RepRapPro Mendel heatbed assembly

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Introduction | Frame assembly | Y axis assembly | X axis assembly | Z axis assembly | Heated bed assembly | Extruder drive assembly | Hot end assembly | Power supply | Wiring | Commissioning | Printing | Multi‑colour/multi‑materials | Colour Printing - Slic3r | Colour Printing - RepRapPro Slicer | Maintenance | Troubleshooting | Improvements

A notice about these instructions!

Since 1st April 2013, the RepRapPro Mendel has been superseded by the RepRapPro Tricolour and Mono Mendel.
If you are building the new machine, use the official, and most up to date, instructions on the RepRapPro wiki here


By the end of this step your RepRap machine should look like this:


And in addition it will have an aluminium plate on top of the red circuit board.


You will need the following tools:

  1. M3 Allen key
  2. Cross-head screwdriver
  3. Multimeter
  4. Soldering iron and solder


Hardware Quantity Reprappro-mendel-bed-components.jpg
Click the image for a high-res version.
Lasercut insulator 1
PCB bed heater 1
Glass printing plate 1
Foldback spring clips 4
M3 x 40mm screws 3
30mm springs 3
M3 nuts 13
M3 washers 16
10K glass bead thermistor
(arrowed; this is very very small;spec here)
twin ribbon wire 1.1m
20A wire 1.1m
3mm LED 1
1k resistor 1
Aluminium heat spreader 1
M3 x 10mm or 16mm countersunk screws* 4
3mm dia. Heatshrink (usually black) 30mm
5.5mm dia. Heatshrink (usually grey) 30mm
Cable tie 1

*Either works fine.

At this stage you will have a long piece of 20A wire. Use all of it. As the last step in these instructions you will cut it to length.

Get the twin ribbon wires by stripping them off the wider ribbon cable supplied with the kit. If that ribbon cable has a colored stripe down one side, strip the two wires furthest from it. If it has no coloured stripe, strip from one side, then put the remaining ribbon flat on the bench and run a felt-tipped marker down one side:


This will make the wires in it easier to identify later. Set the wider ribbon aside for later use.

Step 1: Main assembly


Use the countersunk screws together with four nuts and washers to sandwich the heated bed PCB between the aluminium plate and the MDF insulator.

The zig-zag heater track on the PCB faces up towards the aluminium plate.

Make sure that the PCB solder terminals, the rectangular cut in the insulator, and the notch out of the aluminium plate all line up.

Take care not to break the lasercut springs in the corner of the insulator. (These are to allow for differential expansion.)


Here is a view of the underside. Use a pencil to mark + and - as shown. This will help with the wiring.

Step 2: Wiring


Bend the LED's legs and resistor legs as shown above and insert them from the MDF insulator side. The longer LED leg goes to the end marked +.

The LED and the resistor are in series, and the two of them together are in parallel with the main heater element - the zig-zag pattern of the PCB.


Solder the LED and the resistor as shown on the aluminium plate side. You may find this easier if you prop the bed up on something clear of the bench.

Trim the wires flush with the top of the PCB using side cutters. Make sure that the connections do not project higher than the thickness of the aluminium plate, or they will foul the glass plate which will go on top of it.

Locate the 10K Glass Bead thermistor. This is very small and can get lost in the folds of the bag. It looks like this (M3 nut for size comparison):



Solder the twin ribbon wires onto the thermistor as shown above. Stagger the connections. Use the insulation stripped from the short connection to insulate the thermistor wire.

Tin the ends of the wires, then tin the wires on the thermistor, then sweat them together.


Insulate the joins with a length of heatshrink.

Strip the insulation from the other ends of the wires and use your meter to check the resistance. It should be about 10K ohms, though the exact value will depend on room temperature.


Bend the thermistor at right angles and tape it to the bed. Make sure it goes through the hole in the insulator and the hole in the PCB and touches the aluminium plate. If you like you can add a little heatsink grease to improve the thermal contact between the thermistor and the plate.


Pack the hole in the insulator with something fluffy: cotton wool or a bit of scrunched tissue both work well. Put tape over the hole. (The LED and resistor are the wrong way round in this picture - ignore them.)

Lead the wires to the edge of the bed where the two cable-grip holes are and tape them down. Use the holes on the side you marked -. Note the strain relief loop; don't run the wires straight.


Bare the ends of the high current wire. Put heatshrink on such that it covers only a couple of mm of the bare strands.

The picture shows one wire complete, the other ready to be wrapped.


Push the wires through the bed as shown and bend them flat against the tinned areas of the PCB (you may need to drill the holes out with a 3mm diameter drill first. Do this from the tinned side to get a clean hole). The wire with the black stripe goes to the side marked -. Solder the wires. Be generous with the solder (it's giving both mechanical strength and high current carrying capacity), but again make sure that the join is lower than the thickness of the aluminium plate.

Put the glass plate on the aluminium and make sure that none of the wires nor the LED stick out and foul it.


Use the cable tie to attach the high-current wires and the thermistor wires. Again, note that the high-current wires are not pulled taught. They have a strain relief loop. Make sure they do not project too far on the MDF insulator side - if they do bend them gently flatter.

Pull the cable tie good and tight (but take care not to break the lasercut insulator), and then clip the excess off it.

Step 3: Attach to the machine


Put the three long screws through the mounting holes with washers under their heads.

Put the springs on. The sequence from the screw head goes:

  1. Screw head
  2. Washer
  3. MDF insulator
  4. Washer
  5. Spring
  6. Washer
  7. Nut
  8. Nut


If you want a superb machine (of course you do), before you mount the bed put a little superglue on the heads of the screws that hold down the toothed-belt clamps, cut two rectangles of aluminium foil about 20 mm x 30 mm and glue these onto the screw heads. These will act as a heat shield, and will prevent the printed clamps from getting warm.

Note that we will shortly upgrade the design of the Y bearing mounts to move the hexagonal nut cavity to the top of them when when they are mounted in the machine. This will make bed adjustment easier.

Either way, pull the nuts into their hexagonal recesses in the printed bearing holders on the Y carriage with a shorter screw with a washer under its head. Take care that the nut and the hexagon are aligned.

Use the last three nuts and washers to mount the bed on the machine. If you have the newer variant the washers go underneath and you need to leave the lower nuts lose for the moment. If you have the older variant the washers go on top, and you need to do the nuts up on both sides of the printed Y bearing holder finger tight.

Adjust the screws to get the bed roughly level (measure the heights of the corners above the Y rods with digital callipers).

Tighten the nuts against the Y carriage to secure the bed.

Adjust the nuts above that set the spring compression so that the bed is held firmly against the screw heads, but can be pushed down with a finger.

Fit the glass plate using the four clips and check that the Y axis moves freely back and forth. Then take the glass off and put it in a safe place - you don't want to break it as you build the rest of the machine...

Finally, take a look at the wiring page and run the fat cable roughly along the route it will take round the machine. Cut it a little long (you don't want it to be too short...) and save the rest - you will use the remainder for the power supply and the other power wire in the machine.

Next step

The extruder drive