Engaging the leadscrew with the gears

[Jeremy PaduanoParticipant @jeremypaduano78634]

Hi all,

More of a worry than a problem, but hopefully someone can advise.

I’m disconnecting the leadscrew from the spindle gears in order to stop the cut whilst the motor is still running – I’m also winding off the toolpost at the same time so that the cutter doesn’t just sit stationary against one spot. No issues so far, but before I start the next cutting pass I stop the lathe and re-engage the leadscrew when there is no rotation. This gives me a lot of stop/start on the motor and for the work as I let things get up to speed each time.

My question is – am I being overly cautious only meshing the teeth on the leadscrew with the saddle gear when it’s all stationary?

Hopefully someone will be able to say “don’t be daft, just get on with it, otherwise you’ll take all day to cut anything”, but until I’m sure…

The lathe is a Warco WM240B – the version with the belt drive.

Thanks for taking the time to read this – thanks even more if you know the answer!

😊

[Jeremy PaduanoParticipant @jeremypaduano78634]

[JasonBModerator @jasonb]

You should leave the leadscrew being driven by the gears all the time and just use the lever on the right of the apron to engage and disengage the half nuts at the end of a cut. This is for metric threads on a metric lathe or imperial threads on an imperial lathe

If cutting metric threads on an imperial lathe or imperial threads on a metric lathe then leave the half nuts engaged and just stop the spindle, back out the tool and reverse the motor to get back to the start

[old martParticipant @oldmart]

The lever on the apron is made for engaging and disengaging the leadscrew when the leadscrew is rotating. You should practice with the saddle right in the middle of its travel to give you plenty of time to see what is happening and stop without crashing.

[Nigel Graham 2Participant @nigelgraham2]

Well, you should engage the change-wheels themselves only when the motor is off and the lathe stationary. It avoids you damaging their teeth or worse, them eating your fingers.

As others say, keep the change-wheels engaged and use the feed-lever on the apron for controlling the cut. I do though put their drive in its neutral setting (with motor stopped) when not needed.

I don't know the Warco lathes specifically but does yours have powered cross as well as long feeds? My Harrison L5 does and it is my habit to move the saddle well clear of the work and chuck, and engage the drive with the motor revs down, to verify I have set the right feed and right direction before starting cutting, be it plain turning or screw-cutting.

(This lathe can feed in both directions on both axes, and cut right and left-hand threads, but the sliding and surfacing feeds are of opposite "polarity" with respect to the reverse gear ! )

[Jeremy PaduanoParticipant @jeremypaduano78634]

Thanks everyone. I think you all saw through my slightly garbled question 👍 and answered what I was actually talking about – using the lever to engage the half nuts. (It’s so much better when I know the right terminology to describe what I mean!) I now know that I can safely engage the lever with everything still running – result 😁

Thanks again everyone. Hopefully at some point in the future I’ll be able to contribute as well as ask questions.

Jerry P.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Jeremy Paduano on 12/06/2021 10:07:54:

Thanks everyone. I think you all saw through my slightly garbled question 👍 and answered what I was actually talking about – using the lever to engage the half nuts. (It’s so much better when I know the right terminology to describe what I mean!) I now know that I can safely engage the lever with everything still running – result 😁

Thanks again everyone. Hopefully at some point in the future I’ll be able to contribute as well as ask questions.

Jerry P.

It's a good question! Although OK to operate the half-nut lever when the machine's running, the same can't be assumed of the gearbox, if it has one.

My WM280 has the usual collection of change gears on a banjo plus a built-in 3-speed gear box, with reverse and so forth. The gear box is crude, and although it can be changed in motion, it's not a good idea. On mine it's sometimes necessary to feel for alignment by turning the spindle to and fro gently by hand. Other lathes have an elaborate gearbox able to produce many different thread pitches. I believe most of these are also meant to be stopped before changing gear, and only the most expensive lathes are happy to change gear at any speed.

Gearboxes are funny things. Gears can be changed at high-speed provided the teeth happen to mesh at just the right time, mostly luck and maybe there's a knack to it. Unfortunately gears are likely to be damaged every time spinning teeth don't quite mesh properly, and the ill-effects can range from gradually increasing wear due to mild near-misses to a tooth removing, key shearing, axle bending crash. Lathe gearboxes are simpler than car gearboxes. The latter are specifically engineered to match gear wheel speeds before attempting to mesh. Even so I occasionally crunch mine. Blush.

Dave

[Howard LewisParticipant @howardlewis46836]

in a nutshell, the previous advice is NOT to disengage any of the changewheels.

The Half Nuts can be disengaged, to back out the tool, return the Saddle, and re-engage according to the indication shown by the Thread Indicator Dial. This will differ according to the pitch of the thread that you are cutting.

Check the Operator Manual!

As already said, if cutting a Metric thread on a lathe with an Imperial Leadscrew, or vice versa, keep the Half Nuts engaged, back out the tool, and reverse the Lathe to take the Saddle well clear of the work. This will ensure that any backlash between Half Nuts and Leadscrew is out, before the cut restarts.

Do NOT use the Tumbler Reverse for this!

HTH

Howard

[andrew lynerParticipant @andrewlyner71257]

Posted by SillyOldDuffer on 12/06/2021 13:19:08:

Gearboxes are funny things. Gears can be changed at high-speed provided the teeth happen to mesh at just the right time, mostly luck and maybe there's a knack to it. Unfortunately gears are likely to be damaged every time spinning teeth don't quite mesh properly, and the ill-effects can range from gradually increasing wear due to mild near-misses to a tooth removing, key shearing, axle bending crash. Lathe gearboxes are simpler than car gearboxes. The latter are specifically engineered to match gear wheel speeds before attempting to mesh. Even so I occasionally crunch mine. Blush.

Dave

Gears themselves are very susceptible to damage by trying to engage them when moving relatively. Most car and m/c gear boxes are 'constant mesh' type, in which each pair of gears are constantly in mesh and a gear is 'selected' by sliding and using 'dogs' which lock together on a shaft and do not roll together as as the actual gears do. Dogs are very tough and will take all sorts of abuse – clutchless changing up or down used to be easy peezy. A chipped dog is no disaster. The plastic gears on cheaper lathes would never stand up to graunching at speed.

Let's face it, do you save a great deal of time by taking that short cut? It's not like trying to beat the next guy at traffic lights, is it?

[old martParticipant @oldmart]

Most lathes should be stopped before changing any gears to prevent crashing, grinding and damaging the gearing. The exception is the leadscrew nut. The leadscrew nut is intended to be engaged and disengaged while the machine is running as it is impractical to keep stopping before using it. This does cause wear to the nut in particular, but it is unavoidable.

[Howard LewisParticipant @howardlewis46836]

Even constant mesh gears should be correctly synchronised before meshing the dogs. Hence the need for double declutching, or correctly judged engine and road speeds when the clutch is not used. Unless the vehicle has synchromesh to do the synchronising, or an epicyclic gearbox.

You can drive clutchless with a synchromesh box, but the engine and road speeds have to correctly synchronised to avoid excess wear on the synchronising mechanism

(Our daughter managed to drive her car back to East Anglia, from the Hague, when the clutch cable broke! )

But on machine tools, to avoid damage, the gears should be stationary when being engaged. This may involve rocking the chuck to and fro until a tooth lines up with a tooth space in the mating gear.

Howard.

[ChrisBParticipant @chrisb35596]

If the gearbox on the WM240 is similar to the WM280 then gears can be changed with the spindle in motion without any ill effects, as long as the speed is low. In fact, with the lathe stationary most often than not, the gears will not engage and you have to turn the chuck untill the gears align. This has been discussed a while ago and I had posted a video of this in action. The warning on the lathe warns against changing gears at high speed.

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