Hi All..

I’m fishing for any advice or just a sanity check really as I am loosing the will to live with my lathe..

My Colchester student has developed a ‘vibration’ for want of a better word.

Bit of background –

I began to notice it a while ago at higher RPM (up around the 1000rpm area) but put just thought maybe an imbalanced chuck / work piece / belt gone a bit weird etc

The headstock has always been the usual noisy Colchester but when I refurbished it one of the centre shaft dual gears was really ropey, nothing like the others and as the machine had been crashed in a previous life (to the point of shifting the lower gearbox gears on their shafts!) I presumed that it had been damaged and a second hand replacement was fitted.

Recently both of the centre shaft dual gears came up on ebay and looked to be in excellent condition and were ridiculously cheap (about 15 quid each! + post) so they very quickly got bought. After inspecting them and disbelieving the condition I set to fitting them. Prior to this I ran the lathe with no chuck through all the gears as well as varying the motor speed on the VFD to get a a decent base line of the Before.

I then noticed that there was now a pretty significant vibration at about 950 RPM (in top gear with VFD turned down a bit and in second to top gear with VFD cranked up a bit – so was not motor speed or VFD frequency related.

I replaced the gears and .. Wow.. what a difference .. in the noise level and the rhythmic ‘notch’ feel that was coming from the really bad one gone (I tried to avoid that gear when using the lathe).

However.. the vibration was still exactly the same.

This week I decided to find the source of it and so far this is what I have tried / eliminated.

It is not VFD Frequency or motor speed dependant.

Tried running 1 belt – each belt in each pulley groove – no difference.

Checked spindle bearing adjustment (no lift other than flex, no end float and spun free enough etc) – good.

Checked Spindle nose runout – pretty good, couple of 10ths (excellent for an old student!). I did dress grind it a while ago but only to skim off the burr high spots, I didn’t actually grind the spindle so to speak, only touched it enough to make the slightest of sparks but remove the high spot damage caused but its caring and gentle previous owner..s.

There was nothing obvious I could find.

So off with the lid and measured where I could between the sliding gear and the gearbox output gear at the rear. I was getting almost a thou of swing on the gauge.

When I first got the lathe and refurbished it, knowing the type of bearings they use – i.e. ludicrously expensive and no need for Gamet Super Precision unobtainiums, I did mark the position of the bearing cones on the spindle with a sharpie and that’s where they went back on. Never been a problem.

Thinking the spindle must be bent (lord knows how you could bend a spindle like that and I haven’t crashed it.. yet)I removed the spindle, stripped it and spun it using it’s bearing surfaces in some blocks – The only thing I could measure with a tenths indicator were scratches in the surface. No run out anywhere, on any surface or the front bearing mating face.

What I did notice was that the alignment marks of the bearing cones were not in line with what appeared to be a reference mark on the spindle nose front bearing mounting face (a small CLC 46 in a circle with a small X next to it). No bother I thought, it runs true.

So I reassembled the front bearing on the spindle in the same orientation as it was and adjusted it for preload. This however wasn’t easy, it was either on or off, no gradual build up as expected (see further down..). Reinserted the bare spindle into the head stock with only the front bearing cap plate bolted up to hold the bearing correctly.

Put the indicator on it towards the rear – see pic below – .. 4 1/2 thou of throw! So I added the rear bearing and adjusted that up and the throw came back to the original just less than 1 thou.

The high point was right where the keyway is for the sliding gear –

So I removed the spindle, removed the bearing and re orientated the bearing cones in line with the mark mentioned above. Re inserted and measured – 2 1/2 thou.. better.. and the high point is still exactly the same place.

Over the next day or so I have removed and reassembled the thing many vmany times, each time altering the orientation of the bearing alignment marks (but keeping them aligned to each other), marking the spindle accordingly and noting the throw for each point. the best I could get it to is 1.8 thou which for the bearing type is crap really.

What I did notice was that when I first got the thing and referb’d it the nose bearing looked perfect. This time the outer cup was grey and there were visible rotational lines and I can just about feel some roughness with a little finger nail. There was also some very fine ware material on the magnet in the sump. I have now fitted 2 extra magnets in the bottom of the headstock and also fitted magnets at the bearing oil points so now the oil has to pass over a magnet prior to getting to the bearings..

With the bearing looking like it now does, (any other bearing would be binned) I now thought I’ve nothing to loose so.. and don’t shout at me, I know I know.. 🙂 I reassembled the bearings on the spindle but with the cones reversed, applied some extra preload, plenty of oil and spun the spindle with a drill for a good while. The hope was that the miss alignment of the ‘ridges’ might roll them off a bit and aid with setting preload.

Worked out really well, some unpleasantly dark oil coming from it and the result was a much shinier and smoother cup – Could not believe that worked for such a bodge!

Cleaned up, rebuilt and recheck – 1.5 thou. High spot still in the same place.

So.. that looks about as good as I can get. Put the lathe back to functional and test – preload on front bearing much more gradual.

Still vibration but now at 1150 rpm instead of 950.

I suppose I’ll just have to live with it. The down side is that now the spindle nose has well over a thou of runout!

Before I now regrind the spindle nose and bore taper (has the same runout now) and be done with it what am I missing as I am at the end for me really..

The High point is always the same rotational position on the spindle. The only thing that can not alter is the position of the outer bearing cup due to the locating pin in the headstock face so in my head that means that the cup has worn oddly – does that compute?

Sorry for the essay!!

Cheers

Taf