Colchester Bantam Mk1 - Eccentric Spindle, damaged rear spindle bearing

Microns as in millionths of an inch? Or some other increment?
mark

Microns (µm) as millionths of a meter or approx 0.00004" - I might be mixing my units here though the divisons on my new Mitutoyo are 0.001mm, though the accuracy is quoted at +/- 0.003mm
 
I've done some further testing and certain that the motor cutout problem is not related to the bearing seizure - not enough and certainly no adverse heat build up in the bearings and I can still freely turn the spindle immediately after it cuts out. The motor cuts out after 3-5 mins running fairly consistently and then takes a few minutes to cool down before it will run again, so I'm guessing there's some thermal cutout on the Brook Crompton, a twitchy breaker on the rear cover or brake or possibly wiring. I guess it could also be a supply fault, but I run the lathe independently from a static 240V single to 415V 3phase converter and no fault indicated here.

SnakeyJ,

If the motor shutdown persists, buy/beg/borrow/rent a clamp-on ammeter (or adapter if you already have an AC ammeter) and measure the current in each of the three legs. If less than the posted breaker rating, replace the breaker.
 
SnakeyJ,

If the motor shutdown persists, buy/beg/borrow/rent a clamp-on ammeter (or adapter if you already have an AC ammeter) and measure the current in each of the three legs. If less than the posted breaker rating, replace the breaker.

Great thought thanks, I've got a clamp meter at work so will borrow it for the weekend.
 
You're welcome.
 
SnakeyJ,

If the motor shutdown persists, buy/beg/borrow/rent a clamp-on ammeter (or adapter if you already have an AC ammeter) and measure the current in each of the three legs. If less than the posted breaker rating, replace the breaker.

Embarrassed! - The motor cutout was caused by excessive boost setting on my power supply Transwave Static Converter (Transwave STC2HD - 3hp/2.2kW Single to 3-phase 240v to 415v). I had gradually increased the setting up to the highest to give sufficient boost for dual speeds, but ignored the bit about trimming this back down if the boost light stays on.

After suitable adjustment I was able to get the lathe to run for 15-20 mins without cutting out at full speed (1800RPM). I'm not getting a lot of heat from the bearings, but can feel some warmth through the casings and expect I have some room to tweak them further if needed.

The run out measured at the spindle nose is now about 0.006mm or a little under 3/10,000" - not too shabby for a 50 year old lathe and still within the shop permissible limits. I think I will pass on replacing the bearings for the immediate future.

** I found a very useful example of Colchester's test/acceptance certificate for the Mk1 Bantam, which nicely shows the testing method and their tolerance/limits posted here as a 1 page pdf.
 
Missed the edit timeout for this post....

At the end of an unsupported MT4 15" test bar the eccentricity/runout is amplified to about 0.03mm or a little under 2 thou - a massive improvement over where I started at the top of this thread ( <15 thou runout at 6" chuck mounted 3/4" test bar).

The headstock is aligned but only tightened down by two front bolts and rear offset adjuster (I forgot to tighten the rear bolts off before refitting the banjo). I'll tighten these up tonight and cut a new test bar - hopefully these will look a lot prettier than my last attempt and not show too much taper!

** I found a very useful example of Colchester's Standard Metric accuracy chart for the Mk1 Bantam (slightly newer 1976 example), which nicely shows the testing method and their tolerance/limits posted here as a pdf.
 

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Ive found that Bantams often have no visible movement on a tenths gauge.........I know mine which had new bearings fitted by the college has no measurable spindle runout............I spent a day aligning the head to cut exact over 8"...........then forgot and set the head to a taper to resize hundreds of Honda car pistons,which in typical Jap fashion used different cylinder bores in different markets to discourage aftermarket parts and grey imports. I was offered a taper attachment by a friend ,but couldnt be bothered fitting as the cross screw would need to be restored to length....The taper attachment actually came from my lathe,removed by a college teacher before sale......unfortunately he also cut off the cross screw.
 
More about the bearings for any future travellers..

Both are Gamet precision taper roller bearings with a flanged cup - manufactured by 600 Group, Colchester Tools. The bearings are produced in three grades with 1.5-0.5 micron run out.

Front spindle bearing is marked L2350 on cup and X L2350 on inside. Outer diameter is 90mm + 4.66mm for flange. Inner diameter is 50mm.

Rear spindle bearing is marked L7021 on cup and X L7021 on inside. Outer diameter is 85mm + 4.66mm for flange. Inner diameter is 45mm.

The part numbers are not current, but a quick comparison with their catalogue gives front spindle bearing is possibly part 85050 / 85090C and the rear spindle bearing is possibly part 119045 / 119085C (neither of these are definitive and anyone ordering should use the original Colchester part numbers (03-912 Front and 03-913 Rear). I've attached a copy of the current Gamet catalogue if anyone fancies some light bedtime reading!

I've stripped the headstock and double checked the dimensions and have identified the correct part numbers as:

Front: Gamet 111050/111090C
Rear: Gamet 112045/112085C

I managed to get a slight reduction on the price and get both for a shade under £ 600 delivered (might have to turn the heating off over Christmas) ;)
 
How is your Colchester lathe rebuild coming along in the, "Isle of White"?
Every thread I see that includes, Gamet Bearings, I pay attention.
Just curious, it appears you were going to replace the head stock Gamet bearings?
 
How is your Colchester lathe rebuild coming along in the, "Isle of White"?
Every thread I see that includes, Gamet Bearings, I pay attention.
Just curious, it appears you were going to replace the head stock Gamet bearings?

Hi Jeff, I did buy and install the new Gamet bearings (the installing being the least painful part). It's actually a very easy job on the Bantam lathes and plenty of decent advice on bearing preload tension on a number of forums. The lathe runs much smoother and more quietly afterwards - the original bearings having almost 50 years use on them.

I've done a couple of other builds since then, restoring an ill treated RF-45 Mill, a Naerok Drill Press and now an old English lathe (Portass Dreadnought) - but have a queue of jobs waiting for the Bantam once the workshop is cleared and tidied.
 
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