Cutting ACME and other square-ish Threads

In this exercise we will make a 2-10 Acme threaded rod. This is a practical size for use in a number of projects, so the result may be useful to you.  The general instructions can be applied to any square-ish thread, several of which are used on various firearms.

Materials needed:

  1. ACME (29°) Threading gauge.
  2. Standard thread pitch gauge.
  3. HSS tool blank I recommend 5/16” for the thread shown, but size will vary for different threads.
  4. Piece of steel 1/2” diameter and 8” long.

Tools required:

  1. Lathe with threading gears or quick-change gearbox.
  2. Bench grinder with course and fine stones.
  3. Sharpening stone

Grinding the external threading cutter

The cutter is as simple as a standard 60° cutter.  The following assumes that you use a quick change toolpost or other toolpost that holds the bit horizontally.  If you have a lantern / rocker type toolpost you need to adjust the angles.  (The angles listed are those you want to present to the work, the top of the cutter should be horizontal.

Start by setting the tool rest on the grinder to ~4° and at a height such that the center of the wheel meets the center of the work.

Assuming your grinder is mounted level, you can set the toolrest angle by placing an angle block (4° block shown) and adjusting until the top is level.

Holding the HSS tool bit at 14.5°, grind in from both sides.  Once you have the right shape, but before you come to a point, start checking the angle frequently so you can make small corrections as needed to get your perfect 29°   You want a precise fit with the threading gage – not just close.  As you grind the sides, the angle of the tool rest should give you 3*-5* side clearance on each side.  If you try to put this clearance in after achieving the perfect angle you risk ruining the angle.

When checking in the gauge, hold the gauge and cutter in front of a white card to better see any gaps between the gauge and the cutter.

Checking the cutter in the acme thread gauge

Grinding Notes:  As always, grinding is a slow, deliberate process.  Take your time and let the grinder do the work.  You should quench the piece whenever it starts to get hot to the touch.  Putting excessive pressure on the workpiece only loads up the grinding wheel and produces more heat, requiring more quenching and possibly some loss of temper at the cutting edge.  If you’re burning your fingers, or discoloring the tool bit, you’re pressing too hard!  Just enough pressure to keep the surface fully in contact with the grinding wheel is all that’s required, and it really doesn’t increase the time it will take to finish a cutter.  A simple jig can be made to ride against one edge of the toolrest, or in a slot in the rest to help set an angle.  Don’t rely on such a jig for final finishing though.

Now set the tool rest at ~10° and grind the tip back, slowly, until it just fits in the spot marked “10” on the Acme gage.  (This is the pitch, Threads Per Inch – TPI – that we’ll be cutting.) Note that a different thread pitch would require a different grind.

Checking the tip of the acme cutter in the pitch-specific slot on the acme threading gauge

With a sharpening stone, hone the top of the tool.  We’re now ready to set up in the lathe.

Lathe setup
Put a piece of 1/2” stock in the chuck, center it, face the end and countersink for tailstock support.  Extend the piece 7” so you can thread 6” and put the tailstock in the end.

Place the tool in a toolholder and level it.  Adjust the height so it’s on, or just a tiny bit below center.

Set the compound to 14°.  If zero is the compound aligned with the cross slide, we want to rotate the compound 14° so the end points toward the headstock.  BTW, that’s not a typo – we want 14°, not 14.5°.  We do this so the cutter cuts predominately on one edge, reducing the forces that would exist if you cut at 14.5°.

The compound set at 14° for cutting an acme thread

Square the toolbit to the workpiece – your gage will be useful here.

Setting the cutter perpendicular to the work, using the acme gauge.

Now it’s pretty much normal threading.  Run the compound back so there’s plenty of forward motion available and zero it.  Run the cross-slide forward until it just touches the work and zero it.  Make a mark, or zero your X axis dial so you know where you want to stop (I’m cutting mine 6” long).

The depth of an Acme thread is half the pitch + 0.010”.  This is a 10TPI thread, or 0.1” pitch, so the depth should be 0.05” + 0.01” or 0.060”.    With the compound set at 14°,  a little trig (cos 14°) tells us that our infeed on the tool will be 0.970” for every inch we advance the compound.  We want to go 0.060”, but I’ll leave 0.005” for finishing passes, so to get to a depth of 0.055” we want to go until the compound dial indicates 0.057”.  At that point we’ll want to use the cross-slide to go another 0.005” (though not all at once!).

Any time you’re cutting with a form tool it’s a good idea to take ‘spring passes’ – that means that every few cuts you take another cut without adding any feed, just to cut the material that has been springing away from the cutter.  On Acme and square threads I make a spring cut every other pass, because the form is pretty severe.

Set the lathe for 10TPI and the slowest speed you can go while cutting that pitch.  Start the lathe up, engage the half-nut when the dial is at the right spot, and with the lathe cross-slide at zero, make a “scratch pass” – just enough to leave a mark.  Use your standard thread pitch gage to check that the pitch is correct.

Checking the pitch with a standard threading gauge

At the end of the pass, retract the cross slide, return to the beginning of the cut, return the cross-slide to zero and apply light infeed to the compound.   The amount will vary with the lathe and the diameter of the workpiece, but it will be small, perhaps surprisingly.  If you take too much you will get chatter. Keep the workpiece well oiled.

The acme thread, about 35% complete

Repeat until the desired initial depth (0.055” in our case, or 0.057” on the compound dial).  Now hone the top of the cutter with a sharpening stone and feed in, 0.001” at a time using the cross-slide – this fixes the tiny error we introduced by not having the compound at precisely half the included angle of the thread (14.5°).  Although the infeed is tiny, these are heavy cuts, so don’t try to feed too much!  Test your nut against the thread if you have one after each pass, stopping when the fit is smooth, but not loose.

You should now have a ‘perfect’ acme thread.

The completed acme thread

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