Trials and tribulations with a 7x12 lathe

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cfellows:
I, too, replaced the spindle gear with metal, but left the plastic gear on the countershaft. I installed tapered gibs on my carriage, but haven't done anything with the gibs on the cross slide. So far the factory gibs haven't given me any trouble.

Chuck

loply:
Sorry to go off on a tangent but this thread leads me to share an idea I nearly tried.

I'm worried that the increased torque of the new lathe will mean I will break the plastic gears again very shortly.

I was surprised how much work it was to replace those gears and really don't want to do it again!

I designed a new gearbox which used belt drives inside the head instead of plastic gears. Using this method I was able to design a system with 3 speeds instead of just two, using quite wide belts so that they won't ever snap, or at least, shouldn't snap before the belt that goes from the motor to the shaft.

I designed the gearbox similar to a motorbike gearbox - ie with three sets of always-connected belts between the two shafts. The pulleys on the lower shaft are on bearings and are free to rotate, unless a wheel with some studs on it is slid along the shaft whereupon the studs mate with matching holes on the pulley, thereby afixing the pulley to the shaft... If you see what I mean :coffee:

Because pulleys have a slightly lower 'profile' than gear teeth I was able to make the diameters slightly larger too.

My intention was to end up with 3 ratios - with respective maximum speeds of around 400rpm, 800rpm, 1600rpm.

Using plastic gears it would be senseless to lower the ratio to the point where you could use full motor power at 400rpm as you would almost certainly just break the gear again!

For the moment I haven't bothered but at some point I'd love to do this!

modeldozer:
loply,
I was thinking of doeing something similar with the jacshaft reducer and leaving the headgear locked in low. For the torque involved was thinking all metal gears in the head and a "fuse" pulley in the drive to the head, somewhere where it will be easy to replace.

Mannaged to get some of the other tasks done so had a few hours in the shed this afternoon, din not get a lot done so no pitures today. More time programed for tommorw.

Abraham.

loply:
Abraham,

Well just for your interest here is a video I made tonight whilst experimenting with my lathe. I have just finished the headstock upgrades and this is really the first turning I've done.

This video shows me removing 4mm from the diameter of a steel bar in one pass using hand feed, of note is that on the withdrawal of the tool there is NO secondary cutting what so ever, and what may sound like chatter is the tool chattering against the workpiece due to hand feed.

The end finish was pretty good too! Cut was made with a sharp carbide tool so it was a 'liney' finish but pretty damn smooth and extremely consistent. Motor running at full power which was 750rpm in low gear with the modified pulleys.

Also on that note, I have a set of cheap brazed carbide tools. I discovered tonight they are all crap. I reground them to new shapes and angles on my bench grinder and made a BIG difference. Prior to this there was no way I could remove 4mm at a time, in fact it was always hit and miss whether they would cut at all. I found most of them rubbed on the workpiece no matter what as they didn't have enough relief.

The video actually makes the finish look bad so here's a better pic.

The tit on the end of the part is 0.5mm diameter. At one point I turned a 0.5mm piece about 1cm long but then snapped it.

Prior to the upgrades (headstock bearings, motor speed reduced, modified gibs) the machine chattered like crazy and required several 'spring cuts' if you made a 1mm cutt, and it would stall occasionally and generally be dramatic. I usually stuck to removing 0.35-0.4mm from each side of a part max.

I'm totally confident that I could remove from the diameter 5mm or maybe even 6mm now...

Anyway, what I mean is that I would definitely upgrade the bearings and slow the motor speed down! You won't regret it :)

modeldozer:
Loply,

Many thanks for the info and encoragement.

Spent most of thr day working on the lathe (post to follow soon), but am having problems with the saddle, no matter how tight i adjust the gibs the saddle can be turned/pivoted left to right by 0.02mm (holdig it front and back and foring in opposite directions). Do not know if this is to much,. if so, it seems I might have to get it remachined or replaced. GRRRR :bang:

Abraham

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