Skew correction/perpendicularity correction (millkins or millkins_xyz)

I am trying to use software based skew correction on my 3040 milling machine.  I have tried to adjust it manually and it's just badly made and I would have to modify it by re drilling some holes etc. to get it perpendicular mechanically.  Also there is usually at least a bit of extra adjustment that's hard to do mechanically, especially if all the axis need shimming etc. it really takes forever.  I was hoping I could do it in software.

Worst case scenario I guess I could post process the gcode but that will lead to errors in the parts, pretty minor for my purposes, also it's best to address error sources as close as practical to the root.

This entry had me all hopeful but there are a lot of problems with it, mostly it incredibly ancient and obsolete:http://wiki.linuxcnc.org/cgi-bin/wiki.pl?ContributedComponents#millkins_trivial_kinematics_extended_by_XY_skew_correction

I spend about 6 hours digging through layer after layer of issues.  comp has been replaced by halcompile, emc2-dev sounds like it was a module, linuxcnc-dev is another thing but not clear to me even now if it's a module that appends to the rest of linuxcnc or like a whole forked code base that (hopefully) shares something in common with the main user branch of linuxcnc... 

I am using the .iso file from a usb drive, downloaded about 6 months ago.  I think it's version 2.8.
At first I was miffed that you can't compile stuff using the iso, I thought that sort of thing was part of the open source thing, but now I see how big linuxcnc-dev is so maybe that's why...

Also very unfortunate that millkins is not included, but now I am thinking that's probably because it's not really done.  It is very short, not very much code at all and skew correction is commonly needed on cheap machines so I think it would be good to include it.

Turns out millkins_xyz.c has an issue with the code where probably the guy changed the file name at the last minute before uploading it and didn't test it, after modifying millkins.c  .  The component names etc. in the .c file are wrong.  I just changed the file name back to millkins.c before compilation and install.

So after laboriously installing linux-cnc to get halcompile, I was able to run: sudo halcompile --install /home/anthony/Desktop/3040-1/millkins.c

Said it worked ok. I check in the folder where the hal modules are and looks good.

I modified the beginning of the 3040-1.hal file (the configuration is named 3040-1) to read:# Generated by stepconf 1.1 at Sat Aug 12 17:15:10 2023
# If you make changes to this file, they will be
# overwritten when you run stepconf again
loadrt [KINS]KINEMATICS
setp millkins.skew_y 0.100
setp millkins.skew_xz 0.000
setp millkins.skew_yz 0.100
loadrt [EMCMOT]EMCMOT base_period_nsec=[EMCMOT]BASE_PERIOD servo_period_nsec=[EMCMOT]SERVO_PERIOD num_joints=[KINS]JOINTS
.....
Then I modified the 3040-1.ini file to read, in the relevant section:[KINS]
JOINTS = 3
KINEMATICS = millkins


This all took many tries of course, but that's where I ended up.  Sorry if the forum stuff is garbling the hard returns etc.

Now if I double click on the configuration on my desktop, it opens up ok. I can look at the hal configuration and it shows the millkins_y.skew and other variables set the way they are apparently supposed to be.

Ok. So far so good.

But when I move the axis, they all just move in a straight line. Nothing has changed. When the x axis moves the y has to move too in order to do skew correction. It's not moving. Everything is as skewed as it ever is. Ideas?

Here is the component for millkins by the way, it may only work in the exact context of the linuxcnc 2.8 from live install or something who knows but at least you don't have to go through the epic ordeal of trying to compile it: drive.google.com/file/d/1aEv_BwgTv8Liola...dvw/view?usp=sharing  

The wiki is very old and doesn’t get updated that often.
Some of the code on that page is from 2.6 days, I know I add issues with hidcomp mentioned on that page.

So it would that moving the X axis no other axis would move as this is “base” axis.
When moving the y axis there should be some movement of the Y axis.
Have you checked this with an indicator or dial gauge to confirm ?
Depending on the amount of skew the movement may not be noticeable.
The linuxcnc-dev package includes header files and halcomplie and other such stuff needed to build components that are included with the main part of linuxcnc.

I know you probably don’t want to hear it and probably know this yourself, but fixing the hardware issues is the best course of action. It’s a pain but it’s just the nature of beast.

Yeah I guess you are right fixing the hardware is the best, but like I said there is still that last 100 microns or something that is really hard to mop up mechanically on a cheap machine. Better machines have provisions for adjustability so you can tap things into position, but cheap ones you have to use shims which can be pretty finicky and complex as the old ones fall out when you put new ones in, they compress, affect vibration and rigidity etc. etc.

I moved both x y and z and neither of the other axis moves at all over the whole distance of the axis. I just put my finger on the knob that connects to the stepper motor, no motion.

I read up on HAL and I don't quite know how this is possible. The component must not be loaded right or something. I think the information must be being passed through the kinematics module or it would not reach the lower stages and nothing would move at all. However it is not being transformed for some reason. The code in the millkins_xyz(now millkins) module is very simple and I don't see what could be going wrong there.

I dunno mate but for the small distances you are trying to correct you really need to use a dial indicator.

may work for you, i used it some years ago and it worked for me.
look at last post..
forum.linuxcnc.org/10-advanced-configura...y-error-compensation

Yeah that looks like it could work, but I could have the same problem all over again, too. It's not clear why millkins doesn't work and the same problem could arise.

I will have a brief look in there with the hal viewer tools whatever they are called and see if I can figure out why it's not working, it's educational anyway and even if I do adjust things mechanically as I said there is usually some that is hard to mop up.

I know you can *detect* very small errors with a dial indicator but the problem is actually correcting them. doing that mechanically requires shimming for this machine which as I said is very difficult time consuming and is detrimental to the machine structure to some degree.

There are also some things that are not practical to correct mechanically, such as the twist of the y axis, for instance.  Software compensation is very useful, ultimately.