Thursday, November 14, 2013

Machine tools, going automated through the power of Computer Numeric Control

Been away from this for a while mainly due to school and work. I haven't had much spare time, and what time I've had has been spent on other things.

I've read a couple of posts from my good friend The Silicone Graybeard over the last few months that originally spawned this, but haven't had time to finish it.
I've had a lot of folks pick my brain along the way about what kind of tools they need to build their own firearms, and I've often said the same thing to a lot of them, "It's not what tools are in your toolbox, it's what you do with the ones you've got in conjunction with the most important tool of all, the one in your head. Anything beyond that just makes the job faster."
I bought a large lathe and a medium-sized mill instead of relying on lots of files and hard labor, but back when I started this blog, I didn't have a mill, I barely had a lathe, and I didn't have much experience. What I did have was the desire to build a .50 BMG rifle inexpensively, which I did do most of without the large mill and lathe.
Now that I've been doing this for a couple years (I think I started blogging about the .50 in late 2010), I've had the opportunity to pick up a number of different tools and improve some of the ones I already have. Here of late, I decided to upgrade my milling machine to CNC control. This will do the same thing that most any milling machine can do, but it will do it under computer direction and control. It also means that I can design something on my PC, then send it to the computer and let it make the part while I work on other things.
I looked at the cost of doing this, and figured it would be worth the trouble.

For those of you interested, the electronic hardware necessary to do this was about 150 dollars per axis, and the mill has three axis, X, Y, and Z. X is the longest traveling axis on my machine, and has about a foot and a half of travel, good enough to do large projects. Y is the back-and-forth axis, and has about eight inches of total travel. Z is the up-and-down axis that the cutting tool is attached to. This axis has about 5 inches of travel.
Both X and Y axis have acme thread screws that permit movement when the mill is new from the factory. Acme thread is very useful for high torque and high force applications, in part because the thread has a square appearance, rather than angled like the typical nut and bolt threads that you see.
Better than the Acme threads are what's known as "ball screws" and "ball nuts". Instead of having threads that mate to other threads, the "ball screw" has threads that are squared like Acme thread, but have a radius at the base of the thread leade. The "ball nuts" contain a number of small individual ball bearings, and the bearings will circulate inside the nut itself. These bearings fit in the radius of the screw leade, and allow for smooth travel, with very little backlash. I changed my mill over to ball screws for the CNC application. The upside of ball screws, as previously mentioned, is smooth travel and very little resistance to movement. The downside is that the screws offer little resistance to movement, so unless something is used to lock the screws in place, such as a engaged motor, you can literally push the mill table around by hand.

The Z axis is now an odd concoction that uses a rotating ball nut inside a timing pulley to lift the quill up and down. It works, though it is as ugly as a burlap sack of rusty nails.

I have a CAD program for designing my parts, and CAM program for taking the drawings from a simple image to a toolpath via a genre of computer language known as "G code". This code tells the computer what you want it to do, from cutting speeds, rapid movement speeds, cutter spindle speed, turning on coolant pumps, changing tools, etc.

ViaCad is what I draw and model my parts in, and MeshCam is what I use for converting those drawings to tool paths.

We have all at one time or another, seen a AR 15 lower receiver. On this blog, I have previously outlined how to manually machine a 0% lower forging into a finished receiver. Well, I now am able to clamp a forging into the mill and have the machine make all the critical measurements and cuts.

Cutting the top deck on the lower forging.

Cutting the magazine release slot.

Cutting the pistol grip area
Cutting out the magazine well

Cutting the bolt hold open slot on the top deck.

Cutting the FCG pocket

Cutting the front take down pin area.

It took a little while to get to this point. I've broken a few end mills and ruined a couple of forgings getting this far, but the end result is very nice to behold.

So, that's a sample of what a CNC milling machine can do. It has made life much easier when it comes to building simple things like AR15 lowers, and has also made it easier to do much more complex things as well.


  1. Very cool!

    It's strangely entertaining to watch your tools making stuff for you, isn't it?

    1. Yes. Indeed it is.
      It's also largely satisfying, as much time as I put into the machine. 10 minutes here, an hour there, etc.

    2. Oh, yeah. I don't want to think of all the hours and ruined pieces.

      I guess education is always expensive, but still cheaper than stupidity.

    3. Good judgement comes from experience, and experience comes from bad judgement.

  2. Hah! Bad judgement and a pile of ruined parts.

    BTDT, but not with gun stuff.

    My Dad was a Tool and Die Maker, and then went on to be the Bridgeport rep in the area I grew up in. I've seen the evolution of CNC, starting from punched tape made God-knows-how, and now to equipment that lets you design something on your PC, and send it straight to the shop floor, as you've done.

    Good to see guys like you actually using the technology in thw ay that you do.

  3. Yeah, the pile of broken tooling and ruined parts sitting here turns my stomach when I see it. =(

    I've been tinkering with CNCs for quite a while, but on a random basis. This mill is my latest conversion, the last was a cnc plasma cutter.

  4. Replies
    1. Well, when you come build you some rifles, you'll get to tinker with it to your heart's desire.

  5. TRE you never fail to impress.
    You are a machinist at heart, that went to school to get an engineering degree.
    I don't think you know how unique an individual you are.
    Believe me, I deal with highly skooled engineers every day.
    Tre' cool stuff TRE.

    1. Thank you for the kind words, KX.
      I'm not much of a machinist, just a hobbyist with some bad habits, ADD, and some power tools.

      Glad you enjoyed the machinery, though.

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