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!

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

On 25 October 2023 at 14:18 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.

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

On 25 October 2023 at 12:02 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.

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.

<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>

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.

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.