Homebrew PCBs

Hi Jan.  How did you account for backlash in your homebrew CNC rig?  What
about those PCB drilling machines, what do they do for backlash and other
error components?

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>Particularly if the bit is not NEW-sharp!
>  The particular board which needs drilling is actually for the mechanical=
>=20
>part of the project.  It has Thousands of holes, several hundred per square=
>=20
>inch.<<
>I did a little thing like that with MY (Home-brew CNC PCB drill) and did=20
>holes on 0.05" centers in acrylic.  It was very clean; no melting.  I think=
> the=20
>little piece of plastic was about 0.03" thick.  I stuck it down with "Walls=
>aver"=20
>poster tape (double-sided stickum by 3M).  My machine would do about one ho=
>le=20
>per 1.25 seconds, including fairly near traverse.=20=20
>  I've never been to a PC=3Ddrilling operation.  How fast do they drill out=
> the=20
>boards?  I'm looking at buying some kind of CNC machine sistem for light=20
>work: something like the stuff from Taig Tools or Prazi.<<
>The machine I saw had four "quills" turning at about 100,000 RPM (the=20
>"motors" run on compressed air and literally SCREAM as they spin!), and the=
> "peck=20
>rate" is 4 to 5 holes PER SECOND! (for 0.1" IC-pin-rows; more for "scattere=
>d"=20
>holes).  These have "stacks" of one to seven "blanks" under each quill.  If=
> a bit=20
>breaks, that quill will stop and ask the operator if the others should=20
>continue, or shall the one with the busted bit "go get a new bit" and all c=
>ontinue.=20=20
>All automatic.  The danger here would be that the busted-off piece would be=
>=20
>still in the way, so a new bit would possibly get busted, too.  It is wild =
>to=20
>see four quills all in a row stop, go back to the "start point", "dump"thei=
>r=20
>bits in the original holders, move over, "grab" new ones, and whiz back to =
>work,=20
>loosing maybe 5 seconds in all.  Bang, bang, bang, at a rate of 4-5 "pecks"=
>=20
>per second.  Machines like that cost over $100,000, yet pay for themselves=
>=20
>amazingly soon!  Now, this was over ten years ago.  That machine used 5=BC"=
>=20
>black-cover floppies to transfer the programs from the "office" out to the =
>machine.=20=20
>Today, I don't know.  Perhaps 3=BD" floppies the customers bring-in?  And,=
>=20
>drill-schedules "from the office", if still done that way, surely via LAN! =
> I have=20
>not had reason to "visit" there in 20-25 years, so I have NO clue if they e=
>ven=20
>still operate.  At ONE point during these years I have "been gone", their=20
>front office called to inform that they "now did multilayer", so I could br=
>ing my=20
>work to them!  Hadda tell the lady I once did ONLY 2-sided PTH boards with =
>NO=20
>silk-screening, no solder-mask (which they LOVE to do, as there is MORE pro=
>fit=20
>in those "nice details" than the boards themselves!).  So, maybe they are=20
>still going strong.=20=20
>
>Jan Rowland
>
>
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