Scott Burns wrote:
>
> What diameter carbide bit would you recommend for PCBs with
> discrete components such as resistors and caps, plus 74 series
> ICs (or sockets for these ICs)? Would the same diameter be
> used for making thru-holes?

I use a #69 carbide bit (.0292") for my IC, resistor, and capacitor
holes. The leads on these devices are typically in the .022" to .025"
range.

For larger leads, such as square leads on LEDs and some types of
connectors, I use a #59 carbide bit (.041"). The diagonal on dimension
on these devises is typically .035".

I drill my boards on a milling fixture, so I can place the holes within
a thousandth of an inch. That means I can get away with holes that are
only .004" to .006" bigger than the lead size. That makes soldering
easier and quicker. I also tends to hold the components in place when I
turn the board upside down to apply solder.

If you're going to freehand drill, especially with ICs, you may want to
use a #65 drill (.035") for the extra clearance.

All my PCB are short, carbide drills with 1/8" shanks. That makes them
easy to mount in and ordinary chuck (or dremel-style machine), and they
can be changed quickly if you need one or two slightly bigger holes. Buy
extra, especially if you're going to freehand drill.

For occaisonal jobs, you can buy individual number drills from most
industrial supply houses (they may have to order them in). These will be
High Speed Steel, rather than carbide, but they'll do several small
circuit boards before they wear out. I find they usually break before
they get too dull. These drills have straight shanks, so they probably
won't fit in an ordinary chuck. You'll need an adaptor, or a pin vise
(tiny chuck, fits in an ordinary chuck).

Bruce