Question for you pros out there. I have an Avid 48x48 and a customer wants to see if I can drill 4 precise holes in 400 thin sheets of metal. I’m not sure of the exact size yet but he said it could be done on a drill press. It got me wondering if I could use a standard metal drill bit in my spindle and just change the speed depending on the bit size used. Just thought this might save time and not have to line it up every time on the drill press. Can anyone advise if this is possible or if it should not be done for some reason?
Not sure what spindle you have and if it will go slow enough. The drill bit speed might be an issue. I have a drill chuck with a 1/2" shank that I use.
There is an rpm lower limit in Mach4. Some people have lowered it to get the RapidChange ATC working. See here:
The spindle will lose torque at lower rpm, but it should still work fine for small diameter bits.
You might want to look at making/getting some kind of pressure foot:
I have the 4HP spindle
Yes, you can drill nicely with a CNC. I did a video on using standard twist bits https://www.youtube.com/watch?v=bA_ZyFDdtF4&t=295s
You will need to change the limits in both Mach4 and the VFD for your min speed. I’ve run both the 3HP and 4 HP spindles down to 500 rpm, but the smaller bits will work fine at a couple thousand.
For metal you probably don’t want to use a standard twist bit because it will tend to wander without a punch mark and the CNC isn’t very forgiving when it does that 
Use a center drill bit like below and it should work fine since its only sheet metal it should be long enough to go though. It will also debur the top side for you if you set the depth correctly. A pressure foot like Eric suggested will also help if these are big sheets that are not very flat.
