Running a Myford in Reverse?

[Howard LewisParticipant @howardlewis46836]

If the chuck is not firmly home, the acceleration on start up in reverse, with a single phase motor, can slacken the chuck

Hopefully not enough for it to come off.

Often, the start up acceleration, plus cutting forces cause difficulty in removing a screw on chuck

VFDs tend to give a softer start, because of the ramp up to set speed. Even then, a sharp impact on the end of a bar can be needed to slacken it!

Needless to say, for anyone contemplating it, do not engage back gear to lock the headstock!

Howard

[John BaronParticipant @johnbaron31275]

Hi Guys,

FWIW I do tend to use the back gear to lock the chuck when unscrewing it, although the S7 does have a locking pin to stop the spindle being turned when removing the chuck. I've never found the need to use anything more than the chuck key in order to loosen the chuck.

I do have a 125 mm three jaw on a backplate that is a lot harder to loosen than the original Myford 100 mm chuck. Plus it overhangs the bed much further than the slim body Myford one.

[A SmithParticipant @asmith78105]

I screwcut a a very slow speed. If the chuck stated to unscrew when cutting a left hand thread, very unlikely IMO, the work would slow/stop and the cutting tool would very quickly show distress! In that situation, I am certain that I would: 1) have a ruined work peice and 2) have disengaged the clutch within one or two rotations.

Andy

[OldironParticipant @oldiron]

A screw-on chuck, as on the Myford or Boxford type lathes, WILL ALWAYS unscrew unless specially secured.

That statement is absolute rubbish. OK it is possible but I have done 100's of metric studs up to 20mm on my imperial Boxford using reverse and never once has the chuck tried to come off. My chucks are not stuck on and can be removed without much effort. I mostly use 4" chucks so not a lot of weight involved. The only way it is likely to try to come off if someone is stupid enough to throw the lathe into reverse when running at high speed. The VFD ramps down the speed to a stop and then ramps it back up. As far as I see it why would Boxford have sold lathes with reversing motors /switches if they thought that every time you ran in reverse it wouild throw the chuck off. My brother has an ME10 bought new with threaded spindle & reversing motor. If used properly and with respect I do not seeing running in reverse as a problem.

regards

[SillyOldDufferModerator @sillyoldduffer]

Posted by John Baron on 12/02/2021 06:54:06:

Hi Chris, Guys,

[quote]A screw-on chuck, as on the Myford or Boxford type lathes, will always unscrew unless specially secured. I have had a chuck fly off when I accidentally knocked the Dewhurst switch lever over and threw the Myford into reverse.[/quote]

This can only happen on a machine with a three phase motor ! Switching to reverse whilst running forward will not stop the motor, it will just keep running. The single phase motor speed has to slow down enough for the centrifugal switch to drop out reconnecting the start winding.

Wouldn't reversing the phase of a single phase motor's run winding at speed cause harsh braking? If so, that alone could spin the chuck off. I don't think it's necessary for the motor to stop and reverse to unwind the chuck, it's enough for the spindle to suddenly turn slower than the chuck. If the motor suddenly braked I believe the energy stored in the chuck would be enough to unscrew it,

Has anybody other than Chris switched their Dewhurst over at full speed? Does the motor stop suddenly and then reverse, or does it just carry on?

Intriguing. I could be completely wrong!

Dave

[Tim StevensParticipant @timstevens64731]

In my experience, as long as you avoid any process when turning 'backwards' which will shock the system, there is not likely to be a serious unscrewing risk. So, no interrupted cuts, no stock removal of brass with a brand new cutter ground for steel, etc. But if you are keen to add a belt to your braces, in effect, read on:

There is usually room within a Myford-sized chuck to add a flat steel strip with a tapped central hole. Into this hole a threaded rod is screwed, passing back through the spindle to the end, where another thread is used with a thick washer to pull the tightened chuck tighter against its location. This increases the force required to loosen the chuck by mistake, adding to the friction already there.

Of course, this arrangement will get in the way if you need the hole through the spindle for a long workpiece.

Cheers, Tim

[Roderick JenkinsParticipant @roderickjenkins93242]

Perhaps it depends on the motor. If the motor has a centrifugal switch then the starting / reversing circuit is dead when the motor is running. When I had a single phase motor on the S7 I would occasionally catch the reversing knob on my coat and push it into reverse. Had no effect on the motor.

Rod

[SteviegtrParticipant @steviegtr]

From the op's original question then other than the threaded bar down the centre. I have never heard or seen another way to lock the chuck on. I also would never try to use the Myford in reverse for anything that is cutting. Other than when I am using a Die or Tap. Which i usually screw the chuck by hand , very slowly.

Steve.

[Martin DowingParticipant @martindowing58466]

If these screws are small then Jacobs drilling chuck or other small chuck installed on MT2 taper shank can be used with a good drawbar and said drawbar should preferentially have LH threads, albeit unlikely failure of normal setup is rather inconsequential – it will get loose but usually stay in place long enough to react.

An alternative is a Myford collet system, one used to screw on the spindle nose as it can handle up to 1/2 inch and 13 mm.

Tightened with a tommy bar such collet will *not* undo and even if it did by miracle, it is also inconsequential.

Screwed on chuck in reverse is not in my cooking book at all for other purpose than undoing stubborn chuck (on backgear).

Also ML 7 have thrust bearing designed to take pressure of cut in normal R–>L operation.

Operated another way will rely on thrust collar what can be occassionally done but it is not a recommended procedure.

I will tend to operate my ML7 that way while making LH screws (of course spindle turns in normal direction during such exercise).

I am not a safety maniac at all and sometimes work in a way which would cause a heart attack of HSE official but common sense is telling me not to run ML7 or any other lathe with screw on chuck in wrong direction… Neither keeping your eyes in firing line of tool, should it get broken, or meddling with long swarf while machine works is a good idea.

But it is your lathe, so do what you see fit… You may get away with a lot while your lucky star is shining on.

Edited By Martin Dowing on 12/02/2021 22:44:46

[Mike PooleParticipant @mikepoole82104]

We have probably all used the lathe in reverse with a chuck mounted, working between centres will only risk the catch plate unscrewing and it will be retained by the work. The reverse facility is very useful for thread cutting metric pitches with an imperial leadscrew when the half nuts must be not be disengaged. Having reverse on a lathe is not an endorsement that it is ok to turn in reverse. There are plenty of anecdotes of chucks coming undone in reverse just as there are plenty of I have done it and it was ok stories ( me included). If you work in reverse then the chuck unscrewing should be born in mind rather than occurring as a complete surprise. The story has been related of a very large lathe being accidentally started in reverse and the chuck seen to be unscrewing, the quick witted operator quickly flicks it into forward and the chuck jams on the nose irremoveably. The backplate ends up having to be turned away from the nose. Climb milling without proper backlash control is probably something we all do especially for a final finish cut but you should be aware of what can go wrong and the consequences. I add a bit more friction by lightly applying the axis lock but of course this will contribute to wearing the nut and leadscrew. I doubt that a screw on chuck will be encountered on a large lathe aimed at industrial use these days.

Mike

[KWILParticipant @kwil]

Safety in Reverse. This device was shown at MEX some years ago.

Edited By KWIL on 14/02/2021 12:58:33

[KWILParticipant @kwil]

Safety in reverse.

2MT adaptor with dowel

Intermediate backplate fitted with usual machining to hold chuck concentric. Intermediate plate bolted to backplate by 3 small bolts.Hole diameter greater than spindle bore.

In use, fit adaptor, fit chuck,rotate adaptor clockwise using allen key to take up any rotational movement of adaptor in dowel slots, fit drawbar.

It will not spin off now.

Yes you do loose the through spindle hole whilst in use.

[noel shelleyParticipant @noelshelley55608]

Gentlemen, In the context of a screw on chuck and sudden reverse of single phase motor. The start windings dictate the direction of rotation and take the motor up near running speed where the run windings continue to keep it running. This change is normally done by a centrifugal switch ! If the reversing switch is suddenly thrown over the centrifugal switch will have the start windings disconnected and since the run windings are NOT reversed the motor should continue to run in the same direction as before. Doing this with a 3 phase motor would not be so funny. Using a VFD is cheating and all bets are OFF ! Noel

[GeorgineerParticipant @georgineer]

Posted by noel shelley on 14/02/2021 13:39:06:

… If the reversing switch is suddenly thrown over the centrifugal switch will have the start windings disconnected and since the run windings are NOT reversed the motor should continue to run in the same direction as before. …

My lathe is set up with separate run and reverse switches so there is no centre-off. In a spirit of scientific enquiry… oh all right then, in a fit of absent-mindedness… I flipped the wrong switch and hit reverse instead of off. The motor didn't even notice and carried on as though nothing had happened. I can't answer for the theory, but I can vouch for what happens in practice.

Incidentally, the reversing switch is a standard intermediate light switch (cost £2.34 at the time) rated at 10A. I've moved it now to avoid a repeat.

George B.

[old martParticipant @oldmart]

I found a sketch of the Atlas reverse modification using a drawbar. I had posted it on another forum last year.

 

Edited By old mart on 14/02/2021 15:38:49

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