Which pillar drill for drilling thick mild steel angle

[ell81]

On 17 January 2026 at 11:11 ell81 Said:

…

The thicker the steel and the larger the diameter, the slower the RPM right? Or is it only the larger diameter that affects RPM speed?

…

Worth expanding on this, in case ell81 is worried about the range of RPM answers we’re giving.

Mine is the lowest recommendation, about 500rpm, Robert the highest (1000rpm).   Cedric says 900, Noel 750  and  Larry 600.  We’re not wrong, or disagreeing because the answer depends on the tool and on how fast you want to work.

• I recommend 500rpm because my small pillar-drill is on the flimsy side and the motor only has a 50% duty cycle.  I also buy mid-range twist drills and extend their life by cutting metal relatively slowly, which keeps them cool.   I’m rarely in a hurry, and favour the machine.
• 1000rpm is right when getting the job quickly is important.   1000rpm is no problem on a rigid pillar drill with a more powerful motor than mine.  The main disadvantage is that high rpm wears twist drills more rapidly, though that can be counter-balanced by buying expensive and cooling with cutting fluid.

Metals have a preferred cutting speed, which is a combination of two factors.   They are how fast the drill is spinning, RPM, and how fast the cutter is pushed into the metal (feed-rate, or depth of cut).   In practice, I think we all set the RPM first, and then tweak the feed-rate to suit.  Just keep within the metal’s preferred cutting speed and the capability of the machine.

Preferred cutting speed varies with what the drill is made of.   Carbon-steel twist drills should be spun slower than plain HSS and coated HSS can be driven faster.  Carbide much faster than HSS.    Cheap twist-drills are likely to be carbon-steel, best avoided I suggest, even for woodwork.   Carbide drills are expensive, and best spun faster than most pillar drills can do, so bought for special jobs.  HSS is almost ideal.

The maths! The recommended cutting speed of mild-steel with an HSS cutter is in the range 20 to 35 metres per minute.   Depends on the exact alloy, and isn’t critical.    The formula for calculating RPM is:

Where V is the cutting speed,  D is the tool diameter, and π is 3·14.

As a cutting speed of 30 metres per metre is about right for HSS into mild-steel, I use the approximation:

RPM = 10000 / D

Example:  for a 13mm drill, 10000/13 = 770rpm

which I reduce by about a third to suit my weedy pillar drill and inexpensive twist drills, roughly 500rpm.   Anyone doing a few holes in a hurry, or owns a beefier pillar drill than me, can increase RPM by a third, hence 1000rpm.  Or sit in the middle at 750rpm – that’s OK too!

I find RPM = 10000 / D works well as a first approximation but it’s a rule-of-thumb, not the law   Slow down if the machine labours, speed up if the cut goes well.  Slow down to extend tool-life, speed up to remove metal faster.

Dave

[Author]

Posts