My mini lathe won’t latch on

[Paul McDonoughParticipant @paulmcdonough43628]

My mini lathe developed a problem this afternoon after a couple of hours of light use, I came to start the next cut and whilst the motor would start, the relay/ contractor would not latch in, so that after releasing the start button it would stop.

whilst I held the start button in, the motor would run and each of the safety micro switches could be made to stop and restart the drive.

I noticed that the usual ‘click’ when starting wasn’t there.

the lathe is fitted with a brushless motor and drive which still allows change of speed.

i don’t have a cicuit diagram for the machine which is a nuisance but whilst looking behind each of the covers I found that the relay/contactor is built into the start/stop switch unit.

i will spend some time tracing the fault hoping that it isn’t an issue with the pcb controller.

I know that pin 14 on the switch goes live because the motor runs when the ‘start’ button is held down but as yet im not sure where pin A1 goes.

has anyone experienced similar issues with their machine and what the likely issue is?

 

many thanks

 

 

 

[SillyOldDufferModerator @sillyoldduffer]

Almost certainly the switch/contactor  you’ve identified, or a connection to it.   The circuit is something like this:

When the On button is pressed current is supposed to flow through the coil and close all the switches inside.  The Auxiliary Contact bridges the On button, so the contactor stays closed, ie latched ON.   A power cut or pressing the OFF button breaks the current flowing in the coil so all the switches open, unlatching until the ON button is pressed again.

Most likely the auxiliary switch has failed (replace the unit) OR one of the wires connecting power to the auxiliary has come loose.  Fallen off would be nice, maybe a bad crimp.

That the motor runs when the ON button is held down means the contactor coil, other switches, and the rest of the lathe must be OK.

Dave

[Paul McDonoughParticipant @paulmcdonough43628]

Thanks Dave, I am familiar with the standard starter circuit, but I’ve not seen this ‘all in one’ type switch before, the schematic on the side of the switch seems to just show the power contacts of the relay/ contactor not the hold in contact that you describe.

i will check for loose cables, as it’s usually something simple. A new switch if it is the problem would be an easy fix

[Dave HalfordParticipant @davehalford22513]

If you have a safety shield around your chuck it may not be switching properly to hold the run relay in.

[HuubParticipant @huub]

My mini lathe had the same problem after 10+ years of use. I replaced the 5 pin On/Off/Emergency switch and the problem was gone.

I guess, like said before, you should check the chuck safety shield first. When in doubt, you can short the 2 wires that goes to the chuck safety switch.

[Beardy MikeParticipant @beardymike]

Before doing anything more drastic jab at the stop button a few times hard. I’ve got an NVR switch on a drill that sometimes fails to fully release the stop button then fails to hold on.

[Beardy Mike]

On Beardy Mike Said:

Before doing anything more drastic jab at the stop button a few times hard. I’ve got an NVR switch on a drill that sometimes fails to fully release the stop button then fails to hold on.

Sounds like your switches are incorrectly connected, with the stop circuit broken the start switch should not inch the starter.

Emgee

[Paul McDonoughParticipant @paulmcdonough43628]

Thanks for your thoughts.

Having traced the circuit through in the lathe, I now know that the fault was not related to the PCB, hoorah! I removed the switch and tested it in isolation by energizing the coil from the Lucar blade connections on the back of the switch, all I heard was a faint buzz, so I ordered a new one.

Now that I know that I don’t have to rely on the old one I took it apart, to my surprise this was quite easy and unusually, it did not ‘explode’ in a pile of small parts and springs.

I found that whilst lightly marked, the switching contacts are in reasonable shape, but the leads leading away from the coil were simply jammed down the side of the main contacts from which it derives its supply. No weld or solder involved.

I suspect a poor contact at this point.

I guess I could repair it, but with a new one on the way I question the need, spare maybe?

[Howard LewisParticipant @howardlewis46836]

Being Job’s comforter, the new one might be the same!

So my inclination would be to ensure that the connections in the “old” switch are good, and keep as a spare, in case the problem recurs. With a reliable spare, getting under way again will be fairly quick, and the faulty unit can be given the same treatment (Belt and Braces, probably ensuring that the problem never happens again!)

Howard

[Paul McDonoughParticipant @paulmcdonough43628]

“the new one might be the same!”

Oh, I have no doubt.

I’ll solder the coil leads in the old one to the terminals and put it back together.

Can’t do any harm sitting on the shelf ‘in case’ I don’t expect it will happen again :0)

 

 

[Chris CrewParticipant @chriscrew66644]

I had the same problem with one of these cheap push button start/stop switches which I fitted to a Union pillar drill I acquired. It has nothing to do with the type of motor, it is the the plastic around the very flimsy contacts inside the device that heats up and distorts. I dismantled mine to investigate the issue and managed to resolve the problem. You may not be so lucky but it’s easy to dismantle by prising the outer case latches with a small screwdriver and removing the contacts which are mounted on a small plastic yoke. My guess is you will see where one or both have become welded to the yoke with heat instead of moving freely within it. If you can free it off your contacter should work when reassembled but if it’s burnt beyond repair you will need to replace the push button part which is available on eBay. I was lucky that my contacter has been ok since but I acquired a more robust starter unit just in case it fails again.

[Beardy Mike]

On Beardy Mike Said:

Before doing anything more drastic jab at the stop button a few times hard. I’ve got an NVR switch on a drill that sometimes fails to fully release the stop button then fails to hold on.

Likely that wear and tear are taking their toll on the switch and it’s on it’s way out.   Though an iffy switch might last for years, expect to replace it.

Dave

[Paul McDonough]

On Paul McDonough Said:

Thanks Dave, I am familiar with the standard starter circuit, but I’ve not seen this ‘all in one’ type switch before, the schematic on the side of the switch seems to just show the power contacts of the relay/ contactor not the hold in contact that you describe.

i will check for loose cables, as it’s usually something simple. A new switch if it is the problem would be an easy fix

Well done, I see you fixed it, making this comment superfluous.  Might help others though.

The circuit I posted is very common, but it’s implemented in two ways.

1. With separate components, which is what I think Paul expected, OR
2. With some of the circuit pre-built inside the switch.   This is what you have.

In the switch:

U< is the coil, with one end pre-wired to OUT connector 14 and also via a switch to IN connector 13.   This provides part of the circuit I gave within one component, saving time in the factory when assembling the lathe.  No problem with that, except the diagnostician chasing a fault later on has to understand how the switch is used to implement the circuit.   The danger is that having failed to confirm the switch is guilty due to misunderstanding it, one plunges straight into ‘fixing’ a complicated control board.   You can guess how I know!

Blush.

Dave

 

[Paul McDonoughParticipant @paulmcdonough43628]

Yes, having opened the back of the lathe up I definitely expected to find discrete components making up the traditional  starter circuit that i incorporated in my industrial control panels for years.

But clearly this is the new way.

One thing I struggle with in this ‘compact switch design’ is that the buttons on the front face press the armature of the relay directly, this is a bit naff. But hey its the way of the world, its smaller and cheaper and simpler but weirder!

As for probing the PCB controller, its tempting ,but its usually best to check all the simple stuff first, fuses, wires fallen off etc.

Thanks to everyone who chipped in to help.

As the blue button says below this pane, I must:  ‘Submit’

[Author]

Posts