Making a chamber reamer

I need a chamber reamer to cut the chamber for the DI .45 carbine, so now’s a good time to demonstrate making one. The first step is to acquire chamber drawings. These can be found in a number of places. You may have access to SAAMI drawings, drawings from a reamer manufacturer, or drawings from quite a few different books for this particularly old and mature cartridge.

You will also need:

  1. Some method of indexing the cutter by sixths. (Fourths might be acceptable)
  2. A swiveling vise, or means of holding the work at an angle in the X-Y plane.
  3. Some O-1 (oil hardening) drill rod, of sufficient diameter for your intended caliber.
  4. A torch. Oxy Acetylene is great, MAPP is sufficient, and in a pinch a couple of propane torches would probably work.
  5. A magnet, on a stick or rod.
  6. Some oil. Used motor oil is fine.
  7. A drill press is desirable.

Simplify the drawings. There are usually a lot of dimensions which can be used for verifying your work, but which are not necessary to make the reamer. I distill these down to the pertinent lengths, and starting and finishing diameters of each section.

Cut a piece of 0-1 (Oil hardening drill rod) to a length sufficient for your reamer, leaving enough for a shank, and chuck it up in the lathe.

There are four primary operations in constructing a chamber reamer:

  1. Form the chamber shape
  2. cut the flutes
  3. harden and temper
  4. final sharpening

To start, cut a shank. Depending on how you will go about chambering, this may be omitted. I will be using my reamer holder, which accepts a 0.435 shank, so I turn a couple of inches to that diameter.

Cutting a shank for the reamer.

 

 

The hardest part is perhaps planning how to form the shape. It’s easy to cut a taper and in the process lose a reference needed for the next step. The .45 ACP reamer is pretty simple, and you could cut either taper (the body, or the leade) first. (The leade is the entry into the rifling. It’s ahead of the chamber proper, but provides necessary clearance for the bullet). I’m going to cut the major body taper first. I will use the compound to set the angle for the body taper. This is a very small angle. Based on the drawings I have, the diameter at the breechface is nominally 0.481″ and the diameter at the case neck, 0.910″ away, is 0.474″. That’s only 0.007″ in a distance of just over 0.910″. Of course we want a radius, so it’s half that: 0.0035″. Don’t worry about calculating the angle – your compound scale isn’t accurate enough anyway. I do this with the help of a toolpost indicator holder I made.

A toolpost indicator can be used for dialing in a precise angle on the compound.

 

 

Using this indicator, I can adjust the compound to provide precisely the angle desired. To start with, place the compound in the rough position. This is pointing almost directly at the headstock. With both compound screws loose, push the compound away from you (toward the workpiece) without swiveling it. This will take up most of the ‘slack’ between the compound bolt that is away from you, and its hole. This will make it work as a more ideal pivot point as we adjust. Lightly snug the rear compound bolt, leaving the front bolt loose.

In this case we’re cutting a reamer with a cutting length of about one inch. Back the compound off, at least that much so it has room to travel as far forward as necessary.

The length of the main body taper on a 45 is approximately 0.910″. We will run the compound forward 0.900″ while observing the indicator dial. We will then adjust the compound and repeat, until the desired taper (0.0035″ in 0.910″) is achieved. The adjustments are very small and it is handy to have a 1-2 pound lead sinker to use as a tapping tool to make adjustments. This step has the potential to take quite a few iterations depending on how experienced you are with this sort of adjustment. Remember that you are dialing in on a straight shank, so the indicator should read negative 0.0035″. That is, as the compound is advanced, the indicator plunger is relieved, rather than compressed. I have omitted pictures here because the angle is so slight as to be imperceptible to the naked eye. Once set, tighten the compound in position. It is normal for the indicator to move at this point, but the movement is usually symetrical and a quick check after tightening should verify the same angle.

With the compound properly adjusted, and backed up so that it once again has room to go forward the length of the cutting surface, we are ready to put in a cutter. Use a very sharp cutter with a fine, yet slightly radiused tip. Light cuts are best. Because this is hardening material, we want to keep the heat down.

Face the end of the drill rod, and center drill. Put the cutter at the tip of the drill rod, zero the cross slide and move in 0.01″ to start a cut. Advance the cutter using the compound. Maintain a pace advancing the compound that is slow enough to leave a good finish.

After each pass, retract the cross-slide, back the compound to its start position, and run the cross-slide forward to zero. Repeat.

Two passes at 0.010″, starting from a piece of 1/2″ stock put this taper somewhere very close to completion. As we get close, we want to stop and check precisely. To do so, we need to measure the taper, at a precise point (or two). To do this, I coat the piece in layout fluid (Dykem). Once the fluid is dry, I scribe lines where the mouth of the cartridge will be  and where the breech will nominally be (0.920″). I then measure the diameter at those points. Precision is important so I will use a micrometer, rather than calipers. Because I am measuring a taper, I will carefully place the micrometer anvil edge on the scribed line because the face will not lie flat on a tapered part.

The taper marked at two points for precise measurement.

Having some continuing taper past the nominal breechface is a good thing as the actual breechface distance will be determined by our bolt.

 

Leaving room for the leade, cut a square shoulder. The radius at the shoulder should be 0.474″

Once again, adjust the compound to set the angle for the leade. This time we can use the compound as the taper is 3.5*. Cut the leade taper without changing the previously created shoulder.

The form for the leade completed.

Drill and tap a hole in the tip of the reamer for a shoulder bolt. This will be used to mount the pilot. Remember, the shoulder bolt will be larger than the drilled hole, and the pilot will have to go around it and be no bigger than the bore. I tapped 1/4″-28 in this instance.

Polish the piece with some scotch brite. The piece is now ready to remove from the lathe and move to the mill so flutes can be cut.

 

The completed reamer form.

 

 

To cut flutes, a means of indexing is needed. I will use a 5C collet block. The other thing that is needed is a plan. CAD is best for this, although it can certainly be done by hand. You need to calculate the location of the cuts you’ll take to make the flutes. By cutting just shy of the centerline in the Z axis (0.01″ less) you get a negative rake, which will result in a better surface finish. Also, by varying, by a few thousandths, the depth you stagger the flutes which will reduce chatter. I arbitrarily chose 0.375″ as the amount of reamer ‘core’ that I wanted to remain solid, and sketched up the following:

Cut depth to create six flutes.

 

 

Set the collet block up in the mill vise. It will need to be positioned identically each time it is rotated, so a workstop is a good idea.

Collet block and workstop set up in the mill vise, ready to cut flutes.

 

 

Setup the mill zeros. Zero against the side of the form in ‘Y’, the top of the form in ‘Z’ and where you want the flutes to end in ‘X’. Note that the reamer blank is sticking out the left side of the vise, and I will be cutting flutes on the side closest to me. This will result in a normal reamer. If you get the orientation wrong, you’ll have to run your lathe in reverse for your reamer to cut.

When you are ready to cut, take shallow cuts as the reamer blank is largely unsupported in this setup. (I would set this up between centers if it were a long or thin reamer). I took 0.02″ cuts at a time, adjusting the knee up, then moving into Y position, and then X.

The first flute is cut.

 

 

After each flute, rotate the reamer 60° (or one side in a 6-sided collet block) and repeat until all six flutes are cut.

 

All six flutes cut.

With the flutes cut, it’s starting to look like a reamer. The next step is cutting the relief. The back of each flute must be relieved so that only a narrow band is at full diameter. The amount of this relief is easy to overdo. It only needs to be enough not to rub. Too much relief will weaken the cutting surface.

Flip the reamer (and collet block, if that is your indexing tool) over so the reamer sticks out the right side of the vise – we can now cut on the back of each flute. To get my initial depth, I marked the top of a single flute with DyKem (layout fluid). It will be necessary to rotate the reamer slightly to get the desired relief angle. I eye-balled it, so I don’t have a specific angle for you.

The top of the first flute is marked for better visibility when cutting the relief.

 

 

Carefully touch off on the back side of the flute and make a light pass. The total amount to remove for the relief is small, so proceed cautiously.

Increase cutting depth until only 0.01″ or so remains at full diameter.

The first flute is relieved.

 

 

This is the relief depth, and you can just keep your settings and you rotate and repeat with each flute. Note that the part of the flute that cuts the leade will be wider – this is fine.

Relief cuts on all six flutes.

 

 

With the reamer still poking out the right side of the vise, rotate the reamer until the cutting surface of the top flute is vertical. You can eye-ball this with a square.

Aligning the cutting edge vertically.

 

 

Now swivel the vise 5° so that the rear of the vise is a bit to the left of center. This sets up to cut the relief at the front of the flutes. Set your depth by touching the bottom of the mill on the back of the flute that points toward you.

Cut a relief, using the Y axis for motion. Again, leave ~0.01″ flat at the tip.

Cutting front relief.

At this point the basic form is complete and we are ready to harden and temper the reamer. The ideal way to do this is in a drill press with a very slow low speed.

 

Chuck the reamer shank in the drill chuck. Gather up a can of oil (used motor oil is fine) a magnet on a non-flammable / non-meltable stick, and your torch(es). I recommend a standard magnet, rather than one of the rare-earth ‘super magnets’. We will heat the reamer until it is non magnetic, but it’s awkward if you test and then have difficulty removing the magnet from the (hot) reamer.

Materials for heat treating the reamer. Ignore the heat paste, that didn't work out...

 

 

With the drill press turning at the lowest speed possible, heat the reamer until it glows orange/red. You will know it’s hot enough when a magnet no longer sticks to it. Note that there is no point in testing with the magnet until the piece has a good glow. Once the magnet has no reaction (zero) to the steel, it is hot enough. Try to hold it at this heat for a minute or more. Then, plunge the reamer into the oil bath.

Just about ready. The camera flash hides the true glow of the reamer, it was a nice glowing red/orange.

Quenching. Be prepared for smoke and flame.

The oil will smoke quite a bit, and perhaps even catch fire. Be prepared for it. Let the reamer sit in the oil, turning continuously, until it is cool enough to touch. The reamer is now ‘glass hard’ – that is, it is hard, but it is also brittle.

 

Clean the reamer off. If you have used old motor oil, your reamer will have a black patina. For the next step it is necessary to polish down to bare metal on two opposing flutes. Get them nice and shiny, without rouding the cutting edges.

Two flutes are polished so the development of color can be seen during tempering.

 

 

Setup to heat again, but this time you are barely going to heat the metal. Stop when there is a yellow / brown color forming on the metal (do not heat too aggressively or you will pass this point). Machinists call this color ‘straw’ and it’s the right amount of temper (relief from hardening) for tools that are going to cut steel.  When straw is reached, quench again.

The reamer is now complete, except for the pilot.

We’ll see how it cuts when we ream the chamber for the Direct Impingement .45 ACP AR.

If you’d like to learn more about heat treating and tempering steels, Hardening, Tempering and Heat Treatment (Workshop Practice) is my favorite treatise on the subject.

 

This entry was posted in Shop work. Bookmark the permalink.

One Response to Making a chamber reamer

  1. Topos says:

    Admiration and appreciation for your sterling and inspirational tutorial.
    Your methodology is transferrable to many other projects.

    As an ancient machinist told me in my youth, watch and learn from the
    working machinist. You will never find the innovative solutions in a book.

    Standing ovation to you.

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>