Homing/offsets

Hi all,

I own a 3020T CNC which I use for milling cases. The cases are bigger than what the machine can handle, so I need to apply a different workflow, meaning that I would like to home my milling bit to the correct location after having it manually marked.
This requires me to use LinuxCNC in such a way there are no offsets and that the milling bit goes directly to the correct location, goes down and starts milling. I have browsed the documentation, but I could not really find this (I have to admit that I found the documentation very elaborate but complex). My solution might be linked to the "touch off" buttons, but I do not entirely understand what these do in relation to the homing buttons.
So, in short: what needs to be done to have the milling bit start milling at the set position without having to worry about offset values.

Thanks!

BC

I'm not sure how not using any offsets helps you. I would think you would want to set your work coordinate system relative to your work piece, so that your milling is in the right place. But to make moves such as moving your Z to the maximum travel while positioning, the best thing to do is use a G53 command. Assuming your machine is homed in the conventional manor where Z0.0 is at the top of its travel, and you have touched off the G54 work coordinates so G54Z0 is the surface of your work piece. Since you won't know from time to time exactly how far G54Z0 is from the machine Z0, but you want your program to move to the top of Z travel for a positioning move. Here is how you would code that.

Hi all,

you both stress offsets as being important. Maybe my reasoning is faulty, but when I would be able to place the bit where I want my figure to be cut out, I would think an offset is not important. I do see that LinuxCNC decides that the point where the bit goes down, is not the closest point. With a rectangle the bit travels first to the far corner on the Y axis.
The suggestion about manually creating Gcode is perhaps a good one, as it might make it clearer to me what the rationale is about G53, G54 and the whole homing/offsetting deal is.


Cheers,

BC

G53 refers to the "machine" coordinate system. This is set when you home the machine, and ideally should be fixed repeatable poisoning from day to day, never changing. (Why home/reference switches are important)
G54 and the other G5x coordinate systems are "work" coordinates, and are set when you touch them off. They can be easily moved to suit the part you are milling.

What the actual numbers are in the machine coordinates are generally irreverent when running your code, because you actually run using the numbers in the work coordinate system you are using (G54 is the default one).

These are not just Linuxcnc concepts but general G-code basics pretty standard across most cnc manufacturers. (if you can't sleep older Fanuc control manuals usually have 50-100 pages devoted to this subject, and is very boring/sleep inducing reading)

Thanks all!

So if I home an axis in LinuxCNC this is done with the G53 command and when I touch off, this is done with th G54 command? Can these coordinates not simply be the same in order to match my workflow?

I think I'm a bit spoiled by my 3D printer: there I level the plate so the nozzle barely touches it and n ext I choose a good XY point. The machine then starts printing from that exact spot.

Cheers,

BC

You can, but G53 isn't used for homing, just for positioning in the machine coordinate system, such as commanding an axis to move to your home position (not the same as homing.) It is only effective for the one line of code it is issued in.

It sounds like you are making the rookie mistake of trying to use homing as a tool or work-piece touch off. When you do this it takes away Linuxcnc's ability to use soft limits to effectively bound the machines movements, and you can loose day to day positional repeat-ability. What you need to do is find a way to repeatably home the machine to the exact same position when you start the machine. If your machine doesn't have limit or reference switches, the easiest way to do that is usually to use hard stops at the end of travel of each axis. Most hobby stepper machines aren't strong enough to hurt themselves crashing into the end stops or you can jog close, disable the drives and move the last little bit by hand.

bcnx wrote:

Thanks all!
I think I'm a bit spoiled by my 3D printer: there I level the plate so the nozzle barely touches it and n ext I choose a good XY point. The machine then starts printing from that exact spot.

You can do exactly that using the work coordinate systems, that is what their intended purpose is.

For example if your G-code for milling your part is set up so that if X0 Y0 Z0 were the front right corner of your work piece clamped to your table. You would jog the machine so that the tool is at that position and either use the MDI command "G10 L10 P1 X0 Y0 Z0" to set the G54 work coordinates to 0, or use the touch off button on the gui to zero each axis. Then you just run your code. The machine Home position never changes (and never should).

OK, thank you for the hints. I will test this soonest. I'm happy to have read about the possibility to send individual commands through MDI. Uptil now I was manually jogging the bit with the arrow keys and PageUp/PageDown, while counting my keystrokes. Very crude method of course, but I had to get a piece done. Time to hit the GCode tutorial and enter the commands manually. The manual jogging method is worthless when drawing circles, so I need to get past that.

Cheers all, thanks for helping a rookie,


BC