Lathe screwcutting

Well my post of the 7th concentrated on the multi-start issue as well. No point in worrying about finish until the cutter is landing in the right place, but I didn't say so outright!

I wouldn't throw rocks - apart from writers failing to communicate clearly, it's quite easy for readers to miss important points by fixating on other aspects of the problem.

Another occupational hazard of living.

Dave

Straight in or angled topslide?

I have little experience of screw-cutting inserts but assume that the manufacturer has considered this question and intends them to be used either in one way or the other (probably straight in).

Some time ago on another thread I saw Jason B's suggestion of using an internal insert tool at the back of the work with the lathe running in reverse and cutting away from the headstock. As it happened I had recently bought a very cheap holder and insert and tried this with some success. My immediate reservation was that the insert was cutting in the opposite direction to that intended by the manufacturer and I was surprised that the result was an acceptable thread. The need to get the tool to the far side of the work caused me to angle the topslide which I would not otherwise have done.

When using my own HSS bits and holders my preference is for the angled topslide; having control over the helix and cutting angles of the bit is then advantageous.

.

As mentioned on previous threads:

Here is the design for the one I like: **LINK**

https://archive.org/stream/PopularMechanics1976/Popular%20Mechanics-04-1976#page/n105/mode/2up

MichaelG.

It seems very enterprising of MC to publish his design in a US magazine although I imagine PM did the Americanising for him.

He called his design the "swing clear" and it looks as though it would also have "swung up" on the return pass.

A flip up toolholder (similar idea) has been on my white board for many years. But since I haven't got around to making one thus far I suspect I never will. One problem is they don't work too well on internal threads!

Andrew

If you set the top slide to 29 degrees do be sure to measure it from the right starting point. It's a common beginner's error to set it at 61 degrees. Remember that zero is when the cross slide lines up with the top slide and 90 degrees is when the cross slide lines up with the bed (the normal position for most work). Setting it to 61 degrees will cause roughness and what looks like multi start threads.

Martin C

Thanks Martin , I will be giving that method a go when I nail down simple screw cutting.

At present I am looking at the Chinese multifix type threading tools which retract to make life easier. Just need to make sure I can get hold of the Komet cutting inserts they use. Typically I'm getting more help from China than from a firm ten miles away who took over Komet.

A chap on here called Joseph nocci posted about getting the same but no updates as to its usefulness.

Will keep you all posted how I get on.

Mick

Or you can simply leave the top slide in the "normal" position parallel to the bed and when you add cut depth with the cross slide, advance the top slide by half that amount. This moves the tool point at approximately 29 degrees. It has the advantage that you can use the topslide to manipulate the tool position to clean up one or other of the thread flanks after final depth is reached. You can even take the toolbit out for resharpening halfway through a job and then use the top slide to line it back up with the half-cut thread again.

I wouldn't think a retracting cross slide or swing up tool holder worth the effort to make unless, like MC, you were doing an awful lot of screwcutting and time was of the essence. But that's just my personal preference. Winding out the cross slide handle smartly will do the job for me. Just needs a bit of practice.