Lathe screwcutting

[JasonBModerator @jasonb]

Niels, this video shows the cone arrangement and problems with it.

[Niels AbildgaardParticipant @nielsabildgaard33719]

Posted by JasonB on 11/03/2019 20:45:31:

Niels, this video shows the cone arrangement and problems with it.

Jason,Thank You for link.

It is as I thougth but it is nice to see it clearly.

Material looks cast Iron to me and I am using mild steel,and ca 18 mm thick

I have a piece of 16MnCr5 ready.

I do not use QCTP with tool overhang either so I will make one more for a friend.

[Robin GrahamParticipant @robingraham42208]

Plasma, I asked a similar question a while ago – I was getting ragged threads with carbide inserts. The advice I had was to go faster and/or get better inserts. I'm not sure that I've got to the bottom of it yet, but my experience was that better inserts made a big difference, and going faster helped too. I should have been more systematic, but when you hit on something that works for what you're doing it's hard to backtrack.

The 41×1.5 mm thread shewn in my earlier post was cut at 550 rpm with a 4.5mm relief. Buttock clenching territory I know, about 325 milliseconds (guess) to snap the half nuts open, but it can be done. And itb gave a smooth thread.

Robin

 

Edited By Robin Graham on 12/03/2019 00:35:45

[Martin ConnellyParticipant @martinconnelly55370]

This is slightly off topic but is connected to the video and the issue with the tool post loosening. I had a problem with the cam action, i felt it was not locking the way it should so I made a new plunger. I made it long enough for the cam action to go just over the peak of the lobe and so make it lock in place.

Martin C

[Chris TriceParticipant @christrice43267]

I've had a similar experience with an identical QCTP clone as featured in the video. I machined a few thou off the inner surface of the sprung Tee piece where it fits into the tool holder slot so the cam shaft rotated a little further. It makes it pull the holder into the post harder and the extra friction makes the cam less likely to loosen. Having said that, it mostly languishes in a box because both it and the associated holders don't fit the two genuine Dickson posts I have very well and virtually all the rest of my tooling is held in genuine Dickson holders.

Edited By Chris Trice on 12/03/2019 10:12:58

[PlasmaParticipant @plasma]

Hi again.

I set up this morning to cut an M10 x 1.0 fine thread in brass.

I set the change wheels, gear box and found the correct pick up point.

Speed was 292 rpm and the tool is the insert type I mentioned in earlier posts.

Top slide set parallel and infeed by cross slide.

Half nuts engaged at correct pick up point, cross slide backed out, manual return to start point, infeed to 0 plus new cut , and so on and so forth.

Worked perfectly and produced a thread which fits well in a tap cut nut.

Look like back to basics is the way forward.

Any thoughts on retracting tool holders?

Relieved of Barnsley

[Phil WhitleyParticipant @philwhitley94135]

Dear Relieved of Barnsley,

just another point which I don't think has been covered above. Is there a shear pin in your gear train? My colchester has one and it needs to be in good condition and a good fit or the thread gearing will be slightly out of sync with the rotation, check in your manual. I had a problem with mine on the Colchester student.

frustrated of Driffield

(two week enforced holiday after coming off second in a scrap with an angle grinder and wire cup brush!)

[HopperParticipant @hopper]

Posted by Plasma on 12/03/2019 11:18:27:…

…

Speed was 292 rpm and the tool is the insert type I mentioned in earlier posts. …

Good work! You should now have another practice go but knock the speed down to somewhere below 100 rpm to see if the insert still cuts ok. Much easier to screwcut up to a shoulder at low rpm if you can. 292 rpm is great on long jobs etc but could be sudden death at a shoulder if you are not quick off the mark every single time. No margin for error, or tripping over the cat.

[Neil WyattModerator @neilwyatt]

Not sure if anyone has mentioned this before, bu I note the problem was multi-start thread AND bad finish.

If you are a few passes through a thread and lose position the next cut is going to go in very deep so bad finish as well as a multi-start is to be expected.

Neil

[PlasmaParticipant @plasma]

Hi Neil yes you're right. I took heart care with the pick up point this try and everything was fine. Also not setting at an angle seemed to help with the maintaining of infeed.

The boxford has a pull out thread indicator to give different pick ups for different threads so that has to be carefully observed.

That's why I'm asking about retracting tools holders, is it worth the outlay to make the process a bit more streamlined? Certainly you tube shows one being used to good effect.

Cheers mick

[Anonymous]

Posted by Neil Wyatt on 12/03/2019 13:12:37:

Not sure if anyone has mentioned this before, bu I note the problem was multi-start thread AND bad finish.

Yes they did, on page 1. Either nobody read it, or they didn't pick up on the underlying logic, as there was no response.

Andrew

[SillyOldDufferModerator @sillyoldduffer]

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

[egaParticipant @ega]

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.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Plasma on 12/03/2019 11:18:27:

Any thoughts on retracting tool holders?

.

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.

[egaParticipant @ega]

Posted by Michael Gilligan on 12/03/2019 15:13:59:

Posted by Plasma on 12/03/2019 11:18:27:

Any thoughts on retracting tool holders?

.

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.

[Anonymous]

Posted by Plasma on 12/03/2019 11:18:27:

Any thoughts on retracting tool holders?

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

[Martin ConnellyParticipant @martinconnelly55370]

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

[PlasmaParticipant @plasma]

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

[HopperParticipant @hopper]

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.

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