Member postings for David Brown 9

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

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

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

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

I am looking to buy some pillow block bearings. There are a number of suppliers. What I am finding strange is that there is a wide difference in price for what appears to be the same product.

For instance Bearingboys sell an NP40 40mm pillow block bearing for £38.81.

Bearing Shop UK sell an NP 40 40mm pillow block bearing which appears to be identical for £12.60! Am I missing something here, as I have very little knowledge of bearings?

David

I have some 83mm diameter aluminium round bar, how would I go about making a flange from this on my mini-mill?

Also, how do I attach the shaft to the flange?

David

Edited By David Brown 9 on 22/11/2015 13:54:30

I didn't really understand the flange for the table and wooden pulley. I was planning to drill a large hole in the table and fit some kind of bearing in it? How would the flange attache to the wooden pulley and the shaft?

David

Thanks for all the information. Gordon Waite has made a number of grinding tables, one was supported on skate board wheels, the others seem to be on bearings, I don't know where I got the idea of castors from, I just happened to have some!

I will post again when I have got all the parts and start putting them together.

David

The castors are industrial type castors from screwfix, 50mm in diameter, the wheels are rubber, so maybe they would be OK?

David

I missed the post about pillow block bearings. Transmission Developments seem to have no prices on their website. I guess you have to complete an on line enquiry form?

They have pillow blocks with a 17mm bore and 20mm bore, but no 19mm bore, is there any way round this?

David

Assuming the turntable is flat and the table underneath the castor wheels is flat I don't see why the turntable should go up and down.

Grodon Waite in America has a number of youtube videos making mirrors with such a turntable and has done it successfully. I read that 3 castors should be used becuase this guarantees that the tuntable will rest on all 3 castors?

I am planning to run the machine for about 4 hours at a time. Eventually it will have a grinding disc made of concrete or plaster attached to a wooden disc with a lot 2 pennies glued to it freely rotating on a spindle above it,, resting on the mirror, the disc will be 15 inches in diameter. Grinding grit and water (water in small quantities) will be put on the mirror periodically by hand. A large plastic container will be used to stop the water escaping, I think someone used a very large plastic flower pot!

Any suggestions for bearings for the shafts? Also where to buy the shafts and bearings?

Unfortunately I don;t have an old mircowave oven, maybe I can find one that has been thrown out!

I would like to use the 10 inch flat pulley if possible, 10 inch v belt pulleys arn't that cheap. Is there any belt that can be used on a v belt pulley and flat pulley?

I would guess the weight of the mirror at around 10-15 kg.

To begin with I will grind by hand using the grinding disc on the rotating turntable. I will then make the spindle and frame to hold the grinding disc.

David

I should have said that the turntable with the mirror on will be supported on a table underneath it on 3 large castors spaced equally around the edge of turntable and will rotate on the castor wheels.

David

I am in the process of making a machine for grinding telescope mirrors.

I have a 1/2 hp single phase motor the shaft of which turns at 1400 rpm. I need to reduce this to 40 rpm. I have attached a 1.5 inch 38mm v belt pulley to the motor shaft. I will run a belt from this to a 10 inch pulley from an old washing machine. Will running a belt from a v belt pulley to a flat pulley cause any problems?

This should give me a reduction to 254 rpm according to an on line pulley calculator. On the end of the shaft holding the 10 inch pulley I will put a 75mm, 2.95 inch v belt pulley (aluminium).

My next question is what material should I use for the shaft, it needs to be 19mm in diameter to fit the v belt pulley? Steel? If so which steel? I need to be able to cut a keyway in the shaft with my mini mill.

The third question is which bearings should I use? Pillow block bearings seem the easiest, but they are rather expensive. The bearings are going to be attached to a wood frame, could I just drill holes in the wood and push the bearings in to it, making sure they fit tightly?

From the 75mm pulley a belt will got to a 15 inch diameter pulley. This will be made out of two discs of 18mm thick birch plywood, glued together, both chamfered at 45 degrees to form a v. What is the best way of attaching a shaft to this wooden pulley?

At the other end of the shaft will be the 22 inch diameter turntable for the mirror. This i have already made (I have already made a very heavy duty table as well). The turntable is made from 2 discs of 18mm plywood glued together and varnished. What is the best way to attach the shaft to the turntable and how thick should the shaft be and of which material?

I should end up with the turntable rotating at 40.1 rpm

I know I am asking a lot of questions! This is actually a very simple machine, but I do not have an engineering background and this is the most complicated mechanical thing I have tried to make!

David

I will use a hacksaw, it didn't occur to me that 16mm isn't actually that thick!

David

Hi, is it possible to cut through 16mm diameter round bar on a Sieg SX2 mini mill? I am not sure if it is iron or steel (I found it on the street a long time ago and thought it might come in handy!). How could I do this? It is not stainless steel as it is a bit rusty.

I realise I need a metal cutting band saw but I don't have room, finding space for the mini mill was a struggle.

I have an Evolution/Rage circular mitre saw but no way am I going to try to cut throught that thickness of metal with this, the sparks when cutting through much thinner hard metal are scary enough!

David

Thanks for all the advice. Amadeal in London do a 2 inch boring head set, 9pc 1/2 inch boring bars, for a R8 holder, would these be the right size?

I realise buying the pulley would be cheaper, but I can use the boring head set again and again in the future! I am making a mirror grinding machine and will need more pulleys etc so the boring head set is likely to get used again.

David

OK, I guess it's obvious how to sleeve the pulley, I use a bush as Neil Wyatt advised. Bore out the pulley to 19mm and fit a bush, cut a 5mm keyway in the bush. I presume I can do this using 2 different sized boring bars in the mini-mill?

Would I need boring bars exactly 19mm and 5mm? Obviously I don't know exactly how they work!

David

Edited By David Brown 9 on 15/11/2015 20:04:11

I don't have a lathe.

How do I sleeve the pulley to match the motor shaft?

I would need to buy a boring bar, but I am sure it would come in useful. Should I use the R8 taper on the mini-mill? I have added some pictures-they are rather big!

David

I have a step pulley from my drill press, which I don't use much since I got my mini-mill. I want to fit it to a motor I have.

The motor has a 14mm shaft and 5mm keyway. The pulley is slightly too big for the shaft-around 15mm inside diameter. It does not have a keyway.

If I want to cut a 5mm keyway in the pulley I assume I need a 5mm broach. Can I do this in the milling machine (Sieg SX2) by hand, by putting the broach in the chuck? What size keyway bushing do I need?

How exactly do I do it? Also, will it work given that the internal pulley diameter is slightly bigger than the shaft on the motor? The pulley is just the size I want, so it would be a pity not to use it.

Also, how do I add pictures to my post?

David

I have nearly finished a 14 inch mirror (it just needs aluminising).

Next is a 20 inch mirror (I already have the glass blank). I have already seen the thread on Stargazer's Lounge. I think it likely that making the 20 inch will turn out to be a saga for me as well! One thing making mirrors is teaching me is patience!

The 14 inch mirror took around a year and a half, and the mount still isn't finished to my satisfaction-it works-but not very well-another of my projects that needs improving!

So who knows how long the 20 inch will take!

David