Threading Problem Help Request

kvt

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Ok, was in the middle of threading a project, when all of a sudden a bunch of noise and the carriage stops moving. Figure it out and what happen is one of the gears came loose and thus not advancing the carriage.

Now here is my problem. The gear box is still set, but how do I get the spindle and the carriage back in the same sync as they were before so that I can finish up the thread without loosing the whole project.
20161119_181517.jpg
Ideas, suggestions etc. pleas
 
I'll assume that your compound is set at ~30º (or ~60º, depending on how your angular scale is set up).

With the lathe off, manually turn the spindle until the threading dial is at whatever number you were using. Engage the half-nuts. Then advance the cross slide until the tool is near (or slightly into) the existing threads. By moving both the cross and compound, you can get horizontal (z axis) movement of the cutting tool. Play with this until the tool goes into the threads as close to centered as possible, maybe just bit to the right.

Use the cross slide to set up for a light cut and take a pass. If you're shaving just a bit off the right side of the existing threads, that's OK. Then proceed as usual, doing subsequent advances with the compound, to finish the threads.
 
I've had a similar problem in the past and picked the thread back up without too much problem. Engage your half-nut and pull the carriage towards the tail stock. I locked my tail stock in place and looped a couple of bungee cords to the carriage and around the tail stock to provide the pressure. I'm assuming you are cutting an external thread, so the load during cutting makes the carriage bias to the rear of the slop between the half nuts and the lead screw.

Once you have that set up done, position your cutting tool back into the thread. I usually start threading with the cross feed set at zero and bring the compound up until the tool just touches the work, then zero out the compound. So zero out the cross feed when you position the tool to the bottom of the thread. If you remember where your compound was before the incident, set the compound to that setting. Otherwise, maybe zero it out further down the piece where there's no thread or zero it out when it just touches the top of your thread.

All is not lost, I'd guess pretty much everyone on the forum has had your situation and was able to work it out.

Bruce
 
Ok, KInda forgot to mention, Internal threads. I did try to line things back up, but when I took a light cut, It is as if the tool is about 1/3 of a tooth off, In stead of the point of the tool being in the bottom of the current cut it is about 1/3 of the way up the tooth. If I back the cross slide up and advance the compound more to see if I can get it into the proper grove.

Yes I had the cross slide set to 0, and the Compound on 30 and set to 0 when I started.
Thanks

Ken
 
Picked up a thread many times. Turn the compound so it is parallel to the piece and use it the find the thread. Feed with the cross slide or a combination say .005" withe the cross slide and .001" on the compound. Good luck.
Dave
 
All have given good advice. The most important thing when re-acquiring a thread is to take up leadscrew slack in the same direction as when you were cutting the thread. This includes the leadscrews on the longitudinal, crossfeed and compound feeds.
 
Fun, fun.....try it on an CNC and you'll see how easy manual machines are at times.

On an internal thread (let's see if I can explain this clearly), I engage the half-nuts as though I were about to cut the thread, but no tool in the holder. I stop the spindle and making sure I have enough bar hanging out, slide it into the bore and push it into the existing thread. This gives me -Z- alignment. I do this with the crossfeed deliberately "large" so that the threading bar is sitting at an angle in the holder. Using my thumb, finger, or whichever digit fits, I hold the insert in place, aligned with the thread and crank the crossfeed in (smaller). When it gets in position, where it is properly to be clamped down, I lightly tighten one of the toolholder screws and then continue to crank smaller to get the insert tip out of the existing thread. Then I tighten the rest of the toolholder screws. As long as I didn't let anything shift in -Z-, I am pretty close to right on with the lead screw position. I can then back the bar out and if I was paying attention to the -X- position, I should know pretty close to where I am safe to take the first cut. Sometimes I blue the thread, sometimes not. Until you get comfortable with this method, back off sufficiently to clear the thread and while in thread mode, run the -X- axis (cross slide) out until you get contact with the thread. If all is well, you should touch it at the previously noted -X- position. Or close anyway. If not, you've missed something. If you are on, then carry on threading.

This can be a little awkward for shallow threads, as you don't have much time to determine whether you are accurately "timed" with the existing thread, so go slow. Turning by hand may be the solution in some cases. I have worked in repair shops where chasing threads was a nearly daily thing, so I realize it takes a bit of practice, and a 10 pound sack of nerves. But it happens every day in commercial shops, so we know it can be done. I will say it's much, much easier with external threads, and an approach similar to what I described for internal threads will work just as well. Just leave the threading tool loose and slide it into the existing thread before locking it down. All with the lathe set as though you were actually cutting, meaning nuts engaged, etc. Just coast the spindle to a stop somewhere in the threaded portion and do the tool position adjustment.

Clear as mud, eh?
 
Done it once, got a small mirror so I could get inside the hole and play with the cross slide and compound. Really hard to hold and adjust while everything is moving, so slow it down as slow as it will go.

How much effort you put in kinda depends on how important that thread is for your end product. If you just started the thread, it may not make a whole lot of difference, just get close and you'll probably be ok. If almost finished, you get a better profile to align to, but is way more critical to get it accurate.
 
Took a little bit of a combo on a bunch of it including bluing the partial threads so I could tell where I was contacting. Loosened the tool and aligned it up then then tighten it down then played with the cross feed and compound till I was in the right spot then finished up the threads. they seem to work. I will see when I get it all done.
 
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