Kinematic Mount for 3D Printer Bed

Rock solid mount that allows the bed to expand when heated.

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My latest 3D printer, a coreXY machine, uses a kinematic mount for the bed. It allows the MIC6 bed to be leveled and to expand when heated, without creating any lateral forces that cause things to bend.

There are 3 leveling screws. The reference screw sits in a chamfered hole that allows the bed to tilt and rotate, but prevents lateral motion. The pitch adjuster sits in a chamfered slot that allows the bed to tilt and expand, but prevents rotation around the reference adjuster. The roll adjuster is just a screw that contacts the underside of the bed plate. The bed is free to expand, but can't roll unless you adjust the screw. The plate is held down onto the screws by springs.

It has proven to be very stable, and I can drive the printer around laying on its back in my car, then take it out, stand it up, and start printing without any adjustments. No autoleveling is needed.

When it comes to 3D printers, I'm sort of old-school.  I believe the machine should be made solid, and surfaces that are supposed to be flat should be flat- bed surfaces doubly so.  A lot of printers use leveling screws at the 4 corners of the bed even though a bed is supposed to be a plane, and it only takes 3 points to define a plane.  4 screws will cause either or both the bed plate and the carriage plate to bend, so I use 3 screws.  

I've been experimenting with and refining my 3 point bed support for a few years and believe the bed I used in my latest printer, Ultra MegaMax Dominator (UMMD) is about as good as I'll ever be able to make it.

Kinematic mounts are commonly used to mount lenses and beam splitters on optical tables. They constrain six degrees of freedom while allowing very stable positional adjustments.  When applied to a 3D printer bed, they also allow the bed to expand when heated without causing any forces that might flex either the bed or the support structure.

UMMD's bed uses such a kinematic mount that I detailed on a blog page here.

Kinematic Bed Mount
A look at the underside of the bed plate.  The screw on the right is the reference, left is the pitch, and back is the roll adjuster.  Forgive me... I'm forcing myself to learn Fusion360 and I couldn't resist rendering it.

The three leveling screws mount in heat tolerant Teflon or Torlon blocks (yellow, below) screwed to the bed support structure.  The leveling screws roll their own threads into the plastic so they are gripped tightly and don't wobble.  The reference and pitch adjuster screws are turned from the top side of the bed.  The reference screw sits in a chamfered hole that allows rotation and tilt for leveling adjustments.  The pitch adjuster, sitting in a chamfered slot on the opposite side of the bed, stops rotation at the reference screw but allows tilt and thermal expansion.  The roll screw adjusts from below and and just contacts the flat surface of the bed.  That allows the bed to tilt when leveling and for thermal expansion.  The bed plate is held down onto the screws by springs adjacent to each.  The bed is topped off with a 30 mil (0.7 mm) layer of PEI (green, below).  Nothing stands up above the bed surface, so there's no chance of crashing the extruder nozzle into anything.

In this type of arrangement you adjust the pitch first, then the roll, a process that takes all of 1 minute if you're extra careful.  The structure is so stable I haven't had to adjust it since the last time I took the bed apart about 4 months ago.

The bed is heated with a 750W, line powered heater (purple, above) that gets it up to ABS print temperature of 105C in about 4.5 minutes.  An SSR switches power under PID control from the SmoothieBoard controller.  I added a thermal cutoff to protect against failure of the SSR (they fail shorted), and an electrical fuse to protect against wiring failures.  The printer's frame is grounded to the power line ground.

  • Improvements to the design

    Mark Rehorst05/01/2019 at 16:46 0 comments

    Recently, while doing some other work on the Z axis in the printer, I found that one of the PTFE leveling screw blocks had failed- it was getting wobbly.  I decided to redesign the blocks for more stable mounting.

    New PTFE leveling screw blocks

    These blocks sit on the 4040 tee support frame and are held in place with an M4 screw and t-nut.  They are very stable and the thickness of the blocks matches the length of the threaded portion of the leveling screws.

    Here's how they look mounted on the tee:

    Their positions can be adjusted as needed just by loosening the M4 mounting screw and sliding the block along the t-slot it sits in.  

    And a close-up showing one of the blocks installed:

    More details and a link to the CAD file here.

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