Reconditioning a Relmac lathe

[irving2008]

If you want to get it hot you will need to lag it :D

Silly question... Are you faffing about casting because you don't have another lathe to cut the new bushing on? Do you need help?

Robin,

Well I wouldnt call it 'faffing' as such... there is an element of learning a new skill here too :) but you are correct in that currently I dont have access to another (working) lathe.

Even if I did, I don't entirely see how to create a bronze bushing as the spindle is offset in the casting. It would need to be turned down from a solid 1.75" round and bored out to 1.25" approximately 3/16" offcentre. But then the castings would need to be line bored to ensure they are concentric and parallel with the bed as the advantage of a poured bearing is that it isn't dependent on the alignment of the external casting as long as the spindle is in line. Thats not true of the bronze bearing.

Irving...

[Robin Hewitt]

It would need to be turned down from a solid 1.75" round and bored out to 1.25" approximately 3/16" offcentre. But then the castings would need to be line bored to ensure they are concentric and parallel with the bed

It seems very odd that they would have bored the headstock off centre. If the tail stock does not offset, have you checked for signs of jiggery pokery? It may have a history which can be undone, making the replacement bush easier, but you'd probably want a shoulder to take the end thrust.

[irving2008]

Robin,

If you look at the picture below you can see how much of an offset it is... the bearing itself is quite thick and is side spilt. The main reason it is thicker on the split side is because the spindle is keyed to retain the bullgear and when you extract the spindle the key has to pass through the bearing. To give a strong positive drive they wanted to use a fairly substantial key and that meant they needed the bearing to be thick enough to allow a slot for the key to pass through (you can see the slot to the left in the pic, just above the split in the bearing, out of focus tho). But to have that thickness all round would have made it difficult to pinch down without straining the casting, so they offset it in the casting. Thats easy to do with a poured bearing and meant the casting didnt need to be too accurate. I suspect they had a jig that clamped to the bed with a long mandrel with a built-in key and this allowed them to pour both bearings at once and in alignment..

[Robin Hewitt]

If you want to get it hot you will need to lag it :D

Silly question... Are you faffing about casting because you don't have another lathe to cut the new bushing on? Do you need help?

[Robin Hewitt]

Why doesn't the metal around that offset bush look like cast iron?

Is this the back end or is there paint on the thrust bearing face?

Why is there paint inside the bearing?

[irving2008]

Why doesn't the metal around that offset bush look like cast iron?

Is this the back end or is there paint on the thrust bearing face?

Why is there paint inside the bearing?

I don't know, but it is definately cast iron. This is the thrust bearing face and as to the paint... an overzealous repaint job sometime in the past. Obviously the bearing was shot already and I reckon paint got in through the oiler hole and spread around... who knows maybe it made it run smoother for some unsuspecting buyer... I do know that the guy who owned it before me had it 6y and never used it and it was already painted when he got it - this isnt a standard colour BTW, as standard they were painted hi-gloss black like this one:



It going to be stripped right back and repainted black with gold or red detailing (just cos I fancy it)

[Robin Hewitt]

Is it possible this is a fixit bushing inside the original bushing? Suggest you remove the paint :D

[irving2008]

Is it possible this is a fixit bushing inside the original bushing? Suggest you remove the paint :D

Nope...here's a pic with the white metal bearing removed... and all nicely cleaned up. You can clearly see the anchor which stops the bearing turning in the casting.

[Robin Hewitt]

Okay :D If they never machined it it has to be cast :D

You can't preheat the cast iron mould because you don want to detemper the spindle. You can't upend it and pour from the front because you can't remove the spindle to clean it up afterwards.

Suggestions :D

Get a lot more heat in the metal before you pour, howsabout barbeque charcoal.

A deep well around the sprue so you can pour faster.

A damp leather pad on a wood backing that you can put over the well. This is an old goldsmith's trick, steam forces the metal in.

Consider cuttle fish bone to seal the mould and make the well. It is very easy to cut, lets the air out and doesn't burn.

[irving2008]

... You can't preheat the cast iron mould because you don want to detemper the spindle. You can't upend it and pour from the front because you can't remove the spindle to clean it up afterwards...

I wouldnt have thought it would have been hot enough to detemper the spindle... Its been suggested it needs to be at 150degC or so, the bearing metal melts at 380degC-ish, neither of which are hot enough to detemper a steel spindle, agreed?