Hi Bob,
It may be because the process of cutting the boards apart is not part of the
board milling process. Yes, cutting holes in boards for through-hole parts is
part of the business - round holes. The fab house for OSHPark won't make oval
holes. They won't overlap two holes, either, nor make square notches. This is
only a guess, but I'd say that this is because they use actual drill bits to
make those holes, rather than a laser. Drill bits are expensive, and it takes
a lot of time to change a drill bit when you've got hundreds of boards to mill.
It's not something I would want to do, either, without a serious penalty for
the customer for a broken bit due to odd shaped holes and missed deadlines.
And OSHPark seems to use different fab houses for the small runs vs the large
(10+) runs. With the small runs, the boards are connected to each other by
small tabs. Often small boards arrive still tabbed together. I don't know
whether they route the distance between these tabs, or use a laser. On the
larger runs, there are no tabs, and the board edges are smooth, as if they were
cut apart in one single operation not part of the milling process.
NB, I don't work for a fab house, nor have I ever toured one.
Bob
From: Bob kb8tq <[email protected]>
To: Discussion of precise time and frequency measurement <[email protected]>
Sent: Friday, April 28, 2017 1:06 PM
Subject: Re: [time-nuts] TAPR Oncore M12+ kit
Hi
If you go with the PCB approach, the nice thing is getting stuff like
the slot for the D connector done at the PCB fab. That way you
have something that drops right in and works. The downside is that
not every pcb house is happy doing that sort of “CNC work”. I
have absolutely no idea why. They all have to run some sort of gear
to cut the boards apart. Cutting slots or weird holes with it is pretty
trivial.
Bob
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