Redesigning the y-bed?


#1

So I’ve built my Wilson for a while now but haven’t been able to perfect my bed leveling due to a variety of factors, from material to its design.

I’m using 6mm MDF as per the BOM and have a normal 200x300 bed, using the mk2 pcb heated bed, which I’m looking to upgrade to the mk3 soon. Being in SEA, humidity is a problem for MDF and my bed is prone to warping. As for the design, the legs are the weakest point, especially the long sides which I can see have been bent considerably in my case. From what I’ve read so far, 3 point leveling is the better choice. With regards to the carriage, 3 bearings is enough as opposed to 4 too? Would pillowblocks be a better choice?

So I’m thinking of a solution which can allow 3 point leveling, as well as mounting of pillowblocks if possible. What I’ve managed to find so far is this design by andreq: http://www.thingiverse.com/thing:1501382

and this mod of the prusa i3 plate: http://www.thingiverse.com/thing:1369542

I’ve access to lasercutting, tho limited to woods and acrylics as well as a drillpress and the bandsaw. So I’m not too sure where to start tbh, work on a design from scratch using MDF/plywood, or mod a i3 plate like above? Hoping to get some insights here!


#2

FWIW, I have a really weird plan for my heated bed. I haven’t built my Wilson II yet, I’ll be waiting for parts to ship for several more weeks, but I figured I’d share my plan. I decided to build a normal 200x200 bed using a piece of 1/4" cast aluminum tooling plate. From what I’ve read, these are essentially perfectly flat, and when paired with a silicone heat pad will heat up quickly and evenly. I decided to go with only 200x200 because this stuff is heavy - at this size it will be ~1.8 pounds, and if it was the full 200x300 it would be ~2.5 pounds!

I’m going to use a material called aluminum composite panel (brand name Dibond) for the y-carriage (1/8"). This is made from two sheets of aluminum bonded to a core of plastic, which supposedly is pretty rigid but also lightweight. I’ve come across a few posts on other forums buried in the depths of Google that talk about using Dibond for y-carriages. I think I’ll be able to cut this using a jigsaw and a metal cutting blade.

Anywho, my plan is to use 3-point leveling with this setup. I don’t have a sketch yet for my carriage plate, but I anticipate putting 2 leveling points at the back and the 3rd in the front center.

The reason I decided I could get away with a smaller build plate is that I don’t anticipate doing huge prints in ABS for fear of warping. If I want to try something that would use the entire 200x300 bed, I will put a piece of glass that size on top of the cast aluminum plate and hope that it heats evenly enough for PLA.

I’d appreciate any comments from anyone with suggestions or to point out major pitfalls in my design plan. YJL, I hope this helps, or gives you some new ideas!


#3

@chinkel I have used 6mm cast aluminium tooling plate with a silicone heat pad for my Wilson II. The heated bed is 300x300 and the heat pad is 750W driven from the 240VAC mains supply via a solid state relay. I will take some photos and post them later.


#4

@dtbee Oh, very interesting! At that size it must be more than twice as heavy as my design, how fast are you able to print? Did you have to bring your acceleration and jerk way down on the y-axis to prevent distortions in your prints?

I thought about using a silicone heater that uses mains voltage, but I was worried about not knowing how to implement proper grounding of the printer in case something goes wrong. 750W must heat up very quickly!


#5

Here are some photos of the spider, heated bed and the SSR that drives it:

The heated bed plate is slightly larger all round so that the build area is free of obstructions.

I have added strain relief for the wires leaving the heating mat and to comply with UK standards the power for the mat goes through a thermal cutout to protect against failure of the heating control. The device I chose cuts out at 150oC and auto resets at 130oC. The aluminium plate MUST be earthed because the heating mat can induce currents in it.

The spider is a scaled up version of Marty’s design and I had it laser cut from 6mm MDF.

The Solid State Relay is a 50A device. It is grossly over rated but strangely it was cheaper than the 5A device I was looking for (I can reuse it if I ever need a 12kw bed)! It requires 6-30VDC to trigger it so I am using the original heated bed output from my Rumba controller to drive it. The 2020 extrusion upon which the SSR is mounted is also earthed.


#6

Thought I’d chime in on heated bed. I built my own from an aluminum plate (2-3mm thick), and I’m using a bunch of square ‘sand’ resistors glued to the plate bottom with JB Weld epoxy as the heating element. My current rendition of this has 32 pieces of 12 ohm, 10 watt resistors wired in series parallel to give 6 ohms. I’m driving them with a 44 volt stepdown transformer, and I switch the primary with a relay. I tried solid state relays here, and I had two of them fail in a year, in both cases by shorting out. I probably could switch the secondary and maybe avoid an inductive spike from the transformer primary? You could wire the resistors to give 1.5 ohms, and then operate them off of 24 volts. In my case the bed is 200mmx250mm, and the heater is about 322W. (the resistors can be run above their max ratings since they are ‘heat sunk’)


#7

If you were switching the transformer primary then you had a highly inductive load and this causes failure of the Triacs or Thyristors in a SSR by a mechanism known as dv/dt failure (very fast voltage changes). You can fit a snubber (RC) network to prevent this but then you run into problems with leakage current. If you switch the secondary then your load will be purely resistive.


#8

@dtbee, your bed looks really nice. Is the strain relief a piece of PCB with rubber under it screwed onto the plate?


#9

@chinkel. Yes that’s exactly right. 2mm EPDM rubber, one placed each side of the heater mat cables and a piece of 2.4mm fibreglass sheet as the clamp. I have extended the rubber sheet beyond the top of the bed to provide protection against the edge of the aluminium. The materials used should be OK for the temperature range of the bed.


#10

dtbee is quite right. You should switch the secondary. The best way is what is called a zero-crossing solid state relay. It only switches power at the instant the AC waveform crosses zero (between the negative and positive phase.) At that moment, there is zero power supplied to the load, so no transients are generated whatsoever. This has the advantage of being very easy on the switching circuitry, and also is electrically quiet as far as generated emf noise.