I have a 200x300mm PCB style heated bed which is supported at the four corners. What thickness of glass should I use for that size bed?

I'm currently using standard window pane glass from the h/w store and I'm seeing a bit of sag in the middle when printing things that are close to the 300mm length.

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WZ9V
I have a 200x300mm PCB style heated bed which is supported at the four corners. What thickness of glass should I use for that size bed?

I'm currently using standard window pane glass from the h/w store and I'm seeing a bit of sag in the middle when printing things that are close to the 300mm length.

If you intend to have an inductance probe the the glass needs to be less then 4mm.
or that is what understand from probe specs.

I'm using a standard microswitch based probe so that isn't a concern.

I have found that although 3mm glass is O.K. at 200mm by 200mm, above that things get much less good. and even 4mm is too thin at 300mm by 300mm and even using glass is marginal and aluminium becomes the choice. At your 200mm by 300mm I would prefer to use aluminium but would use 4mm glass as second best.

Mike

What thickness aluminum would be good?

I've seen people mention glass sagging in the middle so many times yet it's almost always (like 99%) the rods on your X axis that are sagging not the glass. The only reason we really use glass is because it's almost always very flat and level where are metals can bend.

My machine (Wilson II) uses 10mm rods that are 500mm long on the Y axis which is the 300mm one. I guess there could be some sagging there, not sure how I would check.

Edited 1 time(s). Last edit at 11/20/2016 09:37PM by WZ9V.

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WZ9V
What thickness aluminum would be good?

1/4" cast aluminum tooling plate supported on a 3 point leveling system eliminates the need for glass. It is flat, easily leveled, and thermally conductive so it heats up very evenly.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Going back to your original posting you say that your bed is supported at four corners.

Quote
WZ9V
I have a 200x300mm PCB style heated bed which is supported at the four corners. What thickness of glass should I use for that size bed?

I'm currently using standard window pane glass from the h/w store and I'm seeing a bit of sag in the middle when printing things that are close to the 300mm length.

If a flat plate has supports on all four corners then it is quite likely that opposite corners are causing the bed to adopt a saddle shape - sagging in the middle.

I would still recommend using a metal bed, but going to a three point mounting could also get the flatness to a livable level.

Mike

Yes a saddle shape is what I'm seeing along the 300mm Y axis. It's most noticeable on parts that use most of the 300mm distance. On shorter parts it is much less noticeable.

The project I'm working on (P-47 wing section) needs a flat base so that they can glue together without gaps. A saddle shape as described above would perfectly explain why I get gaps at the leading and trailing edges when joining two center section pieces together.