dimensional accuracy

jay_s_uk

On a delta, the z dimension is controlled by the steps per mm.
The X and Y axis are controlled by the arm length.
I wouldn't use M579 and would instead tweak the arm length and e steps to suit.
Bear in mind that a delta loses accuracy as you move out from the centre of the bed.

T3P3Tony

@Dugee axis steps/mm should be calculated from the physical components (motors, pulleys, belts) that the printer uses. they should not be tweaked from the correctly calculated settings, especially not on a delta.

Use delta calibration (especially if you have a z probe setup right) to ensure that the M665 and M666 settings are as accurate as possible for your delta. In my experience that gets you to good dimensional accuracy.

Only after than then you can use M579 to adjust the scales, you can also use :
https://duet3d.dozuki.com/Wiki/Gcode#Section_M556_Axis_skew_compensation
if required.

I would only use these after everythign else is a good as physically possible though.

dc42

@Dugee said in dimensional accuracy:

Finally got my 1st calibration cube from my upgraded Monoprice Mini Delta. Small machine, but there is definitely a lot of potential lurking in there. Some parts very much built down to a price, but these are easily replacable, like bed clips and drive gears. Delta calibration and bed mesh has come in to 2 decimal places pretty easily and after working through the config files to get it operational I am very pleased so far, so firstly thanks for the help I have had from this site, directly and by searching old posts.

On to my question - the MPMD FB groups aim for dimensional accuracy via adjustment of the steps/mm of the steppers, but the guide for calibrating Delta printers on the dozuki recommends using M579 to do this. Admittedly MPMD uses a customised version of Marlin, not RepRap, which doesnt support M579 so they may use this for that reason.

What are the pros/cons of the two methods of adjusting the scaling this way, and which should I use?

Thanks in advance.

Adjust the tower steps/mm (keep all 3 the same) to get the correct Z height. To avoid first and last layer effects, print 2 otherwise similar test cuboids of different heights, and measure the difference to calibrate the steps/mm. It should be very close to the theoretical value.

After doing that and setting the correct steps/mm, re-run calibration, then print a noughts-and-crosses/tic-tac-toe pattern. Check that the lines are straight. If they are, use M570 to adjust the X and Y scaling if necessary. If they are not, you will need to adjust the configured rod length, recalibrate, and repeat.

Dugee

@dc42 I'm finding that auto calibration is giving me a delta rod length of 8-9mm longer than measured, so that whilst I am getting good theoretical calibration numbers, when I start a calibration print of about 100mm diameter the head is constantly pushing on the bed, trigger my bed switches, particularly when when the head is opposite the towers.

I'm planning now on deleting my config-override file and only attempting new calibrations that don't recalculate my L dimensions.

jay_s_uk

Calibration type 6 or type 8 are recommended for a delta. It doesn't seem to work as well when it's also auto adjusting the arm length.
And are you running 2.04 RC1?

DocTrucker

I'm about to revisit this on one of my machines.

Calibration will end up correcting for material shrinkage as well machine setup errors, or perhaps simply the fact some slicers think you want to print at 100% dense whereas in reality it is easier to print a little lower than it is to cope with occasional overbuild.

I'm doing...
e-steps
Nonlinear & linear extrusion correction
Then filiment diameter correction based on a vase printed at a moderated speed.

I would handle all of the above with gcode changes.

Then I plan to do scaling and offset checks. I can scale in my slicer but I can only do a generic offset, I can't apply different offsets to x and y. Regardless of trying to understand the reasons for it the spreadsheet wanted different offsets last time I checked.

This will be material and settings dependant.

Dugee

@jay_s_uk yes, I am on 2.04, which was interesting as it seemed to create a few anomalies from 2.03. I know the machine is capable of what I need as I had it dialled in before its original board failed. I think I will stick to the basic calibration and avoid the auto calibration of arm length. I plan on building some new arms soon so it will all change anyway

Dugee

@DocTrucker I spent a bit of time getting good extrusion values, and I have managed to get some good quality prints, just inaccurate. I've been trying to reprint a delta calibration tool as it was basically ovoid when I first did it. I think the auto calibration of delta arm length has given me more problems than solutions.

fcwilt

I spent a great deal of time trying to get my deltas to work as I thought they should.

Then I got an FT-5. Once completed it took only minutes to get much better results.

Deltas can be finicky.

Frederick

dc42

@Dugee said in dimensional accuracy:

I think the auto calibration of delta arm length has given me more problems than solutions.

If you read the wiki page on auto calibrating delta printers, you will see that we recommend using 6 factor calibration, which leaves the arm length alone.