Drilling Stainless Steel requires lubrication at the drill's tip, and flushing
to eject chip s from the hole. Without fluids, chips can bind in the hole, and average roughness of the machined surface can be twice as high as what is possible with a wet operation. Lubricating the point of contact between the drill's margin and the hole's wall can also reduce the torque required from the machine.
A split-point cobalt steel bit is often used for drilling S/S. The bit’s split point reduces the tendency of a spinning bit to rotate away from the point at which you want to drill (walking). The split point also causes the bit to penetrate the metal more quickly, and with less force than is needed with other bits. In many cases, using a split point eliminates the need to mark the hole with a centerpunch (which may work harden the metal).
Regardless of the type of metal you are drilling or the type of bit you are using, reduce pressure on the bit as it exits the work. This prevents the bit from grabbing the surrounding metal and violently twisting the drill and perhaps your wrist.
If your final hole size is more than about 1/4", start with a small-than-required bit size (pilot hole), and use a slower* speed
, and a lot of pressure. As the hole begins to break through, lighten up on the pressure, and speed up the drill slightly. Then switch to the larger bit size and drill again, using plenty of cutting oil.
Pilot holes should be the same diameter or a tiny bit larger than the web (the centre or area that joins the two cutting flutes of the drill bit) again to ensure that the drill does not wander or chatter. E.g. a 1/2" bit should have a pilot hole of about 1/8".
Use Cobalt bits. Tungsten carbide will last approximately 3 times longer, titanium will last approximately 6 times longer, and cobalt should last 10 times longer than HSS bits
* The RPM
setting for drilling depends on the cutting speed of the material**, and the size of the drill bit
. The RPM
setting will change with the size of the bit. Therefore, to maintain the recommended cutting speed, larger drills must be run at slower speeds than smaller drills.
Rule of Thumb:
RPM = (Cutting speed X 4) ÷ Diameter of Bit
** Generally, the harder the material, the slower the speed of the edge needs to be.
APPROXIMATE Recommended cutting speeds:
Stainless Steel: 30 - 50 fpm
Carbon Steel: 70 - 90 fpm
Brass/Bronze: 100 - 120 fpm
alloys: 250 - 300 fpm
HSS Bit on Left - Split-Point Cobalt Bit on Right (pic "7" below)