Why Would I Need A Quick Change Lathe Gear Box?

[ericc]

A QCGB is much easier and faster to use than change gears for cutting threads. Also, there is much less chance of losing gears.

But, if something goes wrong, the change gears can be much more straightforward to fix. I used to be a member of Techshop, a machine cooperative with a monthly membership fee. They had two Jet 1440 lathes. The gear change levers were very tough to operate and engage. Sometimes, the leadscrew would turn erratically, ruining the thread. This almost never happens with change gears unless the gears are terribly thrashed or something is wrong with the banjo. Eventually, the manager told me "this lathe doesn't do threads, didn't you know that?????" It took them several months (paying monthly fees for each month) to fix the lathe. At one point, there were several shafts on the workbench, all covered with rags to keep swarf off them. I rarely saw the tech working on them, but it was pretty intense. Looks like one little mistake and things would get all galled up. The tech was not in a good mood the one time I did see him working. The lathe did get fixed, and was able to thread for a while, but then broke down shortly after. In contrast, I bought a South Bend for a really good price without change gears. I needed to cut one thread, and I had the local public library print out the gears on their 3D printer. I installed the gears, and it worked smoothly the first time, and I was able to cut the threads right away, in steel. I would never have had the guts to tear into that complicated 1440 gearbox. And, it was super finicky.

 

[Robo_Pi]

It does, but as I understand it the threading dial is effectively worthless and you'd have to keep the half-nuts engaged to ensure catching the thread again. I think the threading dial would only be useful within the system that matches the threads on the leadscrew.

It would seem to me that as long as you always use the primary mark (the number 1) it should work for both metric and imperial. I would think it might only be a problem if you used the other marks. Using a single mark would still be better than having no thread dial at all. At least for me. So I may think about looking into making one for my lathe.

I plan on making my own and have looked at several videos at different approaches....one approach is to take a hex die holder and cut off the arms and then turn the OD. make it a slight press fit with Loctite RED applied. Put some set screws in place through the hex and you are all set to go. This is one of my first projects to make figuring it would give me some good lessons to learn at the same time as I get a cool tool.

That's cool. If you make your own tailstock threading tool be sure to post photos of the build. I'd love to see the project unfold.

 

[savarin]

some worthwhile threading info

Cutting Metric Threads

www.schsm.org

Metric Threading

Metric Threading on the Lathe

conradhoffman.com

I cut metric threads on an imperial leadscrew so usually leave the half nuts engaged, I really must explore these methods.

 

[Robo_Pi]

This is all quite interesting. I wasn't even aware of the thread dial problem. My lathe simply came without a thread dial and the instructions were to always leave the half-nut engaged.

I'll have to think about this problem. I read both the articles savarin posted. The second article isn't much help. It actually give a procedure to release the half-nut only momentarily at the end of the cut, but then you need to put the lathe in reverse, and "catch" the correct number and still end up using lathe power to return to the beginning. So that procedure doesn't do what I would want.

The first article talks about returning to numbers that are precise conversions from metric to imperial like 5" = exactly 127 mm, or 10" = exactly 254 mm. So supposedly if you handwheel back precisely those inches you could just reengage using a thread dial.

However another point is that the really important coordination is between the number of times the headstock turns relative to the number of times the lead screw turns. If you had a way of keeping track of both rotations you could then use that information to know when to engage the half-nut. In order to do that you'd need to have counters on both the headstock spindle and the leadscrew. Of course if you have a thread dial then you already have a counter on the leadscrew. So all you would need is a counter on the headstock spindle too.

This might be an interesting project to look into. It's possible that by creating a product that can keep track of both counts it might be marketable gadget. I would imagine that it would almost need to be taylored to specific models of lathes. (I'm thinking mechanically here). Actually a more versatile product would be electronic. Something that can count the rotation of the lead screw and spindle electronically via sensors.

It could have a display on it that counts down from say 5 to 1 when the headstock and lead screw are approaching synchronization again. Then you could see it counting down 5, 4, 3, 2, and when it reaches 1 the mark on the dial counter should be lined up again on the proper rotation of the leadscrew. And you just engage the half-nut for the next cut.

I might be onto something here. An electronic solution to a mechanical problem.

It would glow red when out of sync, then turn green on 5, 4, 3, 2, 1. If you engage on 1 it stays green. If you miss it, it goes back to red again until it comes back around in sync.

I gotta get busy and invent this baby before someone else does.

I didn't even know this problem existed. A problem just waiting for a solution. My favorite kind of animal.

 

[hman]

However, I'm pretty sure my lathe has a metric lead screw. The reason is because there are two gears that only come with the metric lead screw, and two gears that only come with an imperial lead screw

I'm not certain I am following in the discussion about the 2 gears (I suspect my limited experience is at fault), but do you not have a collection of gears which define the speed between the spindle and the leadscrew? It is my understanding that the thread pitch on the leadscrew itself, combined with the gears between the spindle and driven gear on the leadscrew, which defines whatever pitch will result. (edit: in my case, 17 gears of which 2 are duplicates)

I think I can help alleviate the confusion here. Let's give it a shot ...
The "official" conversion factor between inch and metric is the 1 inch is exactly 25.4 millimeters. You can't make a gear with 0.4 teeth, so we multiply the 25.4 by 5 to get the lowest available whole number, 127. It's as low as you can go, because it's also a prime number. If you have a gear cluster with a 127 tooth gear and another gear (usually 120 teeth) on a common shaft, you can transpose between metric and imperial, ie make metric threads with an imperial leadscrew. Likewise, by "reversing" the 127/120 gear cluster, you can make imperial threads with a metric leadscrew.

There are other gear clusters (91/86, 80/63, 47/37) used on some lathes, probably to save space, because a 127 tooth gear is largish. These clusters do not give exact thread pitches, but are OK within about 0.02%.

 

[Robo_Pi]

I think I can help alleviate the confusion here. Let's give it a shot ...

Your explanation appears to be correct to some degree. However there does appear to possibly be a little bit more two it than this. There's actually 4 gears different or 6 gears if you count duplex gears as being two gears each.

Here's the scoop:

Both lathes appear to come with the following gear sets:
24, 27, 30, 33, 36, 42, 48, 60, and 120. All these gears are single gears. These come with both the imperial and metric lead screw.

However, the imperial lead screw lathe comes with an additional two more single gears, 39, and 72. I do not have those two gears and apparently I don't need them since I have a metric leadscrew.

Instead I got a 25, and 75 teeth gears, which do come with the metric lead screw but not with the imperial leadscrew lathe.

But then there are also a set of two duplex gears. Apparently these are the ones that make the main conversion that hman is talking about.

The metric leadscrew comes with a 60/120 tooth and a 125/127 tooth duplex gear set.

The imperial leadscrew comes with a 60/127 and a 120/127 tooth duplex gear set which I didn't get with my lathe because I don't need them.

I also have two gearing charts. One for the imperial leadscrew and one for the metric leadscrew. Obviously I only need to use the metric leadscrew chart since that's the lathe I have. Apparently this lathe can be ordered with either a metric or imperial leadscrew so they just include both the tables in the manual.

I can see from these tables that both lathes will cut the same threads. They simply use different gear arrangements to accomplish the task. So apparently there is no advantage to having one over the other, at least in terms of what thread can be cut.

By the way, this entire conversation has helped me a lot because I wasn't quite clear on this myself until we had this discussion.

But now I have a much better idea of what's going on, and now I also know for certain that my lathe must have a metric leadscrew since those are the gears that came with it. But as I say, it will still cut all the same threads. So in that sense it really doesn't matter. Although it might make a difference if I was to create a threading dial for this lathe. The dial might only work for metric threads and not for imperial threads since I have a metric leadscrew?

I don't know. All I know is that it works when used as the manual describes. Just don't disengage the half-nut until the thread is finished. That's guaranteed to always work. So that's what I've been doing. I just turn the lathe off, and reverse it to get back to the beginning. It works. It's not really that bad. But I can see where I might like to disengage the half-nut if I wanted to work up close to a shoulder or something.

Just as a note on that, I have seen people on YouTube simply turn the lathe off before the end of the cut, leaving the cutter at depth, and then just finishing the thread up to the shoulder by rotating the chuck by hand. That can work too on smaller lathes. My lathe is just barely small enough to allow for that. A much larger lathe would be difficult to turn by hand because you need to turn the headstock gears and leadscrew. That can be a lot of mass and friction on a large lathe. My lathe is just small enough that it still allows for manual finishing if needed. So in a pinch I could thread right up to a shoulder if I need to. Just have to turn the lathe off and do the last few threads by hand.

 

[Susan_in_SF]

You might recall my 3 cents on a lathe without a QCGB and has a flat belt(s) and etc, etc. Moving onto your die holder mounted in the tail stock. It will make threads but not always true to the shaft/project. It’s always better to start a thread with the lathe using your QCGB!!!! And then you can finish up with that die thing if so desire. Using the lathe to get the threads started will assure the “die follows” the unfinished threads instead of the “die leading” with no threads to follow and go astray.

Thanks Chips, I appreciate your 3 cents
Susan

 

[Susan_in_SF]

Your explanation appears to be correct to some degree. However there does appear to possibly be a little bit more two it than this. There's actually 4 gears different or 6 gears if you count duplex gears as being two gears each.

Here's the scoop:

Both lathes appear to come with the following gear sets:
24, 27, 30, 33, 36, 42, 48, 60, and 120. All these gears are single gears. These come with both the imperial and metric lead screw.

However, the imperial lead screw lathe comes with an additional two more single gears, 39, and 72. I do not have those two gears and apparently I don't need them since I have a metric leadscrew.

Instead I got a 25, and 75 teeth gears, which do come with the metric lead screw but not with the imperial leadscrew lathe.

But then there are also a set of two duplex gears. Apparently these are the ones that make the main conversion that hman is talking about.

The metric leadscrew comes with a 60/120 tooth and a 125/127 tooth duplex gear set.

The imperial leadscrew comes with a 60/127 and a 120/127 tooth duplex gear set which I didn't get with my lathe because I don't need them.

I also have two gearing charts. One for the imperial leadscrew and one for the metric leadscrew. Obviously I only need to use the metric leadscrew chart since that's the lathe I have. Apparently this lathe can be ordered with either a metric or imperial leadscrew so they just include both the tables in the manual.

I can see from these tables that both lathes will cut the same threads. They simply use different gear arrangements to accomplish the task. So apparently there is no advantage to having one over the other, at least in terms of what thread can be cut.

By the way, this entire conversation has helped me a lot because I wasn't quite clear on this myself until we had this discussion.

But now I have a much better idea of what's going on, and now I also know for certain that my lathe must have a metric leadscrew since those are the gears that came with it. But as I say, it will still cut all the same threads. So in that sense it really doesn't matter. Although it might make a difference if I was to create a threading dial for this lathe. The dial might only work for metric threads and not for imperial threads since I have a metric leadscrew?

I don't know. All I know is that it works when used as the manual describes. Just don't disengage the half-nut until the thread is finished. That's guaranteed to always work. So that's what I've been doing. I just turn the lathe off, and reverse it to get back to the beginning. It works. It's not really that bad. But I can see where I might like to disengage the half-nut if I wanted to work up close to a shoulder or something.

Just as a note on that, I have seen people on YouTube simply turn the lathe off before the end of the cut, leaving the cutter at depth, and then just finishing the thread up to the shoulder by rotating the chuck by hand. That can work too on smaller lathes. My lathe is just barely small enough to allow for that. A much larger lathe would be difficult to turn by hand because you need to turn the headstock gears and leadscrew. That can be a lot of mass and friction on a large lathe. My lathe is just small enough that it still allows for manual finishing if needed. So in a pinch I could thread right up to a shoulder if I need to. Just have to turn the lathe off and do the last few threads by hand.

Thanks for the info, Robo_Pi.
Now you got me thinking about researching how to cut metric threads.
Susan

 

[Winegrower]

Metric threads: So far, every metric thread I have had to cut has been short, and there has been an Imperial threads per inch that comes close enough. A little bit of 4 function calculator math will show the error percentage this way.

 

[Susan_in_SF]

Metric threads: So far, every metric thread I have had to cut has been short, and there has been an Imperial threads per inch that comes close enough. A little bit of 4 function calculator math will show the error percentage this way.

Cool. I can just use my 20 yr old daughter's TI-84 (I think that is the model) graphing calculator that she used in high school.