Broken lathe

[Fergus OMore]

Deleted- Sorry

[John Rudd]

Norman,

The four gib screws are fine as is the gib strip....

My theory is that there may have been a slight misalignment with the bush and feedscrew nut...Although I did slacken the nut to allow the whole assembly to align itself.......

[andyf]

My theory is that there may have been a slight misalignment with the bush and feedscrew nut...Although I did slacken the nut to allow the whole assembly to align itself.......

John, I think that's a likely theory.

Looking at the Grizzly's online parts list for their G4000 (same lathe, I think), there's a spigot on top of the nut. On my (completely different) lathe, that spigot fits into a recess under the cross slide. If yours is the same, loosening the fixing bolt would allow the nut to swivel, but not to move from side to side as it would need to do in order to line itself up with the bushing in the feedscrew bracket if there was axial misalignment.

When you have made or acquired a new feedscrew, it might be an idea to make the bolt holes in the bracket a bit oversize, to give the bracket a bit of wriggle room.

Andy

[John Rudd]

When you have made or acquired a new feedscrew, it might be an idea to make the bolt holes in the bracket a bit oversize, to give the bracket a bit of wriggle room.

Andy

Andy, The holes I made are already over size....but obviously not big enough

[John Rudd]

Here's the offending leadscrew.......



Here's the new one........



and its new key, I milled a 3mm slot into the shaft with an endmill as I didnt have a Woodruff keyway cutter..needs must again...



finaly the new leadscrew bearing support........



I apologise for the apalling qaulity of the photos...Im useless when it comes to close up shots or its the camera..

[Stilldrillin]

John.
Yes, the old feedscrew's definitely broken!   

Not bad pics. But, definitely the camera's fault. 

Suggest you don't fit the screws, until the crosslide has been wound all the way out.
The holes may need elongating, rather than oversizing.......

David D

[Bluechip]

John.
Yes, the old feedscrew's definitely broken!   

Either that or it's spatially discontinuous ...

Can't really see from here ..

Check it with a meter. If it's more than 10Meg, it's open circuit .. for sure ... 


BC

[John Rudd]

Well after a good scrub and swarf removal session, I'm happy to say we're back together again...

A good dreasing and oiling of mating surfaces/faces and the machine is as good as new or even better...I do recall the crosslide action has always been stiff but its a lot better now, I can turn the handwheel with one hand
Just as an aside, the new lead screw I made from Silver Steel, boy it was hard to cut The new bearing support from a lump of steel I had, but I bushed it with some Phos Bronze reamed to take the lead screw. The shoulder on the lead screw just where is broke, I've added a PB thrust washer, instead of having steel on steel everywhere... Were my choices of materials good or bad?

Thanks to all who have contributed and a special mention to JS for his offer of making a new leadscrew...Shame I'd already ordered taps and dies before seeing his comments...Never mind, I have lh M8 taps and dies for the future should I need them..

[loply]

I've added a PB thrust washer, instead of having steel on steel everywhere...

Forgive my ignorance here, but when you say you added "a" thrust washer, you mean you added only one? And do you mean a proper ball bearing thrust bearing?

I made a similar set up myself for my Chinese mini lathe, but I firmly concluded that it was required to use two thrust bearings in order to absorb the thrust of going both inwards and outwards whilst not having a steel-on-steel interaction somewhere. Otherwise you're surely just clamping the screw between one thrust bearing and a piece of solid steel somewhere?

I've seen several people make similar things using only one bearing though, and wonder what I missed 

[Stilldrillin]

John.

I, for one, am very pleased you got the good result you were hoping for....... 

Well done ! 

David D

[lordedmond]

John

IMHO   that silver steel feed screw unless you have a very good surface finish will wear the nut quickly


Stuart

[John Rudd]

Stuart,

My selection  of Silver Steel was based on Steve Bedair's use of drill rod when he made his new lead screw for his 9*20..

Ok he also made a new nut from Brass.

I'm happy to run with the crappy alloy until needs must...