On Fri, 13 Oct 2000 10:22:22 -0400, you wrote:

>How's this:
>
>1. Build a table that can move a PC board in one dimension in 1/300"
>increments with no cumulative error.
>[Old plotter anybody?]
>
>2. Create a UV LED bar containing LED's on 0.1" centers. Over the LEDs =
is a
>mask containing 1/300"+ holes on 0.1" +/- 0.0005" centers [this is the
>tricky part :-)]. The holes need to be a bit oversize so that =
consecutive
>stripes will overlap a bit.
>
>3: Mount the LED bar over the PC board on the table. The mask can almost=
 be
>in contact with the board.
>
>4: Attach the LED bar to an indexing mechanism that allows it to move
>perpendicular to the direction the PC board in 1/300" +/- 0.0005 steps =
(with
>no cumulative error).
>
>Now, to plot the board:
>
>repeat until done:
>
>Run the board from one end to the other.
>Index the LED bar one step.
>Run the board back.
>Index the LED bar one step.
>
>I figure you should be able to move the board a good 1" (300 steps) per
>second.
>Reversing direction should be about 1/10 of a second, which should give =
you
>plenty of time to index the  LED bar.
>
>It only takes 300/0.1 =3D 30 passes to do the entire board.
>
>Thus, a 5" board would take about:
>
>30 passes/board * 5 " * 1"/second        ; for scanning
>+ 30 passes/board * 0.1 second           ; for reversing
>
>This comes to 150 seconds + 3 seconds =3D 153 seconds.
>
>That's pretty fast.
..erm not much faster than normal laser-print plus UV expose..... and
rather more expensive.

Oh, and 300DPI is not enough. You need 600dpi for decent quality - the
reason being that the normal PCB routing grid is 0.025" , i.e. 1/40
inch. 600 divides by 40, 300 does not, so you get uneven line widths
and clearences.

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