Glass for the heated bed.  I'm building a Prusa i3 with an 8x8 inch

Glass for the heated bed.
I’m building a Prusa i3 with an 8x8 inch build area from a kit. The one thing that isn’t included is the glass for the heated bed. I went to Home Depot last night and they said they no longer cut glass for people because of safety concerns. (Ironic since they sell double sided axes, hatchets, and nail guns). He wasn’t sure, but he thought Lowes no longer cut glass to size either.

So today I stopped at a tiny glass shop in town about getting an 8x8 inch piece of glass and course the first thing he asked was well what kind of glass? He asked if any heat was involved and I said yes so he said then you need tempered glass, regular glass would crack with heat. He said the tempered glass was 1/4 inch thick. I asked about window glass. He said window glass is 1/16th inch thick and would crack under heat even easier.

So my question is what kind and thickness of glass to get? The other question is, do I get 200mm x 200mm in size or 8 x 8 inches in size? It looks like the print area is 200x200mm but it would be useful to use an 8x8 inch piece to have room to put the binder clips. On my heated bed it looks like an 8x8 inch piece might do a little more than touch the edge of the four corner screws. I can ask him to round off the corners. But if I didn’t do that would an 8x8 inch piece actually fit flat without riding up on the edges of the corner screws?

Go to an Ace Hardware and get some window glass. Ask them for the thickest they have. If it breaks, it’s cheap to replace (I’ve had the same piece for nearly a year). I put Kapton tape around the edges, rather than smooth them or have them smoothed.
I can’t help you with the dimensions for your printer, sorry.

I’ve used all sorts of glass. Thicker is better. You can sand the edges with sandpaper at a 45 degree angle to avoid splinters

As of two months ago, our Lowes would still cut glass. Regular window glass is fine, I’ve been using my 2.5mm window glass from Lowes for one year plus. I’ve had it as hot as 120C (not to print on, just to test heater performance). Many people have reported that tempered glass is not as flat as window or mirror glass. 2.5mm will flex, I think 4mm is relatively flex proof.

Lowes still cuts it here… just sayin

Home Depot 8x10 glass pane fits on the 8x8 pcb heater if that’s what your kit uses. They also have a glass cutter, it’s fairly straightforward to score the glass and snap it per those instructions yourself. I haven’t done it, I make do with the provided sizes such as 8x10 and 10x12, but it’s an option.

@raykholo ​ it’s easy to cut glass.
Use a straight support when make the cut. Only bush the cutter once, but firmly. Don’t overdo though, as you can break it. So always use glows, long pants and sturdy shoes. When cut made, put support under glass and make a short push and you should be standing with two glasses.
You might want to practice a couple of times.

@Anders_Jackson Sweet! They cover some of that, namely “One firm cut with constant pressure” on the back of the cutter packaging so what you’re saying rings a bell. Don’t forget eye protection! I trust glass less than when I’m cutting aluminum on my miter saw.

Another option I have used, go to a picture framing place. I used Hobby Lobby. They have some nice thick glass and will cut a piece down for you. I believe I had to buy the next size up pane, but they cut it down to the right size for me. 3 pieces was fairly cheap.

Also keep an eye out on the council cleanup weekends. I picked up a nice big sheet of 6mm glass that I keep carving chunks from. Due to the way I mount it, if I screw up and attach the mounts before I heat soak it properly the glass can crack. I cover it with a sheet of kapton tape so the mess is easier to clean up. Turns out it still works fine even when cracked - at least for a few jobs.

@Mark_Rehorst First I want to thank you for that suggestion. I like the idea of it. I noticed that Alcoa says at 1/4" to 5/8" the tolerance was .015". That is .31mm. Since that is more than the standard layer size, are you just hoping your piece isn’t close to the tolerance?

Not saying the glass I have used is any better. Just wondering.

I get glass from old scanners. I print pla and never need to heat it much usually around 40-45’ but on cold nights I’ll go to 50’ (it’s South Africa it gets about as cold as Florida does here) thicker glass is better but heavier depending on your printer design adding move weight to your bed can affect print quality. Anything over 3mm should be fine 10mm is straight but 20x20 is probably close to 1kg.

Yes, forgot. Harden glass cannot be cut. It will break in millions of peaces, all of it. Search youtub for instructions and examples.

My 1/8" sheet Doesn’t level very well anymore…

@Nathan_Walkner If you are getting quality borofloat from someone like Schott it will be much more dimensionally accurate than regular grade AL. A piece of MIC6 AL 12" sq is about $50 plus shipping from Online Metals. You can get good quality borofloat for about 1/3 the price of a MIC6 plate. MIC6 is expensive because it’s cast, then blanchard ground to ensure dimensional tolerance. That’s why it’s used in tooling applications.

For the OP, Lowes will cut glass, it’s a few bucks for the part. It’s not as thick or has the heat resistance that boro has but it’s cheap and easily available. For an MK2A style heated bed a square 200 mm x 214 mm fits well. It goes to the edge of the heat bed on two sides and leaves room for the screws and solder pads and LED on the other side.

Thank you everyone for the comments and suggestions.