mini lathe speed problem please help

[audioaudio]

Hi,

I hope you can help me as I at the end of my wits with my mini lathe.

It all started some time ago when it would not turn parallel; The upshot was that I believed, rightly or wrongly) that the cause was the need to have tapered roller bearings in it. I changed them and the problem was that the lathe was not functioning properly in that the lathe's speed seemed laboured to get to any speed and if memory serves it was shaking at some speeds. I thought it was the controller board could not handle the preload required to make that type of bearings rigid, so stopped right there and decided that angular contact was the way to go, so I have now replaced the bearings with angular contact ones. They went in perfectly , but I now have the same shaking problem again, which manifests itself at 520 rpm on the low gear range (measured at 92vdc). I originally thought it must be a mechanical problem in the headstock, but I have ruled this out by using a hand held power drill attached to the table next to the lathe and made a small pulley of near the same diameter to turn the spindle with a longer belt I had laying around the workshop. At the same spindle speed (520rpm), there was no shaking.

My conclusion is that there must be a problem with either the controller or the motor, so I have dismantled, cleaned and electrically tested the armature, and it seems to be fine, at least there are no shorts on the coil. I have tried adjusting the presets on the board. but none of them fix this problem (I noted where they were set beforehand by photographing them).

I Should point out that this board blew up last week as I was testing the headstock (one of the thyristors blew I found out it was caused by a loose wire on the forward/reverse switch), but they have been replaced with what I believe are the correct equivalents i.e. S8020L (and I also replaced the diodes with D8020L). It had this problem before the board blew anyway, so do not think this is the reason.

Can anyone offer any help on this please ?

Thank you in advance.

[loply]

There are some potentiometers you can twiddle on the motor controller board, but I'm not exactly sure what they do, I just know other people have solved similar problems by playing with them.

How's the belt tension on your motor? And are the pulleys on the motor shaft and the layshaft lined up and functioning as they should be? They're tight on their shafts etc?

If you're saying the problem remains even after you replaced the motor control board, and doesn't manifest itself if you spin the spindle with an electric drill, then that pretty much narrows it down to your motor and your transmission...

[Stilldrillin]

Hi Audio.
I can't help you directly....... 

But, My Chester Conquest, fitted with Amadeal control box, and belt/ pulley speed reduction. (Full story...  http://madmodder.net/index.php?topic=6602.0).

Pulls like a train, performing faultlessly. Except when running free at full power, Low gear 800rpm, and wearing the 5" chuck.... It "jerks/ lurches/ or shakes".

It doesn't do this, when the 80mm chuck is installed! 

Sorry this is no direct help. But it might be the same problem.........

David D.

[Fergus OMore]

It sounds like a static and dynamic balance problem which could be either in the  motor or spindle.

The rest of the modelling world have these sort of problems but - and I hope that no offence is taken- do many of run at such high speeds

I may be right, I may be wrong but it something which I would look at

[Steve Withnell]

Well I do run at much faster speeds!  My lathe has a top speed of 2500 (its a 5x22) and at times,  I could do to go higher.    Small diameter drills in the tailstock  and 2500 is way to low.   Turning small diameter stock.  However I do have a dynamic balance problem around 1000rpm - I know which pulley is causing it (because I made it!),  but not figured out a method of dealing with it.  I guess I should static balance it first and then see if a dynamic balance problem remains.

[hopefuldave]

Hi AA, sorry about the late reply - been sunning myself by a pool for a couple of weeks!

I agree it could be a dynamic balance problem, although static balancing may be enough to cure it - I know motorcycle wheels are only statically balanced, and the pulleys have a similar "form factor"? To do a good job of static balancing you need really low friction bearings to rotate the parts on - knife edges or similar will do as long as they're dead level and sharp and the temporary shaft's true and a good fit in the pulley bore.

A *properly balanced* lathe should be smooth at all speeds - OK, mine has 2 tons of cast iron to absorb and vibration, but I can just about balance a coin on the headstock at anything between 5 and 2200RPM as long as I don't have any of that awful work stuff in the chuck  The big old BTH motor was specified as dynamically-balanced though, back in 1956...

As a thought (possibly showing my geek roots here...) re: discovering the source of the vibration, there are some free spectrum-analysis utilities on t'interwebby thing, using a cheap microphone as a pickup would let you look at what vibration frequencies you're suffering from - if this matched (e.g.) the spindle speed or the speed of one of the pulleys / the motor, this could point you in the right direction?

Another thought, the "KB-alike" motor speed controls don't produce actual smoothed DC, instead they provide chopped "raw" DC, consisting of a series of pulses at 100 pulses/second (50Hz AC input, 120/second on 60Hz) - if this frequency "beats" against the commutator period or the motor rotational speed it could cause torque variations and hence vibrations at specific motor (and so spindle) speeds?

One possible fix if it's a speed control / motor issue would be to add a SUITABLE capacitor across the motor inputs or a line choke in series, bearing in mind that there will be as much as 220V DC on it *and it reverses polarity with the motor direction* - the cap' would need to selected for a minimum of 300V working voltage, *non polarised* and cope with perhaps a 6A ripple current - all I can think of is the cap's used in high-power (think heavy-metal-band PA system!) loudspeaker crossovers....
A choke could be made from a scrapped power transformer with a secondary winding rated for the current required (say 4A for a 450W motor, up to 15A or more for one and a half Chinese horses on 220V, double the current rating for 110V) with the primary winding *thoroughly* insulated?

The pots on the speed control board are normally (working from the left-hand edge): Max speed; Acceleration (further from the edge); Min speed; a gap; IR Comp; CL (current limit) - speeds and acceleration are pretty obvious (although increased acceleration can make a crackly speed control's effects worse), CL *should be left alone* unless you have a suitable ammeter to set it up, IR Comp controls the current boost with increased load - set too low and the spindle speed varies too much with load, set too high and the speed will "hunt" up and down or surge. *All* the pots are way too sensitive on these boards, I think because they've tried to get a wide range with a single-turn pot!

Just my ha'pennorth,
Dave H. (the other one)