Granite Table tops

[iclark]

Granite won't rust, BUT it will crack and break.

many cast irons will also crack and break. some very easily.
be careful with your older hand planes.

[iclark]

Thanks guys, I get that. So what difference does it make what the material of the bed is? Other then hold all the parts together, it does nothing that wood make one material better then another, right?

some materials are better than others.

some features are more important than others depending on the user.

Mass: between the bed and the base supporting it, you want enough mass to stop the lathe from bouncing around the shop or falling over. more mass generally means less vibration when the object being turned is off-balance.

Stiffness: how much the bed flexes affects how much the tool rest bounces up and down as the materials being turned strikes the tool. it also affects how much the headstock and tailstock bend/sag under load. there have been several threads here discussing the sagging of the SS tailstock when a drill chuck is mounted to it and how to stop the sag.

Machinable: ideally, the bed ways are machined both flat and in a shape that the drive spindle and tail stock spindle alignment (common axis alignment) is maintained at all separations between them. this is similar to the issue of slop for miter slots, but even more exacting for thin spindles.

Energy absorbing: if the bed and base absorb the energy of the vibrations, then they are damped and the risk of resonance is greatly reduced. the trick is to absorb the energy without creating dislocations which eventually coalesce into cracks. there are S/N (strain versus number of cycles) curves that analyze this for materials.

Thermal expansion coefficient: at the least, the bed must not bend side-to-side as the temperature changes. it makes life easier for the designer and the maker of the lathe if the expansion is isotropic (the same in all directions). if the expansion is very different for the bed than for the rest of the lathe, then there is a risk of damaging something unless one loosens all the mounts after each use. not a user friendly feature.

Tough: lots of properties buried here, but basically the bed shouldn't crack or spall when you drop the chuck on it. this tends to favor materials with fine grain structure.

Easy to maintain: probably less important for a production line than elsewhere, but we are part of the elsewhere.

Friction: the "right" amount of friction. it should be easy to slide the parts to the desired position, but then nothing should move once it is locked in place. this is related to the self-adjusting router bit problem.

there are other issues, but I think this covers the biggies. there are some neat materials that would make wonderful lathe beds if we lived in a rich industrial nation. as it is, we get to read ads about the advantages of carbon in the cast iron providing lubrication and the advantage of granite to not rust.

FWIW,
Ivan

[a1gutterman]

Thanks Ivan! Nice detail in your explanation.

[Ed in Tampa]

Ivan
Nice! Now can you do a comparison between Cast Iron, Aluminium, Plate Steel and Granite in these areas so we have the facts to make an informed decision?

[iclark]

Ivan
Nice! Now can you do a comparison between Cast Iron, Aluminium, Plate Steel and Granite in these areas so we have the facts to make an informed decision?

Ed,

that's the sort of thing that I had to do in classes when I did my MS. so I'm a bit rusty on specifics (it's been a while - about 30 yrs). I've been wondering about this, though, so I will put it on my todo list. pardon me if it takes a week or 3.

the other material that has been intriguing me is silicon bronze. I loved it as a valve guide material and keep wondering why I haven't seen it as a lathe bed. I suspect cost is the issue for this material. cost to manufacture was one of the other items that I meant to put on my list.

Ivan