I just double checked, and you’re right. Max focus height is .400" while using PG materials, but once a custom material thickness is entered (and I can use .433" there) then the focus height can also be bumped up to .433".
I just havent done much with thicker materials, and I remembered running into the .400" limit mostly because I was trying to widen the kerf for a project.
Seems to me without the tray in place we are doing something that is neither recommended or supported. It’s just possible. I wouldn’t expect much help from glowforge to enable it, considering the skittish nature of lawyers.
lawyers is a copout on this
if it was evil then it should be permanently attached and not documented anywhere as to having capability or capacity to do it
There really shouldn’t be any need for guessing at all here, the laser has to know where 0 is in order to focus up from that, in order to do that the trays must be all the same height, or within a tolerance the laser can adjust for. If every tray is different, every focus would be different and the streets would run red with the blood of 1000 sacrificed proof grade sheets.
So uhh, can we get the official tray height?
In the tutorial thread @henryhbk measured the height as 1.4".
Later in the same thread Dan commented
If that were the case the FNL would have insisted they add a hall effect sensor and not allow operation without the tray installed. The 
doesn’t work if the software team doesn’t know the zero-point distance. Rather than let people measure, and wind up with these threads, they could simply say the top of the tray to the floor of the unit is x.xx inches, the rest of the math(s) is up to you. Or if that simple, but very important, detail was overlooked, chiming in with a “yes, the height measured by mpipes matches our drawings.”
If you really like to live dangerously, you can put in taller things as long as you don’t engrave/cut more than about 3" from the “top” edge, so that the air assist vent never tries to run over the material. I do not recommend this myself.
See comment above yours and substitute henryhbk for mpipes.
Doesn’t it measure the height before it starts printing? I thought that’s what that scanning step was where it goes over and boops it with a red dot. I understand continuous autofocus isn’t ready yet, but single point focus is a thing, no?
So why can’t it just measure the height, crumb tray or not (as long as it’s within min/max range)? Is this something that’s deliberately disabled for non-Proofgrade materials?
Actually, that’s not true. They advertise the max material thickness as 2", or 1.5" or whatever. That assumes no tray in place. It’s advertised as a feature by the company, not a hack.
Unsure what it is doing internally when trying to measure material height on non Proofgrade. But either way the UI requires you to put in a material height for non-proofgrade before you can Print. It uses that number to correct the bed image. Without it, design placement is way off. Not sure why the scanning step doesn’t do this earlier but hey, I didn’t write the UI.
The material thickness we enter into the GFUI isnt for laser focus, it’s for the lid camera image.
I’m not sure what the laser focus is actually doing at this point because if you put in a scrap or material with parts cut out already, the laser dot doesnt even hit it.
For the GFUI to get the material thickness from the focus laser, I think it would require manual placement of the head by the user to ensure the focus laser actually hits a target instead of the crumb tray or floor of the unit.
The laser focus used to take a reading right before starting the cut in the center of the material, and we ran into some issues with cutouts in that area. Now it takes a shot near the actual cut location, over the material, right before starting. (Something we ran into with the PRUs.)
I’d gladly click a spot on the preview to accomplish this. Later, that same click could be used to bring up a head camera image of that area.
Guess I need to keep a camera handy then, because mine fires into the crumb tray quite often.
It can miss once in a while. Hasn’t affected the cuts that I’ve noticed since they changed it though. It used to crater the print. 
Yes, but I was talking about Focus Height, not Material Thickness. This thread suggests that you don’t need to set the Focus Height, but when I left it blank I got an error.
If you are using Proofgrade, the Focal Point is set automatically for you at the surface of the material. For an Unknown Material, when you enter the thickness for the lid camera to use during placement, that value is automatically populated into the Focal Point slot in the Manual settings.
But you can over-ride that value if you wish to focus the beam at some other point inside the material or over the surface (defocus) the beam.
You always have to set a Focal Point of some kind or have one populate into the field. It can’t be empty.
I think I entered the engrave settings first, with the field blank, then I discovered the material settings. Apparently doing it out of order doesn’t cause Focus Height to be populated.
If you get the error and want to close the pop-out you can just enter the thickness there as well, and it will give you a surface focal point.