Son of MegaMax Gets a New Y-Axis

My second printer, Son of MegaMax (SoM) has been a workhorse at the Milwaukee Makerspace for over 3 years, but there have been a few things I didn't like about it, so I decided it was time to make some changes.

The PEI has been reglued onto the bed surface a couple times and the edges were starting to lift up again.  I also wanted to change from the 450W, 24V heater to a line powered heater that would get the bed up to temperature faster, and eliminate the giant industrial power supply that sounds like a vacuum cleaner.  I gave up on the ball screw drive that has been limiting speed due to a severe mechanical resonance and went back to belt drive.  I also wanted to convert to a kinematic mount for the bed plate.

SoM's bed heater, shortly after it was put into service.

Bed plate just removed from SoM.  Dark spots are scorched kapton where air bubbles got between the heater and the bed plate.  This is how heaters eventually fail...

The New Design

I spent some time modeling the changes I wanted to make and got busy in the machine shop at the makerspace.  My machining skills and time are very limited, so whenever possible, I try to make use of existing parts and materials that will require a minimal amount of machining to make them work.  If you've read any of my other blog posts, you'll know that one of the materials I use a lot is square aluminum tubing.  In UMMD, the pulley supports, motor mounts, and extruder carriage are all made from square aluminum tubing.  I used the same tubing to make the bed supports and motor mount for the new Y axis.

The Kelvin-type kinematic mount uses three leveling screws- one each for reference, pitch, and roll, just like the mount in UMMD.  The reference screw sits in a chamfered hole in the bed plate, while the pitch screw sits in a chamfered slot that allows the bed to expand when heated.  The expanding bed plate is free to slide against the roll screw that simply supports the bottom of the bed plate (no holes or slots).  Springs at each leveler hold the bed plate down on the leveling screws.

Occasionally people ask me why I didn't use a Maxwell-type kinematic mount instead of the Kelvin-type that I used.  The answer is simple: the Kelvin type mount only requires slots/holes to be milled/drilled in the Y direction, just like the motion of the milling machine table.  The Maxwell type mount would require milling three slots, 120 degrees apart.  That would require a rotary table which we have at the makerspace, but it's a real PITA to set it up on the machine.

This is the new carriage plate and bed support designed for kinematic mounting of the new bed plate.  The sphere head screws are the reference and pitch adjusters, and the screw on the left is the roll adjuster.

The tubes used to make the mounts were first cut to a few mm longer than needed, then I drilled the holes, and finally milled the edges to final dimensions.  The angled edges dimensions weren't critical, so I marked the angles on the tubes with a marking pen, then clamped the tube in a vice on the mill table, tilting the tube so that the line I drew was parallel to the top of the vice.  Pieces of wood stacked inside the tubes kept them from collapsing when they were squeezed in the vice.

The springs attach to bolts screwed into the plate on one end and the bed support tubes on the other end.

Springs and the screws that will be used to anchor the springs to the bed plate.  I drilled holes then cut the heads down.

The bed hold-down springs hook in the screw heads.

UMMD has a kinematic bed plate mount and it is extremely stable.  Of course, it moves the bed in the Z axis, not Y, like SoM, so we'll see if the concept holds up in a bed-flinger type printer.

Here's the carriage assembly.  The reference adjuster is on the right, pitch on the left, and roll adjuster at the top of the photo.  The plate that links the three bearing blocks is 2.5 mm thick aluminum that was cut on a band saw (no milling on this piece, though the milling machine was used to accurately drill the holes for the bearing blocks).

Heater mounted on the bottom of the bed plate. The left side tab will sit on the reference screw, the right side tab will sit on the pitch screw, and the tab at the top of the picture will sit on the roll screw. 

This is the reference ear of the bed.  The dark circle in the chamfered hole is where the spherical head of the reference screw contacts the plate.

This is the pitch ear of the bed plate.  The dark lines in the slot are where the spherical pitch adjuster screw head sits.

This is the reference adjuster screw assembly.  The pitch adjuster is identical.  The screw on the side is there to anchor the spring that will hold the bed plate down on the adjuster screw head.

This is the roll adjuster assembly.  The end of the screw supports the bed plate from below.

This is the reference end of the assembly.  Putting the carriage plate on the bearing blocks instead of on the leveler tubes keeps it far from the bed heater.  All the leveling screws are threaded into teflon blocks that won't melt or soften when the screws get hot.

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This is the pitch end of the assembly

This is the assembled roll adjuster.  The knob has 16 ridges, each of which represents 50 um of vertical displacement.

Motor Mount

The motor mount was made from a piece of 2 1/2" square aluminum tubing.  I had to raise it a bit by putting a piece of 1/4" thick aluminum under it so that the belt could easily be clamped to the carriage plate.  The motor mount is held down by two 5/16" carriage bolts that fit into the t-slots in the base plate.  Belt tension can be set by pulling on the motor mount, then tightening the bolts.  I used a 20 tooth pulley.

End Pulley

I had a piece of junk from something I took apart years ago that looked like just what I needed- a milled aluminum bracket with two bearings pressed into it.  The bearings have 1/4" bore, like the motor shaft, so I simply mounted a pulley on a 1/4" shaft that I also had from a junk tear-down.  The only problem was that the shaft height of the motor and the end pulley differed by about 1 mm.  That meant that the belt clamp would have to accommodate that difference.

Y axis end pulley

Belt Clamps

I wanted to mount the belt clamp on the carriage plate, so I calculated the necessary thicknesses of the clamp to keep the belt parallel to the Y axis guide rails.  On the motor side of the clamp, the belt would be 5.6 mm above the carriage plate, and on the end-pulley side, it would be 6.6 mm above the carriage plate.  So I designed a printable clamp with those distances in mind.

At first I designed a one-piece clamp, but thought about it and decided it would be less likely to become a source of backlash if I split it into two pieces.  That way when the bed reverses direction, the belt tension will always keep the clamps in position without introducing any backlash.

New Y axis belt clamps.  Splitting the clamp into two pieces reduced the possibility of backlash.  Steel pins secure the ends of the belt in the clamps and the belt teeth interlock in the slots in the clamps.

Electronics

The original heater was a 24V 450W unit, so it needed a big power supply- 24V at 31A.  That power supply had a fan that sounded like a vacuum cleaner.  Since I have switched to a line powered bed heater, I didn't need the high DC power so I replaced the main power supply with an LRS-220-24, a 24V 8A supply (still overkill) with no fan at all.  Complete silence!

The new, 750 W, line-powered bed heater is capable of getting much too hot, so I added a thermal cut-out for safety.  In UMMD I bolted the TCO to the bed plate, but decided it would be safer to have it attached to the heater.  That way, if the adhesive on the heater lets go (as the adhesive on UMMD's heater is doing now, after about 2 years of use), the TCO will stay with the heater and be able to do its job.  I used the same TCO that I used in UMMD, but in SoM I attached it to the heater using high temperature silicone.  UMMD will be getting modified as soon as I get around to reattaching the bed heater.  When the adhesive eventually lets go on this bed plate I'll reattach it with high temperature silicone.

Here's the heater with the TCO added- it's in the blob of blue goop next to the thermistor at the center of the bed.

Power to the bed is switched using a Crydom D1225 SSR.  It's wired through the 10A circuit breaker that serves as the power switch for the printer, and then goes through the TCO on bottom of the heater.  I used Anderson Power Pole connectors for the heater and thermistor connections to the controller.

Performance

I've been able to crank the acceleration up to 3000 mm/sec^2 and print at 100 mm/sec, and I'm not done tuning it yet.  That's a big improvement over the 40 mm/sec limit that was imposed by the resonance in the ball screw setup that used to drive the Y axis.  Print quality is excellent, as always.

A Few More Changes

I connected the power supply ground to the line input ground and also to the frame of the printer - that should have been in the original build.

SoM's lighting has always looked a little dim, so I added some of the same 24V white LED strips that I used on the top of UMMD.  Much better!

I replaced the Titan extruder with a BondTech BMG.  That means I need a new print cooling fan duct design, so I'll be working on that over the next few weeks.

The BMG mounted on SoM's extruder carriage- the hot end offset from center is different than the Titan, so the print cooling fan would no longer fit.  I'm redesigning that now...