350 newt project

[Beulah]

Lots of people have successfully made their own mirrors, its not easy, but then nothing really worth doing is!

Very true, I have been told I could not possibly do certain things, and have done them.

All the hard work is definetely worth it.

The thrill of veiwing the heavens with your own optics is will have made all of the hard work making the mirror worth while.

John

I get the same feeling when play a guitar I have built. (literally from boards of wood resawn, sawn, bent and shaped with chisels, no pre-manufactured parts except the tuners)

Building something with your own hands is a special experience...it may or may not be cheaper than buying a manufactured item, but it is incredibly empowering when you have success in creating a functional item.

Well John, thank you very much for your enthusiasm and encouragement, it is really appreciated, and I am sure you'll get a few of us knocking on your cyber-door for help!!

What was your favourite scope you built yourself?

[Beulah]

Hi John my blanks were £66 here is a link to the site all the prices are there just click on dob mirrors telescope kits, build your own. newtonian , dobsonian

Paul, are you making a carbon fibre OTA again?

[paul11]

Paul, I see that you have ordered two 360mm blanks, are you thinking of using one as a tool? I mention this as it would be a lot quicker to grind the mirror face up with a smaller tool in order to put the curve in. Some folks use a barbell weight for this hogging process. For a 14 inch mirror a 5 or 6 inch tool would work well, use 80 grit abrasive or courser. The tool is worked centre thru centre inorder to put the curve in. When the curve is just over the required depth make a full sized cement tool by fixing a damn round the mirror blank about 1 inch high. Pour in a mixtute of pure cement and water and level off the top. When this has fully dried remove it from the blank and cover it's surface with cermamic tiles (about 1.5 to 2 inches square should be OK), use epoxy adhesive to stick them down. You now have a grinding tool. You could make two cement tools, the other to be used for the pitch lap, alternatively you could use a thick plywood base for the pitch lap. Just a few thoughts, hope they are of some help. Great to see people thinking of grinding and polishing their own optics.

John

Hi John, yes the second blank was going to be used for a tool but now you got me thinking. When I made my first mirror I used a tile tool and every time I got to the 600 grit scratches started to appear. I thought this was the tile tool but it turned out to be the bevel on the mirror, it wasn’t polished and splinters of glass were coming off and scratching the mirror. Do you find that tile tools ever scratch the glass?, if not I may try a tile tool again and save £66 on the other blank.

[paul11]

Paul, are you making a carbon fibre OTA again?

Yes i will use C/F again i am still working out how strong i need to make it. I am also trying to keep the telescope incl rings to 35lb.What Guitars do you make,i make Classical Guitars.

[Beulah]

Yes i will use C/F again i am still working out how strong i need to make it. I am also trying to keep the telescope incl rings to 35lb.What Guitars do you make,i make Classical Guitars.

Paul, that is awesome, there's not many of us around!!

So far I have built four acoustic guitars and one guitar/cittern. First couple of guitars were pretty rubbish and after a steep learning curve, my fourth belongs to a bloke in North America and my fifth is on loan.

Funnily enough I am trying my hand at a classical for the next build...I'm currently reading Roy Courtnall's book "Making Master Guitars" to see which builder I am inspired by. Romanillos' and Torres' styles really appeal to me.

Good luck with the OTA...35lb for a 14 incher is a challenge, I am sure you will do it.

[Glasspusher]

I use tile tools all the time and rarely get scratches, I used to until I started using a less fine abrasive. I generally finish grinding with a good quality white aluminum oxide abrasive grade 600 (I cannot remeber the exact grade in microns, I would need to check it). Did you notice the scratches during grinding or polishing? If pitch is too hard and/or very cold fine scratching can occur. Hope this helps. By the way I don't make guitars but a do play bass guitar, my pride and joy is a Musicman Sterling.

John

[weareborg]

Where do u get your abrasives from???? I was looking at GoGrit, but the shipping costs are a bit high

Andy

[FraserClarke]

Assuming you're in the UK, two options are;

Galvoptics (already mentioned); telescope grinding and polishing powders and grit

Vacuum Coatings; Telescope making

Beacon Hill Telescopes Beacon Hill Telescopes

I've bought abravises from both Beacon Hill (long time ago) and Vacuum Coatings before; perfectly good service from both.

There are a few others too I believe...

[Glasspusher]

One source of abrasives is from lapidary accessory suppliers, I have see silicon carbide in various grades on Ebay. Might be worth dropping a line to ensure the stuff is well packed and contamination free, explain that is needed for optical work. Remember you do not need the traditional sequence 0f 80,120,220,320, 400, 600...

I regulary use 120, 400 and 600 without any probelms. The 120 and 400 are SiC the 600 is white aluminium oxide. Needless to say that you will spent a lot longer with each grit and need larger quantities.

John

John

[paul11]

Ok guys we are off and running, I had 1 hour on this today, the middle is 1mm deep and I am about 50mm from the sides so I need to concentrate on the centre and get it down at least another 2mm before I let it spread out to the edge.

[weareborg]

Looking good there Paul. Ive been hoping that you was going to post some pics as you go through the grinding process. Ive been doing nothing but read read and more reading on the subject, even though Ive read the same books and webpages over and over again. lol. Can't wait to order my mirror so I can get started on this myself. In the meantime, Ive just realised that Ive lost all my files for my computer GOTO system somehow, so I also need to redo all them. AGAIN. oh the joy of computers. lol

Andy

[Glasspusher]

Looks like a nice start Paul. What size is that tool? Once you have got the curve hogged out you will be able to work all of the time with the mirror face up and that sub diameter tool off centre for general smoothing work. Don't forget when you get to this stage that the mirror will need to be face up on a FLAT surface to avoid astigmatism. The other thing is to regularly rotate the mirror as you are working. I am delighted to see some one making a mirror on SLG, hope others are inspired by your efforts.

John

[paul11]

Thanks guys, John the tool is full size. I am down 2.720mm now and i'm out of 60 grit so i will have to finish hogging out with 80 grit. Also I have decided to make this mirror F4.5 with a FL of approx 1600mm ( so I can get it in and out of the back door a little easier ) so my finish Sagitta needs to be approx 4.9 mm. Here’s 2 more photos you can see the curve on the mirror,,,Paul.

[weareborg]

Excellent work Paul. Only another 2.2mm to go then

How long did it take you to get to this stage???? I'm getting all excited now at the prospect of doing this myself.:glasses1:

Andy

[Glasspusher]

Sorry Paul, that looked like a smaller tool in the picture. It would have been quicker to use a smaller tool working with the mirror face up. I guess you are working with the mirror on top of the tool using a stroke that takes the centre of the mirror in a series of arcs round the tool's edge? 80 grit should be OK for roughing in the curve. If you keep the 'waste' grit you can wash out the residue and recycle what was not broken down. I you will find that a lot of the course abrasive ends up round the glass without actually having done any work! I am a little concerned that you have chosen a full sized glass tool, you will have to be carefull to avoid sticking with the finer abrasives. You seem to be making good progress, please keep us updated.

John

[weareborg]

So do you recommend a sub diameter tool for grinding and polishing then???? What size???

Andy

[Beulah]

Great thread. Keep up the great work, Paul!

Glasspusher - I wanted to ask you a question about the tile tool; I cannot reconcile in my mind the fact a grooved surface such as a tile tool can successfully grind glass - I imagine the tiles themselves would have to have a uniform surface across the tool.

My silly question - what sort of tile? White ceramic tiles you buy in your local DIY store? Do you use the glazed side to grind?

Once I have these questions cleared, I am going to make a start on the tool.

[Glasspusher]

I have attached a picture of a tile tool. A tile tool is made after the curve has been put in the glass, this ensures that the curve on the tool is the same as the curve on the mirror. A mirror blank can be bought with a pre-generated curve or flat, as is the case with Paul's. If flat you have to rough grind the curve in. This can be done by working the mirror on top of a piece of glass the same diameter as the mirror, as Paul is doing. This means buying an additional blank to act as the tool. An alternative is to use a sub diameter tool and work with the mirror face up. The grinding tool should be about 30% to 50% the size of the mirror blank. Once the curve is ground it is time to make a tile tool. For a 350mm mirror I would use a 300mm tool. Take a piece of chipboard, or similar, that is a little larger than the mirror blank and cut a circular hole 300mm in diameter. Place this ontop of the mirror blank and fix a cardboard damn in the aperture about 25mm in height. This forms a mould to make the tile tool base. Put a 300mm diameter circle of grease proof paper in the bottom of the mould to prevent the cement sticking to the glass. Make a thick mixture of cement and water and pour it into the mould. Leave to cure thoroughly. When this cement base is dry coat it's surface with epoxy, I use fibreglass resin, don't forget to add the hardener! Let this cure. Using the same resin stick standard ceramic tiles onto the tools surface. I use 5cm square tiles, they come on a web backing which needs to be removed. Allow to cure. You now have a tile tool with the same curve (approx.) as your mirror. Start grinding with 120 grit to get a close match between tool and mirror.

Tile tools work well as they allow the abrasive to flow between the gaps which makes for efficient grinding. Paul, this also reduces the chances of sticking. When using a glass tool of the same diameter as the mirror sticking can be a problem.

By the way, if you make two cement 'bases' one can be used as a base for the pitch lap. The fact that it has a curve on it is hepful when making the lap. I hope this helps explain some of the mysteries of tile tools!!!

John

[Glasspusher]

Forget to mention, the tiles are attached with the glazed surface uppermost!!

John

[FraserClarke]

You can grind from flat with a tile tool as well, there is no need to use a glass tool first to generate a curve.

Plaster of paris also works as a good base for the tile tool -- though you need to give it several coats of varnish to make sure it doesn't soak up any water

The tiles need to be the solid ceramic/porcelain type with the same consistency all the way through (don't know the formal name I'm afraid). Most floor tiles are like this. You don't want the type with a glaze on the top, and fluffy junk below (as most wall tiles are, for example) -- as soon as you're through the glaze, the tool will be gone... It's pretty obvious when you look at the tile -- you want something that isn't going to change consistency as you grind it down.

Bealuh -- after the first few sessions of rough grinding, the tiles will have a uniform surface across the tool. Any high points will be ground down first, and you'll end up with a nice smooth curve across the surface. The gaps the between the individual tiles have no effect (indeed as John says they make grinding quicker by letting the grit move around).

One thing to watch is that you don't make the tile pattern centred on the tool -- you want the 'middle' tile to be a bit off centre. (though I guess that 'rule' comes straight from pitch laps when polishing, so is it so true with grinding??)

Really must do some mirror making again, it is lots of fun

[Glasspusher]

Unlike cement plaster of paris is not naturally waterproof, I have made tools from it but have found cement to be better, as well as being cheaper and more readily available. American mirror makers often use dental plaster which performs very well, unfortunatley not that easy to get hold of.

John

[FraserClarke]

Thanks John. Not having to waterproof cement certainly is a big plus.

Do you ever have problems with the cement leaking out under the dam? I was paranoid about this when I made my plaster tools -- I think I had about 5 layers of oiled clingfilm on the mirror

[Glasspusher]

Thats another advantage of cement, you can mix it a little thick so that it does not leak under the dam.

John

[Beulah]

Ahhh, you guys are geniuses. Thanks TeaDwarf!

Thank you so much Glasspusher for the crystal clear explanation. Thanks to your input, I am going to get started - I actually have wall tiles and plaster of paris in my possession!

I think I'll leave the tiles to the next bathroom refurb and the plaster of paris for the next modelling project. I'm off to the DIY store for some cement and solid ceramic tiles.

Amazing this - I could never get this information anywhere else.

Thanks Paul for graciously allowing this discussion on your project thread, and apologies if we have intruded.. If I need to know anything else, I'll start a new thread.

[paul11]

Excellent work Paul. Only another 2.2mm to go then

How long did it take you to get to this stage???? I'm getting all excited now at the prospect of doing this myself.

Andy

Hi Andy, incl today i have had about 10 hours and i am down to 4.010mm,if you are going to take on a 14" you need to use 40 grit all the way.