Any suggestions on a good source for 8"x8" (200mmx200mm) borosilicate glass"
No need for borosilicate - just use 3mm thick window glass.
An interesting solution @Theo_Walker but I am looking for a rigid surface with no flex.
So far sources I have found are:
I’ve cracked countless standard glass panes while heating the bed. The mounting surface for them is not 100% flat so it adds extra stress that they can’t handle. That is why I switched to ceramic tile. Flatter than aluminum and takes the heat well, but not as flat as glass
I use 3mm mirrors from home depot and never had a problem. Is your printer in an enclosure? I’m guessing if it’s in open air you are more susceptible to cracks due to thermal delta between the top and bottom surface of the glass.
it is enclosed. It heats up in less than five minutes from 20c to 115c
I have a couple 8x8 mirrors if you want to try one Andy.
McMaster Carr is has 1/8" boro glass for $25
The cheapest price is usually $20
I may end up getting the McMaster ones. I get free shipping 
Try granite tile. Flexes very little and is much flatter than ceramic tile. About $6 for a 12" square at Home Depot.
Andy and I have played with both granite and marble. I had issues with both of them coming apart and leaving stone in my prints. Work amazing while they work, but then the heat breaks the crystal structure down. I think I have some pics some where.
@Andrew_Radovich_BadW if you’re cracking normal window-pane glass, it likely has little to do with how flat it is, and everything to do with not being heated up evenly. Only time I’ve shattered window pane glass is when I’ve adhered an ABS part too it too well and broke shards out of it while attempting to get the piece off.
I need a 400mm square piece. Any suggestions
@B_Gray , I’m guessing 400mm x 400mm ;)–I am looking for a similar-sized sheet, and it looks like Cincinnati Gasket Borosilicate is my best bet. I haven’t gotten a cheaper quote but is still really expensive ($100). Maybe we could buy at the same time and save?
Link:
I use $2 photo frame glass. My bed heats to 110 degrees in a partially enclosed chamber in less than 5 minutes. The only time one has broken was after I left it soaking for a few hours in water. Now I clean and dry them quickly and leave them in the airing cupboard over night before reuse.
ive got about 20 of the window pane glass pieces cut and on the shelf. I’m almost wondering if it is the way they cut them or something that is causing the cracking. essentially, unfinished edges allowing micro cracks to spread
@Ishaan_Gov I still have an issue of finding a big sheet of 1/4inch aluminum for cheap. I bought my taz plates which are over 300mm square for 35 bucks. Wish I knew their supplier
@B_Gray , Is the aluminum for your print bed, bed support or your heat spreader? Also from what I am hearing, you can probably get away with tempered glass rather than spending the extra money for borosilicate
The aluminum is for bed support. I was quoted 177 for a 48 inch by 24 inch 1/4inch 6061 aluminum
@B_Gray As a point of reference the commercial counter single sheet price (which is what anyone without a discount would pay) from my local metal distributor is about US$225 for a 8’ x 4’ sheet of 1/4" 6061T6. Most commercial yards have either minimum of a single sheet or a minimum dollar amount.
The small batch guys are going to be quite a bit more than that as you’ve found. You don’t need 6061 for that app. A piece of 3003 or 5052 will work just as well. Online Metal has them for about US$125 plus shipping.