Really basic lathe question

[slow-poke]

I have lost my fear of asking questions that demonstrate my ignorance of a particular topic, better to ask and learn than remain ignorant......

On lower cost lathes for example my CX706 the lead screw drives the carriage. On upscale lathes there are often two shafts a lead screw and another shaft. Can someone explain what the other shaft is for, functionality, advantages etc?

 

[DavidR8]

The other shaft is strictly for power feed if I understand things correctly.
On really big lathes there will be a third which I think controls the spindle engagement.
Abom79 has one like the latter.

 

[6.5 Fan]

What David said. Doesn't have to be a really big lathe to have the third rod for spindle control though, lots of 13-14 x36-48 have that feature. just costs a bit more.

 

[garageguy]

The second shaft saves wear on the lead screw and half nut.

 

[Brent H]

Lathe “shafts”

- lead screw for threading and sometimes feeding
-Feed screw: drives a separate set of gears in the apron to feed the lathe at particular selection
- third rod, typically a square profile and just lets the user select FWD, stop and REV. The control handle rides along with the apron so you have that control right there.

 

[DPittman]

Just to add to the above..
A single shaft machine uses the leaf screw to drive both carriage travel for turning and threading. It was used as a cost saving method of having a second shaft to drive the carriage travel but on many models that resulted in increased half nut wear also. As a solution to the half nut wear problem some smart cookie came up with the idea of a keyed slot on the lead screw that could be used to drive carriage travel without needing to use the half nuts.

 

[DPittman]

Ha

Just to add to the above..
A single shaft machine uses the leaf screw to drive both carriage travel for turning and threading. It was used as a cost saving method of having a second shaft to drive the carriage travel but on many models that resulted in increased half nut wear also. As a solution to the half nut wear problem some smart cookie came up with the idea of a keyed slot on the lead screw that could be used to drive carriage travel without needing to use the half nuts.

Ha in the time it took me to type my response there were a couple of other responses that made mine redundant .

 

[Susquatch]

I have lost my fear of asking questions that demonstrate my ignorance of a particular topic, better to ask and learn than remain ignorant......

The way I look at it is that it's far better to learn from someone else's mistakes than my own......

On lower cost lathes for example my CX706 the lead screw drives the carriage. On upscale lathes there are often two shafts a lead screw and another shaft. Can someone explain what the other shaft is for, functionality, advantages etc?

I think everyone has covered it except for one nuance. Threading can often demand more precision than simple turning. Therefore saving wear and tear on the leade screw by moving the feed activity to another drive source helps maintain the precision of the threading operations. Furthermore, threading is seldom a high load operation so the half nut is plenty strong enough for that load demand.

Doing the primary turning activity puts the turning load on a gear drive system instead of a screw. Gear drives can wear slowly without affecting precision and take a lot more abuse in the process.

For low volume hobby usage, I really don't think it matters much. Perhaps the size of the lathe also limits the load that can be generated which offsets the need for two separate systems.

That's all I can add.

 

[Susquatch]

Ha

Ha in the time it took me to type my response there were a couple of other responses that made mine redundant .

Ditto! LOL!

 

[Dabbler]

One other tiny advantage to the two driveshaft design... power feeds are almost always much finer than treading. By having the second shaft do power feed, it helps to keep power feeding in the right ranges and gives you a much finer feeds for better surface finish.

Single shaft and keyed single lead screws can also do the job just fine, BTW.

 

[phaxtris]

*Edit* -On most lathes- the half nut does not run the carriage feed on a single shaft lathe, the carriage power is taken from the keyway slot in the lead screw, there is not any wear to the half nut or threads of the lead screw as neither is engaged unless threading, although you could argue the shoulders of the keyway slot may get some wear from the power take off

there is no reason to not buy a lathe because it only has one shaft vs two, as @Dabbler pointed out they both do the job just fine

 

[YYCHM]

the half nut does not run the carriage feed on a single shaft lathe, the carriage power is taken from the keyway slot in the lead screw, there is not any wear to the half nut or threads of the lead screw as neither is engaged unless threading, although you could argue the shoulders of the keyway slot may get some wear from the power take off

there is no reason to not buy a lathe because it only has one shaft vs two, as @Dabbler pointed out they both do the job just fine

Not totally true. On a mini-lathe the only way to PF is with the half-nut.

 

[phaxtris]

Not totally true. On a mini-lathe the only way to PF is with the half-nut.

i didnt even know those mini lathes had power feeds, always an exception i guess

 

[YYCHM]

i didnt even know those mini lathes had power feeds, always an exception i guess

Guess it depends on what you consider PF to be. Engaging the half-nut to attain a PF scenario accomplishes the same thing. At least that's how I used it....

 

[phaxtris]

Guess it depends on what you consider PF to be. Engaging the half-nut to attain a PF scenario accomplishes the same thing. At least that's how I used it....

did yours come with the gears to get proper pf speeds ? or did you just use the highest tpi gear setup it came with ?

 

[Dabbler]

Not totally true. On a mini-lathe the only way to PF is with the half-nut.

Didn't your lead screw have a keyway slot for PF?

 

[Larry_C9]

On my old machine that just had the lead screw the power feed for the cross slide came a gear in the carriage drive from the keyway in the lead screw but for the longitudinal power feed for the carriage you had to use the half nut and I wore a couple of them out. On my new machine that has the separate power feed shaft it drives both the power feeds.

 

[YYCHM]

did yours come with the gears to get proper pf speeds ? or did you just use the highest tpi gear setup it came with ?

I hated my change gears with passion and only changed them once to try threading. For the most part I used the default gears which were rather fast for PF turning. But it was PF per say.

 

[DPittman]

Not totally true. On a mini-lathe the only way to PF is with the half-nut.

Yes and the same with the old Atlas 618 lathes. I replaced half nuts on that lathe for that very reason.

 

[phaxtris]

ok i will revise that, Most lathes do not use the lead screw threads for pf, Most use a slotted keyway in the leadscrew

@Larry_C9 what lathe did you have, thats a very curious setup