Homebrew PCBs

Zoran,

There are calculations we used for the various feeds and speeds but 
most of this was done automatically by the machine. What you need to 
know is the chip load for the bit size you are using. The chip load 
is how many mils (.001" = 1 mil) the bit goes into the board per 
revolution. Your feed rate is then calculated by multiplying the chip 
load by the spindle speed. For example, a bit requiring a 3 mil chip 
load at 60,000 rpm equals 180 inches per minute feed rate. 

There are more calculations required to arrive at the surface feet 
per minute for a bit size etc. I don't know how to post these 
equations in this message because they are complex. There is a chart 
with all of this information on it and perhaps I can photograph or 
scan it into the Photos section. The bottom line on all of this is to 
just get your machine going any way you can and adjust the speeds for 
what works and sounds good to you. 

The Excellon drilling machines have a built in software page called 
a "Diameter Page" which contains drill bit size and feed rate 
information. This chart can be user edited to get the results you 
want. The machine looks at this chart and adjusts the spindle 
parameters as needed. These charts are set up in ranges such as .020" 
to .030" etc. Every size in this range will have the same feed and 
speed. If needed, the Diameter Page data could include a feed and 
speed record for every drill size you wanted to enter.

Some CNC drill programs contain the feed and speed data from the CAD 
system and the drilling machine will use that info. The problem with 
this is I never once saw a CAD system that could get even close to 
having the correct feeds and speeds. So we always edited that part of 
the program and let the machine take over. 

An experienced machine operator will always tweak the spindle speed 
while the spindles are drilling until everything sounds right. You 
can actually hear the difference as you change the speed so you just 
keep tweaking until the spindles are "singing". The only time we got 
out all the charts and did calculations was when a customer was 
hanging around. This way they thought we really knew our stuff and 
were getting their money's worth! As soon as they left we went back 
to "winging" it!!  We bent a lot of rules in those days to get the 
jobs out the door. It really was a lot of fun back in the old days. 
Anyone in the PWB business can tell you similar stories.

Tom






--- In Homebrew_PCBs@yahoogroups.com, "Zoran A. Scepanovic" 
<zasto@y...> wrote:
>   Hi all,
> 
>   I'm on the way of making PCB drillin machine, and I would like to 
get some
>   info  on  the  spindle speeds and feed rates for different drill 
diameters
>   needed for drilling FR-4/FR-10 boards.
> 
>   (Maybe Tom has the answer!)
> 
> -- 
>  Sincerely,
>  Zoran A. Scepanovic
>  zasto@y...
> 
> [Non-text portions of this message have been removed]