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Downwind

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Joined: 09/09/2009
Location: Australia
Posts: 2333

Posted: 12:03pm 02 Jan 2010

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For some time now i have been trying to track down a gremlin with my home made little cnc machine that shows it ugly head when drilling pcb's.
I would get a slight miss alignment of the holes to the pads at times but if i sent the machine back to a earlier drilled hole it was still spot on, so not missing steps, and at times it would go back to spot on after a few holes of miss alignment. It made no sense.
The decoding program used for co-oords to G code was checked and no reason for this could be found there.

After watching it drill a board with over 1000 holes in it i noticed a pattern of of miss alignment as it finished one section and started another, even though the board was 6 identical circuits on one sheet.

Then the penny dropped it was not the cnc machine it was the printer causing the problem.
As i print the artwork off and use positive resist board any printing errors are duplicated onto the board.

A quick check by printing two identical matrix images and reversing one on top of the other showed not all lined up the same.

There is little i can do about the problem but aleast i can stop looking for it.

If i send you a board some time where the holes are slightly miss aligned to the pads then dont blame me, blame the crap engineering in printers now days.

Pete.Edited by Downwind 2010-01-03

Sometimes it just works

Tinker

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Joined: 07/11/2007
Location: Australia
Posts: 1904

Posted: 02:38pm 02 Jan 2010

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Pete, reading your message I notice that all the dots are precisely aligned above the i's so your printer is doing its intended job

I think you need a X/Y plotter (do these still exist?) for this job

Klaus

Downwind

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Joined: 09/09/2009
Location: Australia
Posts: 2333

Posted: 03:21pm 02 Jan 2010

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Klas,

Not sure what you mean with the i's and dots.
The problem is the printer dont always print exactly to scale, as the artwork is seen on screen.

When i generate the drill code the co-oords are taken from Expresspcb as a DXF file and is 100% to scale as seen on screen.

The machine drills correctly in the right spot but the pads on the board are sometimes not exactly where they should be. (The pad is out not the hole)

The worst i have had is a 0.5mm difference but this might be for 6 holes out of 200.
I have stopped and reset the tool position before, only to find then i can be 6 holes spot on and 194 holes out.

It zips around at a reasonable pace and the hole is drilled in a blink of an eye, and you tend to notice after and not prior to it touching the board with time to stop it.
Its not a big deal as every thing fits and solder covers up the mistake well.
It just looks no so good and has bugged me for some time why this was happening.
Still beats the hell out of drilling them by hand.

Yes a X,Y plotter would solve the problem which is all a cnc machine is anyway.
A printer like Gizmo operates would also solve the problem if i could pay the power bill for it. $$$$$

Pete.

Sometimes it just works

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