but I've never heard of anyone using an aluminum shroud on a magnum, so I'm not making any guarantee that it will hold up to magnum loads. But I suspect it will.

The Dan Wesson revolvers recess the barrel nut into the shroud, but that requires either using a skinny barrel nut or a fat shroud, and I didn't want either of those things (the skinny Dan Wesson barrel nuts were prone to round their wrench slots, leaving the nut stuck in the shroud). So I let the nut sit outside of the shroud. I don't care for the appearance of the protruding nut, but it's stout and functional.
Image

It still lacks a barrel nut wrench and a scope mount, so it's not ready to test fire yet.

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Re: S & W N-frame Project

Postby mtngun » Tue Nov 21, 2017 3:15 pm

Finished the barrel nut wrench, and started on the scope mount.
Image

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Re: S & W N-frame Project

Postby mtngun » Wed Nov 22, 2017 12:56 pm

Working on the scope rail & didn't like the itty bitty screw (#2 or #4???) used for the S&W rear sight so I made a nut that will take a 6-48 screw, instead.
Image


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