Completing an “80%” 1911 frame.

A sort of odd byproduct of bizarre firearms laws is the “80% kit”. Just to be on the smart side, you should understand that “80%” has no meaning beyond “some of the work has been done for you”. From a legal perspective, a piece of metal either is a firearm, or isn’t. The U.S. government (BATFE) doesn’t recognize anything as being “80% complete” – they only acknowledge that some piece of metal has had insufficient work done to it to be considered a firearm.

In this article, I’ll go over the steps necessary to complete an “80%” 1911 frame from Tactical Machining. I purchased two of these kits and they seem to be a nicely machined start to a standard 1911. Of course it’s hard to know the true quality of the thing until it’s either been fully measured (not gonna happen!) or built.

The work to do looks to involve the following:
1. Cut the slide rails.
2. Cut the barrel seat.
3. Drill the hammer and sear/disconnector pin holes (and countersink).
4. Install the ejector.
5. Install the spring plunger tube.
6. A surprise bit of machining…

The ejector and spring plunger tube aren’t really part of the frame, but since they’re more or less permanent attachments, and each is relevant to this discussion, I’ll include them here.

This article will cover how I go about completing one of these, using a mill and none of the purpose-built fixtures sold for finishing a 1911.

I’ll start with steps one and two, as they can both be done on the mill in the same setup (position). Because this is a great project for someone relatively new to building firearms, I’ll try to include a little extra detail. In any manner of machining there are multiple ways to do something. This is only one, and may not be the best for you if your equipment is different. It’s the best I could come up with, given the tools I have.

Mount an angle plate to the mill table. This will provide a vertical surface on which to attach the frame-to-be. I dial the angle plate in just as I would align the vise, so that it’s face is closely parallel to the mill’s X-axis.

Clamp the piece to the face of the angle plate, using another plate to spread the load across the workpiece to avoid crushing any of its thin cross-sections. Be sure to clamp on parts of the plate that have part of the work under them. I’m going to use a level to get the piece close before refining the position. That only works if the mill table is level, so check that first.

I then use a Dial Test Indicator (DTI) and move back and forth across the piece until the top surface is level. As it gets close, I snug the clamps tighter. I like to use a two pound lead sinker to tap stuff around in clamps or the vise for alignment. It provides a dead blow and won’t mar most workpieces. Once it’s aligned and the clamps are tight, I measure yet again to make sure nothing has moved while clamping.

Next, I will find the sides of the piece, and therefore know the centerline.

My workpiece came in at 0.754″ wide, which is a neat 0.003″ larger than spec – just enough extra for fitting, if desired. So my centerline is at 0.377″.

The rail starts at 0.1″ below the top of the frame and is nominally 0.119″ ‘tall’. A good sanity check is that the cutter should be basically even with the dustcover at its lowest position. To start, I will find the top of the workpiece, with the bottom of the cutter. I use a piece of cigarette rolling paper. Being about 0.001″ thick it’s about as close as you can get without touching the workpiece. I’ll wet it with a little WD-40, so it’s not sticking up, and then slowly raise the mill table until the paper is moved by the cutter.

Here the paper has just kicked out from the cutter. I’ll move the work sideways, not disturbing the height adjustment, then move up a final 0.001″ to compensate for the paper thickness and zero the Z-axis (knee) dial. That established the position of the bottom of the cutter to the top of the work. I’m now going to dial on the distance to the _bottom_ of the rail. which is nominal 0.219″. Here’s where we can make the “sanity check” I mentioned before.

The next dimension to find is where the edge of the cutter is with respect to the rails-to-be. I’ve already established that this side of the frame is 0.754″ directly under the center of the spindle,so I just need to move away one-half the diameter of the cutter. My keyseat cutter has a 3/8″ diameter cutting surface, so I need to add half of that (3/16″ or 0.1875″) to the center position. (0.754″ + 0.1875″ = 0.9415″) I move the Y-axis to 0.9415″ and again make a sanity check, seeing that the cutter is right at the surface to be cut.

As I start cutting the rail, the previous number (0.9415″) will get smaller, so I should figure out the stopping point. On the drawing, the width from the inside of the rails should be 0.628″. Taking the full width of the piece (0.754″) and subtracting the width between the rails (0.628″) gives 0.126″ so each rail needs to be cut to half that depth, or 0.063″ (basically 1/16″). Since I’m starting at 0.9415, I’ll want to cut until my Y axis reads (0.9415″ – 0.063″) 0.8785″.

Because the X-axis cut goes all the way through left-to-right, I don’t have to establish a position in that direction, so we’re ready to cut! I’ll apply some cutting oil, advance the cutter about 0.005″ for a first look and conventional-mill the length. That looks pretty good.

You might have noticed that the pin hole for the ejector isn’t centered in the slot, but don’t worry! This cutter isn’t cutting the full height of the slot, so the slot is not as tall is it will be when It’s done.

From here it’s just a matter of backing the cutter out, returning to our starting position, adding some depth to the cut and repeating the process until we’ve reached the target depth of 0.063″.

Now it’s time to open up the slot to full height. I’ll back out back to 0.9415″, where I started last time, and, since my cutter is 0.1″ tall, I’ll move up 0.019″ and repeat the process to get a full-height slot.

Note that the ejector roll pin now looks correct.

Now I’ll prep to do the same thing on the backside. Since the cutter is already at the high side of the slot, I’ll cut that first, then move down 0.019″ to finish. But first, I need to calculate the starting and ending points for depth of cut on the backside. The backside is at 0.000″, so at -0.1875″ the cutter is even with the back.

Having “completed” the rails, I’ll make some measurements to verify that everything came out as expected. Any time you break a setup, or change tools, you lose some information. In this case, I’d lose the Z height reference if I were to swap tools. You could find it again, but every time you do, you introduce tolerance. So, I check everything I can before removing the key-seat cutter.

Once I’ve determined that there is no more to do with the keyseat cutter, I switch to an 18mm center-cutting ball endmill. This is how I’ll cut the barrel seat. In combing a half-dozen “blueprints” for the 1911, I was unable to ascertain with any great certainty how deep to cut. I _was_ able to ascertain that there are some completely incorrect “blueprints” out there… Having looked at multiple prints and measuring a few complete 1911s, I decided on a depth of 0.075″. The 18mm ball endmill that I’m using is a little off the spec’d diameter, butI think it should work. I set the mill to 0.377 (my centerline) and, again, used a cigarette paper to find the Z-height. From there, it’s just a matter of cutting ‘down’ 0.75″ across the portion of the workpiece from the mag well forward.

On the first pass, you can check that you’re centered, then it’s an easy few swipes to complete.

If you’ve mounted your frame with foresight, you can now try on a slide to see if it fits. Before doing so, a good cleaning, and a light pass with a file along the rails until they feel smooth and burr-free should be performed. Here’s my frame, with the ‘slide’ (actually a .22 conversion kit) that I will use with it.

The last major operation expected is to drill the hammer and sear/disconnector holes. Both holes are referenced from the slide-stop hole, with their respective positions being:

Hammer pin: (2.973″, 0.016″) 0.158″ dia. with a 0.020″ countersink.
Sear pin: (2.602″, -0.252″) 0.110″ dia. with a 0.020″ countersink.

I used a pretty cheesy fixture for this, using the top deck of the frame for a reference against the fixed vise jaw and a piece of furring strip on the bottom to handle the irregular surface at the bottom of the handle. This was sufficient for drilling, but a better setup would probably be to clamp the frame down on a flat plate, leaving the area to be drilled hanging out so as to be able to observe the drill going through.

I dialed in on the take-down pin hole. I used a Co-Ax indicator, but any kind of hole locating technique should work. I set the X and Y axis for zero (0,0) and since we’re drilling, there’s no need to account for cutter radii.

I first moved to the sear/disconnector pin location and spotted it with a spotting drill after visually verifying that the location appeared correct.

I then drilled the hole just undersize, then crept up on the final diameter using wire-gauge (“number”) drills. If you try to drill to size in one pass, your drill will probably cut oversize and ruin the workpiece.

Finally, I finished the hole with a 60* countersink. I’m using a “combination drill and countersink” for the operation. Be sure to use one with a drill portion that is smaller than the hole you are countersinking. I used this because my other countersinks, and most common countersinks, are 82* but these are 60* as called for by the prints.

To cut the countersink, I run the cutter down until it is in contact with the work. Then I zero the knee on the mill, back it off a little (so I’m not starting in contact with the work), then crank the knee up 0.020″. A quick test fit of the pin looks great.

I repeated the same process with the hammer pin, this time using metric drills, with similar excellent results.

In theory, that’s all the machining that should be required, but I found one more bit, which I’ll discuss in a moment. But first, adding the two parts that are pretty much permanent attachments to the frame: the ejector and the spring plunger tube.

I didn’t take any pictures of installing the ejector, because it’s very simple – insert the ejector and pin it in place with a 1/16″ roll pin. I did want to mention though, that this pin is actually much easier to install if you do so before machining the rails. It’s easy to mash up the head of a roll pin if you’re careless, and even easier if it’s in the trough of the slide rail. Install it before machining, and you’ll have a perfectly flush pin end, every time.

The spring plunger tube, I have much to say about. As a high-volume 1911 shooter, this part is responsible for the one serious mechanical malfunction I have experienced: having the plunger tube come un-staked. In my case, the rear (closest to the thumb safety) came loose and the tube cocked out a little bit, locking the safety in the ‘safe’ position. Since then, I do plunger tubes a little differently than most. The generally accepted method of install is to drop the plunger tube in place (making sure the right end is pointing forward (hint: look at the square take-down slot on the frame, the plunger tube shouldn’t overlap it.) and then stake the ‘legs’ in place using some manner of staking tool.

I like to solder mine in place, then stake it. This belt-and-suspenders approach has never failed me.

I start by setting the plunger tube in place, then tracing the outline with a scribe.

I then sand the inscribed area as well as the bottom of the spring plunger tube so the solder will adhere. (Sorry the sanded portion on the frame didn’t photograph well…)
Next, I degreased both surfaced with acetone, then clamped them together with a small (heat-proof) spring clip.

I then liberally coat the surface with soot (carbon). I used an oxy-acetylene torch, with just an acetylene flame, but you could also use a candle, or a little burning masking tape.

The reason for the soot is that it will prevent solder from flowing where we do not want it. Removing the spring plunger tube with a pair of tweezers, I have a nice, crisp clean area.

Looks like I missed a picture, but I carefully dabbed solder flux in the area to be soldered, using the wood end of a cotton swab, then cut off a small piece of solder (about 3/32″ long) and stuck it in place, letting the flux hold it. I carefully replaced the plunger tube and re-clamped with the spring clamp. I used Brownell’s “Hi-Force 44” because it’s a nice low-temp solder and I don’t want to change the temper of the frame. A bit of heat with a MAPP torch accompanied by a slight jostling of the plunger tube and the whole thing collapses neatly into place.

And here’s a better view after wiping off most of the carbon. The plunger tube is soldered permanently in place and there is no solder visible.

Finally, I’ll stake it in place as is normally the case. I use a kit composed of a filler rod to prevent the tube from crushing, a ‘pad’ to distribute pressure across the plunger tube, and a pair of modified vise-grips with a point in the moving jaw to peen the hollow ends of the plunger tube legs.

It should be mentioned that there is a disadvantage to my method: Solder does not do well in a hot blue solution. The solution may eat away at the solder, and the solder may contaminate the bluing solution, resulting in a poor finish. I have no intention of bluing this frame, but if I did, I would either forego soldering, or do it (carefully!) after bluing…

And a surprise…

While test-fitting individual pieces I discovered that the thumb safety could not be installed in the bare frame. The problem is the frame thickness in the area where the safety stud inserts. The wall thickness must be thin enough to clear the notch on the safety.

Examining the frame, a small step can be seen in the area. No such step exists on the working 1911 frame with which I compared it. measuring across the gap, the 80% frame is thicker. Finally, trying a safety from a working 1911, it did not fit either. This is pretty overwhelming evidence that the problem is with the frame.

Standard advice when fitting two pieces is “Modify the cheaper part”, which in this case would be the safety. And it would be possible to modify the safety, but that would be a point of fitment that I have never seen, meaning this frame would always require a non-standard part. Given this consideration, I decided to modify the frame instead. This might seem like a difficult cut, but with a tiny keyseat cutter (mine is 1/4″cutter dia, 0.035″ thick) this should be a pretty easy operation.

I clamped the frame to a 1911-specific bench block and held it in the vise. For this cut, no external alignment is necessary as I’m just going to ‘clean out’ all of the area I can reach through the hole with my cutter. The two important things about clamping for this cut are to leave enough room for the mill spindle to move through the cut without hitting the clamp, and also leave enough room to test-fit the safety.

Next I aligned the cutter with the ‘center’ of the tri-lobular safety cut, finding the location where a 1/4″ diameter cutter could pass through without interference. I called this (0,0).

Next I’ll find the top of the frame with respect to the bottom of the tool, using a piece of cigarette rolling paper, as previously described.

I want a finished thickness of 0.067″ (which I arrived at by measuring my ‘good’ frame.) So I’ll move the knee up 0.067″ plus the thickness of the cutter (0.035″) or 0.102″. (Note, that’s “knee up”, which is equivalent to “cutter down”.)

Finally, I’ll move the shank of the cutter into either end of the tri-lobular cut and a toward the front of the frame.

Retract the cutter and test the safety… (Note safety in previously-unachievable ‘off’ position.)

It works! But just barely. I might cut a thou. or two thinner next time.

That’s it for the work on the frame. Quickly throwing it together (well, and fitting the thumb safety), I put 10 rounds through it without a hitch, indicating that there’s nothing seriously wrong. The thumb safety is a bit tight, so I may work on that a little more at some point, but probably not until I’ve put a few hundred rounds through it to break it in, and make sure there’s nothing else I want to modify before putting a finish on it.

Happy shooting!


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