A lesson on thread cutting....

E

Esmith41

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My first real project on my lathe did end quite as expected. I have a ton of backlash in my crossslide so I decided to make some repairs and parts per Halligan 142’s tutorial on YouTube. I measured and drew a schematic for the part I was making. Surprisingly enough I nailed all the dimensions up to where it was time to cut the threads. I set my lathe up to the correct pitch and cut the threads until I thought they were deep enough. I had no way to check so I removed the part from the chuck and gave it a test fit. The threads looked good but would not fit, too tight. So I chucked it back up to what I thought was the original position only to find it was out by a couple of thousands. It did screw in but was a little wobbly. A few wasted hours and of to buy some thread wires. The next one will be better I promise.9D414C61-CAE9-410D-B522-80C76B4F67EB.jpeg
 
That's how you learn! One suggestion; clean out your chip pan before using the wires so if you drop one it will be easy to find. They disappear in a pile of chips!

Ted
 
Cobra said:
If you're cutting threads where you have to take the part out of the lathe to check the fit, turning between centres will allow you to easily put it back in and match up.
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That's an excellent suggestion, just make sure you put the lathe dog back in the exact same location to keep things in sync. If you do, everything should be all lined up for you... Just verify before actually taking a cut.

Ted
 
Thread wires....stick them in a piece of Styrofoam, don't try to juggle three wires and a mike all at once.
 
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Sorry to hear that. Like was mentioned above, making mistakes is a given in this hobby. The best machinists find ways to hide their mistakes because even the best still make mistakes.

If you are really careful, you can rechuck a job and continue cutting the thread. But as you mentioned, you'll likely be off by a few thousandths. I've successfully rechucked a project and gotten on location again by engaging the half-nuts, manually cranking the carriage toward the tail stock to take up the slop in the half-nuts, then fiddle with the cross feed and compound to get the thread cutter back in alignment. Zero out the cross feed and compound dials, then back out the tool with the compound. Creep up on the threading until you see some chips.

Another couple of methods involve files. For a few thousandths, you could use a 60 deg. needle file to dust in a little deeper. It's nice if your lathe has reverse so you can hold the file under the work to clean up the thread from the chuck end to the tail stock from the bottom side. That way you get good file engagement all the way to the tail stock end of the work. If the OD is too big, hit the outside with a flat file. You could also try thread files like those pictured below. They're made to clean up burrs but would work to reduce your pitch diameter and OD slightly.

Bruce

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Ah thread files, a beginner's best friend.
 
Esmith41 said:
My first real project on my lathe did end quite as expected. I have a ton of backlash in my crossslide so I decided to make some repairs and parts per Halligan 142’s tutorial on YouTube. I measured and drew a schematic for the part I was making. Surprisingly enough I nailed all the dimensions up to where it was time to cut the threads. I set my lathe up to the correct pitch and cut the threads until I thought they were deep enough. I had no way to check so I removed the part from the chuck and gave it a test fit. The threads looked good but would not fit, too tight. So I chucked it back up to what I thought was the original position only to find it was out by a couple of thousands. It did screw in but was a little wobbly. A few wasted hours and of to buy some thread wires. The next one will be better I promise.View attachment 279591
Click to expand...

It was either Abomb79 or Keith Fenner (on Youtube) that showed how to use the compound to reacquire the thread. Basically, you set the compound 90º to the cross slide. Setup the gearing, close the half-nut, use the cross slide to put the tool near the threads, and use the compound to find tune the tuning dimension so the tool matches the threads. Then you continue cutting the thread using only the cross slide.
 
As you sadly found out, it’s not a good idea to remove and or change anything while you are still set-up on a lathe threading operation. You should be prepared to check the thread in progress with a method that does not remove or change anything. Thread gage, thread wires, thread triangles and even a standard nut will somethings work especially in a hobby shop environment. Or be prepared to have the knowledge to pick up the existing thread and not just go for it with the old settings. Good luck…Dave
 
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