Thursday, August 4, 2011

Building the Hi Point drum tower

Okay, so I've been working my butt off since I got back in town, and some of that work has been dedicated to building the necessary fixtures to make the Hi Point 995 carbine drum a reality.
If you're just now tuning in, I took a factory magazine and a aftermarket Promag magazine and hacked them up and welded them together in conjunction with a Suomi drum in hopes of getting a decent hi capacity magazine/drum assembly on the books for the carbine. I took the completed assembly to the range while I was out of town and put a couple hundred rounds through it with fairly good results. I did experience a few Failure To Eject issues, but this was a problem with the rifle, not the drum. Most rounds fed just fine, and I got several long strings of fire out of it before packing up and heading back. I decided that it was entirely too possible to make it function well enough to continue building, but the biggest hangup I had with the assembly was the hacked up factory magazine. The only solution to this is to just build my own magazines!
How is this done? In a variety of ways. One is to simply get a sheetmetal brake and bend up some sheetmetal to the shape needed, do some welding to hold the mag together, form the feed lips, and call it good.
Another option, and the option I'm trying first, is to machine a block of steel to the inner dimensions of a factory magazine, albeit a bit longer than the factory one, and use it as a buck/die in a hydraulic press to form sheetmetal with.  In addition to building this buck/die, I had to machine a female die to press the sheetmetal through. This sounds simple enough, but the front of the magazine has a nice radius to it, and I had to form the radius very carefully by making several light milling cuts at different angles before filing and sanding the die to shape.

To start with, I have a huge scrap piece of 3/4" thick steel  sitting in my garage that is just rusted all to hell. Since I needed the finished piece to be .42" wide and 1.19" deep, I cut a 7" long, 1.375" wide chunk off with my oxy-acetylene torch and clamped it in the mill vice and got to cutting.
I started by milling the torch cut edge flat after getting the steel as close to level and flat in the vise as possible. It really didn't matter if it was perfectly level, since I would be milling every edge eventually. This may sound haphazard, but it really just depends on the order in which you mill. Mill the top edge flat, rotate the part 180 degrees, mill the bottom edge flat, rotate the part 90 degrees, mill flat, rotate another 90 degrees and mill the last surface flat. This worked fine, and I wound up with a nice die with squared surfaces.

After getting all the edges squared off in relation to each other, I then milled the block down to the 1.19" depth I needed. This took a little bit because the only end mill I had big enough to cut a 3/4" width in one pass was pretty dull, so I had to take very light cuts and move the part very slowly under the cutter.
After I had a good depth, it was time to face the block down to the .42" width. For this, I used a big carbide tipped lathe tool in my fly cutter. I love using a fly cutter, because I can face off a very large (wide) surface in one pass. It took a while to shave the die down, since I'm limited to taking ~.01" deep cuts at a time, but I got it there eventually.

Here's the fly cutter finishing a pass at the die.


The finished fly-cut piece. You can see how rusted and jagged this block was before I machined it by looking at the end of the piece hanging out of the vise.

A comparison of the machined piece sitting on top of the heavily corroded steel stock I cut it off of.
 After I took this picture, I trimmed the ends off of the die and milled them square as well.

Now, with the width, depth, and length all where I wanted them, I had to form the radius on the nose of the die. This consisted of me doing several hours of set-up wherein I made very light cuts (.005") at several different angles down the length of the die. After the cuts were made, I used a medium-coarse file to do the initial blending, and finished it up with 60 grit sandpaper. These pictures aren't the best, but you get the idea. The radius turned out very nice, much better than the pictures show, of course.





The female die was just a block of 1.25" square steel I had laying around that I started milling out a slot in with a .5" end mill, right up to the point my end mill got dull, hung up on the part, and snapped. So, a new end mill is on order so I can finish the female die and press out a couple magazines on the hydraulic press at work. All I have to do is put a piece of greased sheetmetal between the two (the grease helps the metal slide between the dies and keeps it from galling or getting stuck to either die), lower the hydraulic ram into place, and press the sheetmetal through the female die. Once it's pressed through, I whack the remaining edges with a hammer and fold them over the back of the male die and trim to fit. I will then pull the dies apart and tack weld the back of the mag body. After the mag body is completely shaped, I will slide the male die out of the mag and then fully weld the back of the mag. From there, it's on to forming the feed lips of the mag, which is another operation all by itself.

This might seem like a lot of work versus just bending up a mag on a metal brake, but the key here is consistency. I want to build an identical part over and over, and it's pretty tough to do that on a small shop brake.With a hydraulic press and dies, I can make the exact same part over and over again.

So, I'm now sick of running the milling machine, and my arm is pretty worn out from cranking on the milling table handles several hundred times over the last couple days. I am going to take a couple days to rest and wait on an end mill to arrive.

More as I build it or break it!

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