Drill Number / Decimal Cross Reference
Drill Letter / Decimal Cross Reference
How to choose the right drill speed
Quick Summary: Some approximate starting advice is to set the spindle speed between 700-1000 rpms for steel, above 2000 for aluminum, and slow down from there if you get discolored chips or heavy drill bit wear. In most cases the drill press will not be able to supply enough power or speed to follow the below recommendations. A 3/8″ drill bit drilling mild steel at the recommended speed and feed could require around 1 hp. Going slower usually doesn’t hurt, and will prolong tool life.
Drill press spindle speeds depend on lots of things: the type of material being drilled and its hardness, the hole size, the type / hardness of the drill bit and its sharpness, whether or not a cutting / cooling fluid is used, and the rigidity of the drill and clamp, among others. Also, most speed recommendations are geared towards manufacturing environments where machining time is very expensive. As drilling speed increases productivity goes up, but tooling also wears out faster. The recommendations seek a balance between these two concerns, but this balance is not determined with the pocketbook of someone running a hobbyist or prototype shop in mind.
So, for the hobbyists shop, where longer tool life is probably more important than machining time, and where pushing the speed limit may ruin a valuable prototype, reasonable advice might be to start off at about 75% of the recommended drilling speeds. The “First Guesses” below are already a little slower.
You’ll typically see large ranges of recommended speeds for various materials, and some discrepancy between different sources. This is partly due to the large influence the material hardness has on how fast it can be drilled (harder –> slower). Even if the material and its hardness were precisely known, the large number of other factors would require some experimentation. If the chips are smoking, turning brown, or the outer edge of the drill bit is chipping, go slower or add some cutting oil / coolant. (a decent guide to cutting fluid)
In general, a slower-than-recommended spindle speed won’t hurt anything except in the case of extremely small drill bits, say smaller than 1/16″. With small bits, it’s hard to feel resistance from the metal, and therefore, very easy to push down faster than they can remove metal. Using recommended RPMs (spindle rotation speed) mitigates this risk. A tip for drilling extremely small holes is to drill down to the depth stop, and then move it down a 16th of an inch, and repeat. This ensures that too much metal isn’t chewed off too quickly.
Feed Rate: This is how fast the drill bit is pushed down. For reference, the recommended rates go from .001″ per revolution for bits under 1/8th” to .007″ per revolution for 1/2 bits. This, of course, isn’t very useful if you’re lowering the drill bit by hand. In general, push hard enough to create a continuous chip (note some materials just won’t form one–like cast iron), but not so hard that the chips are turning brown or the bit itself is chipping. Slight discoloration of the chips is OK. Don’t push as hard right when the bit is about to break through, this will reduce the likelihood of it grabbing and tearing the metal.
Surface Feet per Minute (SFM): Speed recommendations are usually given in SFM, which is the speed a cutter can be pushed in a straight line. On drill bits, the fastest cutting rate is at the circumference, and its rate of travel is equal to the RPM (Revolutions Per Minute) of the bit times the circumference (pi*Diameter). So, the translation between SFM and the RPM speed of a drill bit is:
Rotation Speed (RPMs) = (3.82) * SFM / Dia. SFM = (.26) * RPM * dia.
|RPM recommendations based on First Guess Speeds
for various drill bit diameters
|1/8 (.125″);||1/4 (.25″)||5/16 (.3125″)||3/8 (.375″)||7/16 (.4375″)||1/2 (.5″)|
|low carbon steel, up
to 275 Brinnel hardness
|high carbon / alloy
steel, up to 275
|aluminum and alloys||200-300||250||7639||3820||3056||2546||2183||1910|
|brass / bronze
high strength bronze may
require 70 or less
Some considerations for using the above table:
- First, note that the speed recommendations for small bits in aluminum are ridiculously high compared to the max speed of around 3000 RPMs on bench-top drill presses. These numbers are more just for reference, it’s fine to go slower, just don’t push too hard on the small bits.
- “First Guesses” are based on more typical materials and hardnesses.
- If the material has been hardened, the recommended speed will be substantially lower. If the hardness is above 300 Brinnel, starting at 20-30 SFM isn’t a bad idea.
- Note that these values are recommended for HSS (High Speed Steel) drill bits, not carbon steel ones. HSS gets its name because it is able to maintain a reasonably hard cutting edge even while it is red hot. If you’re using carbon steel drill bits (unlikely), cut the recommendations in half. If you’re using carbide tool bits, up the speeds by 2 to 3 times.
- Adding cooling / cutting fluid may allow speed increases, too, and should be used in any case on steels. Cutting fluid will almost always increase the quality of the cut.
- If the hole is more than 3 diameters deep, consider drill as much as twice as slow since it will be much harder for heat to escape.
- Other site’s speed recommendation tables (note the differing opinions!): here | most comprehensive | Wilton drill press instruction guide (very good)
Finally, make sure the machine is spinning the right direction. Mills are often left in reverse…