I was looking for a convex lens to replace (or augment) the flat one in my cheap bike lights. Of course, I can’t find one the right size, but glass ones are cheap as chips that are slightly larger.

What are my chances of being able to somehow grind or cut the edges off one? I saw a bloke using a diamond wheel to cut glass for stained glass windows, but I’m not sure how that would work with a half convex lens.. Anyone know about this kind of thing? Can I use a dremel and grinding wheel to grind glass? Will I die (of inhaled glass fragments)?

Glass on a bike light sounds like a recipe for disaster! Especially if there are any imperfections that would initiate a crack (there will be with a diy grinding!).

Cutting glass is hard. Cutting glass in a circle is really hard. Cutting it free hand even worse!

If your local opticians still makes its own glasses (may don’t) you could see if you could cross their palms with silver and what they might come up with. They might even have some plastic lenses?

*nods and looks thoughtful*

You can trepan it if you are determined –

You need:

A steel can that is the diameter you want to cut;
A spindle of some sort;
A supply of coarse silicon carbide powder;
Water;
Wax;
Patience.

Cut the end off the can to get a thin-walled cylinder. Mount said cylinder on the spindle so that it runs true.
Dip lens in wax to protect it and stick it in place, centrally under the spindle.
Press the open end of the can against the lens and rotate it while maintaining pressure (backwards and forwards, or round and round – it doesn’t matter).
Slowly drip a mixture of water and carborundum powder onto the point where the glass and the can meet.
Keep going.

You get slightly neater results if you cut half way through from each side (provided you can accurately match up the cuts).

[video]http://youtu.be/gJQBIQDvOjk[/video]