how to cut a double lead thread on an engine lathe

[Granary]

How do I cut a double lead thread on an engine lathe? It's been years since I've done this and I'm not sure about what I should do.

[DareBee]

Index your part 180 degrees eg. use a faceplate to drive your lathe dog after cutting the first thread remove part and place dog in opposite slot.
If it is a normal thread pitch you can engage split nut half way between for the second thread. Eg if your thread requires engagement on 1 or 3 cut the second lead by engaging on 2 or 4.

[HuFlungDung]

Surely, it can be done.

The easiest method would require that your compound be turned parallel with the lathe Z axis. This means your plunges would be straight in at 90 degrees.

So you can cut one thread with the compound dial set at zero, then crank it along one thread pitch to cut the other one. Remember the pitch is half the lead in this case, so if you are cutting a double start 8 pitch thread, then each thread's lead is 4 threads/inch, but the difference in the start position for each thread would only be .125"

In actual practise, if you don't like the straight in plunge, you can jockey the compound back and forth a wee bit for alternate cuts, to lessen the chip load per cut. Keep track of the backlash and always end with the compound right on the zero mark, so you know that you haven't lost position.

The other method involves studying the gear train at the end of the lathe. Hopefully, you will find the drive gear off the spindle or the driven gear on the quickchange has an even number of teeth. So you mark one tooth when done the first thread. Slide the gear off the shaft, and turn either the spindle or the quickchange enough to turn exactly one half of a turn, so the marked tooth will now go on in the opposite position to what it did before. This should give you the new start position for the second thread.

Edit: Oh yeah, lathe dogs and faceplates, forgot about that, LoL

[Geof]

Originally Posted by Granary How do I cut a double lead thread on an engine lathe? It's been years since I've done this and I'm not sure about what I should do.

It has been years since I did this as well but I remember this is what the chasing dial is for. For a single start thread depending on the ratio between the lead of your leadscrew and the lead of your thread you engage the halfnut always at the same number on the dial, just at the even numbers or anywhere; which it is depends on whether the ratio is odd, even or not a whole number.

Anyway you can find out by trial; take a light cut at 1 on the dial then keep your tool clear of the work and engage the halfnut at the other numbers. You will soon find which pair of numbers give you the two start thread.

Of course if your leadscrew lead is the same as the lead you are cutting then you have to do the work piece indexing or gear indexing method. Or find a friend with a CNC lathe.

[MrWild]

Originally Posted by Geof It has been years since I did this as well but I remember this is what the chasing dial is for.

BINGO. Lots of ways to do it, but the chasing dial is probably the easiest. Never thought of the face plate/lathe dog way, but that'd work also if your lathe doesn't have a chasing dial.

[DareBee]

I know I havn't used a face plate for turning between centres since highschool and I have never cut a multi start thread (I always buy them as T-rod), but I did remember that tidbit of theory.
The only time I use a faceplate is with really odd parts that get clamped to it such as you would do to a mill table.

Do any of you people use a face plate for between centre turning?

I always turn a centre and drive on the chuck jaw.