Jet VS motor surges?

[Darrin]

I drill on low speeds and only happens when drilling and not turning.
This thing doubles and dare I say triples it's speed when drilling occasionally.
I've been turning for years and never had this happen before.
Any ideas?

 

[magpens]

Wish I could help ... I don't know what's wrong. . All I can say is it seems something wrong with the electronic circuitry that controls the speed.

You say it is a Jet VS, so the VS would indicate it has electronically variable motor speed ... VS = Variable Speed, I assume.
But the fact that it only happens when you are drilling (not turning) is very puzzling because the machine does not know you are drilling.
All the machine knows is that it should be turning the headstock spindle (or not) and at what speed, per your selection on the speed control knob.
So the fact that it acts erratically only when you are drilling is weird, because it is all a headstock turning activity for the machine.
But ...... then you say you drill at LOW speeds ... ONLY at low speeds ? . Drilling at lower speeds than what you normally turn at, I assume.

If that is true, then it is the low speed end of the "variable speed". . I guess that fact might mean something to a technician who knows how it works.

Perhaps if you were turning a pen at the same low speed as you use for drilling then the erratic behavior might also occur when turning.
Might be worthwhile trying that, because the technician might ask you about that.

How old is this lathe ? . I think you need to contact the Jet company. . Be sure to tell them the model no. and the serial no. and when/where you bought it. . It is very unlikely that the company you bought from would know anything about the cause of this problem. . So unless it is still under warranty they won't be able to help you.

Does the lathe have any speed control features in addition to the electronic control ? . Can you change the belt position ? Gear change levers ?

If there are mechanical means to change speed, then there could be some issues that are dependent on whether your are drilling or turning.
I say this because when you are drilling you are putting horizontal pressure on the headstock mechanisms. . And that could affect the mechanical parts of the speed control. . I kinda think that is an unlikely scenario .... but I don't know how the Jet lathe works in that degree of detail.

 

[Dalecamino]

Replace speed controller. Replace circuit board. Fixed!

 

[jttheclockman]

It is in the controller board. You are changinging torque when drilling and not so much when turning

 

[Darrin]

It is in the controller board. You are changinging torque when drilling and not so much when turning

This was my assumption because of the drilling.

 

[KLJ]

Could it possibly be that while drilling you are standing in a different position and your body is hitting the speed control ?

 

[Gary Beasley]

Have you noticed the drill binding any while drilling? I would think because you are going very slow and the motor is making very little torque any binding will cause the speed to drop further, is detected by the vs board and it tries to compensate by increasing the torque. My Nova does this in spectacular fashion if there is a small catch that pulls the speed down, it responds by spinning up much faster briefly and scaring the bejesus out of whoever is near it.

 

[Woodchipper]

Buy a new lathe- that will definitely cure the problem. Or...check with the factory and see if they have a solution or maybe a recall on the control.

 

[monophoto]

As an electrical engineer, my approach to troubleshooting this problem would be to start with two principles - Occam's Razor which states that the simplest explanation is most likely the correct explanation, and basic motor theory.

What others have said is correct - there could be a problem in the speed control. But before going there, you might want to check to see if there is a less drastic cause.

This lathe uses a DC motor, and with DC motors, the speed is a function of the product of the voltage applied to the rotor multiplied by the voltage applied to the state. However, this is likely a permanent magnet motor in which case the rotor flux is fixed by the permanent magnets inside the motor. So speed fluctuation can only result from variations in the stator voltage which comes from the speed control box. I think the perception is the the speed is surging from normal to some higher point, but is it possible that over time the 'normal' lathe speed has actually become lower; in that case, you may have simply compensated by turning the knob to force it to speed up, but what is now happening is that it is intermittently returning to that higher normal speed. That would imply that the normal voltage being received by the motor is reduced from what it should be, but is intermittently popping back up to its normal level. Strange - yes, but that could result from a loose connection between the controller and the motor.

My lathe (a Turncrafter) is similar to the Jet, and the connections to the speed control is by means of a plug. A few years ago, I added an external reversing switch which put an additional plug into that connection between the speed control box and the motor. Occasionally, if one of those plug connections works loose, the additional resistance in the circuit causes the voltage delivered to the motor to drop causing a drop in 'normal' speed.
So - before concluding that you need a new speed control box, check for loose connections. If that's the cause, the fix would be much less expensive!

 

[jttheclockman]

Yea what he said!! The control board is going bad.

 

[randyrls]

All the above advise is good to try. I would first check the speed control knob itself. The Speed control knob is a potentiometer, and these are NOT dust proof and not of the highest quality. UNPLUG the lathe before doing anything! A quick shot of compressed air or tuner cleaner may clear up your problem. Turn on the lathe at low speed and SLOWLY increase the speed to see if there is a spot where the speed changes drastically. The pot itself may need to be replaced. This is a common fault with mini-metal lathes.