marlin firmware and half height prints

[bigmikeinil]

i have a nwreprap prusa i3 (nwreprap.com)
originally i misread the instructions and installed the sprinter firmware and managed to get it printing fairly well. i picked up a discount reprap extra large lcd full graphics screen with controller, and was trying to get it working (still nothing but working with the guys at http://www.makergeeks.com/xtsm12lcdra1.html on that problem)

anyway on to the problem, sprinter was running a max height of 100mm for printing (found the setting to fix that) but i upgraded to marlin (https://nwreprap.com/downloads/marli...-i3-250000.zip) for the more integrated screen support hoping that it would help with the lcd screen. in the configuration.h file it shows the correct settings for build platform (200x200x200) but when i print my 20mm test cube it comes out reliably at 50%. before switching to the marlin firmware my only height problem was when it tried printing over 100mm. am i missing something with the marlin firmware?
specs
ramps 1.4 controller
software being used:
slic3r
pronterface

any help would be appreciated!
Mike

[Roxy]

Are you saying the calibration cube is printing at 50% of the size it is supposed to? If so, first make sure you didn't scale it down accidently when you sliced it. But if not, you can try changing the line:

#define DEFAULT_AXIS_STEPS_PER_UNIT

in your configuration.h file. There will be 4 numbers on the line. X,Y,Z & E. If you double each of the X,Y & Z numbers, the print should scale up to be twice as big.

[bigmikeinil]

x and y output is good but i get about half the height i should be ie 12.8mm instead of 20mm

[bigmikeinil]

just so you see what i mean the green cube is 20mm the rest have been printed since changing the firmware:cubes.jpg

[Roxy]

OK.... So just scale the 3rd number. Try doubling it, recompile, flash the firmware, and re-print.

Let us know how it goes....

[bigmikeinil]

i double the numbers in the 3rd position from 3993.9 to 6k and the shafts spun in the coupler, so i dropped it down to 5k and got a vase print that was supposed to be 79mm tall and got this:

[Roxy]

OK... That isn't all bad. It maybe you just need to tighten down your coupler's so they can't slip. If there is slippage, there is no way the movements will be accurate. You have to get that cleaned up first.

But if that isn't what is going on, your stepper motors might not be able to keep up with the higher step rate. If making your couplers tighter doesn't prevent slippage, some of your other numbers might be close to the limit. If so, Go find these lines. Your actual numbers are going to be different than my numbers (that are posted here).

#define DEFAULT_MAX_FEEDRATE {500, 500, 5, 40} // (mm/sec)
#define DEFAULT_MAX_ACCELERATION {9000,7000,100,700} // X, Y, Z, E maximum start speed for accelerated moves. E default values are good for skeinforge 40+, for older versions raise them a lot.

If we are only messing with the Z-Axis, we only want to mess with the 3rd number. Make the 3rd number 1/4 of what it currently is for both of these lines. The reason is we don't want to be right at the edge.

And Doing a quick calculation... We need to change that 3rd number in:

#define DEFAULT_AXIS_STEPS_PER_UNIT

to something like: 5000 * 79mm / 56.7mm = 6966

[printbus]

I agree with Roxy. For comparison, here are the numbers MakerFarm uses as their values, and their printers can likely be assumed to be close to what you have.

#define DEFAULT_MAX_FEEDRATE {250, 250, 2, 22} // (mm/sec)
#define DEFAULT_MAX_ACCELERATION {1000,1000,5,1000} // X, Y, Z, E maximum start speed for accelerated moves.

These are considerably slower than what appear to be Marlin defaults that you're using now.

On the Pololu driver boards, I assume you're referring to Pololu Black as the higher current driver board? I think the only difference is a more efficient heat dissipation scheme on the board. IMO, unless you've noticed the heatsink on the driver board running hot (well, I'm assuming nwreprap has you put heatsinks on the driver chips), buying the fancier driver board won't get you much. You can always set the Z-motor driver for 2x the setting of the drivers with one motor if you haven't already.

If you go to the Acme screws, remember you'll have to go back in and readjust the Z value in the DEFAULT_STEPS_PER_UNIT since they have a significantly different thread.

Have you been in touch with nwreprap at all over this, since they say the firmware files they provide are pretailored for you?

[bigmikeinil]

just finished printing my cube... its now shorter by about 1.5mm than the last one!

[printbus]

Bigmikeinil, I'll be interested in seeing how the printer works out for you. It was one on my short list of printers I was looking at.

According to the nwreprap site, the printer uses M5 threaded rods. That should put the steps per unit in the Z-axis the same as my MakerFarm i3v. FWIW, I have a configuration.h value of 4000 for Z in DEFAULT_STEPS_PER_UNIT. I agree with Roxy that if you think you were seeing the coupler loose at a value of 6000, it's probably still loose at any other value - just not as noticeable. The web site also says it uses Pololu stepper motor drivers on a RAMPS board. Was adjustment of the trimpot on each motor driver something addressed in the assembly instructions?

Have you went through the process to calibrate your extruder for the filament you're using? That'll affect the fourth number in the DEFAULT_STEPS_PER_UNIT values. The lower part of the vase appears like you might be under-extruding.