Homebrew PCBs

In a message dated 5/7/2003 6:36:46 AM Central Standard Time, 
spoothammer@... writes:

> Is Grizzly a good source? Tool steel or carbide? 
> 

Bob:  I have been reading mail from latest-received, on, so I just got to 
your good question:   HSS ("tool steel") drill-bits will last perhaps a dozen 
holes in G-10 or FR-4 (glass-fiber-reinforced PCB material) before they are 
so dull they will not work anymore.  IF you could find some tiny "mill-bits" 
sufficiently small for pattern-routing/milling, I am sure they would go dull 
so fast that you'd soon assume they "CAME dull from the source"!

The ONLY thing that works sufficiently well on "glass epoxy" (the light-green 
PCB-stuff) is TUNGSTEN-CARBIDE.  Professional PCB drill-bits from, what, #80 
up to perhaps 1/4" dia. all have 1/8" shanks so that the auto-load PCB-drill 
machines can handle all kinds of drill-sizes efficiently (i.e., "fast!").  

I have seen solid-carbide "router bits" in various mail-order catalogs, but 
never having used them continuously every day, just buying a few "to have on 
hand if ever needed", I can't now tell you from whence!  Check catalogs of 
MPJA, JDR Microdevices, Enco, Harbor Freight, and of course, those I have 
forgotten at the moment!

I have seen TWO forms of these bits for PCB-work for EDGE-milling:  The 
"fluted" kind, similar to those used for metal-machining, with two, three, or 
four flutes, some parallel, and some "spiral", both "up-cut" and "down-cut".  
And a second kind, the "rotating file surface" kind, that is, with a kind of 
"knurled" surface that kinda "abrade" the material away.  I can NOT compare 
the function of the two, as I have never used either for PCB work (well, I 
DID use a two-flute router-bit, once, to mill "key-slots" between the 
gold-plated edge-connector fingers).  I assume you realize this latter kind 
must be rotated at least at 25,000 RPM (better:  100,000 RPM or more!) to be 
effective.  The "feed rate" for the mill-bit type would have to be such that 
the linear move per revolution would be no more than about 0.002".  Thus, for 
any rate at all, you'd have to spin the bit at least 25,000 RPM!   I have no 
clue what kind of TIP-cutting this "file-type" have, if any.  

For the "mill-bit" type, you should use "center-cutting", as the bit will 
very often (mostly!) have to be "plunged" onto the blank, rather than 
"starting from the edge".  It may-well be best to use only "two-flute 
center-cutting"!

Yes, I realize bearings and motor-types for quills become a problem for 
avocational purposes even at that 25K speed!  The price of good air-motors is 
outrageous, and high-frequency AC (electric) motors cost fortunes, when the 
400 Hz or whatever power-source is considered.  (I did that in '84, and it 
works (still!) wonderfully, but "never again"!)   A "laminate trimmer router" 
by Hitachi or Milwaukee is a possibility, using a 1/8" collet-adapter, but 
such a machine is somewhat cumbersome to change bits rapidly and carefully as 
required by delicate carbide drill-bits!  Also, the EMI generated by one of 
those "universal motors" would probably be a serious problem for 
computer-controlled stepper-motor drives!  UGH!

It almost seems that some "ol' boy out there" in the machine-manufacturing 
business might do well to design/offer a neat little 100-W. air-motor with 
air-bearings, a 1/8" collet, and a design-RPM of, what, 100,000 RPM, running 
on 90 PSI, 4 CFM "shop air" for $100 each, not including compressor!    (is 
90 PSI @ 4 CFM about equal to 100 Watt????)

Discuss?    Jan Rowland



[Non-text portions of this message have been removed]