Hi Chris,
Thanks for the suggestions. Your method sounds pretty good. The
boards I was complaining about were stretched so much that this
technique would not have helped. I drilled the holes by hand, and
only then realized that the sockets wouldn't fit in the holes. Dumb.
I've seen several people post about making large quantities of
boards. I'm really curious what kind of boards you guys are making.
All of my boards so far are for personal use. Most are one-offs,
although there are a few, like stepper motor driver boards, I've made
in multiples.
Kevin
--- In Homebrew_PCBs@yahoogroups.com, "Chris Horne" <chris@...> wrote:
>
> Kevin,
>
> the issue with the prints stretching is a real one but as you say
only
> really critical when using a CNC drill
> I use a CNC'd micro mill for most of my drilling now and quickly
> discovered this.
> The best mitigation I can suggest is to index near the centre of the
> board, that way the errors are halved to each edge. It can make the
> difference between being in the pad or outside it..
>
> First off I always have a pad at the centre or near the centre of
the
> board...
>
> Then I have an MDF board fixed with double sided tape to the mill
table.
> I hand drill two of the pcb holes by hand about 1/4 of the distance
> from the side of the long edge of the board.
>
> I then align the first board on the MDF and push a couple of broken
> off drawing pins into the holes.. they then act as indexing pins
for
> subsequent boards.
>
> To hold the boards to the MDF I use a couple of old drawing board
> spring clips with the rounded edge cut off... They slide easily
under
> the MDF in the gap left by the thickness of the double sided tape.
The
> advantage of these clips is that they clamp firmly, are very low
> profile and very very quick to change boards.
>
> I generate the G-code for the drilling with the origin on the centre
> pad of the board. I often measure between the two pads which are
> furthest from each other on the actual resist layer on the boards
and
> add a correction factor so that the G-code is generated to be the
> average size of the board... it has been up to 2% out on occasions.
>
> I load up the G-code zero the drill in on the centre pad, set the
> origin to the centre of the actual board.... and awayshe goes...
>
> A bit of a performance but gives good repeatability..
> I do runs of 50 or 100 boards without having to re-zero anything..
> and I keep the mdf for the next run of that board.
>
> One of the tasks i keep reminding myself to do is to wire up a
decent
> buzzer to the driver board and get mach3 to sound the buzzer when
the
> board is finished.... I tried getting mach to play a wav file but I
> run the machine unattended and need the alert to be in the house,
not
> the workshop...
>
> I must admit to still using and Iron, I haven't got around to using
a
> laminator but I reckon if it aint broke why fix it..
>
> Chris (-=spiyda=-)
>
>