Gallery, Projects and General > How to's

Single point threading on the lathe

<< < (7/9) > >>

NickG:
I need to read through this thread more carefully!  :bang: :scratch:

I don't understand this not disengaging the half nuts thing? Judging by my questions you can probably tell I haven't cut many threads. I thought if I engaged the half nuts to start cutting on 1 of the numbers on the thread dial indicator, I could withdraw the cross slide at the end of the thread disengage the half nuts and wind the carriage back, put the extra cut on then re-engage the half nuts on the number I started on?

Nick

Darren:
I don't understand why the need to retract the tool......the old books I have read suggest this is only done to save time when cutting long threads.

It seems to me that the fact that the tool may take a little off when reversing is neither here or there as you're about to take more off with the next cut anyway...

spuddevans:

--- Quote from: Darren on August 29, 2009, 11:43:37 AM ---I don't understand why the need to retract the tool......the old books I have read suggest this is only done to save time when cutting long threads.

It seems to me that the fact that the tool may take a little off when reversing is neither here or there as you're about to take more off with the next cut anyway...

--- End quote ---

Maybe in a really good quality lathe ( I dont have one, so I'm just guessing ) there is very little backlash on the leadscrew. But on my little C2 lathe I can (with the motor turned off) engage the half-nuts and then move the carriage about 1mm in either direction. Also when the lathe is running with the halfnuts engaged and I then reverse the motor, the chuck will rotate anything between 1/4 and a whole turn (depending on what change gears are set up) before the carriage will actually move in the reverse direction.

All of that would add up to a ruined thread. It can also shorten the edge-life of your threading tool as it is rubbing the work but in reverse, this is pretty fatal for carbide tools, they really dont like to be run backwards against the work. ( go on, just ask me how I found that out  :bang: )


Tim

No1_sonuk:
Leaving the half nuts engaged also means you don't have to mess about working out which number on the indicator to use for which thread, or if you can use it at all.
I'm seriously considering taking the thread indicator off my lathe.  I don't use it, and it's getting in the way.

My lathe has electronic speed control.  That means I can run it forward slowly for the threading, then retract and run it in reverse at higher speed to get the carriage back to the start.

I just need practice getting the angled compound thing right.  :scratch:

HENNEGANOL:
My thanks to Arnold and Bogstandard for pointing out my obvious mistakes, at my age I should know better.

Referring to my comments regarding cutting metric threads on my Myford and the requirement to leave the the half nut engaged at the end of the cut and reverse the saddle.  These are the instructions given by Myford when using the metric thread cutting kit on a lathe with an imperial lead screw.

Another tip that I was taught was to put a chalk mark on the chuck and another chalk mark on the lead screw and when the two marks coincided with their respective reference points to engage the half nut.  

Bearing in mind that there is a large difference between a Turner and a person who can use lathe! I must confess that I tend to rough out the thread in the lathe and then finish it either using a die, die nut or a hand chaser if they are available.  Thereby saving time and ensuring that a reasonable thread results.

Having re-read Geo Thomas'es "The Model Engineers Workshop Manual" I would reccommend that anyone who requires further information with regard to screw cutting, reads chapter 12 which contains some very useful information.

Navigation

[0] Message Index

[#] Next page

[*] Previous page