Cutting internal threads on the lathe

In this exercise we’ll cut some internal threads. Internal threading is very much like external threading, but with two additional challenges:

  1. Getting over your fear of cutting internal threads.
  2. Remembering to dial the cross-slide in the opposite direction to back out of the cut at the end.

As you can see, that’s only one ‘real’ challenge, and it’s not a difficult one to overcome.

Required Tools:

1.A bench grinder with course and fine stones
2.A sharpening stone
3.A lathe with change gears or a quick- change gear box
4.A 60° threading gauge (‘fishtail’ gauge)
5.A thread pitch gauge.

Required Materials

1.A threading tool – these are numerous in design and any internal tool will work, they may be:
1.Ground from a single piece
2.A High Speed Steel (HSS) bit, appropriately ground and placed in a boring bar
3.A carbide insert type. (Not recommended for your first attempts, unless you don’t mind ruining $25 inserts when you mess up…)
2.A piece of metal with a hole in it to bore


Let’s start by preparing a tool. If you’re using a manufactured tool, you can skip down to lathe setup. We’ll detail creating a tool to fit in a boring bar. We’ll use a commercial boring bar, but don’t be afraid to make one, they’re no more complex than a piece of bar with a hole cross-drilled through it and and a center hole drilled and tapped to hold the cutter in place. Generally speaking you want to use the largest boring bar that will fit into the hole to be threaded – but remember you must leave room for the threading tip as well! If you make a bar, it should be cross-drilled for a specific size toolbit, select a size that is wide enough to cut your full thread. One nice thing about grinding internal threading tools is that they generally use a smaller tool bit, so grinding is faster.

Gather your boring bar, tool bit and fishtail gauge. Your tool bit will be quite a bit too long, but leave it that way for now – it’ll give you something to hold on to while grinding.

Fishtail gauge, boring bar, and untrimmed HSS tool blank

Set your grinder tool rest at about a 5° angle, which will impart side clearance, and holding the cutter at about 30° grind in from both sides until you reach a point. Check the point in the fishtail gage and correct the angle until it is exactly 60°.

Checking for an exact 60°

When you have achieved a perfect 60°, it’s time to add the front clearance. If this were an external cutting tool, the front clearance produced by grinding the angle would probably be sufficient, but this tool will go inside a hole, so some additional front clearance is necessary at the base of the tool (not all the way to the top). This ‘compound front clearance’ will allow threading inside a smaller hole. How much depends on how tight the hole to be threaded is. You just want to make sure that the bottom of the bit doesn’t contact the work.

Additional front clearance an internal tool

The additional front clearance is necessary because the part gets closer to the cutter above and below the cutting tip – the opposite of what is the case when cutting external threads.  The following illustration shows good (left) and poor (right) clearance.

Appropriate internal clearance (L) and inadequate clearance (R)

Now cut the bit to a reasonable length (just enough that the tip sticks out of the boring bar fully and the back is flush, or a little proud. Install the tool in the boring bar so it points back toward you when it’s in the lathe. (Your mom probably told you never to point cutting tools at yourself, so make sure she’s locked out of the shop before continuing.)

Completed internal threading tool, mounting in toolholder and read to cut.

Lathe setup

Set the compound to 29.5°. That is, if 0° is pointing straight away from you, the compound gets rotated until the far end is 29.5° away from that centerline and pointed toward the headstock. The important thing here is that it’s a little less than 30*, use the graduations provided on your lathe if it has them – it doesn’t have to be super accurate.

Lathe compound set to 29.5° for internal threading

Run the compound forward, so you have enough room to back into the cut.

Now setup the workpiece. You may be threading an existing hole (in which case you’ll need to center it) or drilling a hole in order to thread. If you drill, centering is taken care of. You may need to use a standard boring bar to open the hole to the desired size. If you don’t know what hole size to start with, consult a reference like Machinery’s Handbook. Even if you’re cutting a non-standard diameter, there are tables that tell you what diameters are appropriate for a given thread engagement. If you are cutting a standard size, you want your hole to be as big as the “minor diameter” to start with. Remember, unlike external threading you’re making the thread bigger with each pass.

Give some thought to setting up the workpiece. There are two basic internal threads, through and blind. The through thread is the easiest, because it’s not critical where the cutter stops,  anytime after you’ve cut through the work is fine. If your lathe is big enough to ‘swing’ it, cutting an AR-15 buffer tower is one of the easiest internal threads you can do.

Cutting the threads on an AR-15 buffer tower. This is a very simple internal thread to cut.

When cutting through threads, be sure there is room to stop before the boring bar crashes into the back of the lathe chuck. Blind threads are those that stop somewhere inside the hole, and it’s more critical that you stop where you should, to avoid crashing the cutter into a ‘step’ if the diameter is smaller after the thread ends.

The internal threads on this silencer tube are an example of a blind thread.

Cut a thread relief the depth of the thread at the back of the hole. If the part allows it, make it 0.1” long to give you plenty of time to stop the lathe.

With the workpiece set up and ready to thread, install the boring bar in the lathe. Make sure it extends deep enough to cut the thread depth, but not much deeper. (The less the boring bar sticks out, the more rigid the setup).  Set the cutter height at or just a bit below center. Now square the cutter to the workpiece. The fishtail gauge can be used for this, though I sometimes cheat and just square the boring bar to the work.

Aligning the cutter the work, using a fishtail gauge. In this case I'm aligning to the outside of the piece to be threaded, although the threads will be cut internally.

Now it’s pretty much like normal threading. Run the boring bar inside the bore, dial the cross-slide back until the cutter touches and zero it. Double check that there’s enough clearance at the bottom of the cutter. Run the cutter back to the thread relief, or stopping point and zero the X axis. Now bring the cutter out, dial the cross-slide to zero.

Set up the lathe to cut your desired thread pitch and engage the threading dial. At the end of each cut you will want to simultaneously disengage the half-nut and back the cutter out of the cut. To do this have one hand on the half-nut and one hand on the cross-slide dial. Now, if you’ve cut a lot of external threads your hand knows which way to spin the cross-slide for an external thread, but this is backwards and those habits can mean the death of the cutter (especially if it’s carbide) or the workpiece. You must remember to run the cross-slide forward. A tip that works for me is to draw an arrow on the cross-slide dial indicating which way to turn, make sure your hand is ‘cocked’ to turn in that direction before engaging the half-nut with each pass.

Drawing a temporary arrow on the cross-slide dial is a good reminder to people with strong external threading habits.

That’s it, we can start threading. With the cross-slide and compound at zero, watch the threading dial, engaging the half-nut at the appropriate place and take a scratch cut. Verify the pitch is right and you’re ready to go. Use the compound to apply feed (remember you’re backing it up rather than running it forward) a little at a time. Keep the threads well oiled for a smooth cut. Finishing is no different than external threads: pay attention to the crest of the thread and keep making passes as long as there’s a flat on the crest. As you get close to the point where the flat goes away, start testing your mating piece after each pass. When you’re getting close, take a spring pass after each cut to eliminate the spring of the boring bar. If you don’t have a mating piece you will need either a plug gage (standard mating part), internal thread micrometer, or you’ll have to cut the internal part to match.

Once you can cut internal threads you can make any number of things, and you won’t have to buy large, expensive taps for those projects where internal threads are needed.


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One Response to Cutting internal threads on the lathe

  1. Vusi says:

    Hi I’m trying to get a standard chart to guide on cutting internal and external threads sizes. For intense when cutting internal thread for M30 external thread.
    Please clarify for me the purpose of setting the compound slide to an angle of 29.5 degrees.

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