Homebrew PCBs

My T-Tech has the hole and slot.

when I sell it and get my home unit running, I plan on making 
multiple locations so as to spread out the wear on the machine.

Dave




--- In Homebrew_PCBs@yahoogroups.com, ballendo@y... wrote:
> 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