Creality Ender 3 Pro print head drops in standby

Hi allI've had an Ender 3 Pro for 3 months or so and it's been great, printing night and day :) I have a minor but annoying issue with it. When the printer is on but not doing anything, sometimes the print head / print bar will drop down and 'mash' the hot end into the print bed. It seems pretty easy to get the Z axis to start moving and as soon as it does it just drops down until it hits the bed. My workaround is to put something on the bed to stop it dropping all the way by getting under the bar at a suitable height. My question is whether there is a way to perhaps provide some sort of 'motor lock code' at the end of a print (maybe in the gcode) so that the hot end is held in place? Does anyone have a code command for that? I have a Pi running Octoprint controlling it, so if there is a Pi based way of 'jamming' the Z axis, that would also be of interest. I'm thinking of some sort of direct output from the GPIO pins, maybe triggered from the finish of Octoprint job and kicking of some Python task, or an IFTTT trigger, or something. Any input would be of interest CheersEd

Here is a couple ways that may help.

When the printer is just sitting if the steppers are disabled then the chance is higher that the Z-Axis will drop.
I suggest that you should home your printer when setting then the steppers will be engaged.

This 2nd oprion requires that you change the lead screws from a T8 8mm pitch per turn lead screw to a T8 2mm pitch per turn lead screw.
This is the option that I decided on, however it require that you change your firmware configuration.

These are my changes for my 3D Printers

// Z Axis 3200, / 8mm pitch leadscrew = 200*16 /8 = 400 :: (STEPS_PER_REVOLUTION_Z / PITCH_OF_Z_ROD)
// Z Axis 3200, / 2mm pitch leadscrew = 200*16 /2 = 1600 :: (STEPS_PER_REVOLUTION_Z / PITCH_OF_Z_ROD)

//My leadscrew steps per revelotion changed form 400 to 1600 shown below.
#define DEFAULT_AXIS_STEPS_PER_UNIT { 80, 80, 1600, 94.4962144 }

Now that the new leadscrews are in place I have never had the dropping occur.

Thanks for that : makes sense that a shallower thread would be less likely to slip. I've printed and fitted a lead screw stabiliser today, not really expecting it to make much of a difference (it didnt). But I've had an idea around a stepper motor providing a physical interrupt to the motion of the lead screw. A few cascaded plugins might be needed :) But i'll investigate the alternative lead screw : depends a bit on the cost as I already have some steppers and driver boards floating around.

I have posted about this before.
T8-8 Lead-screw Drop with Gravity
https://youtu.be/yFHEIdUf2Lw

Image of three lead screws T8-8mm pitch (4 start), T8-2m pitch (single start), and All thread 1.25mm Pitch.
All turned exactly 360 degrees. I started with a cheap 3D Printer it came with the all thread.

Attachment 14609
Now the All Thread worked but it had it own problems.
1st it was not a very true lead screw and wobbled.
2nd it always squeaked always needed cleaned always accumulated dirt.
3rd the threads were gummed up and even a little dirt caused problems.

The T8-8 (4 start) lead screw always needed me to make adjustments to the Z-Axis.
It was always out of sync with the other Stepper on my dual stepper Z-Axis Prusa i3 type Clone.

However on a single Z-Axis system like my MPCNC the T8-8 (4 Start) lead screw works fine.
The Homing auto-corrects any drop in the Z-Axis.

I would Never go back to a T8-8 (4 Start) lead screw again for a Z-Axis bearing weight.
It took me a while to learn this so I have a few of these lead-screws.
Including a set that was given to me, I ordered a T8-2 lead screw they sent me a T8-8 lead-screw then a replacement saying keep the other.

keeping stepper motors enabled and powered on when the machine is not actually printing, will cause the motor to get really bloody hot.
So sounds like cliff's screw change makes more sense :-)

Quote:

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Originally Posted by curious aardvark

keeping stepper motors enabled and powered on when the machine is not actually printing, will cause the motor to get really bloody hot. So sounds like cliff's screw change makes more sense :-)

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Seems like I already have 8 x 2 threads already. So I think I'll need to create something to block the z-axis rotation. I'll keep you up to date once I am done.

This is probably correct You have a T8-8 which is a 2mm pitch 4 start lead screw.
This is different from a T8-2 which is a 2mm pitch single start lead screw.

The difference is the if you rotate both a single turn the T8-8 will rise 8mm and the T8-2 will rise 2mm.

Attachment 14634
The Left lead screw is a T8-8 turned one complete revolution or 8mm rise
The Right lead screw is a T8-2 turned one complete revolution or 2mm rise.

You will also note the trapezoidal threads are at a different pitch on each.

Quote:

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Originally Posted by Roberts_Clif

This is probably correct You have a T8-8 which is a 2mm pitch 4 start lead screw.
This is different from a T8-2 which is a 2mm pitch single start lead screw.

The difference is the if you rotate both a single turn the T8-8 will rise 8mm and the T8-2 will rise 2mm.

Attachment 14634
The Left lead screw is a T8-8 turned one complete revolution or 8mm rise
The Right lead screw is a T8-2 turned one complete revolution or 2mm rise.

You will also note the trapezoidal threads are at a different pitch on each.

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I'd better go and do some careful measurements.