Mini-Lathe Repair

What a timely thread! Or the cause of my trouble... A harbinger, in fact (Dic. one who harbingers) My super mini lathe has just stripped the leadscrew gear. Removing it reveals a jammed leadscrew, so I slept on it. My task, should I accept it, is to take down the leadscrew and find out why. The carriage moves freely.

Oh dear Geoff, I hope this thread is not a harbinger of doom and start a spate of mini-lathe problems..

In your case the good thing is you should quickly find the problem, at least you dont have to strip down half the machine to find the problem...good luck with it.

Ron

Edited By Ron Laden on 08/12/2018 11:13:18

Geoff, with regard you jammed lead screw, I had similar on my SC3 and found it was the idler gear assembly. The idler gears are mounted on a steel bush running on a steel shaft Needless to say if not lubricated frequently the two seize. They are a bugger to get apart, but judicious use of a wedge(flat screwdriver) and hammer will separate. Then just try and remember to lubricate frequently. Of course alternatively could be just one of the bearing supports seized.

I have recently had a similar problem with the leadscrew gear on my Amadeal cj18 lathe. I have been thinking of fitting a motor to the right hand end of the leadscrew and was trying out options and forgot to tighten the retaining screw on the end. When I started the lathe the collar tightened up and seized the leadscrew. There was a bang and the plastic leadscrew gear shattered.

Luckily easily fixed. I ordered a replacement from Amadeal (£7 inc postage) and it came in the post the next day.

Update:

It's the LH leadscrew bearing. Seized. Can't imagine why, it's had the same lubrication as the other end bearing. Maybe it was tight to begin with. Anyway, now soaking in penetrating fluid (Brake fluid + 3-in-1)

A picture of the metal gear set I have ordered, not as coarse as the original metal gears there is 50% more teeth. So fingers crossed that these will run well and not be too noisy, they are said to be much quieter than the old set.

Edited By Ron Laden on 08/12/2018 20:25:28

Success! Bearing freed off with a combination of brute force and persuasion.

After soaking all night in penetrating fluid, (The shaft, not me!) it moved a little, but not much. I then warned up the bearing with a butane torch, and heard a 'crack' noise, which was promising. Then, resting the shaft vertically in the vice, loosely, not gripped, so the bearing rested on the jaws, I whanged it with a lump hammer, protecting the end... A sliver of light showed where the leadscrew was shouldered next to the bearing. Thus encouraged, I applied my 75 mm gear puller to it, and it began to move! Then I just wound it off. The bearing area was black, suggesting lack of lubrication, so I polished it, made sure the oil holes were clear and, feeling & seeing no obvious problems when held up tho the light, reassembled it in the reverse order, as the Haynes manuals say.

Now, before I was so rudely interrupted, what was I doing?

My mini lathe did not have ball oilers in the bearings for the Leadscrew! . So when making an outrigger bearing for a Handwheel for the Leadscrew, fitted a ball oiler to that and to the one at the Headstock end.

Copied what Neil did with the Apron on his mini lathe, to keep swarf out of the gears. Made a cover, and fitted a ball oiler to that as well. HJaving sealed the perspex cover to the Apron,the gears now run in an oil bath.

Howard

I greased my gears

My mini lathe leadscrew has a ball oiler at one end and a simply oil hole at the other.

The SC4 has so many ball oilers I worry about not getting them all... I'm not 100% convinced they are all needed although having them on things like the tailstock barrel does no harm and means I don't have to extend it to blob some oil on once a year

Neil

The metal gear set arrived, quite a difference when compared to the plastic set. I probably wont get them fitted until tomorrow, it will be interesting to see how they run and sound though I,m not looking for a silent machine.

I changed the plastic gears for metal ones earlier in the year

Aluminium?

I used T6 on my backgear and its been great

As previously mentioned though, the plastic ones are a good weak link in a disaster

You should look into that, or it will just happen again.

It could be that the leadscrew is out of alignment and thus being forced either upwards or downwards when the halfnuts are engaged and the carriage approaches the headstock end of the bed. This would put undue pressure on the bearing (and the carriage and bed ways, not to mention the poor old halfnuts.)

Observe the leadscrew when engaging the halfnuts with lathe switched off. See if you can see deflection as the nuts engage. There should be none.

When you put your leadscrew bearings back in place, if they have the usual bearing mounts that bolt to the bed casting, leave them finger tight, then put carriage at the left hand end of the bed, engage halfnuts and tighten up the leadscrew mount bolts at that end. Then repeat process at the tailstock end. This should set the leadscrew in line with the closed halfnuts along its full length.

I've come across one mini/micro lathe whose leadscrew was so out of line with the bed and the halfnuts' line of travel that the carriage was jamming halfway along the bed. In that case, the leadscrew mounts were cast integral with the bed so some eccentric bushes were made up to set the leadscrew to the correct position. Fiddly but worked.

Ady, in one way its good that the plastic gears act as a weak link but what is not so good is that the weak link is in the wrong place i.e. inside the head. Having to break the machine down to remove the head and then having to strip the head is far from ideal.

I am hoping that the steel gears will be a fit and forget and using Jasons idea of a poly belt adjusted with a little slack will offer a weak link but outside of the head. Yes I will need to make a new motor and layshaft pulley to suit the belt but hopefully that wont be a problem.

What I am annoyed about is me breaking the lathe in the first place, having stalled it (twice) whilst parting off, though I doubt I am the first and guess I wont be the last.

Ron

Edited By Ron Laden on 12/12/2018 14:27:59

The lathe is back together with the metal gears fitted, it is a bit more noisy compared to the plastic gears but not horrendous and I am happy with it. A couple of things I sorted with it stripped down, one of the three small wires to the DRO sensor had broken, must have happened when I removed it. Looking at the solder joints they were pretty feeble so I resoldered all three. The motor was around 10 degrees or more off the horizontal when mounted with the belt riding to one side of the motor pulley. On refitting I adjusted and aligned the motor with the belt running central on the pulleys.

Its good to have the low speed range back, the drive is still on the timing belt but I need the lathe running to make the two new pulleys for the poly belt.

Just noticed that I have gained a few revs, the DRO always read around 2505 in high range, it now reads 2610.

Must be the new gears I cant imagine its anything else.

That is at the top end. What about the minimum attainable? Low range might be different, too. Might be a case of roundabouts and swings!

Thank you, Hopper. The lathe is back together, but I have to replace the leadscrew gear (stripped teeth). I bought a set of metal gears, but the one needed was bored too small for my leadscrew, and the keyway is too small. I opened up the bore, and am filing out the keyway to suit the key. Otherwise, everything's fine.

I shall align the leadsrew as suggested, the bearings bolt on to the bed.

Geoff

Low range bottom end goes down to 11 rpm but I suspect around 30-35 rpm to pick up some torque.

You could always play about with your new pulley sizes to get a bit more mechanical advantage at lower speed, as you say with the motor running at snails pace you can easily stall the spindle.

A fair wallop of force would have been applied to the motor via the belt in the short time before the plastic teeth in the gearbox broke. The crash might well have pulled the motor sideways. The mounting bolts allow movement for alignment purposes and aren't designed to hold strong sideways forces. Likely a good thing: if the motor hadn't turned on the mounting you might have bent the axle or put a ding inside one or more bearings. Shock loads can do surprising damage.

A slight worry about fitting metal gears is answering the 'what breaks instead' question. 3D-CAD packages often have force modelling capability. Anyone know if it's possible to model the forces experienced by a crashing mini-lathe with one of them? Could it be done in Alibre Atom3d?

Dave

Edited By SillyOldDuffer on 13/12/2018 15:18:18