General structure and design
After the initial trials of a very Reprap “Mendel” style Repstrap, I decided to alter the design to fit my additionnal needs:
protection from projections and splinters when using the machine in a substractive mode (cnc), as the machine will be standing less than 50 cms away from my head this seems essential a better usage of the available space, by having the Z axis slaved to the Y axis and not the other way round (thus making it closer to the Mendel design actually) more stable axis platforms even if at lower speeds : I had no way to ensure the stability of the belt driven versions so I switched back to a threaded rod approach So I switched to a boxed enclosure based structure, made out of L shaped aluminium rails, with 5 mm thick plexiglass protection screens bolted onto the frame. The axis remain essentially the same, save for the Y axis, which can now use the full available height.
Since I had no adapted belts but had an additional stepper motor left over from the printer stripping operation, each threaded rod that moves the axis has its own motor , reducing complexity and necessary parts greatly
Custom, low cost electronics
Here is the general overview of the requirements and what i came up with.
versatile : needs to handle not only the standard reprap electronic elements (stepper control, opto end stops, extruder and build platform heating and temperature control etc), but also have spare i/o ports for the upcoming 3d laser scanning elements and more support the Pololu stepper drivers standalone as much as possible : this meant a frontal control panel with lcd display, and sd card reader as cheap as i could
since a single AtMega328 , even with port extensions would have been pushing things a bit on the speed side (as the stepper drivers require very fast operation , in the microsecond range for example) i decided to use two chips , communicating via I2c one AtMega controls the steppers exclusively, while the other “master” chip is in charge of all the rest (no problem since all other tasks are a lot less time critical) The master chip is connected to: a 74hc4094 shift register to control a group of 4051 muxers/demuxer for a total of 32 additional i/o ports (up to 64 available) a second, chained 4094 shift register controls the front 2X16 LCD an addtitional 4051 that checks for the states of a 4X4 button matrix 3 TIp120 transistors to control the extuder head and heated build platform heaters the completely diy SD card reader (using the SPI ports) The end result :
Yes, it is quite complex, and requires a custom firmware (hybrid 5D), but for now the results are encouraging , and its very cheap at around 30/40 euros (not including the stepper drivers of course)!
For reference : here is what the protoboard version looked like : VERY MESSY!
The Doboz repstrap printing :