# Why Would I Need A Quick Change Lathe Gear Box?



## Susan_in_SF (Mar 4, 2019)

Hi Guys,
As many of you already know, I have too many lathes (6", 9", 10").  I was happy having a South Bend 9A 9" lathe, then got the free Logan 200 10" lathe.  Ideally, I would like to keep both lathes.  The Logan does not have a quick change box. 

Awhile ago, I bought this tailstock tap/die holder from a machinist online, Neil.  On his website, there is a video on how to use his gadget.



			https://www.neilsniche.com/lathe-tailstock-tap-die-holder.html
		


Anyways, if I were to use this tool, would it matter if my lathe had a QC box or not?

Forgive me if the answer is obvious. 

I am debating which lathe to keep if I were forced to keep only one due to lack of space (the Bridgeport mill literally takes up a thirc of my garage).

The Logan seems to have more mass than the South Bend, but is lacking a QC box. 

Thanks,
Susan


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## lordbeezer (Mar 4, 2019)

Nothing wrong with a Logan But I would keep the south bend..


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## SamI (Mar 4, 2019)

Given that the tap / die holder is mounted in the tailstock and not the tool post then you wouldn't be using any power feed therefore it wouldn't make any difference whether your lathe has a QCGB or not.

That being said, I wouldn't give mine up!  Being able to change feed rates without mucking about with change gears is a huge bonus for me.  Also being able to change between thread pitches could be useful although, on my lathe at least, you have to use change gears when going from your usual feeds to thread pitches.


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## T Bredehoft (Mar 4, 2019)

Should you want to chase threads without the  Quick Change, you'll spend considerable time manually changing the gears which regulate the feed rate of the saddle. It took me over an hour yesterday, (caveat) new lathe must made gears fit shafts, etc. After a while you'll wonder if you really want to chase threads.


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## bill stupak (Mar 4, 2019)

If you need to single point threads on a shaft supported by a center or threads longer than the tail stock die holder can accommodate then a QC Gearbox would be an advantage over having to change gears.


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## markba633csi (Mar 4, 2019)

Hi Susan, quick change boxes are/were a real advantage for production use where lots of threading and feed speed changes were required, back in the old days, pre-cnc.  Not so critical for hobby use in my opinion but they do save time and are less messy than individual gear changes.
mark


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## benmychree (Mar 4, 2019)

Change gears get your fingers horribly dirty.


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## yendor (Mar 4, 2019)

A QCGB is like any other improvements done to machines over time.

Just like Presliced Bread and adding flip tops to Canned Beer, they just make things a little easier.


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## Mitch Alsup (Mar 4, 2019)

I often hog out metal using 0.008"/turn at 360 RPM and then take the finish cut at 0.001"/turn at 600 RPM.
This is best facilitated with a GCGB.


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## dtsh (Mar 4, 2019)

If you don't mind calculating change gears and you have a set that does what you need, there's no requirement for a QCGB.
I personally prefer change gears, but all of my work are one-offs and the time necessary to change gears is insignificant.

When I was figuring out the gears I would need to cut metric threads on my Seneca Falls lathe, I read "The Calculation of Change-Wheels for Screw Cutting on Lathes" by D. De Vries, 1908 (pdf readily available online) and used what I learned there to write a short ruby script to pick gears based on the threads I needed, both metric and imperial/customary. I've used lathes with a QCGB and it's often an advantage, but I don't find it to be a requirement....for what I do.


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## Susan_in_SF (Mar 4, 2019)

benmychree said:


> Change gears get your fingers horribly dirty.


LOL!


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## BtoVin83 (Mar 4, 2019)

benmychree said:


> Change gears get your fingers horribly dirty.


plus you could chip a nail


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## savarin (Mar 4, 2019)

I work mainly in metric and my lathe is only a generic 9x20 but the qcgb is essential for some thread sizes along with the change gears.
My thoughts are if you dont have it available there will probably come a time when its needed for one simple job and you wish you had it.
I dont know anything about your lathes though.


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## ttabbal (Mar 4, 2019)

My delicate fingers can't be allowed to get so dirty with the change gears. Nitrile gloves are very nice to keep my sensitive hands clean.


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## Winegrower (Mar 4, 2019)

I admit to never having a lathe without a QCGB.  Here’s some logic for you:  All of my stuff is for me, not for pay, so the easier and quicker the lathe is to use, the more fun it is.  I like the quality of finish coming from power feeds, and occasionally I have to try a few different feeds.   Changing gears is just work.  Ergo, gotta have QCGB.   Also applies to QCTP, DRO, tapping heads, collets, spin indexer, rotary table, big...  oh, sorry.   Me stop now.


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## drivermark (Mar 4, 2019)

It really boils down to 1 word....   Convenience.


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## epanzella (Mar 4, 2019)

I can do all SAE threads and feeds on my lathe without swapping gears. I just have to swap gears for metric threads so I avoid them when ever possible. I had an old change gear P & W lathe years ago. It was such a PIA changing gears that I just used the wrong feed most of the time. Of course I had to use the right gears for threading but I just can't imagine going back to that.


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## Robo_Pi (Mar 4, 2019)

Susan_in_SF said:


> Anyways, if I were to use this tool, would it matter if my lathe had a QC box or not?
> 
> Forgive me if the answer is obvious.



That can all depend on what you might want to put threads on. 

If you wanted to thread the inside of a large metal lid or cap it might be difficult to find a tap that would do the job.   It would need to be a very large diameter bottom tap.


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## Eddyde (Mar 5, 2019)

No QCGB, Imagine a bicycle without a derailleur. To change gears you would need to stop, get off the bike and move the chain manually to the desired gear. Now your hands are greasy and you've lost time...


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## Chipper5783 (Mar 5, 2019)

Which to get rid of? The 9A or the Logan?  That's sort of like which hand should you have cut off?  In a do or die situation, I'd keep my right - but I'd try really hard to keep both.


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## Robo_Pi (Mar 5, 2019)

By the way, that threading tool looks pretty easy to make.

Apparently all you need to do is make the following parts:




My tapping dies are hexagon anyway, so I wouldn't want round die holders.

I think I'd design the knurled sleeve differently.  I'd make it quite a bit larger in diameter.

In fact, you could probably just put the tapping dies right inside the the end of the knurled sleeve instead of having separate die holders.

I can start with just three pieces.


The tailstock arbor
The Knurled Sleeve with built-in die holder.
And a stop handle.

I could even start with just a round piece of stock for the tailstock arbor and hold that in a tailstock chuck

It's getting simpler by the moment.


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## Robo_Pi (Mar 5, 2019)

To make this design truly professional you'd need to design in some type of slip-clutch between the stop handled and the knurled sleeve.   That way  you could tap right into a shoulder and instead of crashing the knurled sleeve would simply turn with the workpiece and break free from the stop handle.


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## rock_breaker (Mar 5, 2019)

Not having to be a weight lifter for the cast iron gear guard on my Clausing 100 MKIII is my best reason
Have a good day
Ray


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## Susan_in_SF (Mar 6, 2019)

Chipper5783 said:


> Which to get rid of? The 9A or the Logan?  That's sort of like which hand should you have cut off?  In a do or die situation, I'd keep my right - but I'd try really hard to keep both.


Lol!!


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## Susan_in_SF (Mar 6, 2019)

Robo_Pi said:


> By the way, that threading tool looks pretty easy to make.
> 
> Apparently all you need to do is make the following parts:
> 
> ...


When I visited Neil in Arizona (the machinist selling this lathe tapping/die holder tool) he surprised me when he told me he doesn't make the tools.  He designed it and pays someone else online to make it to his specs in bulk.  I learned a lot from him from my 1 visit....


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## Robo_Pi (Mar 6, 2019)

Susan_in_SF said:


> When I visited Neil in Arizona (the machinist selling this lathe tapping/die holder tool) he surprised me when he told me he doesn't make the tools.  He designed it and pays someone else online to make it to his specs in bulk.  I learned a lot from him from my 1 visit....



Don't tell anyone, but I am desperately in need of additional income.  

So I'm trying to come up with a product to sell.   My criteria is that it needs to be easy to make so that I can make it myself.   And small enough so that it's also easy to ship.   I have several ideas on the burner.   

One is to make wooden garden train bridges.   These are the large G-scale or Garden Train scale bridges.
Here is my design.   I'm thinking of making them in 2 foot sections and the customer can just order how many sections they want for a bridge the size they need. 

The 2 foot basic section:




A 4 foot bridge - two sections:




6-foot




8 foot, etc. 




Front view




Perspective view




 I really should be out there in the shop building these right now.  

I have done some work toward building them.   I've been making up jigs and fixtures to make all the individual trusses mass production.   So this is a project under construction. 

The above drawings were done in Google Sketchup. 

I really need to get some finished and put them up on Etsy or eBay, etc.    I won't be making any money until I get off my butt and actually build them.


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## Susan_in_SF (Mar 6, 2019)

Robo_Pi said:


> Don't tell anyone, but I am desperately in need of additional income.
> 
> So I'm trying to come up with a product to sell.   My criteria is that it needs to be easy to make so that I can make it myself.   And small enough so that it's also easy to ship.   I have several ideas on the burner.
> 
> ...


Call me slow, but your bridges don't look easy to make, lol.
If you have a photo of a similar item made by someone else, i'd love to see it.
Btw, wouldn't this be more of a woodworking project?
Good luck, Robo_Pi.  At least you have made some sketches of it.
As for me, easy to make and ship would be an item fitting in a small flat rate USPS box.  
Susan


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## Susan_in_SF (Mar 6, 2019)

COMMENT moved to Bridgeport posting


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## hman (Mar 7, 2019)

Susan_in_SF said:


> When I visited Neil in Arizona ...


Well, durn, Susan!  I live in the Phoenix area myself, and know Neil from the Valley Metal club.  Fantastically nice guy, isn't he?  I would have really enjoyed meeting you.  Give a shout next time you plan a trip out here.


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## Chipper5783 (Mar 7, 2019)

About the Bridgeport / VFD / motor thing.  Here with I share some opinions. You are power constrained - 20A @ 120V.  Nominally 2400 watts.  The VFD will not achieve the voltage change (120 -> 240).  You have a 3000W transformer (single phase - I assume).  Confirm the VFD is rated at a 240V input (which it probably is).  So you connect the 120V side of the transformer to your supply (via a single pole disconnect) and the high voltage side to your VFD inputs.

Run your existing BP motor (mainly because it has the smaller current draw, it works fine for the machine - and it is ready to go).  Keep leads as short as you reasonably can - that will reduce the harmonics (voltage spikes).  The duty cycle on a manual mill is generally extremely low (few hours and generally lightly loaded) - in practical terms it is going to last a very long time.  It will be up to you to operate the motor such that you keep a reasonable RPM so the cooling fan moves a bit of air (i.e. keep the motor speed up above 45 Hz if you are loading the motor).

I have a drill press with a 3/4 HP, 1200 rpm, 3 phase motor.  It has class B insulation.  I acquired a nice little VFD (vintage mid '90s) - it probably does not have the features of the newer VFDs - the drive and the motor are quite noisy.  The VFD is close to the motor, the leads are short and I'm running the start/stop buttons into the VFD controls.  The drill press performs very well.

Of course a higher rated motor would be nice, but the likelihood that the motor with class B insulation will fail because it is on a VFD is extremely small (if the motor insulation is that weak, it was probably close to failure anyway).

Let us know how you make out.  David


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## Susan_in_SF (Mar 7, 2019)

hman said:


> Well, durn, Susan!  I live in the Phoenix area myself, and know Neil from the Valley Metal club.  Fantastically nice guy, isn't he?  I would have really enjoyed meeting you.  Give a shout next time you plan a trip out here.


Hman, this is awesome.  Now we have an excuse to go out and get some drinks!  I visited AZ also since my best friend from high school now lives in Phoenix.
Here's a crazy pix of Neil showing me how he "rides" his horizontal bandsaw, lol!



Hey, I guess when you're older, comfort comes first.


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## Susan_in_SF (Mar 7, 2019)

Chipper5783 said:


> About the Bridgeport / VFD / motor thing.  Here with I share some opinions. You are power constrained - 20A @ 120V.  Nominally 2400 watts.  The VFD will not achieve the voltage change (120 -> 240).  You have a 3000W transformer (single phase - I assume).  Confirm the VFD is rated at a 240V input (which it probably is).  So you connect the 120V side of the transformer to your supply (via a single pole disconnect) and the high voltage side to your VFD inputs.
> 
> Run your existing BP motor (mainly because it has the smaller current draw, it works fine for the machine - and it is ready to go).  Keep leads as short as you reasonably can - that will reduce the harmonics (voltage spikes).  The duty cycle on a manual mill is generally extremely low (few hours and generally lightly loaded) - in practical terms it is going to last a very long time.  It will be up to you to operate the motor such that you keep a reasonable RPM so the cooling fan moves a bit of air (i.e. keep the motor speed up above 45 Hz if you are loading the motor).
> 
> ...


David,
I moved my comment to the Bridgeport posting.  I accidently typed it under my lathe posting.
I read your reply, but will read again.  I will post my reply under the Bridgeport posting.  Sorry for my mixup.  I was typing quickly in my car - parked but still hurrying.  I really appreciate your insight


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## stupoty (Mar 7, 2019)

Susan_in_SF said:


> Hi Guys,
> As many of you already know, I have too many lathes (6", 9", 10").  I was happy having a South Bend 9A 9" lathe, then got the free Logan 200 10" lathe.  Ideally, I would like to keep both lathes.  The Logan does not have a quick change box.
> 
> Awhile ago, I bought this tailstock tap/die holder from a machinist online, Neil.  On his website, there is a video on how to use his gadget.
> ...




I am on the third lathe I have owned (I still have the other two they were a lot littler)and it's my first with a quick change gearbox, I would never purchase a lath without a quick change gearbox now.



Stu


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## MAKEITOUTOFWOOD (Mar 7, 2019)

Sell them both and look for a lathe that better suits you.


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## chips&more (Mar 7, 2019)

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.


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## Robo_Pi (Mar 7, 2019)

I don't even have a QCGB and I still go to the trouble of manually changing gears just to use the threading feature of my lathe. 

My lathe is also not "threading friendly".    I have no thread dial on the lead screw.   So the only way to thread is to keep the half-nut engaged throughout the entire process, which means that I need to reverse the lathe between every cut.  I can't back up using the handwheel.  

I'm looking forward to the day when I can buy a lathe that has a QCGB and a thread dial.   In the meantime I've been cutting nice threads with the set-up I have.  It is a pain though having to manually swap the proper gears out.  But I'm getting that process down to a science so it's not too bad.


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## dtsh (Mar 7, 2019)

Robo_Pi said:


> I don't even have a QCGB and I still go to the trouble of manually changing gears just to use the threading feature of my lathe.
> 
> My lathe is also not "threading friendly".    I have no thread dial on the lead screw.   So the only way to thread is to keep the half-nut engaged throughout the entire process, which means that I need to reverse the lathe between every cut.  I can't back up using the handwheel.



I do it much the same way. I have a threading dial, but it's not always useful as I tend to cut metric threads as often as US Customary/Imperial, which is why I mostly prefer change gears. On the 1903 SF lathe. a 12TPI leadscrew and gears (18,24,26,30,36,36,42,44,46,46,54,60,66,72,78,84,96) allows me to match an absurd number of threads/feeds. I don't argue that a QCGB would be desired in a production shop where time == money, but shaving a minute or two off production time won't save me anything.

As for the threading dial, I suppose one could be fabricated if you can't find a used one; however, I don't find it to be a required accessory.


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## Robo_Pi (Mar 7, 2019)

dtsh said:


> I do it much the same way. I have a threading dial, but it's not always useful as I tend to cut metric threads as often as US Customary/Imperial, which is why I mostly prefer change gears.



That's an interesting point.   I know my lathe will cut both metric and imperial threads as I have cut both already, and the gear charts that came with the lathe include gear combinations for both metric and imperial thread cutting. 

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.  My gear set only includes the two special gears for the metric lead screw.  So I guess my lathe has the metric lead screw?  I could probably check the lead screw pitch with thread gauges to find out. 

Would this then make a difference is I added a thread dial?  Obviously if I have a metric lead screw I would need to make a thread dial with a metric thread to match the lead screw.   But what then?   Could the thread dial still be used when cutting either metric or imperial threads?  Or would it only be useful for cutting metric threads?

Sorry for my ignorance, but I really don't know how that would work. 

I'm assuming that if I get a lathe with an imperial lead screw that comes with a thread dial it would also cut metric threads?   I don't even know how that works. 

All I know is that my current lathe will cut either metric or imperial and I've been cutting both.   So far I've done an imperial 3/8-16, a metric M8 x 1.25 and a M10 x 1.5 and they all came out perfectly.   These were all covered in my thread gear charts.   It looks like I can do most imperial and metric threads.


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## dtsh (Mar 7, 2019)

Robo_Pi said:


> 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.  My gear set only includes the two special gears for the metric lead screw.  So I guess my lathe has the metric lead screw?  I could probably check the lead screw pitch with thread gauges to find out.



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)



Robo_Pi said:


> Would this then make a difference is I added a thread dial?  Obviously if I have a metric lead screw I would need to make a thread dial with a metric thread to match the lead screw.   But what then?   Could the thread dial still be used when cutting either metric or imperial threads?  Or would it only be useful for cutting metric threads?



I think it would matter. I'd need to think and double check, but if your leadscrew were say 2.0mm pitch, the threading dial would only be useful for multiples and some divisions thereof. If the dial were divided in say quarters, I _think_ it would match in 0.5mm steps.



Robo_Pi said:


> Sorry for my ignorance, but I really don't know how that would work.


I'm guessing and hope if someone actually knows they will pipe up and inform us both!



Robo_Pi said:


> I'm assuming that if I get a lathe with an imperial lead screw that comes with a thread dial it would also cut metric threads?   I don't even know how that works.


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.



Robo_Pi said:


> All I know is that my current lathe will cut either metric or imperial and I've been cutting both.   So far I've done an imperial 3/8-16, a metric M8 x 1.25 and a M10 x 1.5 and they all came out perfectly.   These were all covered in my thread gear charts.   It looks like I can do most imperial and metric threads.



Mine has a tiny brass plaque with some imperial/customary threads, but I use a script I wrote to select my gears, usually in 3 or 4 gear combos where 2 gears are ganged on a spindle, driving the other two, which are connected to the spindle and the leadscrew. I've cut a variety of threads in both measures so I know it's accurate within tolerances.



Where the top large gear engages the middle gear, which is locked together with a second, smaller gear that engages the bottom gear which is connected to my leadscrew.


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## Firstgear (Mar 7, 2019)

Robo_Pi said:


> By the way, that threading tool looks pretty easy to make.
> 
> Apparently all you need to do is make the following parts:
> 
> ...


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.


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## ericc (Mar 7, 2019)

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.


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## Robo_Pi (Mar 7, 2019)

dtsh said:


> 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.




Firstgear said:


> 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.


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## savarin (Mar 7, 2019)

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.


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## Robo_Pi (Mar 7, 2019)

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.


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## hman (Mar 7, 2019)

Robo_Pi said:


> 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





dtsh said:


> 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%.


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## Robo_Pi (Mar 8, 2019)

hman said:


> 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.


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## Susan_in_SF (Mar 9, 2019)

chips&more said:


> 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


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## Susan_in_SF (Mar 9, 2019)

Robo_Pi said:


> 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:
> 
> ...


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


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## Winegrower (Mar 9, 2019)

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.


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## Susan_in_SF (Mar 10, 2019)

Winegrower said:


> 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.


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## Susan_in_SF (Mar 10, 2019)

Heh, heh, heh,
I found an older man who is willing to sell me his QCGB off his Logan 200.  I told him I was broke, and asked if he would hold it until the end of the month, and he said he would.  Tool hoarding is real


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## Ulma Doctor (Mar 10, 2019)

Susan_in_SF said:


> Tool hoarding is real


you can say that again, sister!


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## lordbeezer (Mar 10, 2019)

I don't remember ever seeing a 200 with qcgb..


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## Susan_in_SF (Mar 10, 2019)

lordbeezer said:


> I don't remember ever seeing a 200 with qcgb..


Dear lord,
The Logan 200 lathes were originally sold without the QCGB's, the QCGB from other specific models (from 10"-12" lathes) could be installed on the Logan 200.  
Here is a screenshot of tubalcain with his Logan 200 and obvious add-on box.


So, the old guy (not tubalcain) had an ad up offering a Logan 200 with QCGB, and I sent him an email asking if he were willing to sell the box separate from the lathe.  To be honest, I wasn't expecting him to say yes.  He is also willing to mail it out to me.  Sometimes, you really can't go wrong with asking


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## Chipper5783 (Mar 10, 2019)

QCGB?  Yes or no?  I really like having a QCGB, for all the reasons others have already mentioned.  Of course it is a great convenience for threading.  I used to have a regular job come in needing 3tpi - my selection only went as coarse as 4 tpi - so I got a change gear and away I went.  I desire to get the few change gears to finish out the metric thread pitches that are not available with the 127T conversion on my Imperial machine.  In other words, even with a QCGB, swapping gears may be needed from time to time.

There may be more to the story.  Martin Cleeve wrote an excellent book "Screwcutting in the Lathe" - he knows much more about threading than I, and most of the individuals here, ever will.  It seems his goto lathe for threading, a heavily modified Myford - did not have a QCGB.





Me?  I'll keep my QCGB.


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## lordbeezer (Mar 11, 2019)

Kool..you may have to modify your lead screw..drill and tap your bed..looks like a fun project..


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