Sherline motor problem

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lensman57

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Hi,

I have a Sherline 4500 lathe with an older Sherline motor, internal brush type.
Recently the motor does not stop when I turn the dial all the way to stop it but keeps going at about 250 rpm. The motor speeds up with no problem and goes all the way to max speed once the dial is turned right.
Has anyone come across this problem?

Many Thanks,

A.G
 

Tin Falcon

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Just wild guess here but i would check the variable resister/ pot on the motor control. sounds like the wiper slipped. but like i said just a guess check the simple stuff first.
Tin
 

lensman57

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Hi Tin,

I have suspected as much but have not checked it yet, is it better if I just replaced it with a new 5k potentiometer?

regards,

A.G
 

Tin Falcon

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the pot is a lot cheaper than a motor or control board so worth a try. you can troubleshoot it with a VOM if you like.
Tin
 

rhankey

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It does sounds like the speed adjustment pot. As Tin suggests, if you have a multi-meter, they are very easy to check. Disconnect the three wires, and if it is rated at 5K ohms, you should get that across the two outside legs. And if you measure between the middle and either outside leg, you should see it go from 0 to 5k ohms as you turn the pot from one extreme to the other. If you need to repalce the pot, be sure to match the specs for resistance and current. Robin
 

lensman57

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Thanks to Robin and Tin for your help.

I will find out about the resistance of the pot for sure but I think its 5K,
I tried to order a couple from Sherline Direct, they wanted $80.00 to ship to the UK.
Will try Maplins or similar to find a Chinese one that maybe suitable.

Regards,


A.G
 

John Rudd

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Lensman,
If the speed control pot is similar in function to the generic mills and lathes, a standard pot with switch from Maplins will not work..

What you may need to do is keep the switch part of the existing, in circuit and wire in the new pot to act as the original one....
Make sense?

The pot on my minimill failed and I ended up fitting a new one from Mappies without a swtich and modding the switch side of things....


could be that the min speed pot on the control board needs a slight tweak?
 

Swede

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Have you checked the set screw that holds the knob itself on the potentiometer shaft? If it slips clockwise, that can cause a similar problem.
 

lensman57

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John Rudd said:
Lensman,
If the speed control pot is similar in function to the generic mills and lathes, a standard pot with switch from Maplins will not work..

What you may need to do is keep the switch part of the existing, in circuit and wire in the new pot to act as the original one....
Make sense?

The pot on my minimill failed and I ended up fitting a new one from Mappies without a switch and modding the switch side of things....


could be that the min speed pot on the control board needs a slight tweak?
Hi,
I have checked what is on offer from Maplins and they are not suitable, the closest they have is 4K7 which is not exactly 5k. I have been in touch with MillHill supplies and they are doing their best to find a replacement pot for me, after all this, there is still no guarantee that the thing works anyway as the problem could be with the electronics in the control box or even with the motor or the brushes. The motor is the older version with the brushes concealed and it is one hell of a job changing these brushes as I will have to do a lot of pulling and pushing just to get the shaft out.

Thanks for all your help so far.

regards,

A.G
 

Hilmar

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I would use the 4.7K , not that fare from 5 K
Hilmar
 

HYTECH

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lensman57 said:
Hi,
I have checked what is on offer from Maplins and they are not suitable, the closest they have is 4K7 which is not exactly 5k. I have been in touch with MillHill supplies and they are doing their best to find a replacement pot for me, after all this, there is still no guarantee that the thing works anyway as the problem could be with the electronics in the control box or even with the motor or the brushes. The motor is the older version with the brushes concealed and it is one hell of a job changing these brushes as I will have to do a lot of pulling and pushing just to get the shaft out.

Thanks for all your help so far.

regards,

A.G
The motor is fine. It wouldn't work at all if it was the problem. The controller should cut power and as long as the motor is running it has power.

Jasen
 

Don1966

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The guys are right the speed pot could be your problem, but could also be the minimum speed trim pot that has changed value. Some of these trim pots due drift time to time. Also if you are trying to replace your speed pot. Go to a radio shack and look for a 5 watt 5k linear pot that is if you have aRadio Shack in the UK. If not go to your local electronic store these pots do not cost that much. Make sure it is a linear pot. They do sell audio taper pot and you don't want that. give the pot a try and see what happens. Your controller is working because you can get full speed, just not minimum speed. I have worked on many types of drives these are just smaller and less expensive. If you have the manual on your machine it should show the controller and you could try to readjust the minimum speed pot it would not hurt, just make a mark on it before you adjust just so you can return it to it original position if it does not help.

Regards Don

 

lordedmond

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4k7 is one of the preferred sizes therefore they are easy to find ,they follow the same pattern as resistors

use the 4k7 its the min end that has failed on your current one and will be the same on the 4k7

out of interest was the original one marked 5k or
have you measured it ? if you measured it you could have measured a faulty pot


BTW a switched pot is a switched pot any will work if the r is similar
 

John Rudd

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Sorry I beg to differ...

A switched pot is not necessarrily the same as a switched pot............In this case when the switch part is operated it opens when the pot is turned clockwise...Conventionally a volume control on say a radio works opposite.. i.e the contacts close to apply power.

The switch part forms part of an interlock to prevent the machine restarting if the power fails or the Em stop is depressed. you need to turn the pot back to off to restart the machine.

This is the reason why the Maplins offering is of no use and why I chose to replace mine with a pot without a switch and modify the interlock circuitry.

 

lensman57

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Don1966 said:
The guys are right the speed pot could be your problem, but could also be the minimum speed trim pot that has changed value. Some of these trim pots due drift time to time. Also if you are trying to replace your speed pot. Go to a radio shack and look for a 5 watt 5k linear pot that is if you have aRadio Shack in the UK. If not go to your local electronic store these pots do not cost that much. Make sure it is a linear pot. They do sell audio taper pot and you don't want that. give the pot a try and see what happens. Your controller is working because you can get full speed, just not minimum speed. I have worked on many types of drives these are just smaller and less expensive. If you have the manual on your machine it should show the controller and you could try to readjust the minimum speed pot it would not hurt, just make a mark on it before you adjust just so you can return it to it original position if it does not help.

Regards Don

Hi Don,
I checked on Sherline's site and the value of the pot is 5K but they do not give any current or power rating for the pot. The ones on ebay and from Maplins are 2W at best so there is a doubt. I have checked inside the controller box and there are two trim pots, I think one is for speed and the other one is for torque ( at least thats what they are in the Chinese lathe ) but they are not marked so I may have a crack at it.

Regards,

A.G
 

lordedmond

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well they must have altered thing then just been to check on my potentiometer stock ( thought I better give the full name as check my pot stock don't look to good ) all mine are spdt so can be wires as you wish

as I do not own a mill lathe with that arrangement I cannot comment in more detail ( the one I did have when back asap due to it self starting ( with the pot wound off ) and in this state ( without the e stop in) would motor against hand pressure on the chuck, a quick look at the PCB and sketch up showed up some poor design . ) hence the prompt return for cash back


Stuart
 

Don1966

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Lensman57 the 2 watt pot will work the system draws very little current. All you are doing is applying a voltage reference to the controller. The control pot and the minimum speed pot are in series and usually the max speed trim if you had one, both trims pots are small. Mark both pots if you have only two any try to adjust both one at a time to see which one does what. You will not hurt anything and you my be right one for torque and one for minimum speed. If you notice nothing return to original position. Some of these controllers have four trim pots on them usually torque, minimum , maximum speed and stability. The torque is usually adjusted at minimum speed this give it a current boost to maintain rotation.

Regards Don
 

lensman57

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Don1966 said:
Lensman57 the 2 watt pot will work the system draws very little current. All you are doing is applying a voltage reference to the controller. The control pot and the minimum speed pot are in series and usually the max speed trim if you had one, both trims pots are small. Mark both pots if you have only two any try to adjust both one at a time to see which one does what. You will not hurt anything and you my be right one for torque and one for minimum speed. If you notice nothing return to original position. Some of these controllers have four trim pots on them usually torque, minimum , maximum speed and stability. The torque is usually adjusted at minimum speed this give it a current boost to maintain rotation.

Regards Don
Hi Don,
Thanks to you and all who replied with your suggestions, will do as advised tomorrow and let you know the results, for now I am off to celebrate the Persian new year as it is the first day of spring so a happy new year to all as well.

Many Thanks,

A.G
 

lensman57

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Hi to everyone,
I managed to fix the problem with the motor. It was indeed the Min Speed trim pot inside the control box, it needed to be turned back a bit and it did the job. So thanks to all the guys who helped with their suggestions, by the way I wrote that the control box had two trim pots but as I properly opened the box and moved the wires around I discovered that there were indeed 5 trim pots marked as min, max, accl and two other ones that I can not remeber.

Anyway best regards to all,

A.G
 

Troutsqueezer

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Late to the party, I know.

Trim pots generally do not turn by themselves so the question is: Why did it? Or why did it change in value? In either case, you might continue the hunt for a replacement pot to have on hand if this one does fail completely.

If unable to find something (should it be necessary) you can always check the other four pots to see if they are of the same value and taper. If so, measure the resistance they are set to, replace them with resistors of equivalent value, then use that pot as the replacement. Guess you could do that for any of the pots, if they fail.
 

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