Building the C-Bot 3D printer: Part 18 : Software Day 3 : Tuning movement settings

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Update:  Since authoring this post I have switched my electronics to RADDS, and my firmware to Repetier.  See the “Part 31 post” for the latest on it.


Now that my z-stage is moving down and up, and I can ‘auto home’ my printer through the LCD, it was time to get the movement settings and max travel extents calibrated.

Total time:  About two.5 hours.

Tuning Movement Settings:

You need to make sure that when the g-code tells your printer to go 100 mm’s, it really goes 100mm.  Not 90mm.  Not 100.1mm: 100 mm.  On bots like my Makerbot Replicator (1) this is already done for you.  But when building your own machine from scratch, you need to figure all this out.  Otherwise you get ovals where you should get circles, and rectangles where you should get squares.

First I had to get some info for my hardware:

  • Stepper :
  • Stepper Driver:
  • XY Pulleys:
    • GT2
    • Teeth : 16
  • Belts:
    • GT2
    • Pitch : 2mm (distance between each tooth)
  • Lead Screw :
    • 8mm ACME
    • Pitch : Listed at 2 (distance in mm between each peak), but this value was incorrect for the below calculator.  Info below.  The value needed in this instance was 8 (the distance in mm traveled with one revolution).

Armed with those numbers, I accessed the “RepRap Calculator” to get me the base values for my bot.  I wanted to see if they’d match values Mason had already pre-plugged into Marlin (that mine should very closely match), but I wanted to understand how this system worked:  In Marlin’s Configuration.h, I found the line with ‘DEFAULT_AXIS_STEPS_PER_UNIT’ with an array of four values plugged into it {x, y, z, extruder}.  Based on the calculator, my ‘belt steps per mm’ came out to 100, which was around what Mason had given me.  However, the ‘leadscrew steps per mm’ were 4x (1600) what was listed in the firmware (400).  After a bit of research, I realized the calculator wanted to know the distance the lead screw travels in one revolution, not the tooth-to-tooth pitch (2mm).  Armed with my calipers, I figured out one revolution traveled 8mm, and plugging that into the calculator gave me a value of 400.  Success!

Now for the tuning:  I found this great guide (on the Marlin Configuration page) by Neil Underwood that goes through the manual process of telling your bot (in my case, via the LCD) to go 100mm, and then measure with calipers to see how far it really went.  Do maths, enter new values into firmware, re-measure, over and over.  Not hard, just a bit time consuming:

My final values:

#define DEFAULT_AXIS_STEPS_PER_UNIT {100.2166,  100.20469,  401.770921, 145.5687675196672}

Gotchas:

I just hit one major problems:  Very often the Rumba would reject the upload from the Arduino IDE.  At a certain point, it took me half an hour to get three uploads.  It turned this already slow process into an incredibly slow and frustrating process.

I’ve been programming Arduino’s for years and never encountered anything like this (but never on a 2560 like the Rumba uses):  Sketch would compile, but when it would go to upload it would timeout, saying this:

avrdude: stk500v2_ReceiveMessage(): timeout

Searching the internets, I found many other people with complaints (not Rumba specific, Arduino Mega specific), but no solutions.  Finally, I read one post (which I’ve lost the link to) that had this issue.  The fix?  Simply swapping USB ports on your PC between uploads.  Suddenly I was back in business.

Setting the max travel extents

For each axis, there is an endstop telling the printer to ‘stop moving’ when it hits it.  Most printers (like mine) only have one endstop per axis, although you could setup two per axis (min & max) and probably skip this step.  If the endstop stops the toolhead\build platform going in one direction, what stops it going in the other?  The firmware.

In Marlin’s Configuration.h, there’s a section called “Travel limits after homing”, where you can setup these constraints for X, Y, and Z_MAX_POS.  To start, I edited these values to be something much larger than my current volume, since I was going to dial this in manually on my own and didn’t want a small value to stop me before the true extent was reached.

After re-uploading the firmware with the ‘big’ values, via the LCD I manually drove each X, Y & Z axis to what I considered was the ‘safest max position’: Just before a X/Y gantry would hit something on the opposite side with maybe 10mm to spare, or about 50mm above the lead screw flexible couplings so as to not cause any binding.  I copied those numbers down from the LCD and plugged it back into the firmware.  My final values are:

// Travel limits after homing (units are in mm)
#define X_MIN_POS 0
#define Y_MIN_POS 0
#define Z_MIN_POS 0
#define X_MAX_POS 320
#define Y_MAX_POS 315
#define Z_MAX_POS 530

So my X & Y are clearly over a foot (304.8 mm), so I have no problem filling my whole 12×12″ bed.  But the Z is just under 21″, 3+” short of the 24″ build height I was after.  This is due to a few reasons:  I have maybe an inch before the z-gantry hits the flexible couplings on the leadscrews, and my E3D volcano easily sticks down over 2″ from the X/Y-gantry : If I did a bit of redesign on the extruder mount I could probably move it ‘up’ more and get 2+” back, and I’d be closer to my 24″ height goal.  But 21″ isn’t too shabby to start… 😛

Other Notes:

In my previous post I showed how my Y-endstop was at the rear of the bot:  This actually blocked the gantry from moving all the way back, and since the extruder is mounted on the front of it, I couldn’t get all the way to the rear of the build platform:  I moved it to the front corner instead, and got that clearance back.  Now my  endstops are all Xneg,Yneg,Zneg, when before they were Xneg,Ypos,Zneg.  In the firmware, the settings are thus:

// ENDSTOP SETTINGS:
// Sets direction of endstops when homing; 1=MAX, -1=MIN
// :[-1,1]
#define X_HOME_DIR -1
#define Y_HOME_DIR -1
#define Z_HOME_DIR -1

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Building the C-Bot 3D printer: Part 17 : Electronic & Software day 2
Trouble in paradise: The Replicator quit extruding...
    • David
    • October 19th, 2015 8:48am

    Thanks a lot for posting your build progress and details for values and parameters. Your blog has been helpful to me while I’m building my own C-bot

  1. @David
    Great! Hope you have a good build.

    • David
    • October 23rd, 2015 5:14am

    Did your Y endstop work when it was at the rear of the printer? I can’t get marlin to Y home when the endstop is set as 1 rather than -1. The X endstop is at -1 and it homes ok. My endstops are at Xneg Ypos Zneg. Thanks

  2. It did, I had no problem with it on the rear, or front, where I eventually moved it. You just have to remember to change where it’s plugged in the mainboard. See my post on it here: http://www.akeric.com/blog/?p=3197
    You may have to update the firmware too (Configuration.h), to change the homing direction. This is mine:

    // ENDSTOP SETTINGS:
    // Sets direction of endstops when homing; 1=MAX, -1=MIN
    // :[-1,1]
    #define X_HOME_DIR -1
    #define Y_HOME_DIR -1
    #define Z_HOME_DIR -1

    See the bottom of this post:
    http://www.akeric.com/blog/?p=3209

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