Myford ML7 Leadscrew

[Harry BroadbentParticipant @harrybroadbent75717]

Hello,

Does anybody know the absolute maximum Rpm that the myford ml7 leadscrew could spin if my spindle speed is 900 Rpm?

Cheers

[John HaineParticipant @johnhaine32865]

Do you mean before it risks seizing up or as limited by the change gears?  The latter must be limited by what gears you have and what lowest reduction ratio you can arrange – either with gears or rigging a toothed belt.  For the former, keep it lubricated and don’t run for too long!

[Brian WoodParticipant @brianwood45127]

Hello Harry,

If you will list the change wheels you have and the number of teeth on the spindle gear that would drive the gear chain I will try and work it out for you

It may take a little time so communication by e-mail would be preferable. Mine is    wood_y@btinternet.com

Regards   Brian

 

[Harry BroadbentParticipant @harrybroadbent75717]

Thank you guys, I am wanting the max speed achievable, I have all the change gears.

Cheers

Harry

[DC31kParticipant @dc31k]

How fast is fast enough?

As I understand it, the column thread on a Quorn is 1tpi. I believe that was thread milled on a Myford, so it is possible to achieve 8:1 speed increase (7200rpm) just by consulting historical documents.

[Brian WoodParticipant @brianwood45127]

Hello Harry,

This request has changed and is in completely uncharted territory. To quote the maximum speed achievable is a complete unknown. I for one would not even attempt to establish it, much less test it. You have now moved away from how the question was worded at the start of this thread which implied that you wanted to know what might be the maximum theoretical speed.

The answer to that is in the handbook. Assemble in reverse the gearing for the finest feed quoted at 0.0018 inches which is 555 tpi. Run that at a spindle speed of 900 rpm gives by calculation the completely ridiculous leadscrew speed of 500,000 rpm! Even if it was possible to move things with that degree of ratio increase it would act as a brake and lead to motor burnout as one of many scenarios of damage to the lathe.

Are we just wasting our time with this as to my mind it is unachievable anyway? Why would anyone want to set it up in the first place, let alone try to measure it before everything seizes up, which will happen rather quickly?

Sorry to be so critical but the more I think of it the more unrealistic it all becomes, particularly as I have no idea why you want to know.

Regards   Brian

[Brian Wood]

On Brian Wood Said:

 

The answer to that is in the handbook. Assemble in reverse the gearing for the finest feed quoted at 0.0018 inches which is 555 tpi. Run that at a spindle speed of 900 rpm gives by calculation the completely ridiculous leadscrew speed of 500,000 rpm! Even if it was possible to move things with that degree of ratio increase it would act as a brake and lead to motor burnout as one of many scenarios of damage to the lathe.

Are we just wasting our time with this as to my mind it is unachievable anyway? Why would anyone want to set it up in the first place, let alone try to measure it before everything seizes up, which will happen rather quickly?

Sorry to be so critical but the more I think of it the more unrealistic it all becomes, particularly as I have no idea why you want to know.

Regards   Brian

Surely the finest threads would lead to the lowest lead screw RPM as for 555 TPI you would need to turn the chuck 555 times for every 1″ of travel, where as for 1 TPI you would need to turn the chuck once for every 1″ of travel which implies a leadscrew RPM considerably faster than for 555 TPI?

[Brian Wood]

On Brian Wood Said:

Assemble in reverse the gearing for the finest feed quoted at 0.0018 inches which is 555 tpi.

Except that it uses the 12t fine feed gear, which I do not think can be assembled in reverse.

Also, please review your 500 000rpm calculation. 555tpi from an 8tpi leadscrew is a reduction of about 1:69. My calculator says that 900 x 69 is a far more sedate 62,100 rpm.

 

[Brian WoodParticipant @brianwood45127]

Benedict,

I did say assemble the gear train in reverse to gear up the leadscrew instead.

Brian

[Brian WoodParticipant @brianwood45127]

dc31,

Well spotted, I just got lazy and took the reciprocal and multiplied the two things together. Mia culpa, and I put my hand up to forgetting entirely the 12 tooth pinion at the start of the gear train which would make it impossible anyway.

Despite all that, 62,100 rpm is far from modest and I can’t see leadscrew bearings, gear teeth or much else surviving the experience if Harry has the courage to risk his lathe in trying it out.

It is all becoming an exercise in nonsense really.

Brian

[Brian Wood]

On Brian Wood Said:

 

It is all becoming an exercise in nonsense really.

I was under the impression that intellectual effort for no purpose was one of the hallmarks of a curious and civilised society.

Turn the problem on its head and consider the possiblity that you might want to drive the spindle by turning the leadscrew (c.f. the Quorn column thread). How slowly could you make the spindle go using standard change gears?

With a powered spindle held in the toolpost, it provides a way of making things with a very long helical lead.

[Harry Broadbent]

On Harry Broadbent Said:

Thank you guys, I am wanting the max speed achievable, I have all the change gears.

 

FWIW, I read this as: what is the maximum leadscrew speed achievable by selecting gears from a full set? (Which would presumably lead on to a discussion of which gears to select).

Seems a perfectly reasonable question to me.

[Nigel Graham 2Participant @nigelgraham2]

Harry –

I think before we get too aireated it would help if we know why you are asking, if it’s not just a gear-ratio puzzle.

Calculating how fast you can make the leadscrew spin for a fast-running spindle is a strange thing to do, but easy enough as a pen-and-paper exercise with a list of change-wheels and a calculator.

Brians’ calculated 62,100 rpm leadscrew rotation equates to nearly 11 feet per second traverse, so obviously not for any sensible machining operation.

Trying it practice is best reported to the RSPCL.

.

Are you intending <8tpi screw-cutting or the long-pitch thread-milling DC31k cites?

For that it is normal to “drive” the lathe manually with a hand-wheel on the lead-screw, power isolated and countershaft belt loose; because the speed-raising change-wheel ratio puts very unfair loads on the machine if run under power – and then in lowest pulley, back-gear.

Even the manual for my Harrison L5, a larger and much beefier machine with 4tpi screw, warns you to take threads of 4 and less tpi very carefully; and even advises this should be done only by an experienced turner.

 

[Peter SansomParticipant @petersansom44767]

The 500,000rpm of the lead screw for .0018″ is incorrect, it equates to a leadscrew of 13rpm.

As the leadscrew is 8 tpi, cutting an 8 tpi (.125″) thread has the leadscrew turning at the same speed as the spindle.   You would never cut a 8 tpi thread at 900 rpm, the carriage speed would be 900 x .125″ = 112.5″ /min. This equates to 1.875″ / sec or 47.625mm/sec.

That carriage type of carriage speed you equate to a high risk of damage to the lathe and of injury to the operator.

From a theoretical point of view the limitation would be the bronze bearings, gear load and clasp nut as well as the associated lubrication of these items.

[Harry BroadbentParticipant @harrybroadbent75717]

Thank you for all your replies.

I didn’t realise that the initial question was so vague, therefore Brian I apologise.

 

To give some more details on the problem I am wanting to rig up a stepper motor to my leadscrew, much like Tony Jeffree. When reading about the Tony Jeffree CNC upgrade, for an ML7, I realised that he just used the smallest motor he bought because it worked.

I want to know the maximum speed the leadscrew would turn for cutting threads, as per the Myford thread cutting guide. This will then allow me to buy the smallest and more importantly cheapest stepper motor.

 

I will then link my motor up to an LCD screen with a key pad so I will be able to enter a thread Pitch and it will turn the leadscrew at the correct speed.

 

I have already built a speed sensor using a hall effect sensor so I can get an accurate number for the spindle speed.

 

 

 

[John HaineParticipant @johnhaine32865]

<p style=”text-align: left;”>Buying a stepper motor the torque is more of a price determinate than speed.  On my S7 I have an Nema 23 on the Z axis with 2:1 reduction, albeit quite a long one.  Also run the motor from the maximum possible voltage for best speed response. Don’t stint on the driver either, use a modern digital drive and avoid the old TB6600 type. Happy to give more details if useful.</p>

[Brian WoodParticipant @brianwood45127]

Thank you for the explanation Harry. I had always hoped there was more substance to the question than “what if ” sort of thinking.

Over to others now as the topic has moved completely from a pure clanky’s viewpoint into fields outside my pay grade.

Best of luck in your endeavours

Brian

[Mark RandParticipant @markrand96270]

I would note that you would suffer whipping of the leadscrew long before suffering bearing failure. Running a long shaft with limited support at high speed needs some calculation if one is to avoid catastrophe. Stick a long bit of 5/8″  or 3/4″ shaft in the chuck, with tailstock support to see what I mean.

[Howard LewisParticipant @howardlewis46836]

I would have thought that if the intention was to screwcut, then the chuck would be rotated at a low speed.

Possibly using a Mandrel Handle, certainly using Back Gear. irrespective of how the Leadscrew is driven.

Trying to withdraw the tool when the chuck is rotating fast calls or very good reactions.

The question suggests a wish to screwcut a very fine thread (Irrespective of whether Changewheels, Norton box or ELS is used. )

How fine a thread do you wish to cut?

By purely mechanical means, my lathe will cut 112 tpi, but working up to a shoulder, or down a blind hole, I would most certainly use a Mandrel handle, or, at apinch, under power using back gear and minmum belt speed.

Cutting screw threads at speed, under power is the realm of the old Cri Dans, or CNC control, or spindle speed and traverse.

Howard

[Nigel Graham 2Participant @nigelgraham2]

Isn’t there a flaw here?

The finer the thread the slower the leadscrew with respect to the spindle.

For 112tpi the leadscrew rotates at 1/14 times the spindle rpm.

The OP asked how fast the thing can be driven, but later reveals wanting to use a stepper-motor drive for screw-cutting. That of course still needs be referenced to the spindle rpm – electronic drives are out of my league so I don’t know how this is achieved, and so that the thread re-engagement is correct for every interval of the cut – but the fantastical speeds being talked of are still way out of reality on a lathe like a Myford 7.

The leadscrew is still normally rotated at no more than the spindle speed anyway. Cutting threads coarser than 8tpi threads on a Myford or similar, under power from the spindle, is asking for expensive trouble.

Non-NC lathes were sometimes fitted with an “Ainjest” or similar half-nut trip mechanism, allowing fast screw-cutting, but there we are talking of hefty industrial machines and production times.

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