Homebrew PCBs

----- Original Message ----- 
From: "Alan King" <alan@...>
> 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

:) Good one. No doubt you're right for something that small and into a 
larger chunk of something more substantial. For 24 mil holes in 1/16" FR4, 
though...

I'm having trouble forming complete thoughts this moment; running off a 
fever here in winter wonderland. Here's a dump of what flickered across the 
forebrain on reading your thoughts.

Something very tiny will break before it can cut sideways, while something 
rather larger will cut rather than break. Somewhere in between is the 
continental divide between "woefully fragile" versus "bulletproof". The 
threshold size is smaller for: faster cutter speed; thinner material; 
smaller angular misalignment; slower feed rate;  softer material; sharper 
tool edge. Small is good; it's the smallest hole your tool can drill instead 
of break. I'll put forth the notion that, various reasons, the size is 
rather sharp defined, and more or less fixed for a given tool. #73 is as 
small as I care to drill, and is on the bulletproof side of that line on my 
drill.

Flexural stress varies as the cube of the diameter for small deflections. A 
#73 is about twice the diameter of your #80. So, considering geometry alone, 
it sees about 8 times less stress for the same misalignment. It is also 
stiffer, requiring more side force to deflect a given distance, and so has 
stronger cutting action.

3" per minute sounds significantly big, but to what do we compare it? A #67 
bit at 30k rpm is moving two flutes 500 fpm on the surface. 3 ipm is next to 
nothing in comparison, if comparison is reasonable.

Radius of gyration, and buckling limits... It hurts my head to think about 
right now, but is very likely the physical failure mode.

Resonance... Definitely significant if present (but similarly impossible to 
frame into a meaningful thought this moment; will try again in the morning).