Thread: Lathe Issue Question

Hi Guys,

I just picked up a new to me, secondhand AL330A lathe. It has something wrong with it when you turn it on. It sounds a bit like the motor has power, you can hear it, but the spindle wont turn in the high gears until you give the chuck a little start with your hand then it will go. In the low speeds it starts, and the mid speeds it slowly starts and winds up to speed. Can anyone diagnose the problem from this description. I'm hoping it's some small electrical issue I can quickly get round.

Cheers

Hi,

Could be a number of things. My lathe does exactly the same thing, I have been living with it for now but it's a PITA especially when you need to use a high speed. Sometimes I have to warm the oil up in a lower gear and then try the higher speeds. While I have not looked at it yet, my peanut brain just assumes that the start cap is faulty. There is no doubt that it could be any number of issues, all relating to the motor, damaged or over heated windings etc but when I'm ready to look at it, I will be checking the start cap first.

Actually, the only flaw with my theory is that it has two start caps, one for forward and one for reverse. Only issue is that it behaves the same in forward AND reverse. Surely both caps can't be faulty at the same time? Sorry, just me thinking aloud!

Check your start capacitors and go from there...... Other people, more electrically minded will no doubt come up with other theories too.

Simon

As mentioned, if it has a start capacitor it could be that.

If it doesn't have a start capacitor it could be the start windings that have burnt out, but the run windings are ok.

Or, if it has a centrifugal switch that switches the windings from start to run once the motor reaches speed then it might be playing up.

Originally Posted by simonl

Hi,

Actually, the only flaw with my theory is that it has two start caps, one for forward and one for reverse. Only issue is that it behaves the same in forward AND reverse. Surely both caps can't be faulty at the same time? Sorry, just me thinking aloud!

Check your start capacitors and go from there...... Other people, more electrically minded will no doubt come up with other theories too.

Simon

Are you sure that's correct Simon?

Generally dual start caps are wired in parallel and work in both directions same as a single cap.

Seems odd to me.

Rob

is it better when it has warmed up? While I'm a supporter of the starting cap theory (or perhaps the switch), my previous lathe had trouble starting in top speed until it warmed up. I think it was a combination of bearing adjustment and lubricant viscosity.

Michael

Originally Posted by tbonesmith

Hi Guys,

I just picked up a new to me, secondhand AL330A lathe. It has something wrong with it when you turn it on. It sounds a bit like the motor has power, you can hear it, but the spindle wont turn in the high gears until you give the chuck a little start with your hand then it will go. In the low speeds it starts, and the mid speeds it slowly starts and winds up to speed. Can anyone diagnose the problem from this description. I'm hoping it's some small electrical issue I can quickly get round.

Cheers

hi thonesmith
you said the motor sounds like it has power. have yo checked to see if it is turning?
It sounds like it may be a motor cap problem but have you checked the basic mechanical
set up. belt tension ect.
aaron

Hi Rob, Ah see I don't know everything! I assumed that the two caps were one for forward and the other for reverse! You say they are normally parallel then I'm good with that……... I'm good.

Hi Michael, when I mean I warm it up, I mean it goes from being impossible to start to starting with some help. I replaced the bearings a few years back and took a fair amount of time and effort to make sure they were nicely adjusted. With the gears in "neutral", the spindle with the 3 jaw, spinning it by hand will see it do a couple of revolutions before stopping. The bearings don't even get warm, in fact I may need to re-nip them up a bit. Manufacturer recommends Mobil DTE light but I use ISO 32 hydraulic oil. I'm pretty sure it's the motor.

Haven't been loosing much sleep over it as I have a nice 3 phase motor lined up for a VFD conversion anyway. After a certain other project is finished!

Cheers,

Simon

Originally Posted by azzrock

hi thonesmith
you said the motor sounds like it has power. have yo checked to see if it is turning?
It sounds like it may be a motor cap problem but have you checked the basic mechanical
set up. belt tension ect.
aaron

At a glance it all looks fine, belts are in good condition, and tension is right. The spindle is free as well, out of gear and in gear everything feels just normal. I'll set it up tomorrow and change the oil and run it for a bit and see if it clears up, I doubt it will though.

Originally Posted by tbonesmith

At a glance it all looks fine, belts are in good condition, and tension is right. The spindle is free as well, out of gear and in gear everything feels just normal. I'll set it up tomorrow and change the oil and run it for a bit and see if it clears up, I doubt it will though.

Do you have a muti meter

No, but I can buy or borrow one, my neighbour is a sparky.

Sounds like mine. Starter capacitor problem for me.

For my money, either something is mechanically tight, or the motor caps are failing.

As gearing is applied between the motor and spindle, there will always be an overall power loss. If you are gearing down to a lower speed, the torque available at the spindle will increase in proportion to the speed reduction, gearing up to a higher speed and the torque available at the spindle will decrease in proportion to the speed increase. For Oz mains power at 50Hz, the normal motor speeds are either around 1440RPM or around 2850RPM depending on motor winding configuration.

Say you have a 1440RPM motor in your lathe, and you have selected a spindle speed of 280RPM (hyperthetical). This implies a mechanical speed reduction of 5, and a torque increase of 5x. In practice, the drive train will cost some power and torque, so you have an overall torque increase (motor shaft to spindle) of perhaps 4.

Now say you select a spindle speed of 2880RPM (hyperthetical). This implies a mechanical speed increase of 2, and a torque reduction to half motor torque. Again allowing the same mechanical loss factor of 20% lost, 80% transmitted, the practical torque available at the spindle would be around 0.4 time the motor spindle torque.

If there is anything tight in the drive system, motor, intermediate, headstock gearbox, or spindle bearings, feed gearbox and leadscrew bearings etc when not isolated from the spindle, overtight belts etc, it will have a far greater effect when high speeds are selected than when low speeds are selected.

Single phase motors need a start winding and a run winding to operate. The start windings operate in series with a capacitor to force a magnetic phase shift in the motor windings to establish sufficient torque to start the motor, and to determine the direction of rotation. The start winding carries significantly more (4-6x) current than the run winding, but is generally switched out of circuit with a centrifugal switch once the motor attains about 70 to 80% of its nominal speed so the higher current normally flows for a second or so only. For motors over typically 2HP, a second unswitched capacitor is often used with the start winding, allowing a much smaller current (about .5x run winding current) to flow through the start winding whenever the motor is energised. The run capacitor and start winding causes a magnetic phase shift in the windings, enhancing motor torque and hence power.

If either the run or start capacitors are deteriorating, the motor start and operating torque deteriorate, causing the motor to have poor starting performance, or to fail to start unless given external assistance as the OP has to do to start the motor with higher spindle speeds selected. If there are excessive transmission losses in the drive system, there can be starting difficulties even if the capacitors are functioning properly, due to the excessive load placed on the motor. Continuing to operate in this manner will accelerate the ageing of the capacitors as they become overloaded.

For motor reversing, the same general configuration and components are used (i.e. there is not one cap for starting in one direction and another for starting in the opposite), but the relative phase of the connections to the start and run winding networks is swapped the magnetic phase shift caused by the start winding is in the opposite direction, causing the motor rotate in the opposite direction.

Originally Posted by tbonesmith

No, but I can buy or borrow one, my neighbour is a sparky.

laughing ask him very nicely to check it

Originally Posted by malb

For my money, either something is mechanically tight, or the motor caps are failing. As gearing is applied between the motor and spindle, there will always be an overall power loss. If you are gearing down to a lower speed, the torque available at the spindle will increase in proportion to the speed reduction, gearing up to a higher speed and the torque available at the spindle will decrease in proportion to the speed increase. For Oz mains power at 50Hz, the normal motor speeds are either around 1440RPM or around 2850RPM depending on motor winding configuration. Say you have a 1440RPM motor in your lathe, and you have selected a spindle speed of 280RPM (hyperthetical). This implies a mechanical speed reduction of 5, and a torque increase of 5x. In practice, the drive train will cost some power and torque, so you have an overall torque increase (motor shaft to spindle) of perhaps 4. Now say you select a spindle speed of 2880RPM (hyperthetical). This implies a mechanical speed increase of 2, and a torque reduction to half motor torque. Again allowing the same mechanical loss factor of 20% lost, 80% transmitted, the practical torque available at the spindle would be around 0.4 time the motor spindle torque. If there is anything tight in the drive system, motor, intermediate, headstock gearbox, or spindle bearings, feed gearbox and leadscrew bearings etc when not isolated from the spindle, overtight belts etc, it will have a far greater effect when high speeds are selected than when low speeds are selected. Single phase motors need a start winding and a run winding to operate. The start windings operate in series with a capacitor to force a magnetic phase shift in the motor windings to establish sufficient torque to start the motor, and to determine the direction of rotation. The start winding carries significantly more (4-6x) current than the run winding, but is generally switched out of circuit with a centrifugal switch once the motor attains about 70 to 80% of its nominal speed so the higher current normally flows for a second or so only. For motors over typically 2HP, a second unswitched capacitor is often used with the start winding, allowing a much smaller current (about .5x run winding current) to flow through the start winding whenever the motor is energised. The run capacitor and start winding causes a magnetic phase shift in the windings, enhancing motor torque and hence power. If either the run or start capacitors are deteriorating, the motor start and operating torque deteriorate, causing the motor to have poor starting performance, or to fail to start unless given external assistance as the OP has to do to start the motor with higher spindle speeds selected. If there are excessive transmission losses in the drive system, there can be starting difficulties even if the capacitors are functioning properly, due to the excessive load placed on the motor. Continuing to operate in this manner will accelerate the ageing of the capacitors as they become overloaded. For motor reversing, the same general configuration and components are used (i.e. there is not one cap for starting in one direction and another for starting in the opposite), but the relative phase of the connections to the start and run winding networks is swapped the magnetic phase shift caused by the start winding is in the opposite direction, causing the motor rotate in the opposite direction.

nice post malb. that would of taken me a long time to type

as said above it sounds electrical....I'd be checking the electrical side first to make sure all is okay there

eg capacitor, Motor windings, Centrifugal switch if it has one, wiring continuity capacitor/motor etc etc