How to "offset" Double sided PCB if I don't have abolute "home" on my CNC ?

[Calico]

Watching on pcbmills video, when milling the other side require offsetting X.
the problem is, I don't put any limit switch on my CNC and the "offset" require "home" coordinate.

How can I solve this without installing any limit switch on my CNC ?

is there any other way ?


cal

[Bubba]

I use Pcb-gcode in eagle to make my files. Therefore, I flip the board such that the bottom X0,Y0 is located on the lower left corner of the board and when flipped to to the top, the SAME corner of the board is on the lower left of the board.
So make a fixture to hold the board and then you can locate the origin points for each side. I make my bottom fixture location as G54 and the Top as G55 and thus have two different origins.

Hope this helps.

[Drools]

I'm in the same boat as Calico, I have several double sided designs that I would like to try but when flipped in eagle the image always ends up in the negative range and my router 0.0 position is the limit it can go. If I had my router setup so that 0.0 was somewhere in the middle of the routers range then flipping would not be a problem. Lining up the board perfectly would be the next issue.
Bubba, would you mind maybe doing a brief how to on the process you are using?

[Bubba]

Drools,
Will do, but it will be later today as I have a meeting to go to right now.

[Bubba]

As promised, attached is a pdf of MY system of doing this type of operation.
I hope it helps and if I messed up, let me know and we will do what we can to rectify it.

[Drools]

That does answer a few questions. Thanks Bubba!

[Calico]

Bubba,
that's very simple step G54 ... g55 offset command.

I have tried it and works.

just one little problem, how you pin point the X0, Y0 with the center of the spindle ? witth accuracy 0.1 mm ? or close ?

because I have to do this procedure everytime I mill PCB or the next day when I turn on the computer right ?

the Mach3 can not remember the 2 coord. reference which is G54 and G55, or other words I can not command my spindle to move to G54 or G55 right ?
I hope I'm wrong ?

so I have to type in G54 G92 X0 Y0 and G54 ..... every time I do milling right ?
because I turn off my CNC .

I hope I'm wrong

thank you

[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...