Good morning

[EmgeeParticipant @emgee]

Posted by Emgee on 13/08/2020 10:06:37:

As Andrew says the Bantam 2 speed motor is controlled by a 2 speed switch/contactor arrangement, 800rpm spindle speed in low range with max 1600 in high range. Full wiring details in the manual.
From memory there are 6 selectable speeds by lever in each of the ranges, lowest being 32rpm in the 800rpm range.

Tom

If you want copy of the manual send me a PM (click on Message Member at bottom of message) with your email address and I can send you a copy.

Mistaken on the number of selectable speeds in the above post, chart below provides the correct number and speeds available. Threads available and feedrate speed charts in my picture album.

Emgee

[Tom Walker 1Participant @tomwalker1]

Thank you Emgee. Yes, Ive got my head round the gearbox, so all good there.

I was initially thinking 16 speeds would be rather good, till the infintely variable scenario of the inverter came along.

Can I ask one more…..if you run an inverter with the variable speed capability, is there anything to tell you what speed the lathe is running at?

Thanks.

[EmgeeParticipant @emgee]

The spindle speed cannot be calculated by the VFD as it will depend on set Hz and the gear selected.
As you have 3 phase electric supply with a 2 speed motor you would have a good choice of spindle speeds between 32 and 1600rpm, whichever speed is selected you will have the full torque available that the motor produces, using a VFD that will not be the case without changing gear selection.

Emgee

[Tom Walker 1Participant @tomwalker1]

Thank you, with a VFD, how do you select the correct surface speed for machining, if you dont know the RPM of the spindle? I must be missing something…

Edited By Tom Walker 1 on 14/08/2020 21:32:18

[Harry WilkesParticipant @harrywilkes58467]

Hi Tom

It,s fairly easy to fit a rev counter to your lathe but that bridge can be crossed when up and running, but going back to your VFD and it's setting if the lathe came with the VFD fitted and assuming you have not tried to reset them and it's the same motor them the VFD should be setup.

H

[Anonymous]

Posted by Tom Walker 1 on 14/08/2020 21:02:29:

…if you dont know the RPM of the spindle

Simple arithmetic. You know the speed of the original motor and the speed of the spindle at each gear setting. So the gear ratio for each setting can be calculated. For any given setting of the VFD you know the output frequency. Knowing that, and the number of poles on the motor, you get the motor speed for each frequency, less around 5% for slip. The motor speed plus the gear selected will give you the spindle speed.

Andrew

[Tom Walker 1Participant @tomwalker1]

I can put it back on single phase Harry, because I kept all the bits and bobs. Literally an hour faffing about.

I am embarrassed to admit that I always considered single phase inverted to be a very poor second to 3 phase.

This is probably because 30 years ago, I had a mate who got a lathe going on an early form of inverter which was basically useless. Times have moved on, Im just trying to weigh up the best way forward, being in the lucky position of being able to use either. Decision time!

[Tom Walker 1Participant @tomwalker1]

Posted by Andrew Johnston on 14/08/2020 21:49:15:

Posted by Tom Walker 1 on 14/08/2020 21:02:29:

…if you dont know the RPM of the spindle

Simple arithmetic. You know the speed of the original motor and the speed of the spindle at each gear setting. So the gear ratio for each setting can be calculated. For any given setting of the VFD you know the output frequency. Knowing that, and the number of poles on the motor, you get the motor speed for each frequency, less around 5% for slip. The motor speed plus the gear selected will give you the spindle speed.

Andrew

Sorry Andrew, Im just a simple farmer. I sort of understand what your saying, but does every spindle speed selection require one more calculation other than the diameter of the workpiece? I dont know what information you get from the VFD box, is there a read-out of some sort?

[Joseph Noci 1Participant @josephnoci1]

Posted by Tom Walker 1 on 14/08/2020 22:01:04:

Sorry Andrew, Im just a simple farmer. I sort of understand what your saying, but does every spindle speed selection require one more calculation other than the diameter of the workpiece? I dont know what information you get from the VFD box, is there a read-out of some sort?

 

 

Tom, if you do not know how many poles your motor is, etc…Set the gearbox to the lowest speed = I see 36RPM?

The inverter has a display in 'Hz' – the frequency at which it is supplying the AC voltage to the motor.

Set that to 50 Hz and stick a large bit of masking tape to the chuck periphery to mark position. Run the lathe and count how many time the tape passes a set point in a minute – more or less…If you get close to 36 turns then all is easy – whatever gearbox setting you select will now give the speed on the chart. If you increase the frequency of the inverter output, the motor will increase speed in direct proportion.

If you set the inverter to 60 Hz the speed will be 20% faster , Double the speed at 100Hz..

If you obtained a very different count of chuck turns at 50 Hz, say like 72 turns, then the motor poles are quite different – lets get into that one if your count differs from 36…!

Also, don't stress about the motor slip – you would have ignored that anyway had you a 'conventional' motor setup of the lathe – you would simply set the gearbox and get on with it!

Joe

Edited By Joseph Noci 1 on 15/08/2020 07:19:10

[Tom Walker 1Participant @tomwalker1]

Thank you Joe, so effectively you calibrate the lathe once, then have one factor, which is the reading on the inverter in hertz, to apply to the gearing chart.

[ANDY CAWLEYParticipant @andycawley24921]

Posted by Tom Walker 1 on 13/08/2020 19:33:05:

Well Tim, Im not an engineer, so "weedy" was my definition of choice!

Im used to my machinery starting rapidly.

Thanks, Tom.

I should say so Tom with Hisso power😄❗️

My simple way of checking rpm is to chuck a length of threaded bar and put a nut on it. Mark a length on it, set the lathe going , grab the nut so that it runs between the two marks and time it. From the distance and the pitch you can calculate the number of revolutions.

The longer the length the more accurate but beware of the potential of a long thin length flailing if you rev it too mutch. To be safe I usually have the end of the rod inside the taper socket.

See you at Mallory❓❓❓

[Tom Walker 1Participant @tomwalker1]

Ha Ha ay oop Andy, fancy meeting you here!! Good way of social distancing eh..!

I knew you would have some mad scheme for checking revs…sounds lethal…cant be at Mallory very sadly. Hope you have a great race, if you ve still got a hand left to steer with!!

[SillyOldDufferModerator @sillyoldduffer]

Tom gets my sympathy – calculating RPM when the lathe has lots of gears is a pain, though one of the advantages of VFD speed control is most of those gears are redundant. I like Andy's suggestion of timing a nut running along a thread, but having to do that for every speed or gear change would soon get old.

Buy a tachometer. They come in various forms : hand-held (where the operator points a pistol at the spinning object), and direct measuring panel-mount or boxed. Not the Inductive type sold for IC engines because they detect ignition pulses which lathes don't have!

Instead a Hall Effect or magnetic tachometer like this example:

The rise and rise and rise of VFDs in modern times is 'quite interesting'. Their advantages are such that it's common to connect 3-phase motors to a 3-phase supply via a 3-phase VFD. Big savings in professional installations by exploiting their energy management features; the electronics provide a multitude of control options.

Much more to VFDs than a box of tricks allowing 3-phase motors to be run off a single-phase supply in a shed. I certainly wouldn't write VFDs off because a mate failed to get one working 30 years ago.

Dave

[Anonymous]

Posted by Tom Walker 1 on 15/08/2020 07:34:23:

…..so effectively you calibrate the lathe once, then have one factor, which is the reading on the inverter in hertz, to apply to the gearing chart.

Got it in one. Most, but not all, VFDs have a display that can be programmed. I've got two VFDs on my CNC mill, one displays frequency and the other rpm.

Personally if I was in your position I'd bin the VFD and use the 3-phase supply. It makes life a lot simpler, and gives you full power over the speed range without having to think about it. I assume that the lathe is a geared head? So It's fairly quick to change speed. Cutting speeds are not written in stone and are not critical. If you're somewhere in the ball park that's fine. To some extent the speed range needed will depend on tooling. If you use HSS tools then the low range of speeds will be fine. For insert tooling it might be a little low at the top end. Having said that my lathe goes to 2500rpm but I mostly use 800rpm and lower. For small work I might go to 1200rpm or rarely 1700rpm. I've never used the 2500rpm range in anger. If nothing else it's very noisy, which is a problem in a quiet village location.

Andrew

[Tom Walker 1Participant @tomwalker1]

Thanks Andrew, it is a geared head machine, as you say. I think I will leave it on 3 ph for the moment with the single speed motor, and keep my eye out for a 2 speed motor and switch gear.

Thanks for all the information and help, this site is an invaluable resource.

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