Chinese lathe Half Nuts availability

Double starts are usually reserved for big machines and for moving the head on big boring mills, not for threading. Made a number of those over the years, 4" dia. 16' long. not much fun. That was a long time ago though.
 
There was a guy in the states who rebuilt my half nuts for a reasonable cost many years ago. I found him on the home machinist forum , maybe somebody there still does this.
 
dogzilla said:
Double starts are usually reserved for big machines and for moving the head on big boring mills, not for threading. Made a number of those over the years, 4" dia. 16' long. not much fun. That was a long time ago though.
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pmatuk sent me a contact in the US that does this work.
 
bearhunter said:
No you're right, it's not a double lead thread. Brain Fart happened.

Just finished pulling the "carriage group" It's heavy and care needs to be taken not to drop it.

The lead screw nut and the half nut look fine, with very little wear. What was wrong was that the thread indicator dial gear does show a lot of wear and it looks like it may not be tracking the lead screw, which is also fine and has 8TPI, which I thought was strange, seeing as it's a Chinese made lathe.

I have a rotary table for my mill and can grind proper shaped cutters for the right form, angle and pitch.

Likely making up this small gear is going to be finicky and tedious for me, so time consuming. I have lots of time.

The follower gear is SOFT, as it should be when riding against a harder lead screw. I can either use brass or some cast iron material I have on hand.

One thing I did learn from this was that parts can be made up at a few different places, both in Canada and the US. Pricey.

Companies that actually were able to source spare parts a year ago, no longer have access to those parts. They blame Covid of course.

I have another Grizzly type lathe but it's equipped with a quick change gear box. It's fully capable of threading the material I would have used to make up new nuts.

The one good thing about my particular back gear lathe is that the blocks holding the nuts accept "round shells" which are held in place by screws.

I took some measurements and I'm going to make up a spare set next winter. If I get ambitious, I'll make up a set for the other lathe as well.

Always something new and interesting to challenge my capabilities.

Thanx again for all the information, which saved me days of searching on my own. Very much appreciated.
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Keep in mind that the gear that tracks the lead screw for the threading dial, does not need to be much more complicated than you can cut with a chisel and files, and it will work as well as if it had all of NASA's machine shops working on it.

I have seen a fair few that were 3D printed.

Really, all that matters, is that you get the right tooth count, and it fits where it is supposed to.

If you want to just order a gear, measure the outside diameter, count the teeth, add 2 to the tooth count. Divide the tooth count plus two, by the diameter, to produce the Diametrical Pitch required. DP= tooth count+2/OD. To mesh with a standard Acme Thread form, you want a 14.5 degree pressure angle.

Chinese or otherwise, 8TPI is a pretty bog standard thread pitch for an inch lead screw. Lots of easy conversions via gearing, to produce the ratios used for creating inch size thread pitches.

Bigger machines will use a 4TPI thread, typically.

Any thread that can be evenly divided (no remainder) by the number of threads per inch on the lead screw, can be cut by simply engaging the half nuts, and ignoring the threading dial. So in your case, 8, 16, 24, 32, 40, etc. TPI
 
I'm lucky in that I have both a horizontal and vertical indexing system for my milling machine.

It was pretty easy to copy the original gear, once I got the angle of the teeth correct.

The lead screw has square cut threads and likely I could have gotten away with just using a square cutter, but I ground the cutter so that the top of the tooth - addendum circle - is smaller than the part of the gear that rolls into contact - pitch circle.

The first attempt wasn't spectacular, the second attempt was much better and a bit of clean up with a fine wire brush on a Dremil tool smoothed it all up and it works like a charm.

Thanx again trevj, I do appreciate your tips. You've had a lot more experience with these things than I have.
 
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bearhunter said:
I'm lucky in that I have both a horizontal and vertical indexing system for my milling machine.

It was pretty easy to copy the original gear, once I got the angle of the teeth correct.

The lead screw has square cut threads and likely I could have gotten away with just using a square cutter, but I ground the cutter so that the top of the tooth - addendum circle - is smaller than the part of the gear that rolls into contact - pitch circle.

The first attempt wasn't spectacular, the second attempt was much better and a bit of clean up with a fine wire brush on a Dremil tool smoothed it all up and it works like a charm.

Thanx again trevj, I do appreciate your tips. You've had a lot more experience with these things than I have.
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Welcome!

Glad to pass on some of what I have learned in the last 35+ years of dicking about with metal.
 
If you haven't checked already try aliexpress. Lots of crazy stuff on there. High risk of not getting what you expected but typically pretty cheap costs. A bunch available if you can find the right one.
 
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