ML7 Main Shaft Slipping?

[Captain BarnaclesParticipant @captainbarnacles]

Good Evening All,

I inherited an ML7 about 5 years ago and with one thing or another I have only just got it set up and almost operating. It's been in storage for almost 5 years and before that it saw very little use in the preceding 10-20 years or so.

I'm an absolute beginner so I apologise if my terminology is incorrect or my question has an obvious answer, I have searched for an answer but I couldn't find an exact match for my particular problem.

My query is this. I powered up my lathe and tried turning a piece of 1" brass bar that I had laying around. As soon as the cutter made contact with the workpiece the chuck slowed right down.

I powered down and looked inside the headstock. When I turn the chuck by hand the pulley (and the gear immediately to the right of it) weren't rotating but the large gear (that engages with the back gear) and the small one on the end of the shaft (that engages with the lead screw gears) were.

I presume that something is not right with the shaft or pulley but I can't see any grub screws on the pulley that would lock it to the shaft. Am I missing something? The last time I used it I don't recall having this problem.

Thanks in advance for any help.

[Captain BarnaclesParticipant @captainbarnacles]

[Swarf, Mostly!Participant @swarfmostly]

Hi there, Captain,

Someone else is probably going to be a faster typist than I am! However, here goes:

Do you have the User Instructions booklet with your ML7? (I suspect that isn't its full title – I don't have mine to hand.) If you don't, it used to be downloadable from the Myford web-site.

The term you need to research in the book is 'back-gear'. There is (or should be) a little segment of gear that will engage with the exterior of the large gear-wheel (aka 'bull wheel' ) and is locked in (or out) of engagement by an Allen-headed screw. You need a shorter-than-standard Allen key to manipulate the gear segment.

If you lock it out of engagement, you also need to operate the back-gear lever (immediately to the left of the headstock front) – this puts the lathe in back-gear, i.e. low gear. With the segment engaged, the bull wheel is locked to the pulley so one drives the other directly.

There are some pictures on-line, I don't have a reference to hand just now.

Have a look at the book and at the hardware and hope for a 'light bulb moment'!

Best regards,

Swarf, Mostly!

Edited By Swarf, Mostly! on 09/03/2020 20:45:24

[Swarf, Mostly!Participant @swarfmostly]

Belay the last message!!!

I think the segment actually engages with the small gear on the three-step pulley.

The User Instruction book is your friend!

Best regards,

Swarf, Mostly!

[John OlsenParticipant @johnolsen79199]

Ok, Swarf mostly is probably right, the little toothed segment that should be engaged when you are not using back gear is probably not engaged. You need an allen key with a very short leg to work it. When you are in backgear it is in the out position and for normal work it slides in to engage with the gear.

However, another thing that can happen is that the aluminium pulley comes loose on the bronze sleeve that is part of the small gear. If this is happening you will be able to see that the pulley moves relative to that gear when you put a load on it. The only way to fix that is to dismantle the whole shaft and use loctite to improve the fit. I had to do this to mine, it has given no trouble since. You don't want any loctite getting inside the bronze sleeve locking that to the spindle of course.

John

[Captain BarnaclesParticipant @captainbarnacles]

Great info chaps, many thanks. I searched and found a PDF copy of the manual so I had a look through that and found the general bits and pieces that you referred me to. From there I found a YouTube video by Steve Jordan called "Changing Myford ML7 Headstock Spindle & Bearings Assembly Part 1" where, at about 5 mins in, he shows the toothed part clearly and all became clear.

I'm really hoping that it's as simple as that and that I don't have to pull the whole shaft assembly apart – I'll do it one day but for now I'm just itching to get turning

Thanks again for the help, I'll report back on my findings tomorrow.

[Captain BarnaclesParticipant @captainbarnacles]

I'm pleased as punch! It worked and I'm back in operation again. If I want to use the back gear do I need to disengage that tooth again? I tried engaging the back gear (without running the motor) and I couldn't turn the pulley, chuck or anything else by hand, it felt completely locked up.

Thanks again for the help,
Paul.

Edited By Captain Barnacles on 10/03/2020 20:47:54

[Nigel BennettParticipant @nigelbennett69913]

Yes, that's right, Paul; if you engage the toothed segment and the back gear it will all jam up. Engaged = direct drive, disengaged + operating the back gear lever = back gear.

[Captain BarnaclesParticipant @captainbarnacles]

Posted by Nigel Bennett on 10/03/2020 20:50:51:

Yes, that's right, Paul; if you engage the toothed segment and the back gear it will all jam up. Engaged = direct drive, disengaged + operating the back gear lever = back gear.

Many thanks for the confirmation Nigel. I have fitted a new three phase motor with a VFD so hopefully I won't have much need of the back gear, very useful to know how to use it correctly in the event that I do need it though.

Paul.

[not done it yetParticipant @notdoneityet]

Hi Paul

Remember the motor power decreases as the VFD is reduced below 50Hz and also the motor cooling is reduced as the fan slows, while the motor efficiency likely drops as well. Some have cooked their motors due to this.

[SteviegtrParticipant @steviegtr]

Mnnnn. Not sure about that.

[not done it yetParticipant @notdoneityet]

Not sure about what? If my post, just consider the power when switched on but rotation at zero RPM!! That should help you to think before posting and getting it wrong. Also just consider the cooling as the speed approaches zero!! And also think about the winding current as it approaches a resistive element codition (ie with little inductive back emf)!!

[Martin of WickParticipant @martinofwick]

CB

I always think VFDs should come with a Govt health warning along the lines of:

'This apparatus is no substitute for applications where mechanical reduction of shaft speed is available'

NDIY points out that it would not be wise to use the VFD at 5 Hz to run the spindle at say 50 RPM to carry out some high torque operation – most of the power will be absorbed in heating up the motor windings and very little delivered to the tool/workpiece interface.

So I am afraid you will need to get used to mechanically setting the spindle speed to the desired rate for the job and then, if required, fine tune with VFD adjustment. That way your motor and VFD will have a long and healthy life.

 

Edited By Martin of Wick on 11/03/2020 09:24:27

[Swarf, Mostly!Participant @swarfmostly]

Posted by John Olsen on 09/03/2020 21:03:53:

Ok, Swarf mostly is probably right, the little toothed segment that should be engaged when you are not using back gear is probably not engaged. You need an allen key with a very short leg to work it. When you are in backgear it is in the out position and for normal work it slides in to engage with the gear.

SNIP!

John

Thank you John, you're most kind. I ought to have checked the book before typing. I've been kept out of the workshop for a few weeks by one of the 'frailties of age' and my mental picture had faded a bit!

Best regards,

Swarf, Mostly!

[Robert Atkinson 2Participant @robertatkinson2]

The oft quoted "most of the power will be used up heating the motor windings" or references to reduced back EMF or torque are at best simplistic and do not reflect the basics of VFD operation. A correctly set up VFD will reduce the appled voltage proportionall with frequency maintaining the current and thus the torque. Resistive heating is I2R so remains the same regardless of speed. Thus the reduction in power with reduced speed is purely mechanical, less speed at the same torque is less power. Reduced cooling due to lower fan speed can be a problem but normally only if running at full load for extended periods. A external fan like those used in PCs can help with this.
When setting up a VFD and motor the motor should always be running at the highest speed possible so get a 2 pols motor and ad use pulley/belt reduction to get the maximum spindle spee you want (or max rating or spindle) with the motor runng at 60 or 65Hz. Two pulley ratios can still be an advantage even with a VFD if you do both high and low speed work.

Robert G8RPI.

[Neil WyattModerator @neilwyatt]

Any decent VFD will model the temperature of the motor and reduce, then cut power if there is the danger of overheating.

Not 100% reliable but a useful safeguard.

Neil

[Martin of WickParticipant @martinofwick]

Theoretically, RA is quite correct, but has not taken into account the man machine interface. Some VFD users may not be aware that the reduction in speed afforded by a VFD (or other speed controller) leads to a reduction in power available at the cutting edge. Under conditions of low speed (and low power) it is quite easy to overload the motor with the resulting increase in current drawn and consequent temperature rise.

I was hoping to point out to the OP (in my simplistic way) that a VFD is not a miracle device that means an end to any requirement for managing belt or back-gear settings.

Well, it pays to read the VFD manual to check for any software controls, likewise some new motors have an embedded thermistor that the VFD can monitor to prevent overheat as another safeguard.

However, the fundamental principle remains, in the first instance use the mechanical means of spindle speed control and use the VFD to fine tune or for other adjustments. The aim being to keep the motor at or close to the design frequency where possible.

[Captain BarnaclesParticipant @captainbarnacles]

A most interesting discussion, and very illuminating. I was aware (to some degree) that VFDs are not ideal for running motors at low speeds and that belt/pulley changes and/or use of the back gear are still necessary for such work. I also knew that motors could overheat due to the reduced cooling fan speed but I am now much wiser about the additional factors that make the VFD "less than ideal" for low RPMs.

With my limited turning experience I envisage I'll mostly be doing small diameter (<25mm) work and taking light cuts so I'll be running at pretty high speeds most of the time and keeping the motor close to it's design requirements. I'll be slowing down for threading and some boring operations I expect but I now appreciate the benefits of mechanical speed reduction over simply dialing down the VFD.

Thanks chaps.

[Cornish JackParticipant @cornishjack]

Make life simple – fit your VFD to a TriLeva – works a treat!

rgds

Bill

[Howard LewisParticipant @howardlewis46836]

Having mentioned that engaging back gear and the toothed segment locks everything solid (Basically two different gear ratios engaged at the same time )

DON'T be tempted to use this as a means of holding things while you use a mallet and bar to remove a chuck that is jammed on the mandrel! That way lies disaster. You may get away with it a few times, but eventually you will break one or more teeth from the gears.

By all means tighten the belt, in the highest speed position, out of back gear, (in the normal driving configuration ) to add the inertia of the motor before using the mallet on a piece of bar across the chuck jaws.

Alternatively, grip a piece of hexagon bar in the 3 jaw chuck, and use the mallet on the end of a correctly sized ring spanner.

HTH

Howard

[SteviegtrParticipant @steviegtr]

Constant Torque Characteristics This setting enables the output of a constant torque based on the frequency, according to the V/f characteristics that represent the proportional relationship between the output frequency and the output voltage. However, the output voltage is proportional from 0 Hz to the base frequency, it is constant independent of the frequency, from the base frequency to the maximum frequency. This setting is suitable for cart, conveyor, overhead traveling crane, and other applications where a torque is required, independent of the motor rotation speed.
This is a setting in the Omron inverters & many more. Not sure in the cheaper versions if this is available. Obviously constantly running at a very low freq would cause overheat if full torque was being demanded constantly of which a workshop lathe probably does not.

Most of the inverters in factory use that we & others installed ran between 35hz to 65hz for the motor control on a production line. They never had any trouble with overheating at those figures.

Steve.

[Howard LewisParticipant @howardlewis46836]

Daft question!

Is 0 Hz Zero volts, or full voltage DC ?

Howard

[Howard LewisParticipant @howardlewis46836]

Now it won't let me edit / delete the stupid question!

Howard

[SteviegtrParticipant @steviegtr]

Posted by Howard Lewis on 11/03/2020 17:44:05:

Daft question!

Is 0 Hz Zero volts, or full voltage DC ?

Howard

I would not know that. Maybe someone else will. Some of the inverters are quite amazing in what they can do. If that is what you are referring to. The cheap imported inverter's look pretty good but I have never even seen one , so not sure what can be done with them in terms of parameter alteration. .

Steve.

Edited By Steviegtr on 11/03/2020 17:50:23

[Martin of WickParticipant @martinofwick]

|……so not sure what can be done with them in terms of parameter alteration.

Not much apart from the basic operating ones setting ramp time, under/over voltage and current, carrier . That is why the supplied manual is only 2 sheets of A4! You could play with the Hz/V relationship as min mid and max Hz/v is user selectable, but I would be a bit wary.

There are 'factors' to modify torque at low speed, but no explanation. suspect they may only increase pulse width. If you ramp it too high it will cause the motor to 'cog' (feel like it is running over a washboard) at low speeds.

 

 

Edited By Martin of Wick on 11/03/2020 18:59:09

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