When my first prints of my raygun prototype warped up from the print bed, I knew I was going to have to bite the bullet and build the heated build platform. Here is the result. RESULT!
I’ll write about what I did, what I’d do differently next time, and how to hook it all together.
If you compare the new photos in this post with the one from the previous post, you immediately see that the warping has gone.
The PLA has stayed firmly stuck to the blue tape on the build platform, with no lifting at all. In fact, even after everything had cooled down, this print was more difficult to remove from the bed than any earlier ones.
(Btw, I use a small stainless steel artist’s painting knife to remove prints from the bed. The edges are thin and slightly flexible without being sharp. So usually I get away without damaging the blue tape.)
The heated bed components
The MakerGear Mendel Prusa kit came with a big red printed circuit board, called the “Heated Bed PCB”. It was designed (so it says on the silkscreen) by Joseph Prusa, who designed the printer variant as well – it is named after him. He uses a different system now, but he proudly claims his personal printer is never the same two weeks running. He is always modifying and experimenting.
One side of the pcb has a big ‘Heated Surface – do not touch’ warning. The other side has the tracks that do the heating, and space to solder on two surface-mount LEDs, a surface mount resistor, and pads for connecting the leads to the RAMPS controller board. The LEDs and resistor are optional. There is also a thermistor for temperature regulation, but no obvious place to mount it.
The online instructions for this are, um, challenging – if you can find them at all. The pages on the MakerGear website are for earlier, different, heatbeds and for the Mosaic which is different again. The ‘Differences from standard Prusa‘ file covers this a little, and I recommend you read it (especially for the photos), but I could have used a bit more help.
Board orientation and cable location
You’d think that you would want the warning side up, so you could read it. But that puts the LEDs underneath, where you can’t see them. And no, the PCB is not translucent at that point, so they don’t glow through from underneath.
Most importantly, you need to fix the thermistor somewhere, so the RAMPS can tell when the bed is at its correct temperature. The print bed is in constant motion, so you need some form of strain-relief for the wire that leave the printbed and connect to the RAMPS. Surface mounting wires onto PCBs is fragile, and won’t last long if the soldered joint flexes at all.
The kit comes with two lengths of red wire with crimps at each end for fitting into pushon connectors. I only found one unused connector, that fitted the T1 thermistor pins on the RAMPS board. I plugged the crimps into that connector, and twisted the wires together so I could tell how much length was available. This helped me decide which way round to mount the PCB.
After much pondering, I decided I needed to mount the heatbed PCB so the surface mount components and the connecting wires were nearest the RAMPS board, which means they are coming off the side of the printer. The photo below shows the terminal strip that the thermistor and heater cables connect to. It also shows that the wires from the terminal strip to the RAMPS go underneath, between the printbed and the y-carriage, and out the back. I’ve taped them to the underside of the print bed. That way there is no flexing of the joints going into the screw terminal, and nothing sticking out to catch on the RAMPS board as the y-carriage whizzes backwards and forwards.
I chose to mount the PCB with the ‘hot surface’ warning on top. Although I’m covering it with blue tape, I wanted to have the heater tracks underneath the pcb in case of massive nozzle crashing damage. If I get the print height wrong and scrape the nozzle along the surface, it’ll only scrape off some warning, not cut my heater tracks.
Soldering the cables and the tiny surface-mount thingies
You have to cut the single piece of green-insulated high temperature wire into two pieces, for the connection to the heater tracks. Strip and tin both ends of both pieces.
Solder the wires to the tiny square tabs, as the ‘differences’ file shows. I carefully and gently scraped the squares with the tip of a scalpel until the copper was nice and shiny. Then I tinned them with a 25w soldering iron, because my 10w fine-point couldn’t heat the pcb up fast enough. This was my first attempt at smt soldering, so you don’t get photos from me. Please read a how-to on smt soldering if you haven’t done this before.
You don’t have to attach the LEDs and resistor, you won’t be able to see the LEDs anyway if you mount the board the way I do. Life would have been simpler for me if I had known this. I cracked one of the LEDs by applying too much downward pressure while trying to get it to seat against too large a blob of solder.
If you do decide to use the LEDs and resistor, use your multimeter to check that you haven’t got a solder bridge under the resistor. The resistance from wire to wire should be around [Update: 1.6 ohms] not zero!
Testing the heater
You can test the heatbed at this point if you wish.
- Put the ends of the green wires into the white screw-terminal block.
- Put one end of the black twin-core power wire into the matching holes in the terminal block.
- Put the other end of the twin-core into the D8 connectors on the RAMPS board. The + goes to the + sign (Dur!) and the – goes to D8.
- Ensure all screws are done up firmly
- Turn on your computer and the printer power supply.
- In Pronterface, connect to the printer. Set the Bed temperature (in the bottom left of the window) to 60. The setting doesn’t matter because you don’t have a thermistor attached yet anyway, so RAMPS will just run the heater at maximum.
- If you installed the LEDs and resistor, one of the LEDs should light, but not both of them, unless you oriented them wrong.
- Listen to the powersupply strain, and notice the bed getting warm.
- Reset the printer, and turn the printer’s power supply off.
Carving holes in your nice flat print bed
Having the heater tracks on the underneath of the PCB means the LEDs, resistor and connections are also underneath. We don’t want them pressing up against the underside of the PCB, because that would make the bed uneven, and cause all sorts of problems. I carefully cut hollows and grooves in the top ply of the print bed, so the components would lie beneath the bottom of the PCB.
You can see the grooves for the wires in the photo at left. You can also see I was not careful enough, because a couple of wiring grooves are unused – I put them in the wrong place.
You can also see that the green wires don’t connect to the terminal block. This is a mistake! Don’t do it the way I did, do it the way I wished I had. See the next section, on the Screw Terminal Block, where I explain.
Because I was too lazy to remove the printbed (and have to re-level the whole thing? Nooo!), I had to be careful not to use too much downward pressure while drilling and carving. I started the hollows with an electric drill, just enough to break the surface. Then I used some very sharp lino-cutting tools to cut the grooves and lines to shape without going below the first glue layer of the plywood. I was afraid if I cut too deep, I might change the flatness of the build platform.
The screw-terminal block
All along I’d been wondering how to connect the wires so they don’t flex, and don’t catch on other parts of the machine while the bed is moving. In the end I decided to use another piece of the screw terminal strip. I think I’m going to buy some more to keep in my toolbox – it is so useful.
By the time I thought of the terminal strip I had already soldered the black cord to the red plugs, as written in the ‘Differences’ file. I’m now wishing I’d just taken the ends of the high-temperature wires from the heatbed straight into the terminal strip, next to the thermistor wires. Then plug the ends of the thick black wire into the other side of the terminal strip, and off to the RAMPS. That would have saved me some iffy soldering – I don’t like soldering wires onto connectors without some sort of hole to loop the wire through, so I’m not happy with the red plug and socket.
Steps for thermistor installation
This is how I did it. I’m sure there are better ways, because this was no fun, and I prefer elegant solutions. I couldn’t find a writeup anywhere, so at least you can laugh at mine, while you do whatever the obvious better technique requires. I still can’t think of an obvious better technique, so if you can, comment!
- Ensure you’ve already mounted the LEDs, resistor and heater element wires, and cut the hollows and grooves for them.
- Lay the thermistor on the build platform with the legs hanging far enough off the side that you’ll be able to put them into the terminal block (see previous photo for guidance). That shows you where you need a hollow for the thermistor and a groove for the leads.
- Draw around the thermistor and leads in pencil. Put the thermistor somewhere safe – it is tiny and almost invisible, don’t drop it on shag pile carpet.
- Carve the hollow and grooves. Don’t go below the first ply.
- Lay the thermistor down on the sticky side of a 1″ wide piece of kapton tape, with the flatter side of the thermistor up, where it will contact the PCB. Ensure the insulation on the wires is firmly up against the glass body of the thermistor.
- Lay the tape-and-thermistor on the plywood build platform in the hollow and groove, sticky side and thermistor on top. Push the thermistor down into the hollow, so the tape bunches around it a little. This is hard, because if you touch the tape, it won’t want to stay in place.
- Put the heated bed PCB back in place on the build platform. Get the mounting holes aligned on the side opposite the thermistor, before gently lowering the pcb onto the thermistor and kapton tape. Press the PCB down to ensure the tape has stuck to the underside.
- Gently lift the PCB up, and lie it thermistor side up on a flat surface. Burnish the tape down onto the PCB, starting around the thermistor so the tape presses it against the underside of the PCB. Avoid bubbles as much as you can.
- Put the PCB back in place again. Take special care that all the wires are in their grooves, and that the PCB is laying flat against the build platform. If all is well, bolt it down with the mounting bolts and washers. (Mounting it this way is sub-optimal, in that as the board heats, it expands, and because the corners are fixed, the only way to expand is to bow upwards. People have tried all sorts of ways to fix this, but mostly its not too crucial.)
- Connect the clear-covered thermistor wires to the screw terminal strip. Connect the free ends of the red wires you prepared earlier to the corresponding holes on the terminal strip. Yes, it’s another waste of perfectly good crimps.
- Connect the green-covered heat bed wires to the screw terminal strip. Connect one end of the black twin-core to the corresponding holes on the terminal strip. I don’t know why polarity should matter here, but get it right anyway.
- If you haven’t already done so, screw the terminal block to the side (or underneath, if you are really clever) of the printbed.
- Run the black twin-core and the red twisted wires between the print bed and the y-carriage, and tape them to the underside of the printbed as they exit at the back. Ensure no wires stick out to the side where they could catch on the RAMPS or the RAMPS wiring.
- Plug the connector end of the red wires into the T1 thermistor pins next to your hot-end thermistor on the RAMPS board.
- Plug the bare end of the black twin-core into the D8 connector on the RAMPS board. + goes to +, and – goes to D8.
I think that’s it.
Here’s a photo showing the twisted red wires heading off towards the ‘T1’ thermistor connector.
The two heavy black wires directly under it are the heatbed wires, with – on the left and + on the right.
The medium-weight red and green wires on the left are the hot-end nozzle connections. The unused D9 connector in between is most often used to run a fan, but I haven’t done that yet.
I hope that’s enough (along with the ‘Differences’ file) to get people going. Please comment on any confusion I may have caused, any better ways of doing things, any warnings you may have.