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How the Pros Finish AR-15 Lower Receivers for Profit

Nov 14, 2016   //   by Bob Warfield   //   Blog, CNC Projects, Cool, Manufacturing, Techniques  //  5 Comments

AR-15 Lower Receiver CAD Fusion360

AR-15 Lower Receiver in Fusion360…

We have a lot of readers who are interested in firearms, and I’m a shooter myself, so I thought I’d pass along this fascinating video about how a pro shop goes from a forged 80% lower to a finished AR-15 Lower Receiver:

This video was done by Palmetto State Defense, LLC, and it shows a number of operations as well as their fixtures for finishing 80% forgings on their VMC.

I love to watch and try to learn things from videos like this.  On this particular video, I saw a number of things that I found interesting.  Here are my thoughts as I watched it, stream of consciousness style:

  • In-process probing–cool!  Before any machining starts, the probe comes in and identifies the exact location of all the key features on the forgings.  This no doubt makes the process more accurate, more efficient, saves time on setup too,and likely accomplishes some mistake-proofing too (what the Lean Manufacturing crowd calls “Poka Yoke“).
  • Always interested in fixture design.  There are 4 receivers here including one in a machining vise, two laid down on either side on a plate fixture, and one in a 4th axis.  They say that unless you use every square inch of your machine’s travel for each setup, you’re leaving money on the table.  Whoever designed this setup must agree.
  • What’s with the rubber flap guarding the toolchanger?  Looks like a user add-on to keep chips and other contaminants out of the tools.
  • About 2:36’ish it fetches a twist drill which is used to make two pilots on the threaded buffer hole in the 4th axis.  It then interpolates the buffer hole with an endmill and also makes a hole below it flat-bottomed.  Wondering whether it was worth the cost of the tool change to do this or if it’d been faster just to use the end mill for everything?  I am guessing the issue was the smaller hole and how slowly you’d have to plunge without the pilot, but I don’t know that for sure.
  • At various points I wondered about order of operations.  Could the position of tooling be shuffled in the changer to make toolchanges faster?  As it moved around on the fixture, could the order of ops be shuffled to reduce travel times?  I expected the 4 lowers to be in various stages of completeness, but for this video, they all seemed virgin.  How do they get finished as they move through  a multi-setup process?  Does the next step involve shuffling them to different positions in the fixtures?  Is that done on another machine?  Do they have 4 machines so everything proceeds in a logical flow?
  • About 9:43 looks like one end of the mag well was chain drilled.  Why not chain drill both ends at this time?  Is it because the other ends has different radii in the corners perhaps?  Also, not that the milling of the deep magazine well is done from both the top and the bottom.  This limits the required stickout of the endmill and minimizes tool deflection challenges–nice!
  • At 10:23, we do a toolchange to pick up a broach.  Yes!  I am fascinated by these broaching ops.  The magazine well on the AR-15 requires broaching out the sharp corners.  Being able to do broaching on your CNC Mill or Lathe can be a valuable time saver.  I’ve written about broaching on CNC machines before, and CNC Broach Tools has sent CNCCookbook a broach to play around with, so I’ll have more to say about them in the future.  Without a tool like that, you would have to send the receiver out to a broaching service, do it by hand on a press, or for some shops, EDM it out.  I count something like 14 or 15 passes with the broach on each corner, so it’s taking a fairly light depth of cut.  The broaching op is the final op in the video, and it took about 1/3 of the total cycle time.

Please don’t take my comments as critical of this machining process.  It’s a very sophisticated setup, and I’m just curious about it.  I would love to hear your own thoughts about it in the comments.

Being an entrepreneur, I’m always curious about the economics of an operation like this.  Palmetto sells the forged 80% lowers that are the starting point for $79, presumably at a profit.  They sell the finished lowers for $149, presumably for an even larger profit.

The question in my mind is, “What was the cost to machine the lowers?”  In the end, it’s a 15 minute cycle time to finish 4 receivers, so it’d be a function of this machine’s hourly operating cost.

We can use G-Wizard Estimator’s Machine Hourly Cost Calculator to estimate an hourly cost.  I don’t know which model Haas is shown, but a base VF-3 lists new for $66,000 on the Haas site.  Here is the Machine Hourly Cost Calculator from G-Wizard Estimator set up to estimate hourly costs for that machine:

hourlycosthaasvf3

Hourly Cost Estimate for Haas VF3, about $45/hour…

I get an Hourly Cost Estimate of about $45/Hour by doing the following:

  • Fire up G-Wizard Estimator.
  • Go to Setup and selected the Haas VF-3 machine profile.
  • Clicked the “Calculate” button next to that machine’s hourly cost.
  • Plugged in the base price from the Haas web site.  Note:  You probably would want to up that with some initial tooling.  Also, I left out financing to keep it simple.
  • Went over to eBay and looked for Haas VF-3’s.  There was a 10-year-old listed for $45000, so I put down a 10 year life and trade-in value of $40,000.
  • Put down $5000/year in consumables for coolant, electricity, and such.
  • Put down $30/hour for an operator.  You might pay more or less, they might work different hours or shifts, and you might have more than 20% downtime if you shop isn’t busy or if the machine has problems.
  • Tacked on minimum 30% markup.

So, the machining cost for these lowers is about $12 by that estimate.  That’s probably low as most of the assumptions I’ve made in the Calculator were probably not conservative enough and we have no allowance for setup time either.  But, even if that hourly cost is too low, there’s pretty good margin on these when you consider the $79 forged blank they started with had some profit built-in too.

One last thing that I know someone will bring up if I don’t–the conventional wisdom is that you make AR-15 lowers on a Horizontal Machining Center for best margins.  Since it seems clear these folks are making money doing this on a VMC, it just underscores they did a great job setting up this job on their VMC.

If you’d like to try G-Wizard Estimator, it’s free to folks with a valid G-Wizard Calculator subscription or trial while in Beta Test.  Just head over to the GW Estimator product page and sign up, but be sure you’ve either got a free trial or subscription to G-Wizard Calculator first.

 

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How the Pros Finish AR-15 Lower Receivers for Profit
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5 Comments

  • Oh, that’s some good stuff! This is something near and dear to me. Agreed, the most efficient way to run forgings is on an HMC, but these guys are doing a great job running one piece flow on their VMC.

    A couple of items I noticed (and I know I’ve already forgotten several others), each fixture station is one operation of a four step process:
    1) center bottom = buffer tower plane (semi critical dimension) and front plane – allows consistent locating for step 2.
    2) center top = top plane of receiver and front/rear pin holes – these are critical dimensions, mag release slot and first op of bolt catch slot
    3) 4th axis = was well implemented, and the drill op before boring the buffer tower probably avoids launching out a slug of unmachined stock.
    4) vise fixture = last step with drilling, machining and broaching the mag well. this was well done. I agree with the question about the step over on the broach, it looked awfully small.

    A lot of thought and programming time went into this setup, and big kudos is due them. It’s easy for us to Monday morning QB the setup. That said I think there are still some efficiencies to be found in the operation saving even more time. Once each station is filled it looks to me that every time the green button is pushed, they will have a complete part coming off the machine.

    Thanks for the post Bob. Oh, and don’t machine that model in the top picture unless you eliminate the happy hole above the safety hole.

    All the best,
    Josh

    • “Happy Hole”, eh?

      LOL. Grabbed the first CAD model I could find on GrabCAD and loaded it into Fusion 360 for the screen shot without realizing the auto sear pin hole was there.

      Good eye!

  • As long as you’re going to CNC it, why not start with a 0%? I’ve seen a 0% casting for under $25 that’s pretty much already the right shape, unlike some I’ve seen where they want even more for just a block of 7075. The one I saw even had a pdf of the machining sequence and really detailed step by step if you want to do it on manual machines.

  • This process shown in the video produces ONE 80% lower per 15 minute cycle not FOUR. All four operations per 15 minute cycle are required to complete one part. So, 60 minutes machine time to get four completed parts.

  • The video is that of a $10 raw forging being made into a 80% receiver. And due to using a VMC they are making one complete item per hour! That is pretty slow and inefficient. I helped a friend out in the business several years back, he would machine 72 lower on a 4 sided tombstone using an Mori HMC. This took all night and required 32 tool changes – he also made the lowers from billet.

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