Single point threading problem

[basalt]

I have a small Craftex 7 X 8 and when I am threading I hold my thumb down on the carriage handwheel to take all the backlash out thereby forcing the leadscrew to do all the work. This came about when I noticed that when cutting small threads the cutter didn't always go back into the same place. It is due to the fact that there is not very much mass in the carriage and it tends to float. I trust that with all the excellent advice from the other guys your threading problems are over.

 

[No-ladder]

Thought I would bring you guys up to date.
I usaully count my good days a opposed to the bad ones, normally I am sick more often than not. But enough with the sob story. Tonight was a good one. The first I felt like going to the shop, since first post.
I made a extended dead center, had a small enough shank that I had room to back up to start my cut.
I used another HSS that I had success with before, basically just a narrower profile but still at 60 degrees
Checked and rechecked tool height
Used thread cutting oil every pass
Checked gibs, about as smooth as it gets, nice firm twist to get things moving. So thought I would give it a go.

Made about three passes, and missed again, everything following skim up that point. So here is my process, turn in cross slide to touch.
Zero both compound and cross slide, turn compound in 3 thou (I not sure what the math says at 29 deg) catch any number, (i have been using only the odd numbers)
lock in half-nuts
run to my out, just prior to shoulder.
Release half nuts
Back out cross slide 50 thou
Crank carriage back withthe wheel. (is this where I am messing things up?)
Cross slide back in 50 thou
Repeat the above steps

 

[eightball]

An indicator on cross slide would tell you if your getting all your backlash out . Some chase dials just don't repeat either. (Read my previous post) I would have to assume that your not getting all the backlash out. Try backing out further after each pass

 

[DMS]

I have a feeling you have your compound turned 30 degrees from parallel, rather than 30 degress from perpendicular. I made the same mistake on my first attempt at threading (and fortunately, I had watched a video where the guy did the same thing). The result looked like a barbed tubing fitting. Point the compound so that it is perpendicular to the bed axis, then rotate 30 degrees from there.

Also, as far as never opening the halfnut when cutting metric threads, it's a bit more than that. You can open the half nut when cutting metric threads if it is a metric lathe (metric leadscrew). You can open the half nut when cutting inch threads on an inch lathe (inch leadscrew). If your cutting metric on inch, or inch on metric (at best, they are both an approximation), don't open the halfnut.

I am also having similar problems with threading.
I have checked the points as mentioned above and things improved but I am still having problems (maybe?)
I'm cutting 8mm metric threads so have set the change gears as 30-120-127-30 with the gear selector in 7.
According to the charts this should be giving me an 8mm x 1mm thread.
Doing all the usual things, not disengaging the lead screw, backing out and running in reverse, advancing the compound etc.
The result was an appalling thread. ie. looking at the V's it was like this \|\|\|
I use the fish tail gauge to set the bit at 90' to the shaft so decided to forgo advancing the compound and just use the cross slide backing out-reversing-advancing in a little more so the bit cuts on both sides at the same time.
This produced what appeared to be a reasonable result but was a bit too tight so I cut another pass that unfortunately cut a little too much so the thread is a little loose but still usable. (just)
(this is only practice before the real thing)
What I would like to know is how come advancing the 29.5 degree set compound produced such an awful thread but advancing straight in with the cross slide appeared (to my beginners eyes) produce a better thread.
Thanks