I keep shattering glass printbed surfaces on my MendelMax printer.
I suspect that it’s caused by the combination of using plain glass from old flatbed scanners, and that the glass is 1-inch bigger than the heater on all 4 sides.
I made this custom sized heater by taping down nichrome wire, to the back of a piece of aluminum.
I used a little spreadsheet to figure out the lengths and how many paths in parallel, and the end result came out to within about 0.5 Ohm of my calculation. Not perfect, but close enough that it’ll work fine.
One thing I did notice is that it heats much sooner on the side where the power is connected. I’ll probably wire it so the power connections are at opposite corners.
You can’t solder to nichrome, so I did my best to mechanically trap the nichrome under a little fold of the brass, time will tell if this is reliable.
Here are a few blurry phone-camera pictures.