Newbie Question about Variable Speed on lathe lead screw

[OaklandGB]

Not wanting to hijack an earlier post about using variable speed treadmill motor to run a lathe lead screw, I thought my newbie question might have a simple and direct answer.

Wouldn't a variable speed driven lead screw have a real potential to mess up on threading or other ops that are dependent upon an absolute relationship between the lead screw and the chuck? Especially if tool forces come into play, or slight variances in motor rpm tolerances exist?

Maybe I missed the whole point of the need for a separate variable speed on a lead screw? That's probably why I'm a newbie and not a machinist.

[J Tiers]

Two different functions.

1) Threading, which keeps the gearbox/change gears.*

2) Turning feeds, where variable speed is very helpful and useful

Obviously one does not change entirely over to pure variable speed, unless one has no wish to ever do any threading.

* I would not classify the "Electronic Lead Screw" as a "variable speed", since it is simply substituting a "servo" for the gearbox

[OaklandGB]

J Tiers,
OK now that makes sense to my beginner's mind. Geared lead screw maintains thread cutting relationship with the work's turning, hopefully zero variation to get consistent and expected threads. The turning work speeds otherwise may be varied according to material and tool used. Better than having to do a gear change pit stop to adjust.
Thanks for confirming.

[BCRider]

Oakland, if you were considering this you sure don't need the treadmill motor for such a project. They make way more power than needed.

What I have seen in other threads is an automotive power window or windshield wiper motor and gear assembly run by a 12volt supply with a separate power control to regulate the power to the motor and thus slow it down. These motor units easily make enough torque to turn the lead screws. And for those with the fully manual gearing changes or the semi manual change boxes it's a nice option to consider to avoid frequent gear changes under the end cover. You just need some way to disengage the drive from the lead screw when threading.

[J Tiers]

................... You just need some way to disengage the drive from the lead screw when threading.

Easiest way is to drop both the QCGB levers, or adjust the "banjo" out of mesh, depending on how the thread gearing is set up .

[BCRider]

Easiest way is to drop both the QCGB levers, or adjust the "banjo" out of mesh, depending on how the thread gearing is set up .

I was thinking of the other end with the motor feed unit. But yep, no point in spinning all the thread gearing when power feeding.

[OaklandGB]

BCRider,

Nope, not thinking of any changes to my cheap lathe's lead screw drive. That would be way beyond my pay scale......$0 USD ($0CDN) which is commensurate with my experience level.

Just trying to get an understanding of what the other poster on this subject was trying to accomplish. I was just mentally stuck on how variable speeds might mess up making threads. Tiers noted the differences I wasn't thinking about. Still learning here.

[Bented]

* I would not classify the "Electronic Lead Screw" as a "variable speed", since it is simply substituting a "servo" for the gearbox

A servo driven NC lead screw is about as variable speed as it gets, I use 25+ year old lathes that will feed at 100 inches per minute if you program it that way. They will also feed in inches per revolution from .0005 to the maximum rapid speed of the Z drive, in this case 1" per revolution at 100 Rpm's. Doing so has little use and will not be terribly accurate
I consider this "variable speed".

New lathe arriving tomorrow, an entry level CNC machine with 400 feet per minute Z rapids.

[J Tiers]

A servo driven NC lead screw is about as variable speed as it gets, ..........

One "sets" the feed, or tpi, as opposed to "twisting a dial", and it does what you ask, just as if you put in a change gear, no questions, no nonsense, you get what you set...... A variable speed, with a dial you "kinda adjust for what you want". And, you "sorta get what you asked for". It's "kinda-sorta"

The dial is "variable speed", the NC is "settable feed".

[Bented]

One "sets" the feed, or tpi, as opposed to "twisting a dial", and it does what you ask, just as if you put in a change gear, no questions, no nonsense, you get what you set...... A variable speed, with a dial you "kinda adjust for what you want". And, you "sorta get what you asked for". It's "kinda-sorta"

The dial is "variable speed", the NC is "settable feed".

Virtually every machine that I have ever used also has a feed "dial", rotate the dial clockwise and the feed increases, counter clockwise slows the feed.

[J Tiers]

"Can" is different from "Did".................... you do not HAVE to do that for all feeds.........

[wierdscience]

Well on my 9x20, the problem I had was. If I had the lathe setup to produce my most commonly used threads( 24,20,18,16,12,) on the QCGB, the resulting feed rates in that rage were .020,.022,.026 etc, which are too coarse for normal turning. The only way to get a finer feed range was by swapping gears around in the gear train between the spindle and QCGB.

So my solution was to mount a milling machine table feed to the end of the leadscrew. Since that feed has FWD/N/REV built in and Neutral is free wheel, the leadscrew and QCGB are unaffected.

So yes, a means to disconnect the electric feed is desirable, but doesn't need to be anything fancy. It could be as simple as a Lovejoy jaw coupling and a motor mount that slides to disengage the coupling for threading.