Finally caught up on unread threads and still didn't see this answered for you. Most of this info is scattered in various threads, so I'll post what I know to directly answer your questions.
- From your experience, is the glass warped/being warped by the heat?
Glass can technically have some surface out of flatness depending how it was manufactured, but I have never seen any warped enough to affect a print. It shouldn't warp when heated, no at the low temps it is heated to on a printer. What you are probably seeing is the lack of uniform heating across the plate. Someone posted a FLIR comparison
of glass and aluminum on the heated bed to show this issue.
- What are the pros and cons of MIC6 vs glass?
MIC6 is a cast, precision milled/ground sheet of aluminum. The casting helps prevent stresses in the surface so it remains stable when machined, thus allowing a high precision (i.e. flat) sheet to be made easily. Aluminum has a much greater thermal diffusivity than glass, so it helps disperse the heat across the entire plate faster and more efficiently. Again, look at the FLIR images linked above. Essentially, MIC6 provides a better thermal, flat sheet than glass, but it is heavier and it is more expensive.
- Can you order it from a local aluminum supplier cut to size? Is it milled to size?
There are many suppliers for MIC6 and I'm sure some can cut it to size. Most often they saw the sheets, but you can mill it if you can anchor the sheet. Might be best to have a supplier cut one slightly oversize and you mill that to fit the bed. McMaster sells smaller plates than a full sheet.
- Without PEI, does MIC6 require bead blasting?
I'm not an expert on printing on bare MIC6, but I do not believe it is required. MIC6 does not come as a polished surface. I'm sure the bead blasting helps in part removal and gives parts a nice bottom finish. I think I read someone simply sanded his with 1000+ grit sandpaper.
- What is the function of PEI?
- Why would you need/want PEI?
- Is it just a more resilient Kapton equivalent?
PEI is a good neutral plastic that adheres to almost everything you can print with. You can put PEI on your glass bed and never use hairspray, glue, ABS slurry, etc. again. The key with PEI is keeping it clean as even finger print oils can prevent adhesion. A quick wipe with alcohol prior to printing is all you need. If you lose adhesion after many prints, you can take some fine sandpaper to it and get a great surface again. PEI is also rather resilient to scratches and such when removing prints. I wanted PEI because it is cleaner and easier than most other solutions and I am happy with it.
M114 - Get Current Position
Also, why do I get no response from my printer when I do a G90 G1 X 15. ? (the only command I've tried) and how do i read the current absolute position relative to the z height offset? I know how to read the z offset already. Do you guys use the makergear quick start for z offsetting?
should work. I see no reason the printer would provide a response to a G1 code. I use the Quick Start app for initial setting and leveling but I'll use Octoprint to make any fine adjustments (there is an EEPROM editor plugin that makes it way too easy).
Hope that helps! Welcome to the community!