Material for pulley shaft, and bearings

[Ian S CParticipant @iansc]

There are 4 quite useful cast iron discs, one in each corner of a car, tucked in behind the wheels, they usually have 6 bolt holes, and one large hole in the centre.

Ian S C

[David Brown 9Participant @davidbrown9]

Unfortunately I don't have any old cars, or bits of cars lying around or I would be looking at using the bits (it's hardly surprising as I don't drive!). Even if I could buy a second hand stub axle I doubt I could get it home on public transport! It's a nice idea, I wish I had a big yard full of old machinery!

So far I have figured out I need 2 shafts, one 20mm diameter and one 35mm or maybe 40mm. Would EN1a mild steel be OK for both shafts? The 20mm shaft I will need to machine, the other one possibly just drill some holes in it.

Two pillow block bearings for the 20mm shaft. One pillow block bearing for the 35mm shaft and a 4 bolt flange bearing for where the shaft will go through the table.

Will the pillow block bearing hold the shafts vertically by themsleves, obviously the shafts will rotate with the bearings so do I need an attachment to stop them sliding out?

Also, I still don't know how to attach the shaft to the grinding table and wooden pulley. I appear to need flanges to do this, but should I try to machine these, or buy them?

I forgot the most important question, before I buy anything. Maybe this is obvious! I am assuming that a 20mm shaft will fit a pillow block bearing of 20mm internal diameter, is this correct? Or do people buy the shafts slightly smaller or the bearings slightly bigger, or machine the shafts to fit? It is probably a silly question!

David

Edited By David Brown 9 on 23/11/2015 19:56:53

[Mark CParticipant @markc]

You might try searching for "trailer hub" (internet and ebay). Might be just what you want ready made?

Mark

[MikeKParticipant @mikek40713]

Looking forward to seeing your progress on this, David.

[Howard LewisParticipant @howardlewis46836]

I would have thought that to grind/polish a mirror for a telescope, the bearings would need to be of much better quality than commercially available Pillow, or Plummer Blocks.

A cautionary tale

Many years ago, a supplier of shell bearings began supplying bearings which failed in minutes, wrecking the engine. The bearings showed striations, which by the time we had managed to find an engine that had not totally destroyed the evidence, were providing fatally deep (0.00001&quot grooves for oil drainage.

A visit to the supplier showed that the broaches had been ground on a surface grinder "converted" using commercial plummer blocks. The vibration from these was being ground onto the broaches, and so mirrored in the bearing alloy in the shell bearing.

Try commercial pillow or plummer blocks, but be wary of what might result!

Howard.

[David Brown 9Participant @davidbrown9]

Actually, my impressions from reading on line are that the requirements are not so precise. Grinding a mirror is rather different to producing accurate metal components or tools.

Basically, a grinding disc of 75% of the mirror diameter when placed in the correct postion above the mirror blank which is rotating (the disc also rotates) will tend to grind the mirror spherical (concave) meaning that the curve could eventually be imagined as part of a huge sphere. The disc has either tiles (mosaic) stuck on it for the grinding or I have successfully used two pennies! The mirror has first been 'hogged out' to nearly the correct depth. The grinding disc is made of cement or plaster formed to the shape of the mirror.

I do not have the scientific or mathematical background to explain why this is so. However, I have successfully ground a 14 inch mirror by hand and am now using it to look at the stars. The techniques for machine grinding are somewhat different, but I have read up on these on the internet from experienced mirror makers. Finer and finer grinding grits are used.

I intend to use the machine for rough and fine grinding. What is called 'hogging out' which is getting the mirror down to the correct approximate depth I will do by hand, possibly with a rotating turntable if I have got the machine working by then. This is because the stroke is straight across the centre of the mirror for this and I do not know how to make a machine to do this. There are various videos where people use angle grinders but this is far too risky for me!

What is called 'figuring' which is the final stage of the process when the sphere is turned in to a parabola I will also do by hand, as this needs a more complex machine.

David

[David Brown 9Participant @davidbrown9]

As this thread has got rather long I think I will make individual new posts about how to fit the bits of the machine together. I have almost finished asking questions for a while (until I get some more components)!

David

[David Brown 9Participant @davidbrown9]

Actually, one more question! The 35mm shaft to which the turntable will be attached. Will EN1a mild steel be good enough for this? I was recommended to get precision ground mild steel for this.

I am planning to get the metal from Metal Supermarket in Acton, London. They have 'bright steel' round bar EN1A, EN3B, EN1A (leaded), EN8, EN16, EN24, EN19

But they do not appear to have precision ground. Would EN1A be fine or would one of the others above be better?

This shaft I may only need to drill some holes in (to attach it to a flange etc.) However, if it does not fit the bearings exactly I may of course need to machine it. Should a 35mm shaft fit a 35mm bearing-or is it not that simple?

David

[David Brown 9Participant @davidbrown9]

Sorry, to make it more complicated I have now noticed that Metal Supermarket have 'engineering steel' precision ground bar EN8, EN16 and EN24 would one of these be better for the 35mm shaft?

David

[Ian S CParticipant @iansc]

I have used quite a bit of 35 mm steel in 35 mm bearings, can't quite remember the steel spec for the mild steel(something like 1020) the other steel was 4140 which is a heat treatable grade, and we welded it (stick) with out further heat treatment with out too much trouble.

Don't they have car wrecking yards in your part of the world, we even have one in our town (pop about 2000).

Ian S C

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