@stellator Your first illustration is a Kelvin type kinematic mount. Both Kelvin and Maxwell mounts accomplish the task of allowing the bed plate to expand laterally without bending any mounting screws or causing the plate to flex or lift. The main differences between them are that the Maxwell mount reference point is at the intersection of the three grooves while the Kelvin mount reference point is at the hole or cone in which a ball sits. The other difference is that the Kelvin mount axes can be aligned with the X and Y axes in a machine making it especially easy to tram the bed because adjustment to the roll doesn't affect the pitch of the plate. With a Maxwell mount the axes are usually aligned 120 degrees apart so adjustments at any one point affect both the pitch and roll of the bed, making tramming a little trickier.
This is the bed support in my printer, designed as a Kelvin kinematic mount. The axis with the two spherical screws is aligned parallel to the printer's X axis. One of those balls (the reference) sits in a hole in the bed plate, the other (the pitch adjuster) in a groove that runs parallel to the X axis. The third point (roll adjust) is just a screw that touches the smooth bottom surface of the bed plate. The bed plate has ears for the three adjusters/support screws and is held down on the screws by springs. The reference screw doesn't normally get adjusted when tramming the plate. Tramming is done with a piece of paper between the bed plate and extruder nozzle. Simply adjust the Z position of the nozzle near the reference screw until it just catches the paper, then move the nozzle along the X axis until it is near the pitch adjuster and adjust until it just grabs the paper. Finally, move the nozzle near the roll adjuster and adjust until the nozzle just catches the paper and it's done. The last time I trammed my printer was almost 2 years ago.
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The reference screw sitting in its hole in the bed plate:
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Here's the whole thing:
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All three adjusters are mounted in PTFE blocks that grip the screws firmly but still allow them to be turned for tramming. They also don't mind the heat from the bed plate. If you make anything like this, don't use nylon instead of PTFE. you won't be able to turn the screws (I know, I tried). There's a reason they use nylon in nylock nuts! You don't need to tap the PTFE blocks. Just make slightly undersized pilot holes and thread in the screws. They will roll their own threads into the PTFE.