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Material for pulley shaft, and bearings

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David Brown 920/11/2015 19:15:55
75 forum posts
4 photos

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

David Brown 920/11/2015 19:23:37
75 forum posts
4 photos

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

Chris Evans 620/11/2015 19:34:32
avatar
1960 forum posts

How much work has this contraption got to do ? Choice of materiel may vary if it has to do a lot of work.

KWIL20/11/2015 19:48:53
3426 forum posts
66 photos

"Table rotating on large castor wheels", what is to stop the table moving up and down on such wheels, leaving ripples in the mirror blank?

Frances IoM20/11/2015 20:17:54
1164 forum posts
28 photos
http://www.mirrorgrindingmachine.com describes a somewhat similar homebrew mechanism
David Clark 120/11/2015 20:35:45
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3357 forum posts
112 photos
10 articles

I would suggest silver steel for the spindle. I would not recommend a vee belt on a flat pulley. Probably two flat pulleys slightly bowed in the centre would be best at low speed.

duncan webster20/11/2015 21:10:19
3527 forum posts
63 photos

For a lightly loaded shaft like this, EN1A (now known as 230M07) would be more than adequate, and a lot easier to machine than silver steel. Model engineer suppliers usually stock EN3 (070M20), but they call it GCQ as they don't seem to understand steel specs. Might be a daft idea but can you cannibalise an old microwave oven, they used to have a slowly rotating turntable, modern ones don't, a least mine doesn't

Adrianh20/11/2015 21:45:32
11 forum posts

You can purchase very cheap pillow block bearings from Transmission Developments in Poole.

I have a selection of surplus gearboxes and motors if you contact me, as well as a lot of other engineering items and materials, face plates etc - scaling down my day job!.

If anybody has any requirement let me know

David Brown 920/11/2015 22:08:18
75 forum posts
4 photos

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

David Brown 920/11/2015 22:23:53
75 forum posts
4 photos

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

Adrianh20/11/2015 22:39:56
11 forum posts

19 mm bearings don,t really exist - I guess what you need is 3/4" - this is not a prefered size

If you want a turntable to run true and smooth you cannot use pillow block bearings as they have to much clearance and runout and are not suitable for much side loads.

You need an arrangement with angular contact or taper roller bearings similar to a vehicle wheel unit. To support a table perimeter use small proper ball races ( 2RS sealed type) directly in contact with the rim. You have a choice of very cheap/poor quality chinese bearings or quality bearings for not much more money.

PM me with your details if you want parts/help

KWIL20/11/2015 22:44:14
3426 forum posts
66 photos

Microwave turntables have "micro power" motors driving them and with load you will be placing on the drive, would not move at all.

John McNamara21/11/2015 00:35:58
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1314 forum posts
113 photos

Hi David

My understanding of the process is that the grinding disk is allowed to sit by its own weight on the glass being machined while it is given a lateral orbiting motion by an eccentric.

The turntable does not have to have high precision bearings, anyway the vertically mounted shaft loaded up with the weight of the turntable glass and grinding disk will tend to centre any end play in the pillow block bearing unit. It will only be a thou or two anyway.

Castors however are not made to any degree of precision, the table may wobble noticeably. It might be worth looking at child's scooter wheels (Or roller blade wheels which are smaller)

https://www.google.com.au/search?q=scooter+wheels&biw=987&bih=588&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiB6fChnqDJAhVDOKYKHUCGDK0Q_AUIBigB#tbm=isch&q=roller+blade+wheels

**LINK**

I have used these in the past for a project they are quite well made and have dual ball bearings available in various degrees of hardness from enthusiasts shops. The ones I purchased had very little run out you could check this in the shop. They are able to take the weight of a person so no problem with strength. They are mostly 8mm bore you would need to provide high tensile bolt and a mounting bracket.

It is desirable that the wheel has a line contact with the underside of the table, wide wheels will only cause drag for the same reason a car needs a differential. Wheels with a semicircular profile are common and provide almost a line contact. If you have access to a lathe you could make your own, however the bought ones may be less expensive, Industrial plastics and bearings have a cost.

Regards
John

Edited By John McNamara on 21/11/2015 00:37:19

Georgineer21/11/2015 00:55:43
524 forum posts
31 photos

If I can respectfully disagree with David Clark 1, Vee-flat is an acceptable and successful drive arrangement though it would take me a little while to find a printed reference.

My ML4 lathe has this arrangement with a 2" motor V-pulley driving an 8" flat countershaft pulley. It has worked without problems since my father set it up in the late 1960s.

There is a picture of it on the Practical Machinist forum. I don't know if it's permitted to post links here, but a search for the thread title 'Myford lathe' will bring up a thread started by Harrydiculous (wonderful name!) and there's a picture in that.

George

David Brown 921/11/2015 08:05:39
75 forum posts
4 photos

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

David

Ajohnw21/11/2015 11:29:02
3631 forum posts
160 photos

People don't generally run the table on castors. If you google mirrormatic mirror grinding and just telescope mirror grinding machine you will see photo's of various arrangements that are used. Personally I wouldn't pay for the mirrormatic design unless you want to actually make the same thing. You'll find designs on youtube too.

Most use pillow blocks. They are cheap on ebay in the UK. Some designs have used oil light bearings pressed into a hole. These are cheaper but if you have a shaft running in 2 of them it can be tricky to get them lined up. A lot easier using pillow blocks as 2 just need mounting on a flat surface. Pillow blocks will take some angular miss alignment but it's important the the final output shaft runs dead true as any rise and fall in the table the mirror is mounted on will be transferred to the surface of the mirror to some extent. There are 2 types of pillow block, The usual with mountings at right angles to the axis of the bearings and the other that can be fastened to a flat surface with the shaft passing through. The advantage both offer is that they can be moved around a bit to get the shaft they carry running true. That will also help get a V belt running on a flat pulley. You might be better off using a flat washing machine belt and another flat pulley with a rim on the smaller one that you could make yourself.

If you want a 22" table I would suggest a 35mm shaft for that. Precision ground mild steel or the stuff used in cnc machines would be fine for that. Cast iron pillow blocks, the first that came up on ebay were £7.69 each. You could probably use the pressed steel ones elsewhere but 19mm shafts is a bit if a problem. The few thou away from 3/4" ones shouldn't be be a problem in real terms.

You will need to make a flange to attach the table to that and another for the ply pulley. For the table I would suggest say 6in dia with the length of the hole for the shaft around twice the diameter of the shaft. The 4 bolts for fixing. A better idea might be to buy an aluminium disc off ebay and attach a smaller flange to that and the disc to the table. Less material to machine away and largish discs 5 or 6mm thick are sometimes cheap on ebay. A square would do as an alternative, say 250mm or more.

I've never made one but have been looking around at them for a long time as I am sorely tempted. Very high quality mirrors are rather expensive. Personally if I was making one I would add more shafts and arrange for more than one speed. That can be done by using say two 3 step pulleys reversed one on a shaft driven by a reduced drive from the motor that is easy to get with cheaper pulleys. At the motor you might get away with a 2" or 2 1/2" driving a 7" with a rather tight belt. It would be worth you watching the Gordon Waites video's on youtube. Search quartz mirror. He gives a lot of detail on that one. There is also one on retouching. I may not have spelled his name correctly.

John

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David Brown 921/11/2015 14:54:38
75 forum posts
4 photos

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

Ajohnw21/11/2015 15:41:52
3631 forum posts
160 photos

I've been gathering bits for some years to make one. The speed reduction is always a problem and big pulleys seem to be hard to find cheap in the UK so I recently added a motor with a gearbox on it to the collection. They do crop up pretty cheaply on ebay at times. In my case it ended up as being 3 phase 240/440 but I do have an inverter about that I could use left over from something else.

My reason for going that way is that I need it to be rather compact. I don't think much use can really be made of the inverter drive to change the speed but from use on a lathe I'm pretty confident it could be run from 40 to 60 hz without any problem and if the motor did get too hot and let any of it's smoke out it would be because it just wasn't big enough.

The catch with just using belts and pulleys is that as the speed goes down and the torque goes up the smaller pulleys need to get bigger and bigger so that the belt doesn't slip. Things are a bit suck it and see in that area including what power of motor to use. There will be information about on the web about pulley sizes and the torque they can provide without slipping. Not an area to worry about too much but does need some accounting for.

Worrying too much though is a good way of finishing up doing nothing.

John

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David Brown 921/11/2015 19:19:49
75 forum posts
4 photos

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

Georgineer22/11/2015 10:05:21
524 forum posts
31 photos

Following up my post about vee-flat drives, I have turned up some on-line design guidance from Carlisle Power Transmission Products. It isn't what I was thinking of, which is buried deep in my bookshelf, but it is at least up-to-date.

http://www.c-rproducts.com/downloads/pdfs/Industrial_vbelt_drives_design%20_guide.pdf

The vee-flat (or as they call it, v-flat) notes start on PDF page 277, which is printed page 276.

George

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