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Still messing around with how exactly it'll look, but I think I'll take advantage of the CNC mill we've got here at uni and make the replacement buttplate from aluminum.  Metalworking is something I'm still pretty new to, so I'm trying to figure out how I could get this done.  Due to it being elliptical in shape, a vice wouldn't hold it in place very well.  I figure what would likely work best is machining the outline of the piece out, but keeping a few tabs here and there so it's still  attached to the stock material (and thus, the vice).  Later, I could just machine those bits out and remove what's left by hand.  Wanting to just make sure this sounds plausible before I get too far into this.  Plan B is just to bolt it onto something else that I'll put in a vice since I'm drilling a couple holes in this anyways.

 

We've not got anything for knurling here, so I'll probably just give it a smooth surface or something, or perhaps just thin horizontal lines.  Below is just the tab idea mocked up in 3ds max (ignore that checkering pattern, won't be what I give it).

 

74pCf4t.png

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I went to the range with a friend of mine the other day. It wasn't difficult to decide which rifle to take. While we were there, his AR was having some problems, and we needed some tools to take it do

I found a new paperweight for my desk. This will considerably augment the document anchoring capacity of my 37mm M59 projectile.  

I seem to have acquired something fun  

Leaving tabs is the usual method - I'd recommend finishing the surface of the buttplate and the holes before cutting the circumference, and leaving the tabs for manual removal with a hacksaw and file. If you try and remove the tabs with the mill it'll probably move, and an unrestrained part next to the endmill will end up with a nasty ding which'll ruin your finish.

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There are several methods you can do this with, including the one that you have shown above.

 

The method you are considering will leave several tabs that you have to remove by hand or by machine. You can make those tabs fairly small, too. It will leave you with hand finished spots on your part.

 

The second method is to take your work and drill the initial holes in it before you do any other machining. You can then take your work and bolt it down to a fixture plate, and machine the rest. This might require removing the vice from the mill table, and I don't know if people would frown upon that where you are. You might also be able to mount the fixture plate in the vice, it depends on the equipment. Also, you are then expecting those two screws to take all the side load from machining.

 

The third option is to use a work piece thick enough to clamp in the vice and still have enough material sitting proud of the jaws to machine your part. You can machine the entire part in the first set up, and then make a set of soft jaws that will contour the part for when you flip it over and face the back down to the correct thickness. The downside is that you waste more material, and it takes more time, but you will have machined every surface to exactly what you want.

 

All of these methods are fairly common practices in machine shops, and there are definitely several more that I am missing.

 

Additionally, you could get most of the checking done with a .45 pointed chamfer tool. It wouldn't cleanly terminate at your boundaries, but it would be close.

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I'll get a good chance to ask about the vice on Monday, since bolting it down would save myself the trouble of getting that all filed down just right. Tabs sounds like the safest bet since I wouldn't need to worry about finding some stuff to keep everything snug and in place, so I'll go with whichever I decide would be less annoying to deal with.   

 

I've already ordered my little bar of aluminum, so working on an extra thick piece is out of the question now.  The idea did cross my mind before, but that would be extra money I don't need to spend. 

 

As for the chamfer tool, that's another thing I'd need to ask about.  I know that I don't have that available for me at the moment, but I could probably either buy the thing myself or get the uni to get one.  Pic below is what I've got to play with, mostly just a bunch of flat and ball endmills.  I know we have all sorts of drill bits laying around that I used last semester on just an oldschool vertical mill, so I probably have a few more drill bits than just the one listed below.

XmELH4q.png

Not entirely relevant to this all either, but I'll get to measure the stock a week sooner than I thought, which is good for me since April is my last month of classes before finals and summer break.

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Trying to figure out how exactly I'd go about making the toolpaths for that pattern, still pretty new to CAM stuff in any form (3d modeling less so).  The surface is a lot like that of a golfball, so I suppose that'd be an easy way to find tutorials.  I'm working in MasterCAM, but I know solidworks is easier to find tutorials on and is something you here are more familiar with.  May try to grab a copy when I swing by our CAD department.

 

2e22cf6b-b140-4b11-9574-e94035f54dea_zps

 

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My CAD skills pale in comparison to those of some others on this forum, but I did just this evening finally perfect my model of the Official SH Velocity Messiah and Sturgeon's Ballistic Waifu:

YRU1zgO.png

 

Doesn't look like much more than a couple of sketch->revolves (and it isn't), but it is more accurate than any previous model I had made of it, thanks to some new data I found on the russianet.

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1 hour ago, Sturgeon said:

My CAD skills pale in comparison to those of some others on this forum, but I did just this evening finally perfect my model of the Official SH Velocity Messiah and Sturgeon's Ballistic Waifu:

YRU1zgO.png

 

Doesn't look like much more than a couple of sketch->revolves (and it isn't), but it is more accurate than any previous model I had made of it, thanks to some new data I found on the russianet.

Do we get to have a description of this thing before we all set out to make a shrine to it? Or do we just grab the image you provided and start printing out pillows?

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1 hour ago, Xlucine said:

 

How does that thing work? Chambers ought to be close fitting, so shouldn't there be very little stretching of the case?

 

Chambers actually tend to be fairly loose, that groove thingie is basically extra slack in the case so that it doesn't have a breech when it its hits peak pressure of 65,000 PSI or whatever.

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2 hours ago, Xlucine said:

 

How does that thing work? Chambers ought to be close fitting, so shouldn't there be very little stretching of the case?

 

1 hour ago, Sturgeon said:

 

Chambers actually tend to be fairly loose, that groove thingie is basically extra slack in the case so that it doesn't have a breech when it its hits peak pressure of 65,000 PSI or whatever.

 

Chamber tightness and case fit in the chamber is a surprisingly complex topic.

For a manually-operated weapon the chamber can be much tighter around the case.  If the case gets a bit sticky, the person working the bolt just works it a bit harder.  A manually-operated weapon probably isn't going to fire enough rounds between cleaning sessions that the grunge will build up to the point where that's a problem anyhow.

Automatic weapons tend to have slightly looser chambers, and machine guns looser chambers still.  Machine gun chambers are so loose, in fact, that it's generally not a good idea to re-use brass that's been through a machine gun.  The chambers are so loose that there can be significant stretching and thinning of the case around the case head area.

In general though, cases get significantly distorted during firing.  I know that if I take a once-fired piece of brass and try to chamber it, it doesn't fit without some shoving.

 

6x49 needed that expansion groove because it was going to be quite high pressure, used in machine guns, headspaced on the shoulder, and made of mild steel, which isn't as stretchy as brass.  On a rimmed cartridge the shoulder basically becomes the expansion joint.  If it had been made of brass rather than steel, or if it operated at a more typical pressure, they probably could have gotten away without having the joint.

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Got a chance to go to the range today, 1897 worked great and I put a box of 25 through it.  Glad to have a gun now with cheap ammo that I don't need to obsess over keeping its finish in shape.  

 

There is a plug thing in the magazine, so I can only fit two shells in there and one in the chamber until that gets taken care of.

 

For the buttplate, the rubber is so old and crusty that it has more or less glued the screws in.  Going to have to knock a lot of that off to do what I need to do.

 

 

EDIT: Plug removed, holds 5 or 6 in the magazine now instead of 2.  Buttplate removed, crusty rubber sits at the bottom of my trashcan in 10,000 different pieces.  Screws were partly rusted and too wide to be removed without destroying the rubber.  Probably will buy replacements for those, as well as a magazine cap screw since I've realized my gun is missing one.

Edited by ApplesauceBandit
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I think I'll be dropping that golfball knurling idea.  Not experienced enough in Mastercam or Inventor to find out how to do that on a curved surface.  Unless I happen to find some tutorial by luck, I'll either keep it smooth or just do some horizontal lines.  Going to be busy for the weekend and need to machine this by the end of next week.  

 

I also realized how much of a PITA it would be to get the outline of the plate machined out perfectly to size, especially when the sides are slightly tapered in, made of rubber, and the plate itself is curved.  Going to make it a little oversize and file it down by hand afterwards.  Seems to be how it gets done for most other DIY buttplates.

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Hopefully going to get the plate milled out later this week, all that's left is finishing toolpaths.  And of course I find out at the last minute, but I came to realize that I could get something very close to that knurling through plunge roughing with a ball end mill.  I've not got enough time left in the course to do anything more fancy than knurling over the whole surface, so I'll leave it at that and pretty up the edges by hand.  Before I actually run the program on my aluminum, it's going on a piece of foam first. 

 

A little workflow thing to give an idea of how I plan to approach this:

 

Drill holes/counterbore → Mill out the profile that will rest against your shoulder → Add knurling over that entire surface → Machine the rest of the plate's countor, leaving a tab on either end → Have fun adding a fillet and perfecting the shape with dremel, files, and sandpaper.

 

 

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