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