Homebrew PCBs

Stefan Trethan wrote:

>
>If you feed slow enough for the RPM, and board and spindle is solid, and  
>there's not too much runout, the drills won't break, of course. But  
>carbide drills are about the most brittle drill one will ever work with,  
>and usually the smallest too, so it takes some attention to get it right.
>
>  
>


  Well, haven't even read all the replies yet.  But runout is not the 
only critical factor.  You can spin absolutely true, and still be 
putting a heck of a strain on a bit.  If your drill isn't 100% in line 
with the motion, and I mean well beyond what most people would probably 
consider acceptable, you'll be flexing the bit like crazy.  1/10,000th 
off is 3" of flexing back and forth per minute at 30k RPM..

  If your bit is say half a thousandth off of where it should land for 
vertical, the tip will cut a hole there.  Soon as that tip cuts a hole, 
the bit is locked in, these things have just about zero side cutting 
ability.  An X narrows the gaps between the lines as you go up, assume 
as you drill that gap narrows to .4 of a thousandth off to where it 
should be, and the tip is locked in way over at .5.  That would average 
to half of a ten thousandth off.  That's 1.5 inches per minute you're 
flexing the drill, and at 500 Hz.  Really it's 3" per minute, 
considering it goes one way then the other as it rotates.  And that's 
with zero runout to start, runout has nothing at all to do with the 
drill being even a tiny bit off line of the motion.  And compared to 
near zero if you get it much closer, which bit will fail first?

  And you're not likely to even begin to see errors of this small 
magnitude.  I can't see it myself.  But for sure, if I chuck up an 80 
bit after aligning by a .025 or .030 first, I can certainly see how 
crappy my alignment on the first bit was by how badly the 80 will flex.  
And go back up after aligning by them, and everything is that much 
smoother and nothing breaks.

  I'd personally think very very few in this group might have the tools 
to align their rigs statically to even come close to what's really 
needed for these bits now that I've done some math on it.  If you're 
just aligning your axis fairly well, and then checking runout, you 
probably aren't even that close to where you should be, at least not 
close enough for very small bits that need to have zero flexing.  It'd 
probably take a laser interferometer to do it reliably with a general 
measuring tool.  The numbers that you think of as great for machining 
aren't even close to good enough for these fine bits to not be seriously 
flexing, even if you can't see that few ten thousandths of error by 
looking at a .030 bit while it's running, it's working very hard on your 
bits.

  But with 32 boards here to round off and do some internal cutouts asap 
things just got kind of busy..  Need at least 6 or so built within a day 
or two now, have to get samples to a couple of people, hopefully before 
Christmas which will be a stretch.  While it's an anxious time waiting 
for them to get here, it's very relaxing to have a pile of excellent 
boards in your hands without having to make them!  :)

Alan