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I bought a 101 year old mini-lathe (Drummond B type) and from what I've seen and read online it should be possibe to bring this lathe into the 21 century.

In my most extreme vision I would like to direct drive the main spindle and later direct drive the less torquey lead screw so I then would have 2 independent systems. This should later give me the option of precisely controlling the speed so could I could cut any pitch of thread by setting the speeds accordingly before I engage the half nut. If you are still with me, most VFDs have external control options so by using a Rasberry Pi, pre-determined parameters could be implemented by a push of a button (or will write a program taking user input and outputting correct speeds.)

My electrical knowledge is not excellent but I'm learning. In my setup I would like -

  1. Constant torque
  2. Varying speed between 50 - 2500 rpm.
  3. I’m in the UK so VFD input voltage is 240v 60Hz single phase.

There are also other factors in play such as -

  1. Start up torque (should be 1.5 times running torque)
  2. Efficiency
  3. Power factor

I would like to know what torque range can I achieve on single phase VFD as I would like / need about 200 - 300 Nm at low RPMs (300 - 400 at start up) (this occurs normally in traditional motor systems (low RPM = more torque) but the VFD const torque is just confusing me now.)

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    \$\begingroup\$ If you’re in the UK then your mains supply is 230V 50 Hz \$\endgroup\$ – Frog Jun 4 at 9:55
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    \$\begingroup\$ What power? 300-400Nm ??!! Are you serious, that's more than 100kW . Why do you need speed between 50 - 2500 rpm if you have a gearbox? The 4 pole AC motor is typically 1420rpm. \$\endgroup\$ – Marko Buršič Jun 4 at 10:14
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    \$\begingroup\$ You could mount a Volkswagen 2.0 TDI 150HP engine to have such torque, it would be cheaper. \$\endgroup\$ – Marko Buršič Jun 4 at 10:20
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    \$\begingroup\$ This is not a discussion forum. It is intended for one specific electrical engineering design or theory question per post. This question presents a broad outline of a project and requests "any help would be greatly appreciated." I am voting to close the question, but I will offer some further comments. \$\endgroup\$ – Charles Cowie Jun 4 at 10:26
  • \$\begingroup\$ Machine tools tend to require constant power over the speed range. Electronic motor control tends to be better suited to provide constant torque. Changeable gear ratios and other mechanical speed changers are more suited to provide constant power. You will need to consider more carefully what you need and figure out a compromise. \$\endgroup\$ – Charles Cowie Jun 4 at 10:37
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Belt drive is actually quite important for safety on these lathes : in the event of a tool jam, the belt slips and you do minor damage to the work instead of wrecking something major (like the operator).

Direct drive would be a pretty bad idea. I know one user found out that chain drive certainly was!

Best starting point would be a <0.5kW 3 phase motor on a VFD; best for overall CNC would be a similar powered BLDC with its dedicated drive electronics.

If you want to drive the leadscrew separately but synchronised to the headstock spindle, fit the latter with some form of rotary encoder (absolute position preferred) and "gear" the leadscrew to that.

Oh and in case you haven't already discovered it, there is a group dedicated to Drummond lathes (disclaimer : I run that group) with quite a lot of resources (drawings for spare parts, modifications, sources for changewheels among other things)

Wisdom on the group is that it's best (especially for metric threading) to leave the halfnut engaged, wind the tool out for clearance, and reverse the spindle to return for the next pass. (The old timers do this with a hand crank, but you could program the BLDC motor drive for that)

Some have been fitted with DRO scales, but none to my knowledge (yet!) with full CNC automation.

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I thought the frequency in the UK was 50Hz, not 60. Nevertheless, I don't think it makes any difference to a VFD.

As for startup torque - you would normally use a 3 phase motor with 230VAC windings with a VFD. Being three phase, you get better torque performance and a physically smaller motor. One thing you haven't told us is how many Watts the motor is. Being a small motor - do you really care about efficiency? Same with power factor. I could understand if it were a 100kW motor running 24/7 as these parameters would translate into real money.

Having a separate motor on the leadscrew suggests you want an 'electronic gearbox' - this means you probably want an AC servo drive - the average VFD does not have the level of control needed. The motor also requires an encoder. Not difficult, but requires the right hardware to achieve it.

A common retrofit on mini lathes is to use a treadmill motor. These are a brushed DC motor and are better suited to providing more torque than an induction motor over a greater speed range. These use a different method of control than a VFD.

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    \$\begingroup\$ It certainly is 50 Hz in the UK. \$\endgroup\$ – winny Jun 4 at 10:56
  • \$\begingroup\$ Ok, thanks every one for ALL your responses, I knew this was a bit of a cheaky ask and would throw up alot of curved balls but you've probably saved me a fair bit of wasted life so I will heed all! I will join the Drummond group, thanks user-18.... UK 50Hz 230v - DOH!! (what she said). Iv seen an old lathe on Utube retro fitted with 3ph motor+VFD and thought great idea, I think he plans on adding a second for the lead screw, if these could be synced as accurately as Iv seen some of these VFDs can (to within 0.1Hz) then again I suppose even a difference as small as this would cause thread error \$\endgroup\$ – Mamba76 Jun 4 at 13:30
  • \$\begingroup\$ I know this is an electrical questions / answers site but whats the highest torque needed to turn say 6 inch cast iron as the speed needed can be as low as 4 m/s surface speed? Where the hell did I get 300nm from lol, just realised my audi 3.0 tdi maxes at 575nm lmao! Think Iv turned up to a gun fight with a spoon again, damn it! Oh yes pulley belt driven allows for slip so definately safer, agreed) \$\endgroup\$ – Mamba76 Jun 4 at 13:42

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