CNC Screw Mapping

markz

Sure, here's the tentative syntax (from the code comment). I randomly picked the M80x's just to be easy to remember/type during debug.

--Enable/disable all screw mapping. Does not clear tables.
-- if no S prints report
M800 S[0|1]

-- create a screw table for srcaxis with one row per destaxis
-- if no arguments it prints a summary report
M801 R"srcaxis" A"destaxes" Sstart Iinterval Ncount

-- set screw table entries
may be called multiple times (once per row) for shorter gcode lines
-- if no data (rows) it prints contents of table
-- each entry is an offset (delta) value
M802 R"srcaxis" Xf0:f1:f2:f3... Yf0:f1:f2...
-- run the screwmap selftest (ifdef'ed just for debug testing)
M810

I considered condensing the Mcode but the destaxes is a list of possible axes that are adjusted by the source axis. So, the M802 syntax can be very very long.

On my machine the X axis has 3 rows of 36 floats and the Y axis has 3 rows of 48 floats. So the M802 is a very long list of numbers. I'm not sure what the gcode limit is but this would be about 3000 characters.

I let the M802 to be called repeatedly to set rows individually for clarity.

--Mark

postscript: I'm obviously not wedded to syntax. I'd just like the script file to be readable.

There are two sets of 'upgrades' possible to the functionality for later syntax thoughts. I don't use that functionality on the CNC.

This can support directional tables (which I don't use but some milling folks do) to support directional leadscrew wear. It can also support backlash as a 1 column directional screw map.

postscript 2: in practice this works well in a script. Create a table, fill the values, create a table, fill ... The only odd error checking is to ensure the table data list is <= the defined length and the result values are monotonic. It would be easy enough to have M801 do both tasks depending on parameters but the interdependencies are worse and I think it's less clear.

markz

Ok, even this M802 line is too long. There seems to be a 200 character upper limit and mine are over 300. I guess I should allow an offset in the M802 command.

dc42

@markz said in CNC Screw Mapping:

Ok, even this M802 line is too long. There seems to be a 200 character upper limit and mine are over 300. I guess I should allow an offset in the M802 command.

Is 200 characters not sufficient to describe the mapping for a single axis? A separate command for each axis seems reasonable to me.

markz

@dc42 Thanks for all the interest. I just ran it quickly doing the following...

The worst axis is X so I copied the old lookup map from WinCNC to excel, converted to mm, then created a script file... which was too big. So, I added the O option and redid it. This worked.

I set the cnc to 116,0 (left of table), set down my yardstick horizontally.. then I disabled screw map, went to 15",0 - a known bad area) took a screen capture of the microscope view, return to 0,0 and enabled screw map then went back to (15",0) and took another screen capture.

I hadn't tried it before - and - first try with no edits. Here's the script and the comparison photo. I know it's a lot of digits, but I'm used to inches. Probably could shrink it a little.

M801 R"X" A"XY" S116 I25.4 N36
M802 R"X" X0.00000:-0.04064:0.00000:0.06350:-0.07112:-0.10922:0.02286:-0.04318:0.00508:0.10414:-0.02794:-0.09652:-0.22352:-0.40894:-0.37846:-0.47752
M802 R"X" Y0.00000:0.01778:0.06858:0.11176:0.14732:0.18034:0.22606:0.29210:0.34544:0.36576:0.40640:0.43942:0.46228:0.51308:0.53594:0.57658
M802 R"X" O16 X-0.49784:-0.27178:-0.25908:-0.29718:-0.15240:-0.24638:-0.26162:-0.05080:-0.08636:0.01524:0.14224:-0.00508:0.05588:0.09144:0.07366:0.24130
M802 R"X" O16 Y0.60960:0.59944:0.55372:0.48514:0.40640:0.30480:0.22606:0.19304:0.15748:0.09144:-0.21082:-0.48768:-0.15748:-0.12954:-0.10160:-0.08890
M802 R"X" O32 X0.25654:0.25146:0.41656:0.78994
M802 R"X" O32 Y-0.12446:-0.09652:-0.04572:0.01270
M800 S1

Photo

markz

@dc42 I've forked and updated the github repo. You can see the code changes at: https://github.com/MZachmann/RepRapFirmware/tree/v3.2-screwmap

and it forks from the 3.02-dev branch (3.2 retail).

Let me know where you'd like to go from here. Implement (pull request)? Move to M-codes? Change syntax?

The only thing I couldn't yet figure out from the code base was how to correctly have the ui update the user position when the screw map definition changes.

Mark

markz

Here's a doc file describing the current implementation of screw mapping.

Screw Mapping in RRF.docx

markz

I've posted a video showing a calibration run (in X). It took about 1/2 hour to gather X and Y error terms and improved the CNC accuracy from .033" to .002".

here: https://youtu.be/wpUOaYwDrvI

bot

Nicely done!

I just had a thought, though. Make sure your yard-stick is straight. You could be compensating for an error in the yard stick.

markz

Sorry, I should have posted that. You're absolutely right.

The yardstick is one of the most expensive hand tools I own since it's so important.

https://www.mscdirect.com/product/details/58700246

bot

@markz Oh!!!! That's a nice "yard stick" lol!!!

markz

@bot Yeah, Starrett would probably not be happy me calling it that.

Phaedrux

I also spit coffee on my laptop when I saw that price.

bot

I suddenly want a yard stick like that... but I couldn't even use such a large one! My print bed is only ~11" x 11". I'll look for it's younger sibling. And possibly metric, too.

markz

@bot Yeah, I wish I had a metric one - if you watch the video I spend a lot of time converting inches. Kinda dumb.

backstory - when I first realized about 10 years ago my CNC had so much error I did this measurement with a normal yardstick, couldn't believe the results (especially the Y error), so in order to root-cause I had no choice.

markz

I've cleaned up the code so setting a screw map entry changes the DRO readout in real-time.

Here are a couple of plots of my CNC error curves with increment=25.4mm (1 inch). Note the 2d-ishness of the whole thing.

To put this in a worst case if I were just drilling holes the largest X error pre-compensation would be about 1.5mm (or 1/16th of an inch for US). Post-compensation error is 2*repeatability + test measurement error (~.03mm).

XAxis Compensation.jpg YAxis Compensation.jpg