Lathe Change Gears - What Are The Best Combinations?

[SeattleKent]

I recently purchased a used PM1127VF lathe. The lathe came with 14 change gears for cutting threads. The manual lists the gear settings to do 21 inch threads and 8 metric threads. Wanting to learn more about threading and change gears I did some searching on the web. I came across this post by jimbojones132. http://www.hobby-machinist.com/threads/cx701-change-gears-oem-gear-list-and-additional-threading-options.40649/. Jim wrote a program to calculate the pitches for all the different gear combinations. Intrigued, I decided to write a program for the PM1127 and the gears I have.

The results were very interesting. Given my set of gears I can produce most any thread in many ways. For example there are 71 gear combinations which produce 14 TPI threads exactly. Plus there are another 119 gear combinations which get within 1%. Need 18.4 TPI? There are four gear combinations which are off by mere 0.049% and 233 others that are within 1%.

Now my question. Are some gear combinations better to use than others? Are there rules about what to do and not to do? For example never have a small gear drive a gear more than twice its size or always put a big gear on the lead screw. Or should I always put a small gear on the lead screw. That sort of stuff.

The book “Screwcutting in the Lathe” by Martin Cleeve hints that there are rules but never explains this fully (at least in the parts I have read). I was hoping people on this forum can help me out.

If my question is not clear, I can add more details.

Thanks for reading.

 

[T Bredehoft]

I wonder if some of the 119 near misses are metric. After struggling to set up a couple of thread gearings, I opted for a feed pitch, (maybe .0028 per rev) and bought a couple of threading dies. If push comes to shove and I need some wild pitch, I'll change the gears, but it's a bear.

 

[epanzella]

. I'm assuming you have a QC gear box or all those combinations wouldn't be available to you. I don't know of any official change gear rules but common sense should prevail. For example. it wouldn't make much sense to use a gear combo that was way to fast and then use the gear box to slow it down again. Of course, the reciprocal is also true. I've found that if you're cutting a thread that's on the threading chart mounted on your machine it's usually a good idea to use the gears they recommend. If you're cutting a thread that the manufacturer doesn't list and you have choices, use the combination of gears that are closest to each other while getting you exactly what you want. My G4003G only comes with 6 change gears so I couldn't make up an outrageous combination even if I wanted to. I only touch the gears for metric threads.

 

[SeattleKent]

I wonder if some of the 119 near misses are metric.

. I'm assuming you have a QC gear box or all those combinations wouldn't be available to you.

Actually, the 190 gear combinations mentioned above is just for the doing 14 TPI. There is no QC gear box on the machine.

This is the surprising thing to me. People get these lathes and see 30ish thread possibilities in the manual. Well, if you run the numbers, there are actually thousands of threading possibilities. Basically any thread, English or metric, is possible. Pretty fun.

 

[janvanruth]

in general one should try to keep the number of gears involded as low as possible as each gear added will cause drag
gears with more teeth are to be preferred as they will run quieter and will be stronger
avoid big ratios if possible as the small gear in the ratio will be under great stress

 

[Wreck™Wreck]

You have 14 gear sets, assuming that each gear has a different number of teeth, or 28 different gears.

That is 14 X 14 possible combinations, 196 in all. With transposing gears, (MM lead screw to inch threads for instance) this is 392 combinations.

If the lathe has one more gear that may be used in the gear train, this gives you nearly 800 combinations.
As far as lead error, .0005 error X 24 TPI over a 1" long thread is .012", this is considerable.

As a general rule turning a thread that has a TPI less then the lead screw lead requires rotating the screw faster then the spindle, turning a 4TPI thread on a machine with an 8 TPI lead screw will rotate the lead screw at twice the speed of the spindle, try to avoid this (-:

 

[David VanNorman]

A couple of years ago there was an article in HSM on threading an with a few gears plus the quick change you could cut almost any thing . Even metric threads close if not perfect.

 

[dave2176]

. I'm assuming you have a QC gear box or all those combinations wouldn't be available to you. I don't know of any official change gear rules but common sense should prevail. For example. it wouldn't make much sense to use a gear combo that was way to fast and then use the gear box to slow it down again. Of course, the reciprocal is also true. I've found that if you're cutting a thread that's on the threading chart mounted on your machine it's usually a good idea to use the gears they recommend. If you're cutting a thread that the manufacturer doesn't list and you have choices, use the combination of gears that are closest to each other while getting you exactly what you want. My G4003G only comes with 6 change gears so I couldn't make up an outrageous combination even if I wanted to. I only touch the gears for metric threads.

I have the same lathe as you. I needed a thread that wasn't in the chart. That's when I learned that those 6 gears plus the gear box gives more than 1,000 different thread pitches. I keep all the possibilities in a spreadsheet for reference.
Dave

 

[SeattleKent]

If the lathe has one more gear that may be used in the gear train, this gives you nearly 800 combinations.

The lathe can handle 4 or 5 gears at a time which is something like 264,000 combinations.

As far as lead error, .0005 error X 24 TPI over a 1" long thread is .012", this is considerable.

Good to know. Do you have a gut feel for when close is close enough? The example you give of 0.0005 for 24 TPI is a 1.2% error which it seems is not good enough. Should I be looking at 0.5% error? 0.25%?

Thanks!

 

[SeattleKent]

in general one should try to keep the number of gears involded as low as possible as each gear added will cause drag gears with more teeth are to be preferred as they will run quieter and will be stronger avoid big ratios if possible as the small gear in the ratio will be under great stress

Thanks - very helpful. These tips will help me eliminate a lot of bad gear combinations.