Poor torque

Hi there, I've just bought a secondhand Chester DVB11GVS to replace my trusty 10" Atlas. The lathe is really nice but seems a little underpowered. Taking modest facing cuts on stock over 1 1/2" can cause the lathe to stall. It seems parting off at a slower speed is going to be virtually impossible. Anyone have experience of this lathe or similar problems to this?

Many thanks Kev

Its because a reduction of spindle speed is done electronically and not by a step down gear box that automatically increases the spindle torque.

Frank

Hi Kev,

As Frank says, using electronics to reduce speed reduces torque, a brushless DC motor will probably be better. It should be possible to mount a countershaft on your lathe so you can get a low speed range with good torque.

Here is a link, yes it is for a different lathe but might give you some ideas.

Something similar has been discussed for Sieg C6 on this forum, here is a link to it.

Thor

I have the Warco WM280VF which is very close to yours and don't have problems at that diameter but what do you call "modest" Its only when doing 9-10" stuff that it can be slowed buy I just run a bit faster and take slightly shallower cuts 0.025" on a 10" CI flywheel is no problem. Mine will also part 3" steel and 4" CI OK.

Firstly make sure you have it in the slower of the two speed ranges which will give a better mechanical reduction from motor to spindle. Then check its not th ebelt slipping.

Edited By JasonB on 11/04/2016 07:42:09

Edited By JasonB on 11/04/2016 07:45:07

Firstly, thanks to all who have replied so far. I did some homework before I changed lathes and realised there were going to be a difference in the drive characteristics between mechanical and electrically controlled systems but there really is I think a problem with either the motor or control board.

I watch you video clip Jason and I certainly couldn't perform a cut like that with my lathe as it is at the moment. Let me give an example of what's happening.

Taking a 0.2mm facing cut on a piece of 18mm thick 145mm square mild steel plate. I tried several passes at various speeds from around 180rpm to over 350rpm using a new tipped tool with lubrication. Across the corners, interrupted cut, all was well. As soon as I was into a full cut the speed slowed and surged leaving a very uneven surface finish as the tool seemed to drag at the slower speeds. This continued until I was maybe 30-40mm from the centre where the cut seemed to stabilise again. I had similar results facing a piece of 75mm round. Very frustrating indeed. I do have a copy of a manual for the control board and I have a modest electrical knowledge. Are there some simple electrical checks I could carry out to eliminate the obvious? eg voltage or current supplied to the motor from the board...

Again thanks to all. Regards Kev

Does sound like something is amiss, did you check it was in the lower speed range? also have a look at the brushes on the motor

This steel fabrication has a bit of 10mm black flat bar on the top which is not the nicest steel to machine. When it gets past the intermittant cut the dia is 80mm, and I think it was 0.020" depth of cut or 0.5mm

And a bit of fun the other day, 1" dia EN1A being reduced to 1.2" dia in one pass so 0.250" or 6.35mm depth of cut. I don't usually go that deep.

Jason, that's pretty impressive! My lathe is in the low range pulley setting so I guess tonight's job is pull the motor and check the brushes. I'll keep you posted as to my findings. Thanks

Even more impressive is Jason reducing a 1" diameter to 1.2" diameter...arry potter got nown on Ja!!

Before you remove the motor, do as Jason says and check the belt isn't slipping. Someone on here recently had a similar problem and it turned out to be belt slippage. If you take the motor out you may find nothing amiss and never be the wiser.

Brushes seem an unlikely explanation - generally they are either worn or they aren't. If they were able to cause a loss of torque in a continuous condition, they would be getting mighty hot.

So I just got back from the lathe having removed the motor and...the brushes were ok. I gave the com a clean and refitted the brushes. Next the drive belt. Funnily enough it was my first thought as to the cause of the problem and I'd given it a tweak on the tension previously. However, after taking it off to remove the motor two things surprised me. How thin and puny it is considering the kind of torque and inertia it has to deal with and how plasticky it seemed. Hardly seems to be made of a suitable material to best promote friction. Anyhow I cleaned the pulley grooves with contact cleaner and did likewise with the belt. I only tested it briefly but it does seem to have improved things. Thank you to everyone for their input it's much appreciated.

It's a Gates Polyflex belt (or should be), will handle more load than a conventional Vee belt

And if you need to buy a spare belt the I can recommend Beeline Engineering in Milton Keynes...no connection et-al...

I bought a spare for my SPG lathe cheaper then anywhere else.

Thanks for the info. I think I'll order a replacement just to be sure

If you want decent torque and no complications in a machine then it must include backgear IMO

All evident here and what I have been trying to find for 5 1/2 years, so far only Jasons touched the sore subject.

In short motor runs full torque and power at 50 to 60hz, drop to 30hz it halves the torque and power so your 1hp would be 1/2hp, so around 175rpm would equate from a 1hp motor literally 1/8hp.

**LINK**p.

This is the reason cnc machines have massive motors usually 4 or 5 times greater than non variable speed.

So when I take the cheap route and vfd my lathe, control panel going down I will be running it at a constant 50hz using exactly as have been doing by using the gears for speeds.

Jon,

If you halve the torque and the rpm then the power is 1/4 of the full speed output.

The VFD system has to lower the applied voltage to the windings to keep the stator and rotor currents below a safe maximum. The main thing opposing the ac is the reactance (X) of the inductive winding on the stator. The stator resistance is normally quite low but the reactance is linear with frequency. X = 2Pi x F x L where F is the frequency in Hertz and L is the inductance in Henry's. As the frequency drops the current would go up as X is reduced, so the applied voltage is reduced to compensate. Torque to a first order is determined by the current flowing in the windings, as it generates the magnetic flux. At high frequency the reactance is now high and so to get sufficient current flowing the applied voltage has to be increased.

The rotor essentially is a massive shorted turn which when the magnetic field is applied by the stator has a very high current flowing in it. It behaves like a normal transformer where the stator is the primary winding and the rotor is the secondary winding. The rotor effectively only has 1-turn and rotor current is essentially the turns ratio between the primary and secondary, just like a static transformer. So if the stator (primary winding) has, say 100-turns then the secondary current in the rotor will be 100-times the stator current in simplified terms. Because of this the rotor current can become high enough to cause overheating.

The rotor induced magnetic flux works with the rotating flux supplied by the stator windings to produce the torque, but only when "slip" occurs, which is always the case with ac motors. If the rotor did manage to get up enough speed to equal the applied frequency then slip drops to zero and so does the torque, hence it slows down until enough slip occurs to produce the torque required to drive the load.

Below a certain rpm the cooling effect is decreased and the rotor begins to overheat. NEMA recommends that a motor should not be operated below 1/3rd of its normal speed to ensure sufficient cooling is available from the internal fan. For applications where very slow speeds are required then an external fan provided by another motor is required.

John its a DC motor on this one not AC

Hi Jason,

Then why was Jon describing an ac motor? I am confused!