Chatter Marks on Single Point Threads

[mksj]

I would check the gibs on the back and the front of the saddle that hold the saddle down to the bed, they can loosen and cause some movement in the carriage. As indicated the half-nut may be worn. Keep your compound and cutter holder as close to the work as possible, may be a rigidity issue. I do thread with only the cross slide, but you will tend to get a better finish with the compound if your lathe is not rigid. It varies. The DOC varies, but I usually start out at around 0.01" for the first few passes, then 0.005" x 2 passes, 0.003" and typically one spring pass at 0.001" or less. Do not take repeated passes with shallow cuts, as the cutter will chatter similar to what you have. Could also be the insert you are using, I pretty much use Carmex (Iscar) or Mitsubishi (VP15TF) slay down type , there is also some geometry differences for OD and ID inserts/holders. I would double check the height of the cutter tip so it is dead on as to height. Try some heavy cutting oil. I typically thread steel at around 120-150 RPM, Aluminum and softer materials at 250-300 RPM. I use an electronic proximity stop system, it allows threading speeds of up to 450 RPM and maintain a repeat stopping accuracy of around 0.0002". I also do not need to disengage the half-nut. When I threaded without the electronic stop, about 150 RPM was the limit of my reflexes (on a good day).

Carmex inserts,

 

[BaronJ]

Thanks for the nice complement. I wind back by hand. I have a lathe handle that is made from a plastic washing machine drum pulley.
If I'm running the lathe, usually in reverse when threading, I have a detachable handle that I insert into a hole drilled in one of the spokes, that I use to wind back. Removing the handle prevents out of balance vibration when running at high speed.

That thread in Nylon 60 Was cut in five passes, about 65 thou in total.

 

[pacifica]

Half-Nut on lead screw not staying engaged fully? Possibly.
I had that problem on my 14x40 Birmingham.
The other thing I o that I picked up off of videos, is that I installed an extension spring to keep tension on my lead screw.
It moves ever so slightly at the tail stock end.

could you show a quick diagram or photo, thks

 

[middle.road]

The half-nuts were becoming slightly disengaged while under power so to say.
Half-Nuts were not engaging / locking down fully.
I re-worked the whole assembly, de-burred, polished and filed everything to get 'em to engage tightly.

 

[Liljoebrshooter]

I remember a thread on another site where the guy had bought a brand new lathe and was having issues similar to this.
He tried many different things. He got several parts from the manufacturer to try.
He ended up putting a rubber shim between the motor and the mounting bracket.
He said it made a huge difference.

 

[rgray]

The return motion is done under power and I don't want to miss the stopping point and crash into the shoulder. I suppose I could thread at high speed then reverse at low speed, but that would be quite annoying to change the backgear constantly.

Do you not lose your timing doing that.? I have by mistake made my first cut at the speed the lathe happened to be set at. Not thinking I shifted to a
lower gear. It then cuts a different path.
I remember a thread on here with someone having trouble with double tracking his threads.
Everyone chimed in trying to help and it came down to he was shifting the gear box and losing the timing.

 

[catsparadise]

If you lose the timing by changing speed that must be down to where the leadscrew picks up its drive from. On my Harrison, the motor drives the speed gearbox which drives the spindle, and the spindle drives a geartrain to the leadscrew, so even if you change speed between cuts, the relationship between the spindle and the leadscrew is maintained.

 

[macardoso]

Do you not lose your timing doing that.? I have by mistake made my first cut at the speed the lathe happened to be set at. Not thinking I shifted to a
lower gear. It then cuts a different path.
I remember a thread on here with someone having trouble with double tracking his threads.
Everyone chimed in trying to help and it came down to he was shifting the gear box and losing the timing.

My lathe is a belt drive, so the gear train starts at the spindle. I can change any of the belts or back gears in between the motor and the spindle without issue. I guess that might be different on a gear head lathe.

 

[chips&more]

I had a similar problem if not the same. I got new half nuts and made a new guide bushing (the bushing that was a few inches away from the nuts that supports the feed screw "on my lathe"). And when I did all that, the problem went away.

 

[middle.road]

Yeah, that changing speeds during threading is a 'gotcha' on some of the gear-head styles.
I mangled a couple of runs before I figured out what I was doing.
When I switched to doing 7/8"-32's with a manual wheel, the first time out I had it in low gear, and that's a lot of hard spinning let me tell you.
Always want it in the highest gear and speed.
Wish it would allow switching speeds without losing track.