Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

We always drilled a maximum depth of five times the drill diameter.
This was the general rule of thumb for carbide bits. Most of the time
we drilled a stack of panels three deep and only one deep if they
were multilayer boards. We sharpened our bits only two or three times
and then sold them off. Most bits need to be resharpened after about
2000 to 2500 hits.

When drilling multilayers we always used new bits and never resharps.
Our bits were used first for multilayers, resharpened and then used
on two layer panels for a couple of cycles and then junked. Many
shops now days are only doing multilayer work so they put out tons of
bits that have drilled 2500 or less holes. These are the bits to get.
For single sided and hobby boards you can just about drill until they
break although the dull bits will make a larger burr.

The original auto tool changers were "pods" located along the front
of the drill table. The idea was for the machine to dump the bit and
then pick up the next one etc. They almost never worked correctly and
many shops just put their machines in a manual tool change mode. Many
newer machines have large capacity "cassette" type tool holders and
these work pretty good, much better than the original pod type.
I found that the machines would break more bits by crashing them into
the tool pods. The bits almost never break during drilling.

In the "manual" mode when a tool run was complete the machine table
would move to the "home" position and stop. The spindles had a
dynamic braking circuit to stop them in about four or five seconds.
These spindle bearings were so good that a new spindle might spin for
two minutes after the power was removed so the braking circuit was
needed.

The spindles were driven by a three phase frequency converter and the
dynamic brake circuit put 7 volts DC across one of the windings to
stop the spindle rotation. All of these spindles used .125" collets
and the plastic ring on the bit was used to set the depth. These
rings were pressed on so that the tip of the bit to the top of the
ring was set for .81". The Z axis of the machine could then be set
for a precise drill depth. Hope this info answers some of the bit
questions.

Tom