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.

I frequently machine parts that are larger than the travels on my VMC. I use an indicator to locate the front face of the rear jaw on the vise and that is usually Y0. I put a dowel in the spindle and move to the X+ end of the part plus 1/2 the dowel diameter and Z down. I slide the material up to the dowel and lock it down. On my LinuxCNC machines I just set the X offset to a number that makes sense for the part. I make sure there is a "feature" that can be reached in both set ups. That feature is used to set the part to the second set up. Offsets are the key to using LinuxCNC for CNC work.

JT

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)

Don't feel out of place, I thought the same thing when I first started. IIRC I must have asked a dozen times why I could not use the G53 coordinate system till someone explained how it all works.

Here is a good place to start to understand G code programming.