Machining custom Acme screws to fix a lathe chuck

This old beast

My dad has always had this antique lathe sitting in his workshop - having inherited it from a previous employer. It hasn’t seen a ton of use over the years, and I never really understood what a powerful tool it can be. It wasn’t until recently when I came across a few machinist youtubers that I discovered that the lathe is capable of some jobs I didn’t expect - such as single point threading, or facing non-symmetrical parts. I also was able to pick up some basic skills with the machine that I didn’t have before.

Cutting some random threads in aluminum for fun.

For all of the time this tool has been around in the workshop, I never really knew that there were any other options for work holding other than the three-jaw chuck, despite there being an old four-jaw sitting on the bench right there.
Once I discovered that the four jaw existed, and learned what it was used for, I decided to fit the one we had available and start trying it out for some different operations.
Not long after digging out the chuck and fitting it to the lathe, I managed to break one of the jaw screws. I think this was due to a combination of age, too much force, and the custom wrench I had to make to adjust the jaws being slightly the wrong size.

The screw in question

This was sort of perfect timing, as I realized that I had all of the tools required at my disposal to fashion a suitable replacement. I had never done any functional single point threading, or tried to turn any parts to a (relatively) precise dimension. This would be the perfect mini-project to try out some new techniques, and level up my lathe skills.

After taking the necessary measurements, I decided that the hardest part of the project was likely to be cutting the thread, so I decided the best approach would be to tackle that first.
Assuming the threads to be made to imperial dimensions, I measured the thread of the screws to be 10 TPI. This was good news as the thread of the leadscrew on the lathe is 5 TPI, meaning I would only need a simple 1:2 gear ratio in order to cut the correct thread pitch.
Next was to figure out the cutting tool- Acme threads have a different shape than standard 60° threads found on most bolts and screws, so I would need the right tool for the job in order for it to work.
Luckily along with this lathe came a drawer full of old HSS tools and scraps, so I would be able to grind my own tool. I could have also just as easily purchased the right one, but I like the idea of being able to make a tool when I need it, rather than having to wait on shipping or run out to the store. So this was a skill I wanted to develop as well.
Grinding HSS tool bits quite a bit of an art in itself, so I’ll just reference the two videos that helped me out the most.

Once I had my tool bit ground, I grabbed a piece of steel rod from the scrap bin and got to threading.

Starting to look like threads!

From following some tips and tricks learned from various thread cutting videos watched, as well as my own experience with taming this old machine, I saw a fairly acceptable acme thread emerge on the part.
I would later realize that I used the wrong notch on the tool grinding gauge, so my thread form isn’t quite right - it’s too skinny. This didn’t turn out to be too much of an issue when used in the chuck since there is always going to be some clamping pressure against the part. If this were used in a lead screw on a machine tool however, it would introduce a lot more backlash then a properly made screw would. Lesson learned, though.

The parts emerge.

Once I was happy with the threads, I used a parting tool to cut the grooves as required. Using a dial indicator against the carriage, I was able to do this fairly accurately. I then parted off the screws(I decided to make two) and took a second to compare them to the originals.

Fresh off the lathe. You can see here how I got the thread form wrong- they’re too skinny. This didn’t end up being a big problem this time though.

The only thing left at this point was to cut off the parting nubs and somehow create the square hole for the wrench to go into. I waffled about how to do this for a while, looking up some approaches to building a cheap rotary broach or to simply filing the shape out by hand.
It took me a while to decide to just try popping them onto my CNC router and attempting to mill out the holes using a small end mill- I didn’t expect the machine to be rigid enough to cut steel. Eventually I decided to just give it a go anyways, and it ended up working out just fine.

I was surprised by how well it went, actually. I was very conservative with the feedrate and tool engagement- starting with a helical bore to the required depth, and then a very gentle adaptive clear out to the required shape, I was done in no time. I didn’t even need to square up the vise, since the orientation of the square to the part really didn’t matter.
Once the square was cut, I gave the parts a quick deburr and then installed them in the chuck to try them out. Of course they didn’t quite fit at first since I left the outer diameter a little big. All I needed to do was put them back on the lathe(and luckily we have another 4-jaw chuck so I was able to get them relatively concentric again), and turn off a couple thou at a time until they fit.

Happily installed in its new home

Feeling satisfied with my work, I installed the newly fixed 4-jaw chuck back on the lathe and started turning some metals just for fun.
This proved to be an excellent mini-project to hone my lathe skills, and I feel a lot more confident about making functional parts on the machine in the future. Look forward to writing about even more fun and interesting machining projects in the future.
Thanks for reading!