[Bubba]

Calico,
Locating the spindle center point can be done several ways, I use an electronic probe or a mechanical edge finder to locate the edges as necessary. Depending on how careful I am, I believe I am within several tenth's (.01+/-mm).
As for doing this process every time you power up, I am not sure, but think Mach3 will remember your settings. However, how sure are you that the machine knows where it is when it powers up? It might remember the offsets and you can set the home point (G53) and be back in business. You just need to try and see what happens. As I said in the beginning, I don't use Mach so cannot comment on that.

Regards,

[Calico]

Bubba,
thank for the tips, I think I will go for the edge finder which is cheaper. nothing cheaper than this right ? but as accurate as this ?

regards
ruddy

[Alfadeko]

I'm not an expert on it, but sometime ago I did a double side PCB, and to save the good reference in both sides I did the next:
- I put 4 vias with a unused diameter ( 3 mm. in my case), in a symmetrical position, in this way:

------------
- O........O -
- ............ -
- ............ -
- ............ -
- ............ -
- ............ -
- O........O -
------------

- I fixes the PCB on a wood base, and mill the upper side and drill all the vias.
- Then, I put 4 calibrated rods on each 3mm. drills, and fix the bottom side with this reference. The position was OK, and I can't remember the accuracy, but it looked perfect.

NOTEs: If you left the rods while milling, take care the tool don't touch them.
Be sure about the symmetry you need in the bottom side G-Code.

I expect it helps you.

[Walky]

Bubba, thanks for the PDF, very illustrative (I know almost nothing about gcode and fixtures, since I'm used to CAM and woodworking). I think it can also be done by using only the left and down fences and setting the zero accordingly, but then you'll need to generate your code based on the the exact Y size of the material so it "fits" the reference space.

BTW, I think the quadrants II and IV on the PDF are swapped.

[martkrui]

Bubba, I just wanted to thank you for your PDF... I didn't understand why the g-code was using the -x range, but you've just made it look really simple how to work in different quadrants.
I'll have some experimenting to do tonight


==edit== After your PDF this makes a lot of sense:
http://linuxcnc.org/docs/html/gcode_coordinates.html

==edit2==
Tested with EMC2. After setting the offset home I had to reload the g-code file. but it works...

[de_structr]

Sorry to bump an old post but I have a couple questions for anyone willing to spoon feed me o.O
When using this method do I need to mirror anything in pcb-gcode.ulp?
Currently, I use pcb-gcode and have it provide gcode for top.etch and bot.etch. Do I need to mirror either top or bottom or just use the default from pcb-gcode.ulp?
For instance, when I mill the bottom of the board can I use the bot.etch from pcb-gcode or do I need to mirror it? Also, default for top.etch or mirror that?
I tried some brds last night and can't quite wrap my head around which side needs to be mirrored.