It could have worked the way you were doing it it's just fussy and so many details to keep track of- takes practice
Me I would have left the halfnuts engaged. Note the number on the cross slide for each pass, crank it in after each pass. Reverse to starting point.
Reverse the carriage way back to remove the plate/chuck to check fit
Cumbersome but less chance to go out of sync
-Mark
Yeah I had assumed that if I left the leadscrew engaged (turning with the headstock) then picking up the thread would be as simple as setting the halfnuts on at a whole number. Or maybe even the same whole number, if we're being picky. Chances are I disengaged the leadscrew at some point, but even if not I probably was tripped up by the Z-slack in the leadscrew.
Maybe fill the threads with braze. Then bore and try again. Practice makes perfect. One suggestion from someone that isn't very smart DON'T REMOVE FROM THE LATHE TILL YOU ARE DONE.
Not removing it from the chuck was the only thing I managed to do right here but I'm proud of it.
I would agree with John York and make a plug gauge. Over the years I've made about a dozen back plates for various chucks. Most were made by repurposing existing back plates. In many cases the existing thread was too large for new lathe. In those cases, I bored them out and made a press fit plug. The plug was then pinned to the plate before boring and threading.
Most of the back plates were made on a lathe that didn't have a thread gauge. The half nuts had to remain engaged, and the carriage was reversed far enough to allow the plug gauge to be installed. By backing it up several inches all the play was removed when the carriage was returned to the forward travel long before the threading tool was engaged in the work piece.
I've just removed this mess from the chuck and will turn up a gauge next. I took one final pass and attempted to fit it on again and no dice, plus the threads look poorer than before and I don't want them scoring up the spindle threads. There might be enough stock on the back end of this to get a (thin) backplate off the back side. I got a large piece of cast iron so that the chuck jaws would be aft of all the turning.
Hello aliamateur. When cutting a thread with a high helix angle you should be aware that as the threading tool progressively cuts the thread then the heel of the tool can rub on the flank of the thread. This will load up the leadscrew and chew up the thread. And yes, I learnt this the hard way.
The threading tool will require heel relief and if you are currently using an insert you will have to change to HSS.
I see, I used a 22ER 5UN insert because I had a separate toolholder already from a package deal. I thought the insert would have that relief built in but maybe they're made for bigger and better machines that don't mind the rub. I'll grind out a 5UN HSS tool if that's the case.
One more thing to double check.
It is hard to tell from the pics (optical delusion?) but it looks like you cutting point might be below center. for external this is fine, but for internal work you want to be around 0.010 above center. If you are below center then as the tool deflects under load it will bend down pushing it deeper into the material which increases the load witch pushes deeper......... By having the tool slightly above center then when it deflects down due to cutting forces it will bu pulling away from the material.
All of your pics show chatter you need to figure that out. change of speed, feed, tool stick out, center height, and some setup things can all contribute to chatter.
One wy to help pick up a thread is to coat the thread with blueing or a Sharpie you can then check to be sure that you are getting contact on both flanks of the thread, picking up an internal thread is a lot harder than picking up an external thread. All of the different grooves indicates that you did not pick up the thread exactly the same each time.
It's probably a bit off center, I eye-balled it in but it seemed close. Definitely chatter is due to low speeds and carbide mixing, that was the major issue I was expecting. I was turning low speed to be able to hit my stop without crashing, probably about 50-60rpm when powered and 10-20rpm by hand for the first 2 passes to feel it out. All way too slow.
There are two sets of grooves, the original deep set, and then another set that sits about 0.030" further away from the chuck, which is roughly the endplay in the leadscrew. So that may have been a big contributing factor to not finding the thread again. Some of the offset may also be due to disengagement of the halfnuts and/or the leadscrew itself in between test fits, hard for me to be sure. Even with those possibilities, there is the thread dial (for fixing engagement of the halfnuts) and I'm sure the collar that engages the leadscrew turning is only keyed to 2 positions, so finding the right one there may have been the fix. But in the future, just gonna leave it all engaged and not worry.
Thanks for the help all, I had some vague ideas about what had gone wrong but not whether these were normal/worth worrying about my lathe being weird. And if there's nothing else too strange and unforeseen waiting for me, I should be able to turn up a plug gauge and salvage the remaining stock on this cast iron or turn up a sleeve for it.
