Variable speed motor for Atlas 12 ” lathe ?

[Rich2502Participant @rich2502]

I have an Atlas 3000 series 12″ lathe with no motor and want to fit a variable speed drive, what hp motor would be suitable for this machine ?

 

I can see too small would cause issues but if you go too big on hp what problems will it cause ?

 

[bernard towersParticipant @bernardtowers37738]

suggest lathes .co.uk for lathe spec and motor spec.

[noel shelleyParticipant @noelshelley55608]

1 Hp would do but I would be tempted to go to 1.5Hp. In modern speak .75Kw to 1.1Kw . And 4 pole or 1425RPM. Noel.

[SillyOldDufferModerator @sillyoldduffer]

What Noel said.

Re the ‘too big’ question, I say ‘with great power comes great responsibility!’

• Having extra power tempts the operator to use it, and the lathe may not be rigid enough to cope.  Cutting creates equal and opposite forces on the lathe that twist the tool-post and cross-slide, increase wear and tear on the bearings and gears, and stretch the belts, etc. etc.  The cost of forcing the lathe outside it’s design envelope is reduced finish and accuracy, and – if the operator is continually heavily handed – he will eventually force repairs.
• Extra bad things happen when, not if, there’s a crash.  A souped up motor will inflict a lot more damage than a standard one.  Rule of thumb, unless the power supply or some mechanical device limits it, the power output of an overloaded electric motor could peak at, say, 5 times it’s continuous rating.   So fitting a 1HP lathe with a 3HP motor would have 15 horses tearing at it, not a mere 5.   Generally speaking, lathes of this size are fitted by the manufacturers with motors in the 0.75 to 1.5HP range, power not chosen for accuracy and long-life.

The description of the Atlas3000 on lathes.co.uk led me  to think initially it was a beefy machine.  Not so sure after reading this:   ‘In the right-hand section of the box was a safety over-load clutch that protected the drive against damage – one of the problems with earlier lathes being the tendency for the cast-in key on the ZAMAK gear that engaged with the leadscrew to shear off.’

ZAMAK eh!  Not the end of the world, but strength-wise, this Atlas may be a sheep in wolves clothing!  Putting a big motor on this particular lathe may not pay.

I don’t think it insane for an owner to go, say, 33% over the designers rating, but doubling the power or more is frankly unwise.   Unless done deliberately to improve productivity, after the owner promises not to cry if the machine eventually breaks.

Oh, and believing oneself to be a careful machinist who will never crash tempts gremlins beyond endurance; they will find a way of causing havoc! You can guess how I know…

🙁

Dave

 

[Clive FosterParticipant @clivefoster55965]

Fundamental limitation of a VFD set-up is reduction of torque at lower speeds. I prefer a 6 or even 8 pole motor, ie 1000 or 720 rpm (nominal), to give more oomph at lower speeds. A VFD goes up above nominal speed as well as down but with a 1440 rpm motor the above nominal range is pretty much wasted unless you do very small work.

Do check the surface speed – diameter table or, better, graph for the materials and sizes you will be using. As a 12 inch to the ft scale worker I rarely bother to go much above 1,000 rpm.

Ideal in my view is to set things up so you pretty much never need back gear and can do almost everything on a middle setting of the multi-speed pulley drive. Best torque transmission when pulleys are of similar size. Here preference for middle ranges. small and large pair always looses out significantly with Vee belts due to the limited wrap angle on the small pulley.

Generally 1.5 HP would be considered low for a 12″ swing lathe but the Atlas isn’t teh strongest example of the breed. It really isn’t an industrial standard machine so lighter cuts only. Honestly best thought of as a 9″ lathe with 3″ of extra swing when you need it.

Clive

[old martParticipant @oldmart]

I have just looked up the motor specs for the Atlas 12″ lathes and 1/2 to 3/4 hp are fitted as standard. If I was thinking about a 3 phase motor and a VFD for one, it would be a 1hp six pole motor which can be run easily at the same maximum speed as the original, but also having the torque at 500rpm. If you want pain free installation and have lots of money, then ready made kits can be the way to go. If you can carry out safe wiring yourself, then the easiest option would be to buy from The Inverter Drive Supermarket who have both motors and VFD’s, some of which have their own pdf with wiring instructions that make sense to non electricians, called “quick start guides”.

Having variable motor speeds is not a lazy persons way out of changing belt speeds, but complimentry to the old series of speeds. You may have to deal with motor mount modifications and shaft diameters needing changing, so take your time checking before chucking out the old motor.

[Russell EberhardtParticipant @russelleberhardt48058]

I replaced the original motor on my Atlas 10″ lathe some years ago with a 0.75 kW motor and VFD from Inverter Drive Supermarket the most significant improvement was in the surface finish on the workpiece as a result of reduced vibration from 3 phase motors.  You needn’t worry about the effect of extra power damaging the lathe provided you don’t over tighten the belts.

Lots of Atlas lathes were built in the UK during, and in the years after, the war. Mine was built in the 1950s by Acorn Tools in the UK and they replaced all of the ZAMAK parts with steel or cast iron.

Russell

[Russell Eberhardt]

On Russell Eberhardt Said:

… You needn’t worry about the effect of extra power damaging the lathe provided you don’t over tighten the belts.

…

Which begs the question: how tight should the belt be?

Too tight risks damaging the bearings or bending the shafts, so always best avoided. And a too loose belt will spoil the finish by slipping whilst cutting, also undesirable.  So we’re looking for a tension that allows the belt to perform normally whilst taking heavy cuts, but somehow slips just before the weakest part of the lathe breaks. And we don’t know what the weakest part is, or what the breaking strain might be.

I have a few books that cover belt-drives in detail.  They were written when industry relied exclusively on belts and line-shafts and much thought went into the subject.  None of my books countenance deliberately slipping belts as a way of protecting machines, I guess because it’s hard to get the balance right in practice.  It’s easier to protect machines with shear pins and other means.

Does anyone have a reference to a engineering book or other source that describes how to avoid damage by deliberately slipping a belt?  My suspicion is that the method is workshop lore rather than effective engineering.

And I suggest fitting a big motor and then loosening the belt to prevent the motor delivering power isn’t logical.  Better to fit a smaller motor and tension the belt normally.

Dave

[Robert Atkinson 2Participant @robertatkinson2]

+1 for Clive’s suggestion of a 6 pole or even 8 pole motor. People seem to have a blind spot about using a VFD to increase motor speed. An 8 pole can remove the need for a layshaft reducing losses.
Yes, torque can be limited by the available voltage but if you double the speed you only need half the torque
for the same power. If you lower the speed you can’t increase the torque (current) above rating because of resistive heating in in the windings an possible reduced fan efficency. S you loose power there.
I fitted a 910 RPM rated 6 pole motor to my ML7 and it runs fine at 1400RPM and above. The bearings etc are identical to the 2 pole model so mechanically good to even higher speed.

Robert.

[noel shelleyParticipant @noelshelley55608]

Daves comments above  and the reference to workshop lore go back to the days of flat belts and line shafts. The way in which a V belt works, unless worn out and running on the bottom of the “V” it would be a brave man ( or a fool ) who would rely on slippage as a torque limiting device ! One might even fit an ampmeter (overscale)in the supply  to observe power being used and experience would tell you when to ease off, or build a slipping clutch into the pulley, set to limit torque, shear pin is fine till it breaks and the ali pin is replaced with silver steel rod. Bit like the 6″nail cut down as a 13A fuse ! Noel.

[Robert Atkinson 2Participant @robertatkinson2]

If running a VFD you can set the current limit to protect the lathe as well as the motor…

[old martParticipant @oldmart]

The six or eight pole motors run slower than the original motor but can usually be run at the same speed with a VFD. The extra poles allow more torque at lower rpm, if you halve the speed, you also halve the power output and speeds below 1/4 are not worth having particularly as there are mechanical gear ratios by the pulleys available. The Atlas lathes are made to run just over 2000rpm top speed and it would not be a good idea to exceed that. The link is worth looking at, particularly the printable pdf of the quick start guide.

 

 

 

https://inverterdrive.com/group/AC-Inverter-Drives-230V/WEG-CFW100C04P2S220G2/

[mgnbukParticipant @mgnbuk]

High pole count motors are a great way of increasing motor torque at the expense of speed – the electrical equivalent of lowering gearing. But there is a price to pay – frame size. High pole count motors are at least one and possible two frame sizes up on a 2 or 4 pole motor for the same power due to the increased number of windings.

Ac motors on inverters are constant torque when the speed is reduced below the nominal & constant power above nominal. Rotor mounted fans typically don’t give enough airflow below nominal speed but absorb a lot more power at speeds above nominal. Best solution is to remove the rotor fan and fit some form of independant blower for constant cooling at all speeds & no high speed power loss.

Fitting an oversized motor to get more low speed torque is a normal tactic. My (currently dismantled & way down the projects list) Denford Triac uses a 2.2Kw spindle motor (far bigger than usually fitted to a machine of this size) with a separate blower to allow continuous use at lower revs without overloading or overheating.

Nigel B.

[old mart]

On old mart Said:

The six or eight pole motors run slower than the original motor but can usually be run at the same speed with a VFD. The extra poles allow more torque at lower rpm, if you halve the speed, you also halve the power output and speeds below 1/4 are not worth having particularly as there are mechanical gear ratios by the pulleys available. The Atlas lathes are made to run just over 2000rpm top speed and it would not be a good idea to exceed that. The link is worth looking at, particularly the printable pdf of the quick start guide.

 

 

 

https://inverterdrive.com/group/AC-Inverter-Drives-230V/WEG-CFW100C04P2S220G2/

An alternative might be to reduce the motor pulley diameter and increase the machine pulley size.  That would allow higher motor rpm for the maximum ‘safe’ lathe speed, while not losing so much torque at slower spindle speeds.

Pulley cost is likely far less than a slow speed motor.  4 pole motors are reasonably cheap and often available on the second hand market.

[Robert Atkinson 2Participant @robertatkinson2]

My six pole motor was a NOS item on ebay at a price less than a 4 pole. At the smaller motor sizes we use the frames are often oversize anyway. Certainly in the range mine came from there was no size difference between 4 and 6 pole of the same power. I used a larger than “standard” pulley on the motor wich further reduces losses and the tendancy for the belt to take a “set”, with associated noise and vibration, if left tensioned and not turning. Using a 6 pole motor was a no brainer for me even if the motor had been more expensive.

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