[HomeCNC]

Don,

This is the way I run all my CNC machines. I have a CNC Mill as well as a CNC router. You must use fixed tooling for this to work. I use Home switches because this is one position that is somewhat repeatable. When you home your machine you are setting the machines coordinate system, which is G53. For the record a coordinate system is a spot that the DRO’s read 0,0,0 I always home in the far positive direction.

After I have the machine coordinate system defined, I then move to the material corner (I use Lower Left) and you can find this by edge finder or other methods. What you are doing here is finding the coordinate offset position as measured from the machines coordinate system. In Gcode standard you can have many different coordinate offsets defined. Machine 0 is G53. The first offset is G54, second is G55 and so on. By finding the edge of the material in X and Y you will have a negative number for both. Next I find the Z position of the stock. I do this with no tool in the spindle or router. I move the Z down until I touch the stock top. This will be a negative number as well. You now have three negative offset numbers for X, Y, and Z. If your Gcode controller is good it should have a place to enter the coordinate offset for G54. You enter the three numbers here.

I get my tool length by clamping a tool in the holder and placing it in the spindle. I move the Z down until I touch the top of the part. I read the Z dro and subtract this number from the Z offset number I got above. This will give you the tool length. You enter your tool numbers in the controllers tool offset database.

Now when you have this data you just make sure that your Gcode has a G54 at the start and a G43 for the tool length offset. When you switch your CNC from machine coordinate system and use the offset your DRO will now read 0,0 when your are at the corner of your part. When you turn on the tool offset and place the correct tool in the spindle your Z DRO will read 0 when you touch the top of the material.

The nice thing about doing it this way is: 1) if you machine off the top of the part you will still be able to do tool changes and not loose where the material was. 2) if you shut down the machine for the night you just home it again and you are ready to go.