Prototype Heated Build Plate

Having a non standard sized Prusa RepRap often means that standard parts simply don’t fit. This was a particular problem when obtaining a heated build plate to print ABS plastic onto. With a assessable area less than 180 x 180 mm, the 210 x 210mm mass manufactured RepRap Prusa heat plate simply wasn’t an option.

So what next, get a PCB manufactured? Cost around £50 including shipping. Make my own PCB? Around £5 each for the pre-sensitized boards, with no margin for error. Or use some PCB strip board as per this post on Thingiverse. I opted to use the strip board method, having managed to trip the safety features in my power supply with the first attempt, I now have a working heated build plate.

So how do you build one for a Huxley sized build area?

Below is a prototype of a Strip Board Heated Build bed that is fully functional and will heat up to 110°c and 60°c for ABS and PLA.

Figure 1. Single Row Series Strip Board

1. Prototype Strip Board Heated Print Plate

Using the idea taken from this post on Thingiverse, a single piece Strip Board, 160 x 100 mm was wired up in strips of one, as shown in figure 1.

Placing a 3 x 100 x 150 mm piece of aluminium on top of this sheet enables the bed to be uniformly heated, and most importantly level. If just the strip board PCB is used, the PCB will bend when heated. A 2mm plate would also be suitable, and is potentially a better solution as the plate would heat up faster.

The Aluminium and PCB are held together using binder clips. This allows the difference materials to expand whilst keeping them firmly together.

A 100K Ω thermistor is used to monitor the temperature and is attached to the top surface of the aluminium.

Figure 2, Prototype Heated Build Platform

This allows the real temperature of the surface to be measured. Some designs place the thermistor on the bottom side of the heat panel, and compensate for the 10-20°c drop in temperature in software. A prototype of this system can be seen in figure 2. Notice how the thermistor is placed, with the trailing wires going across the heated build plate itself and with no strain relief, meaning in its current configuration the build bed can’t be used. Another issue is that this bed isn’t attached to the Y-axis. One possible solution would be to use Kapton tape to secure it.

Instead, in the second revision a larger strip board will be used to allow the bed to be attached using the screw and spring method as traditionally done.

DO NOT USE! Dual Row PCB.

2. What about this power supply tripping business?

Ah, you haven’t forgot that was previously mentioned have you. The very first attempt at producing this circuit used the ‘gang tracks’ together approach, so that they are running in parallel to alter the resistance. A mixture of inaccurate resistance measurements and erroneous Maths resulted in the PC power supply detecting a short circuit and performing a shut down, which took quite a while for the power supply to believe it was safe again…

Attached to the left if how the PCB tracks were ganged together. Do not use this design, or you’ll risk damaging your power supply or worse. It is attached for reference only.

A digital caliper, making sure the X,Y and Z axis are moving as they should.

So you’ve made it to the end of construction, you’ve meticulously ensured that the build bed is level, the X-Axis is level to the build bed and Z-Axis lead screws and that every last nut and bolt is secure. You are itching to turn on the heater and print something but before you do you really should calibrate the whole machine.

So How do you calibrate a RepRap Prusa X,Y and Z Axis?

1.0 Firmware Calibration

X, Y Axis Steps Per Unit (MM)

 float axis_steps_per_unit[] = {80,  80,  3200/1.25, 700};

{X, Y, Z, Extruder}

The default Sprinter / Marlin firmware settings are 80 Steps each for the X and Y axis. In theory this is ideal, but with tolerances in the gears and belts this will never be perfect. We need to measure some real world values. To do this make sure all your axis are homed correctly (So position 0,0,0).

1.1 X-Axis

If you are using stepper motors with a 1.8 degree step (Standard), 1/16 Microstepping and 5mm (T5) belts then the 80 value is the correct starting value. If not, you’ll need to use the calculator on RepRap-Calculator and replace the 80 starting value with the generated starting value. Remember these are the ideal starting values, we are going to modify these nice round numbers to values with a decimal point for maximum accuracy.

With the X-Axis carriage homed (At position 0) measure with a caliper (Or other accurate measuring device) the distance between the X-Axis carrage and the X-Axis Z-Axis mount (Was around ~20mm on my setup). If you have digital calipers, zero them now.

In your host software, instruct the X-Axis to move 100mm (Or larger if your measurement device lets you). Measure the traveled distance. You’ll be able to see that it is not exactly 100mm, but it should be very close, within 95mm – 105mm. If it is outside of this range you need to make sure there is very little slack in your X-Carriage Belt and that the grub screw holding the drive pulley to your stepper motor is tight.

Having measured the traveled distance, re-home the X-axis and repeat the exercise a total of 5 times. If you are getting very consistent results, good, if not make sure your axis are lubricated and nothing is slipping or stalling. You must find out the cause of the issue at this point before continuing.

With 5 good measurements, take the average (Add each value together and divide the total by 5). Use this value together with the RepRap Calculator to find your calibrated Steps. Replace the first 80 value in the Sprinter firmware with this new, very long number (Something like 83.236543). Save and upload the new firmware to your electronics.

The code should now look similar to:

 float axis_steps_per_unit[] = {83.236543, 80, 3200/1.25,700};

Repeat the test again and you should be extremely close to the requested travel distance. You have just Calibrated your X-Axis.

1.2 Y-Axis

Repeat the steps for the X-Axis, just using the Y-Axis instead.

1.3 Z-Axis

Again for the Z-Axis use the previous procedure. However, when determining your ideal starting value if you are using non standard parts, remember to use the Leaded Screw section of the RepRap-Calculator.

The main board with the Step Stick drivers added.

In the last update, it was mentioned that there were a few components left to make the control board complete. It can be said that the board is two steps (ignore the pun) closer to being complete. Four step stick stepper motor controllers have been purchased from china and have arrived in good condition (although to the wrong address!) and assembled and installed to the Sanguinololu.

The next task was find / adapting a 12v power source to the system. This implementation accepts a standard PC atx 2.1 power supply and requires only the 12v auxiliary power connector for the CPU. As it does not look tidy and can be difficult to manage, the extra unneeded cables and connectors have been de-soldered from the PSU PCB. It is a very bad idea to simply cut of cables from the PSU as you may end up shorting the system out somewhere down the line.

The other set of wires left connected are a single ground connection (black) and a power on (green). This is so the PSU can actually be turned on using a toggle switch of some description.

Again at the end of this update the only thing left to do is find two suitable MOSFET’s for the heat bed and hot-end and some cable crimps (The headers and sockets have already been purchased).

Thats alot of de-soldering going on there.

The modified power supply

Just needs two MOSFETs and the stepper motor drivers

For those of you who are not familiar with the Reprap project, it is an open source idea to create machines that can reproduce them self. In non technical language, 3D printers. A 3D printer just like a normal printer receives commands from a data source (PC, SD card, Compact flash, camera etc etc…) however instead of using ink, it uses plastic.

This means that we can print 3D objects designed using a CAD package . So here I begin with my RepRap Journey. I’ve already gone through a lot of the process, getting parts for both the structure and electronics.

Most of the electronics that have been designed to control the Repraps’ various parts are based on the Arduino open source hardware project, and mine is no different. Based on the Sanguino Arduino, the Sanguinolou is a single board solution with all stepper motor end stop and heating element drivers present.

Being the graduate that I am, finances are tight and as such everything that has been done has been done as cost effectively as possible (That means best value, not cheapest, I’ve made that mistake before). I started this by getting a free prototype PCB from spirit circuits, who will mkae a one of PCB for free. However it doesn’t come with soldermask or silkscreen, and will be in a much larger panel with their advertising. I think that is more than fair since it is free and thus I have my PCB. By scavaging around for parts, left over from other projects and asking the right people, the only electronics I’ve bought have been the Atmel 644p Microcontroller, the FTDI USB chip and the 12v ATX power connector.

Notice the bad cuts?

That’s all for the electronics so far, the mechanical side I have based the design on an over-sides huxley (There are a few versions of the Reprap design, check RepRap.org for all the current ones). I am using M8 sized threaded rod for the structure and M8 silver steel bards for the rails.

So far I’ve produced the main electronics board, burned the bootloader for the Ardunio software and uploaded the latest sprinter firmware. I’m waitnig for some stepper motor drivers to arrive from China and for Robotics Common to have their RepRap mendal arrive so the plastic connections can be made.