Strange problem: When starting print, head jams into bed after successful homing/calibration

General Discussion
1

Basic Information:

Printer Model: Multirap M420
MCU / Printerboard: Megatronics v3.1 (AVR2560)

Describe your issue:

I recently acquired a dead Multirap M420 printer with a ton of filament for free. This printer is a very special snowflake, as it is only loosely based on the RepRap design. Because I got 11 rolls of filament with it, and it still uses 2.85mm filament, I at least want to revive it enough to be able to print with one of the two extruders it has. It also has no heated bed either, it still uses the old sticky tape on bed approach.

It uses an inductive sensor for Z probing and optical end stops for X/Y.

I flashed Klipper onto the board and am using a Raspberry Pi 3b with OctoKlipperPi to drive the printer. I pieced together a configuration and managed to home the printer and also do a manual bed level calibration (paper test).

I now have a very strange problem: when I start a print, a G28 is issued to home the printer which works flawlessly, but when it actually wants to start printing, it seems to ignore the Z limit and drives the print head further down, causing it to jam into the bed. Then it tries to print, resulting in either being stuck or scratching the sticky tape off the bed with the nozzle.

What might be the problem here, where can I investigate?

2

The problem starts with you not following the template and posting a klippy.log

3

I thought that this was a general beginner problem and there were more general hints I could take into account where the log wasn’t directly needed.

But the way your answer was phrased proves the point why I left all social media and have been hesitant to even post in forums since then.

4

Oh, come on. Nice try in reversing the facts:

grafik
grafik865Ă—75 10.9 KB

It was you who chose to ignore that it is written in bold, italics and underlined and added with “in all cases”.
It is also you who come here for help and by not providing the relevant information is burning the time of the folks here who provide this support free of charge in their free time.
So, please be so kind to spare us with being pretentiously offended. Ever thought about why this is so prominently required and how often we need to repeat this request?

7 Likes
5

You are right, and I am sorry for that. I want to apologize for my reaction.

A lot of the support questions can be solved by having a look at the klippy.log, I suppose.

In the future I will ask the questions correctly if I have additional ones.

In my case though, I figured it out for myself: I had incorrect parameters in my Cura profile, and the old Cura seems to have a bug to replace travel_speed in the start GCode with something that evaluates to too high values. I had a G1 command that uses a F9000 feed rate which is much too fast for the stepper motor driving my Z axis. This thing can do at most 1500.

2 Likes
6

Correct.

Make sure to have sensible limits in your printer.cfg, e.g.

[printer]
kinematics: cartesian
max_velocity: 300
max_accel: 3000
max_z_velocity: 10
max_z_accel: 150

If you have such limits, then regular G-code commands cannot exceed them.

I do not know this printer but judging from the layout, these values should be sensible. Note that lead-screws react quite allergic to high velocities and high z-velocities usually do not offer any real world benefit.

2 Likes
7

When I look at your example, this actually might be the problem, I have in mine:

[printer]
kinematics: cartesian
max_velocity: 100
max_accel: 1000
max_z_velocity: 100
max_z_accel: 1000

Currently in the Start GCode of my Cura profiles, I hardcoded the F1000 before starting prints.

Nobody does :wink: This is a very exotic printer, and a very special snowflake, too. I think I started my foray into the world of 3D printing on hard mode.

But the more rewarding it is when I finally get something to print. I am currently spitting out calibration cubes, adjusting one value after each print, and they are gradually getting better.

My printer isn’t fully functional, the heated bed doesn’t work and it isn’t completely set up yet, but when I have something that at least prints reliably I am eager to share my configuration and Cura profiles.