ATX Hack & Steps per mm calculations

Encountered an ATX power issue

MK2a heatbed @ 110° +  E3D hotend @ 240°C + powering the Sanguinololu = ATX power source wires melting!

So I decided to solder thicker wires that feed the Sanguinololu directly to the ATX printed circuit board(PCB). Results are good, the wires get a bit hot but don't melt anymore. I also added new spacers to the bottom of the ATX to be  able to mount them to the Pusa's frame.

Next issue: the MK2a heatbed takes 30 minutes to heatup to > 100°C

So I took care of these 2 things: "create direct current paths" to spare the Sanguinololu PCB and "Balance the ATX power supply"...

1. Create Direct Current paths on Sanguinololu PCB

A (modified) quote from cobrageek on the Reprap Forums on this subject:

The pads and traces on the Sanguinololu board are not robust enough to handle the high current involved when controlling the heated bed with the dedicated mosfet. If left as is, the board will heat up in this area, could be damaged and the plastic connectors discolored.
To fix, simply add a direct current path by way of (preferably insulated) wire:

• from the 12v input to the 12v pins on the Heated Bed (HB) connector; 
• from the center pin of the HB mosfet to the two ground pins on the HB connector;
• from the ground pin on the mosfet to the ground input of the board (probably the ground side of the 12v connector)

2. Balance the ATX power supply:

To have a strong and steady 12V on the heatbed and hotend, you need to 'put a load' on the ATX +5V output.
To 'put this load' connect a 12 Volts 50 Watt light bulb to the +5V and GND of your ATX supply.
Check the detailed explanation on 'Balancing the ATX power supply' by Triffid Hunter.


UPDATE: The heatbed now reaches 100°C in 8 minutes (instead of 30 minutes).

Steps per mm calculations

To calculate the Steps per Milimeter for my stepper motors I used the formulas in Triffid Hunter's calibration guide to create a little excel sheet. You could also use Josef Prusa's online reprap calculator to calculate the steps per mm.

I get these results (for now) that I need to configure in my Marlin firmware:
(In "Configuration.h" the variable: DEFAULT_AXIS_STEPS_PER_UNIT {80,80,2560,674.6521728} )

Edit: I have 5mm threaded rods for my Z-axis so there are 4000 E steps for 1mm (and not 2560!)