A Better Potentiometer?

[Andy StopfordParticipant @andystopford50521]

The speed control potentiometer on my lathe is showing signs of failure once again. I’ve found these devices, regardless of the machine manufacturer/supplier, tend to have a tiresomely short life. Does anyone have any recommendations for a type or brand which might last a bit longer?

Any attempt at a general web search gets bogged down with discussions from hifi enthusiasts and musicians (and the latter, in my experience, probably have limited information to impart since they never use a setting below 11).

[Ian PParticipant @ianp]

I suggest using a potentiometer with a conductive plastic track.

Expect to pay well more than £20 but it will last forever.

The larger the diameter the better really (mainly for mechanical reasons as it should have decent spindle and bush.

Ian P

 

 

 

 

[Michael GilliganParticipant @michaelgilligan61133]

Further to Ian’s excellent advice …

Whether or not you choose to buy from RS, the ‘window shopping’ is very easy there: https://uk.rs-online.com/web/c/passive-components/variable-resistors/potentiometers/

Just apply the appropriate filters to narrow your search.

MichaelG.

[Bo’sunParticipant @bosun58570]

Very useful info’ guys.  Thank you.

Warco did replace the pot on my WM250 under warranty, but this is realy helpful for the future.  Having said that, with fairly light usage, under 12 months to failure is pretty poor.  Or was I just unlucky?

[HowiParticipant @howi]

Unlucky probably, had my Chinese lathe 10+ years now, still on original pot.

[JasonBModerator @jasonb]

Maybe it is pot luck what you get, the one on my X3 is still going strong after 16yrs use. The one on the Warco 280 failed after about 2 years and cost a fiver for a replacement from Warco that must have been in there now for 12-13yrs.

[Mark RandParticipant @markrand96270]

I used a wire wound (ten turn actually) pot on the VFD for the Myford. Bullet proof!

[noel shelleyParticipant @noelshelley55608]

My first thought would be to fit a wirewound ! Noel

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, the one on my Chester mini mill is 11 years old, and the one on my Chester mini lathe is 8 years old, both of which are still working fine.

I would say an early failure is just bad luck, as of all the millions that are made, a few bad ones are bound to get through.

Regards Nick.

[Andy StopfordParticipant @andystopford50521]

Thanks, chaps. I decided to give one of these a try (bought from ebay – cheaper than RS):

https://docs.rs-online.com/c0c7/0900766b80030319.pdf

For context I’ve had these fail on three different machines, one twice. In each case they’ve lasted about a year. I don’t think I do anything terrible to them!

[Howard LewisParticipant @howardlewis46836]

Strange!

I must be lucky.  The speed pot on the VFD that came with my lathe in Septenber 2003 is still functioning OK. But ma\ybe I use it less than most?

Howard

[Andy StopfordParticipant @andystopford50521]

The new pot arrived this week – what I hadn’t realised/noticed was that there are no stops on it, i.e. if you keep winding it clockwise it goes to max and then drops immediately to minimum, and vice versa going the other way. Not really what you want for a lathe (in fact, why would you want this behaviour?).

I thought of sending it back, but the old one was failing rapidly, and I wanted to use the lathe this weekend, so I bodged up a knob with a groove and stop underneath, to engage a stop screwed through a backing plate. I didn’t spend an awful lot of time or trouble on it,but it works OK. The potentiometer has a smoother action than the rather gritty feel of the original, and the speed control is much finer, so worth doing, but if you go for it, make sure you don’t get one listed as ‘continuous’ or you’ll have to fool around making a stop arrangement too.

[Bo’sun]

On Bo’sun Said:

…Having said that, with fairly light usage, under 12 months to failure is pretty poor.  Or was I just unlucky?

It depends!  The pots fitted to Chinese lathes (and almost everything else!) are consumer grade.   They’re intended for indoor domestic use, such as a radio volume control, getting relatively light use in a clean environment. Their construction keeps the price down: the spindle simply turns a spring loaded wiper over a carbon track, which is protected by a steel cover, but not sealed.

For consumer use they are both value for money and fit for purpose.  But they are vulnerable to wear, dirt, and condensation which suggests they’re not the best choice for a machine tool.

However, when they go wrong:

• a squirt of switch cleaner often fixes them and is always worth trying.  It blasts dirt off the track, removes grease and water, deoxidises the wiper, and leaves a protective lubricating  layer on the track.
• They are under a fiver to buy, and generally easy to replace – 3 easily soldered wires.

If consumer grade isn’t ‘good enough’ there are plenty of alternatives, but don’t expect them to be cheap!   Mid-range, sealed in plastic, cost about £20.   Next grade up can be had for about £60, and I expect aero-space grade reliability could be bought for a few hundreds.   When the better type fail, they can’t be fixed in-place with a squirt of switch cleaner, and usually gave to be replaced.

I’ve been lucky in my workshop – the ordinary pots fitted to my lathe and mill are still going strong after nearly 10 years.  The workshop doesn’t suffer condensation and I rarely machine dirty materials.

All engineering is a compromise and fitting a posh £60 potentiometer to a £2000 ex-VAT hobby lathe is unlikely to be value for money.  Given ordinary pots have lasted 10 years in my machines, it would be hard for me to justify fitting anything other than consumer grade replacements when the time comes.  On the other hand, if pots only lasted a year, it would be worth going up-market and trying a £20 replacement.

Also check to see if anything going on in the workshop could be causing pots to fail quickly.  Machining cast-iron is likely to cover all the electronics in conductive muck, resulting in expensive repairs.  Workshop problems like condensation or poorly controlled dirt are bad for electronics.

Dave

 

 

[Michael GilliganParticipant @michaelgilligan61133]

Nice neat modification to that knob, Andy

… I hope you will be happy together !

MichaelG.

[Andy StopfordParticipant @andystopford50521]

A match made in heaven, Michael (or Tunbridge Wells anyway).

I agree Dave,  that £60 would be overkill, but £20 might be a worthwhile upgrade, especially if your motor lacks low speed torque. I think the finer speed control could be handy for large diameter stuff, especially parting off, where you’re pushing the limits of what the machine can manage and trying to find the narrow window between chatter and stall.

[Author]

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