Mini-lathe dying..?

I fear my lathe may be on the way out, the speed control is not as it should be.

If I dial it straight up to approx 1250 rpm of more it goes straight there and is stable. At lower speeds it fluctuates and takes about 10 seconds or more to settle on the selected speed. Also when the speed control is turned to minimum it doesnt stop, but runs on at a low speed for quite a few seconds before stopping, so something has changed.

I suspect the board is faulty, there is only 6 month warranty on the electronics which expires next week, so better ring the supplier.

Just my luck, damn it.

Hello Ron,

In the old days, that was the characteristic behaviour of a dirty track on a potentiometer and a drop of switch cleaner into the guts of it combined with sweeping it back and forth over the full travel was often enough to correct the problem

Whether or not you can do that in this case I don't know, but you have little to lose in trying it before blaming the board.

Regards

Brian

Personally, I would contact the supplier first if the machine is still in warranty, as they could consider that attempting to fix it voids the warranty.

(another) Brian

I agree with the second Brian. The supplier may know, or need to be aware of, the problem. There may be a whole consignment with dodgy pots.

Well you have made my weekend Brian.. you were spot on, I have just put a couple of drops of contact cleaner into the pot turned it back and forth a few times and problem solved.

Thanks a lot Brian, very much appreciated.

Ron

Brian,is there no end to your talents ?

A friend of mine came across something similar a while ago,same problem,same answer. It seems these these things are prone to this. One of the reasons why I dislike electronics,apart from the fact that I dont understand them anyway.

Gear boxes and belt drives,I,m OK with them,but the other stuff----------------??

I think it sometimes comes down to the conditions in the workshop which may affect the pot. Can't all be prone to it as my mill one is still OK after 11 years and the lathe 9yrs.

It may well have solved the immediate problem - but for how long? I’ve cleaned pots before now and some have lasted, but others have eventually succumbed ... and sometimes not much later ...

Good luck with your dodgy pot. The enclosure should be sufficiently dust proof, to keep out rubbish for a lot longer than 6 months.

Sorry to ruin your weekend Larry, but the electronics didn't fail. The part that went wrong is mechanical. When you turn the knob, a sprung contact sweeps along a curved resistive track. Being mechanical, it often goes wrong.

Ron's problem on a new lathe sounds like dirt on the track - perhaps metal dust from cutting. My mini-lathe had a hole in the control box where it went over the leadscrew. Bad design because it allowed swarf inside where a streamer of aluminium dropped on the electronics could blow the board! Newer mini-lathes are fitted with a grommet. Might be worth Ron checking that dirt can't get inside the control box.

Dave

It all depends on the type of resistive track in the potentiometer - most likely the pots typically used in these inexpensive applications is a simple carbon-ink/varnish screened or printed onto the phenolic resin base. That type of track has a life cycle at best of several hundred rotations.

Cermet based resistive medium can give around 50,000 rotations, and the best for this sort of application, a conductive plastic medium, capable of around 1 million cycles or more..Same material as is used in the pot in modern car gas-pedal controls. A good quality 10kOhm conductive plastic linear pot will only cost between US $5 and $15 ( the latter a sealed pot - better for this application) , so I really don't understand why the mnfr/suppliers skimp on this item! The drama caused by its failure does not warrant the cost saving.

Joe

Larry,

I can't claim to be other than just another bodger really, but with a few miles under my belt!

There is a PM waiting for you to complete the definition of gear sizes, this covers Module versions

Regards

Brian

Edited By Brian Wood on 17/11/2018 10:54:11

Thanks guys, I did think that the pot is probably not of the best quality or maybe not a good example when manufactured. The electrics box is open at the bottom for the leadscrew as Dave explained but at least there is a plastic cover screwed in to cover the board and all the lower electrics. The pot is situated at the top of the box and there was no evidence of any dirt or swarf visible. There was some swarf in the bottom of the box but not much and none had got behind the plastic cover.

The pot would be really easy to replace and as Joe suggests a sealed better quality one would be a good idea.

I will see how it goes and should it play up again I will replace it.

Ron

Joe

Anyone who has worked producing items in an engineering company will be well aware that overall control nowadays is in the hands of accountants, there is no choice when a range of costs are presented for an item where they all execute the same function, the cheapest item is the one that purchasing will be instructed to acquire. In the case of a budget priced lathe or mill then costs are paramount in the equation as profit margins are very slim.

Dave W ( Thankfully now retired Engineering Quality Assurance Manager )

Dave W - I realise that for sure, but still - The cost of a comeback to , for example, a supplier in the UK who sources these lathes from the east, is surely a lot more than the cost of the pot! And so I would have hoped that said supplier would 'insist' on some level of built-in preventative Quality. Ron's problem with the pot is not the first on this forum,or the tenth, so should not be to difficult to determine where cost of failure lies in a product..

Joe

Tell me about it... happened to my CL300M, I converted it to a VFD and 3-phase motor but used teh same control box, with added grommet.

Neil.

Some organisations learn from this kind of penny pinching bean counting experience in the hardest way.

In the early days of RB 211 production at RR in Derby, electron beam welding was a new process that was used to weld up the titanium alloy compressor drums for this new range of engines. It had the advantage of making very good welds in narrow access areas, and with the process carried out under vacuum, the welds were clean as well.

One drawback was that a sacrificial back plate in matching alloy was needed to mop up the beam as it passed through the target section to avoid impingement on the remote side of the drum. The first production run of a number of drums was sanctioned after successful trials and these were found to be contaminated with a brittle Fe/Ti eutectic that could not be sorted good from bad, it wasn't even magnetic. The only test that revealed it was that it rusted when wet.

Management was hard pressed at the time to meet cost pressures and were NOT impressed to find that mild steel backing plates had been substituted, without reference, in the re-order for material. The whole batch had to be scrapped out of hard, it cost the company the thick end of £1/4 million.

A very expensive mistake

Brian

The pot is actually quite difficult to source because the switch at the back turns off, ie contacts open, as the control knob is turned clockwise. This is the opposite of most normal posts where the contacts close as the control knob is turned clockwise.

Mike

My switch broke after about ten years use, I found a company that made a suitable replacement. I rang them up and they said they only sold trade quantities, so they sent me one FOC! Sadly I don't remember who they were.

Neil

Not sure what resistance your pots need be, but from 1kOhm to 50kOhm, conductive plastic, BOURNS product - also from TE-Connectivity ( both via DigiKey UK )

BOURNS P/N - 56AAD-C28-B15/R81L with a SPDT switch, so ON or OFF at rest - $8.00

BOURNS P/N 3310H-003-104L, same DPDT sw -$13.00 ( a beefier pot)

Odd to use a pot with a switch working that way - They are not the most common which I would think makes them costlier as well. So then why did they not fit a decent one to start!

Joe

Edited By Joseph Noci 1 on 17/11/2018 14:14:29

Hi Joe,

Thanks for the part numbers, RS Components stock the 56AAD part number, cost £8.26. The lathe suppliers replacement pot is rated at 4.7K according to their website. I know little about electronics so dont know if the 4.7K needs to be strictly adhered to.

Ron