Threading on metal lathe.

Basically start off with bits of scrap material and just practice cutting single lead threads.

Cut your OD to 1/2″ or whatever you like, and then install your outside thread cutter on your tool post.

You want the thread cutter edge to be just below the centerline of the part. JUST barely below. Shown here:

above: It’s hard to photograph, but the cutting edge is just below the centerline of the part.

below: Here I’m about a 1/4″ to the left of the face of the part, and I make a scratch pass or touch cut, to “set zero” on my lathe. I’m not actually setting anything b/c I don’t have a DRO, but I use the numbers on my cross slide wheel to know where I’m at.

below: Here’s my cross slide wheel and my “zero” set from the picture above. Yours will be different to this:

above: This arrow below shows where my scratch pass depth was, and where my first thread cut pass will be.

below: I’ll advance the cross slide one arrow for each cutting pass, until the threads are done.

The touch cut from above will be the very left edge of whatever thread I’m cutting. If this part is going to be a barrel, and that thread the outside thread that will engage with the cap, then that is as far to the left as those barrel threads will be cut.

Now back your threading tool OUT away from your part with the cross slide wheel, and move the whole carriage to the right, so the cutting tool on the right side of your part.

You’ll need to engage your lathe’s thread cutting gears, and set them to whatever pitch or TPI you like, but I would start off with single lead threading while you’re learning.

Move the carriage wheel until you can engage the thread nut, and give it a gentle wiggle left and right to be sure the thread nut is engaged all the way. I always start a thread pass 1/8″ or so to the right of my part so that as I advance the carriage and cut my threads, any slop is worked out of the carriage before I actually cut the threads. If you try to start the thread cutter AT the face each time, chances are the slop in your machine will show itself and your thread passes won’t be on top of each other, and will be a mess. (all machines have some slop in them)

Once your thread cutter is in place, thread nut engaged, turn the cross slide wheel until you get back to your “zero” that you made on your scratch pass.

Turn the chuck by hand slowly (NO POWER) until your first thread is formed, and stop at the scratch pass you made.

-Some lathes are stiff, and a little harder to turn, than others. I have my machine set at 1500 rpms (fastest speed), and is the easiest to turn the chuck at this setting. If the machine was set at 72 rpms (lowest speed) I wouldn’t be able to advance the chuck by hand.

Turn the cross slide so the cutter is back away from the part.

Disengage the thread nut.

Move the carriage to the right of your part.

Engage the thread nut (be sure your cutter is not right at the face, but to the right of the face of your part).

Move the cross slide in to the depth you want to cut. I like to cut about .005-.007″ depth per pass so the thread forms cleanly and not all jaggedy.

Turn chuck by hand until the thread cutting pass is done, stopping at the scratch pass.

Repeat all that over and over until you have nice pointy threads. Don’t cut TOO deep because you’ll ruin your outside diameter and your threads will be very sloppy.

It takes practice. Have fun.

threading in Pathpilot:

on a half inch barrel, your external thread tool will start cutting at .510″ Ø, and cut down to .460″ Ø if you’re making 36 tpi (threads per inch).

If you’re cutting a single lead thread, like inside a section (nib unit) or inside a barrel (section) then you don’t need to worry about pitch. Type in whatever thread you want to cut, for example 36, in the threads box in Pathpilot, and it’ll do that for you. Of course your Z start will be zero, and your Z end will be however far to the left you want it to be. Mine is -.250″ b/c I like the way 1/4″ threads look.

Now, if you’re cutting triple start, you’ll need to find the pitch of 36tpi (.0277″). Your first thread operation cut will start at zero, and the second will start at zero plus the pitch. So second thread op Z starts at .0277″, and then add the pitch again for the third thread op, and your Z starts at .0544″.

If you’re cutting 36 tpi but triple start, you won’t type in 36 in the thread box in Pathpilot. Divide 36 by 3 and get 12. You type in 12. So you’re basically cutting 3 separate 12 tpi threads, but only as deep as you would cut a normal 36 tpi.

So you’ll have 3 thread ops at 12 tpi, each one .0277″ apart from the other one, so that you get a nice clean triple start, 36 tpi thread on your barrel.

and when you cut internal threads, don’t think for a second that making the inside major thread diameter .500″ will work. The cap and barrel will be so tight as to be scrap. Always build in a little bit of slop. I like to build the slop where you won’t see it, so inside the cap. So instead of .500″ inside major thread diameter, I go .505″ or .510″. I can’t remember. Play with it and you’ll get sorted out.

and don’t forget in pathpilot, you need to include a lead length of at least .100″, so when your thread tool actually starts to do the cutting operation, it’s .100″ away from the face of the part.

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