Tony,
Commercial PCB mills (LPKF,T-Tech,etc) use two 1/8 dowel pins to
locate the board on the mill. T-tech uses a 1/8 hole and 1/8 wide
slot, while LPKF uses plastic "wavy" inserts into a larger slot. The
plastic wavy insert has the hole for the 1/8 dowel pin. The wavy-ness
allows it to "spring" against the table slot sides.
Anyway, these "fixture" or "registration" holes are programmed, same
as all the others. And drilled first.
This is also the way to get multi layer boards lined up. The holes
may remain in the finished board, but commercially they are usually
cut off when the board is sheared to size.
Both these mfrs. recommend the holes be in the PRECISE center of the
board i.e., splitting it into two equally sized "halves", this way
you can use the "flipped" 2nd side layout without any offset.
At first, this DOES make things easier. But before long, you'll
probably find yourself using any convenient location for the two 1/8"
holes, and drilling into a waste board mounted onto the table to
accomodate their position for both sides (means you may use 4 holes,
for the two pins) As long as your PCB mill zero aligns with your
board zero, things are gonna be fine...
Note that if you duplicate their center of the table pcb mill slot,
or slot and hole, you will HAVE to keep the 2 holes in line with each
other, or you'll have rotation of the layout to deal with. (But they
don't have to be in the center. If they aren't, you'll have to use an
offset to account for the new board position when flipped.)
When I first started making CNC PCB mills, I decided NOT to use a
slot. Because if you have a slot, then the linear travel around that
part of the machine sees all the wear. Using sub tables (You need
entrance and exit material ANYWAY, to do a good job of drilling!),
you can move things around to spread the wear, and thereby maximise
the life of your machine.
BTW, Some pcb mills DO account for rotation. Which makes for an easy
setup. You mount the board, then locate the pin positions, and the
sofware rotates the layout to fit where the bd is ACTUALLY located.
These are usually high end PCB drills/mills which have vision
capability to "find" the pins accurately.
Hope this helps,
Ballendo
--- In Homebrew_PCBs@yahoogroups.com, "kg4wfx" <tony@e...> wrote:
> Hi all,
>
> I'm trying to figure something out I know anyone who has been
> milling for a while probably already knows....
>
> How do you line up the two sides of a double sided pcb when
> cncrouting it?
>
> And also how to make a bigger cut out of the copper? I'm using
> 60deg angle bits, which when cutting the copper opens a very fine
> area between the trace and the rest of the copper - this is fine in
> many situations, but there are times where I want larger spaces
> between traces or trace and remaining copper, so I'm sure there is
> another bit I should be using for it, but I don't know what =)
>
> -Tony