Our magbeds are milled from .250" (6.4mm) 5083 cast aluminum alloy tool plate. Cast tool plate is very flat, and the casting process means fewer internal stresses which can cause warping. Once machining is complete, these beds are probed with a Renishaw Probe on a Haas VF5 mill, and we share this data with you in the form of a "birth certificate", see example below:
The top of the heated bed features a recessed thermistor channel and three attachment points to connect to the Z-yokes. The 9 high temperature neodymium magnets N48SH are recessed into custom milled pockets on the top side of the bed, to keep them as close to the flex sheet as possible.
In addition, they are attached to the bed with countersunk screws. These magnets are rated for temperatures up to 150C. They each have a pulling force of 11 lbs.
With the Buildtak Flex sheets, we see enough holding power laterally that we can actually slide a 300zl printer. Do note that flex sheets vary in magnetic strength, depending on the material and thickness used by the flex sheet manufacturer.
The magnets will likely cause interference with the bed probes. If using a BLTouch, please use the plastic tipped BLtouch bed leveling probe, and avoid probing directly over the magnets.