Mk2B Bed Modification for Anet A8 by FredGenius

Last crawled date: 3 years ago

I upgraded my heated bed to a Mk2B heater pcb and borosilicate glass plate recently, thought I'd share the construction information and the printable parts.
Why upgrade? The aluminium plate is heavy, and because it heats up unevenly (hotter in the middle and cooler at the edges) it warps, making printing difficult, especially when you want a 0.1mm first layer.
The borosilicate glass makes an excellent build surface, and because it has a very low coefficient of thermal expansion it stays very flat.
Using larger, M4 screws to support the bed makes it more stable, and more durable.
So, by replacing the stock bed I've saved approximately 300 grams mass in the y axis, and have a very flat build surface. My print quality is excellent and I have no regrets.
You will need:
1x Mk2B heater pcb
1x 213mm x 200mm borosilicate glass plate
1x 100K NTC glass body thermistor
4x M4 x 35mm hex head bolts (or #8-32 x 1.5")
4x M4 (or #8-32) half nuts
4x binder clips
Kapton tape, and some strands of copper wire (I used strands from some 7/0.2mm equipment wire I had lying around, but almost any wire will do if you can solder it).
Tools:
An M4 or #8-32 tap to re-thread the holes in the H frame
A soldering iron and solder
A digital multimeter
Method:
Choose the adjuster/locknut file, M4 or #8-32) for the hardware you have.
Start by printing the adjusters, lock wheels and caps - you need 8 wheels and 4 caps. Press the half nuts and bolt heads into the wheels.
Next, remove the old bed and de-solder or cut the wires. Also remove the screws and springs, you won't need them.
Take the thermistor and solder a strand of copper wire to each leg, long enough to reach the edge of the bed. Next, tape the thermistor to the underside of the pcb, there's a hole in the centre which is the perfect location. Tape the wires in place so they aren't touching and solder to the original thermistor cable (the two thin white wires).
Test the thermistor with a multimeter, it should read about 100K Ohms at room temperature, and fall slowly when you touch it.
Solder the heater wires to the pads provided. The Mk2B heater pcb is designed for either 12V or 24V supplies, so make sure you use the 12V option. Ignore the pads for the resistor and LEDs, they are not needed.
Secure the cables - I used two zip ties, one around the cables, the second looped through the hole in the PCB and through the first tie. This should provide adequate strain relief.
Next, use the M4 (or #8-32) tap to re-thread the holes at each corner of the H frame, no need to drill out the M3 holes first.
Screw the bolts with the adjuster and lock wheels in place through the H frame as per the diagram, and position the caps on top.
Rest the PCB on the caps, they should locate in the holes at each corner.
Position the glass on top of the PCB so the screw caps aren't obstructed and secure with the binder clips. I find that placing the clips at the sides rather than the front and back edges works best.
Connect your bed and thermistor to the main board, level your bed and you're good to go!
I've been using this setup since June 2017 without problems. The bed heats up really quickly and the glass is the best build surface I've tried. A smear of ABS Juice on the glass is all you need for good adhesion, and the print will come loose when the bed temperature drops below about 40C.
PS, be careful - if you run the nozzle into the bed you'll most likely break the glass.
Update! I've re-designed the caps, they are longer now, and have an M4 thread internally, making them a bit more stable. See below for print settings.
Update! I've added the adjuster-locknut and cap mesh for #8-32 bolts. :)
Update! Check out my differential corner adjusters, 0.3mm per turn! https://www.thingiverse.com/thing:3151276