First mirror project.

[JB80]

Hi all,

Well my 8" glass blank has finally arrived with a bunch of grinding/polishing powders as well. It's a Schott Borosilicate glass blank from Germany and the grinding material set includes silicon carbide grit #80, #180, #320, Aluminum oxyde with specific flat gritshape and tight size distribution Microgrit WCA 25my, 15my, 9my, 5 my, 3 my, polishing powder CER-SKX (a very fine and pure cerium oxide) and optical pitch of 28° hardness.
Which no doubt I'm going to have to container up and label correctly as soon as possible.

This is going to be my first attempt at making a mirror so hopefully I can use this thread as not only a log of what I have done but to seek advice when needed.
I have decided on making a plaster tool and already have the plaster but finding the tiles is proving harder than I would like/make it out to be but I come across some one way or another. I still have a couple of books I'd like to read too and have been recommended books by Howard or Texereau so I will read both. Online stellafane has lots of information too and of course the very knowledgable IIS members.
I don't want to rush things so will have plenty of time but I am keen to get underway, I can at least make the plaster disc for now but I still have plenty of other things to get in order, like as mentioned above the tiles and I need to pick some epoxy up as well.

Infact if I think about it there quite a few things....

Decide on the f ratio - rather important really and seeing as I was planning on using the time making a mirror to come up with a plan for making a dob this is the first major decision in that department too.
Create some space to do this in - We have a pretty good enclosed room that nobody goes into with a stable temperature which I can use but it's full of junk and would take a big cleaning effort so I need to get that in order. I can always do the rough grinding in the garage or somewhere else maybe?
Source a grinding table - I'm thinking a big 44-50 gallon drum that I can fill with water or even a wine barrell but it's likely I'll end up with something more mundane. Maybe a rotating bench.
Make the tool and find the last few things I need to make the tool.
Pick up a couple of minor things like metal rulers and the like.
Read up a bit more to give me a final confidence boost.
Not that much really, I have probably forgotten a few things but I'll be able to get some things happening soon.
Anyway thanks for reading and I hope to add more as I go along.

[omegacrux]

Good luck with it
Will be interesting .
I don't think I would attempt it !

David

[Merlin66]

Get some 25mm bathroom tiles (the ones mounted on a hessian backing, they come in 250mm sq sheets(?)) from Brico.
That's what I used..

[JB80]

David, Thanks. It will be a challenge I'm sure but I don't think it's a scary prospect as I originally thought it would be.
Still daunting.

Ken, I have had a look at our local Brico and couldn't find any. Maybe I was looking in the wrong tile department but I didn't see anything remotely resembling those.
Maybe it's worth checking a different location.

[Rod]

Good luck Jarrod

Grinding outside is ok for #80 grit. I would go inside after that.

You can always get larger tiles and cut them smaller. I like to use a dremel to bevel the sharp edges. You could always get a plate glass tool cut by a local glazier.

I would suggest f6 to f8 as being a good target. Think about how much room you have in a vehicle to transport it to dark sites and let that determine the tube length.

Look forward to hearing how you go.

Rod.

[JB80]

Thanks Rod.

I had read that for the larger grit I could do it outside, it might be a possibility. I'll need to see what sort of table set up I come up with first and how portable it is although I do like the idead of a rotating bench.

I do have a sharpening stone we don't use I can sacrifice and a couple of files for beveling edges, I might have to get the big tiles in the end but I'll have a bit more of a look for the ones on a sheet first.

F/6 and f/8 is where I'm at right now and I'm leaning towards F/6 as I think I want a portable break down type of dob, nothing so ultra portable that it will fit in your hand luggage but like you said something that can fit in the car and still leave room for a few other things, or I could just get a bigger car.

My progress might be slow at firs while I gather up the remaining materials I need but I think once I'm underway the thread will move along a bit quicker.

[DavidU]

This brings back memories ! Make sure everything is hosed off very thoroughly in between grit changes.

[JB80]

This is something that I worry about but so much so that I think I will be extremely thorough with my cleaning, and I'm considering doing the rough grind completely elsewhere from the fine grinding.

[JB80]

Well I made a plaster cast for the grinding tool last night, it's going to need some time to cure though and maybe a bit longer than I'd like given the wet and humid weather we have been having but it's a start.
It's a bit uneven on one side but I can sand that flat once it's cured.

[JB80]

Just a bit of an update, like I said things are moving slowly but I'm nearing the end of all the prep. With my Dad visiting from Oz I can't do a great deal but he has been useful too.
I have the tool all sealed with polyeurethane, simply because it was the easiest way with what I had at hand. It took quite a lot though before it decided to stop soaking into the tool and create an outer coating.

Also I managed to acquire far more easily and cheaply than I expected a nice oil barrel to use as a grinding stand.
Yesterday I made a rough ply table top for the drum that sloots in nicely which I'll give a rough sand and then seal it with the left over varnish.

I think all that's left is too cut and bevel the tiles and then attach to the tool and I need to bevel the edges of the blank before I can get underway.
Oh and I need to clear out the room I want to use, possibly the biggest task of them all.

[DaveGee]

Hi Jarrod,

A good trick to control grit contamination is to varnish/paint your stand before you start and if washing between grits won't clean it properly, just give it another coat of varnish/paint to seal the little buggers in.

Also, for hogging (getting rid of excess glass), I would recommend a steel tool of about 2/3 blank diameter. Below is a picture of one of my hogger tools. Nothing fancy, just some 3mm steel glued to some ply and cross hatched with an angle grinder.

Once you are at depth you then fix the tiles to the plaster using the hogged mirror blank as the former and away you go. Also, there is no need to chamfer the tiles, but make sure you chamfer both sides of the blank.

[JB80]

Hi Dave,

Thanks for the reply. I plan on varnishing the table top made of ply and it should be easy enough to remove and replace for cleaning. The re coating to seal anything in is a good idea too, you can never be too over cautious in cleaning the grit away so I may just do that when I'm under way.

I was curious about the hogging out, I had seen a number of tools ranging in size. I have a pretty heavy pestle from a mortar and pestle set and wondered if that would work. I can probably chase up some steel but don't have an angle grinder.

Most of the things I had read say to chamfer the tiles if you cut them, if I don't have to it'll save some work that's for sure.
I was thinking about doing the blanks edges this weekend if I get a chance.

[cometcatcher]

There's less hogging at higher focal ratios, but it's no big deal really. Long offset strokes hog pretty quick on their own.

[JB80]

OK, something I didn't want happening is I picked up a couple of small chips when chamfering the blank

Since I haven't done any other work I know I can just use the other side anyway but I was wondering if this would grind out if I chose to use that side?
I can't really see that there is any difference between both sides of the blank, no bubbles or deformities to speak of and it would be as much for aesthetic purposes than anything I'm just not sure how much the edges grind away.

Still, lesson learned. I guess I was a touch heavy handed. Slow and carful from now on.
Here is a pic of the chip, it looks bigger than it actually is. There are two more much smaller ones as well, the big one is about 1cm to 1.5cm at it's widest.

[Rod]

Hi Jarrod

Unless it is especially deep I would expect it to grind out OK. If it were mine that is what I would do. If you turn it over make sure you bevel the sharp edges, otherwise you will keep cutting your hands.

Good to see your progress.

Rod.