Hi,
So i finally got this thing going, did the crown plotter yesterday and it was ok.
Tried to cut in plywood today and it was ok i guess, for a 130w dremel
But my Z-axis is not moving correctly, when i`m moving it 10mm with the display or repetier host i can see it only moves about 2-3 mm.
I havent made any changes to the firmware.
I have not used any lubricate on the leadscrew yet, but i think it moves pretty easy with my fingers.
Its about 7mm off, can a dry leadscrew or something with the z-axis alignment is off make that much difference?
I´m using:
Mega+Ramps 1.4
DRV8825, set to 0.7v
Leadscrew = T8 OD 8mm Pitch 2mm.
Firmware = Marlin_V1CNC_Ramps_Dual_2.0.7.2_508-src
Any chance you have a T8-2 lead screw instead of a T8-8 lead screw? It didn’t look like from the video, but I’ve never seen a T8-2 to compare. If this is your problem, then you can use M92 g-code command to adjust the steps. The default steps for the T8-8 is 400, so you would set it to 1600. If this is the problem, and if you keep this lead screw, you’ll have to make further adjustments to the maximum feedrate for Z to avoid losing steps.
Google translate of this German Amazon page did not allow me to figure out which lead screw this is. The MPCNC uses a T8-8 which moves 8mm for each revolution. A T8-2 will only move 2mm per revolution. If this is the problem, as mentioned, there are more potential issues with using this Lead screw rather than the T8-8, but you might be able to mitigate or completely solve those issues by altering values in the firmware.
Edit: It would be pretty simple to figure out which lead screw. Twist it 360 degrees and see how far it rises or falls.
This is what the marlin code looks like now: #define DEFAULT_AXIS_STEPS_PER_UNIT { 200, 200, 800, 200 }
Should i change these values with IDE or with reptier host?
In repetier host- can i send something like that, M92 200, 200, 1600, 200
And save with M500?
The info I’m giving is concepts I’ve learned by reading on the forum, not practical knowledge for this I’ve had to do.
First the DRV8825’s are running at 32 microsteps, rather than 16 microsteps. That is why the your value is 800 rather than the 400 for my Rambo that used 16 microsteps. That means you need to adjust the value to 3200. You have the format of the M92 wrong. Would need to do:
M92 Z3200
M500
While this will get you up and running, I believe this is the wrong fix for the problem. There are too many other issues. Thinking about it, the better fix is to adjust the microsteps for the drivers down from 32 to 8.
If you are going to try some cutting with this adjustment, I’d limit the Z feedrate down to 3mm/s or below to avoid lost steps. You can force this limit with an M203 and make it persistent with an M500.
This solves one issue, which is the cpu load. We used to have problems when Z would step too fast, Marlin would just reset. Marlin has fixed that issue (AFAIK) but the real trouble with 1 start leadscrews is that the motors fight the current when the rotate quickly. This can be thought of as a voltage resisting the incoming voltage. When that gets to really high rpms, the power supply’s 12V is no longer enough to provide full current to the motor and the motor gets weaker. You’re also moving the leadscrew really fast, so the friction can add up and you end up skipping steps.
The thing you need to avoid is rotating the motor too fast. The cpu steps and microstepping options don’t affect that. It is very counterintuitive. Using a 24V PSU will give you higher top speed (and no extra torque) because of this same phenomenon.
You can also look at the end of the leadscrew and see if it is one start or four.
Robert, you’ve been doing an awesome job in these forums. Thank you for helping everybody out.
