8 inch mirror being ground.

maybe a bank holiday then
Seriously though, if you could do it in a week or 2 or 3, then suddenly its looking very viable, now, if only I had the time

There is one point Dave Grennan has not mentioned Dave Lilis, that you should bear in mind. No matter what glass you chose to use, it all expands by some small percentage with changes in temperature. Idealy, if one was to consider grinding a mirror, and to make it more accurate, you should do it in a room that is able to have a more or less constant temprature. A cellar or basement with no heating for example, is an ideal place to do it at home. The cooler the environment the better for grinding.

Any amount of expansion between grinds and wave testing will change between each test, therefore you could end up with a rippled mirror too.

But, as Dave Grennan said, it is best to start with a smaller mirror with a longer focal length. Practice does, after all, make perfect! And the cheaper you start the better!

Seanie.

There is one point Dave Grennan has not mentioned Dave Lilis, that you should bear in mind. No matter what glass you chose to use, it all expands by some small percentage with changes in temperature. Idealy, if one was to consider grinding a mirror, and to make it more accurate, you should do it in a room that is able to have a more or less constant temprature. A cellar or basement with no heating for example, is an ideal place to do it at home. The cooler the environment the better for grinding.

Seanie, its not so much temperature thats important its stability of that temperature thats critical. Excluding serious swings in temperature normal room temperature is fine for grinding and for polishing to an extent. Having said that its virtually impossible to have stable air in a warm room, so yes, cooler is better. However overly cold is as bad as overly warm, as even your body heat will then send thermal air currents around the room. Its at the testing phase that air stability becomes critical. The recommended procedure is of course a cellar or basement but few enough of us have that. The next best thing is to leave your mirror in a west facing room overnight to allow it reach ambient and do the testing first thing in the morning before the sun gets up and starts warming the air and creating air currents.

Any amount of expansion between grinds and wave testing will change between each test, therefore you could end up with a rippled mirror too.

There are a couple of reasons for mirror ripple. This happens at the polishing and figuring stage and can be caused by either using too regular a polishing stroke. Think of how the windscreen wiper on your car can bounce across the windscreen if the windscreen is dry. Well its that on a smaller scale!!. This can also be caused by not pressing the lap correctly or the lap chaging shape because of thermal effects, even the heat from your hands can do this. Advanced mirror makers recommend making a plaster tool for this phase as far less prone to thermal problems. Backing a glass tool with plywood prevents a lot of the heat from your hands deforming the tool/lap.

It is very interesting stuff Dave, and I am glad to see you have really prepared for your little adventure! You'll hardly be at Connaught tomorrow to show some pictures or have a chat about the fine points of how to get a mirror grinding basement into a house, would you??



Seanie.

I would really like to hear some sort of talk on this, its one area I never got into, its great to see someone out there who knows about it
thanks for the info Dave G.

Though I should write an update on how this is going.

As I write this I'm about half way through the polishing phase. I havent really encountered too many serious issues. The biggest problem was making the pitch lap. Its a damn messy business and I had to re-do it three times. It was worth it because the next time I do it I'll know what not to do.

Rough grinding: This was too easy. Just a case of applying the elbow grease using a 'chordal stroke' until the mirror was the correct depth. I simply used the end of a vernier calipers and a steel ruler to measure the depth. 1/10 mm accuracy is fine here as you will refine the focal lenght beyond measurable accuracys at a later stage. Cleaning up every last grain og No80 Carborundum is critical. As should even one single grain contaminate at a later stage it could lead to disastrous scratches.

Smoothing: Some say that as few as two grades of smoothing grits will do. The kit I bought came with 5 so I used them all. The first thing I learned here is exactly when to stop with one grade and move on to the next. A little trick here is to spread a tiny amount of grit on the cleaned mirror. Backlight it with a diffuse led. I use my wifes book reading light. Use an inverted eyepiece of around 25mm as a magnifying glass and compare the grain size to the pits in the glass. If you cannot find pits bigger than the average grain size, then its time to move on.
Right throughout the smoothing process it is essential to make sure you have good contact between the mirrort and the tool. To test this the 'sharpie' test is excellent. Draw a freehand grid of squares around 1 inch in size across the mirror with a permanent marker (in the US a major brand is 'sharpie' hence the name). Grind a short wet or two and wash the mirror. If the mirror and tool are in good contact (ie the mirror is spherical) you will see the marker being bround off uniformly across the mirror. If you see uneven wear, this indicates a problem. Thankfully I did not have any problems at all on this front.

Polishing: As I mentioned making the polishing lap was a right nasty job. However having done it one I will know better next time. I bought high quality optical pitch from Galvoptics in the UK (If you need stuff like this order it early as they take weeks to deliver). I laid out strips of planed wood on a baseboard covered in greaseproof paper to make the mould. I melted the pitch and pured it into the mould. The first problem was the mould wasnt level and pitch flowed over the end and made a right mess. BTW you MUST do this outdoors, apart from the fumes, you WILL make an awful mess and you will not be popular with your family if you ruin the kitchen. Second mistake was I didnt cover the wooden strips with greaseproof. When I tried to remove the pitch strips from the mould they shattered. Ho-hum, I recovered all the bits of broken pitch, re-made the mould (covering the strips in greaseproof), levelled and repoured the mould. Worked out much better this time. I then used a hot knife to cut the pitch strips into one inch squares. Having attached the squares on a grid I drew on the tool I covered the tool in clingfilm and pressed the mirror against it using a 10lb counterweight. Thiis cuses the lap to deform to the shape of the mirror. After 5 hours pressing I removed the mirror and began to trim the edges of the squares which had protruded.

Trimming tips:
To trim use a razor sharp cold knife or wood chisel and a rubber mallet, a sharp bang of the mallet cuts the pitch cleanly.

Dont let the squares touch, if they do you WILL shatter the squares if you try and trim with a chisel. If this happens trim with a hot knife and re-press.

Cover your work area with newspaper, pitch gets everywhere and its a curse to clean.

Of course I made a complete dogs dinner of all of this and had to replace countless squares and re-press the lap a zillion times. Eventually I got there and have been polishing using a mormal stroke for a few hours now. Its nice to see the mirror turning from a frosted grey to a shiny mirror:)

Details of figuring and testing to come once I'm done.