In article ,
Richard Ferguson wrote:
I was going through my Machinery Handbook, and trying to figure out the
optimum drilling RPM. I ended up having to look at one table to get
recommended FPM for the material, and then another table to convert FPM
to RPM. The RPMs seem high to me, so I wanted to run them by you guys
as a sanity check.
For HSS drills in low carbon steel, I read 100 FPM, and maybe half that
for harder steel.
Converting that to RPM, I will list a range of RPMs for a given drill
diameter.
1/4 inch - 1500 to 750 RPM
1/2 inch - 750 to 400 RPM
1 inch - 400 to 200 RPM
Is this anywhere near right?
Pretty close. If you want to convert SFM to RPM precisely, what
you do is:
1) Multiply diameter in inches by Pi (3.14149.....) to get
circumference in inches.
2) Divide circumference in inches by 12 to get circumference in
feet.
3) Divide that figure into the desired SFM to get the needed RPM.
Note that it is common to simplify that a bit by converting
12/Pi (3.8197) into 4 as an approximation, so you divide the
diameter by 4 to get an approximate circumference, and divide
that into the SFM to get an approximate RPM (close enough).
As I keep a good pocket calculator handy -- one which includes
Pi built in, I do the more precise way, as it is almost as
quick.
If so, I need to change the pulleys on my
drill press, as I drill a lot of holes in the range of 1/4 inch to 1/2
inch, maybe set it to 750 RPM or so. It is a pain to change, so I want
to pick a compromise value.
Note that these SFM values (and the resulting RPM ones) are
considered maximum speeds for a good tradeoff between tool life and
number of parts machined per hour. You *can* run much slower, and the
hole will just take that much longer to drill.
The other question relates to pilot drills. I did not find anything in
my Machinery handbook on pilot drills. Does it make any sense to use a
pilot drill for finished hole sizes of 1/4 inch or less?
Not normally -- and even less if you have a set of split-point
drill bits. The primary purpose of the pilot drill is to eliminate
having to force the chisel tip of the drill bit (the blunt line at the
center) into the workpiece. When running a 1" drill bit into a
workpiece on a lathe or drill press, that chisel tip takes a lot of
force (and thus is more likely to bow the drill sideways and enlarge the
hole). If you make a pilot hole with a much smaller bit (just a little
smaller than the length of the chisel tip) the force needed goes way
down, and the job gets easier.
Of course, the same could be said for any size drill bit, but it
gets rather insane to pilot drill for a 0.100" diameter drill bit. :-)
If so, what is
the rule of thumb for pilot drill sizes?
As above -- just a little smaller than the length of the chisel
tip.
Do I need to change RPM
between the pilot hole and the final drill size?
It would make the drilling faster -- but might take you longer
to change the speed than to drill at the slower speed. Mine is pretty
quick to change, so I tend to change when it feels right. (I probably
drill too slow most of the time, anyway. :-) Some of these days, I will
pull the single-phase motor from that drill press, and pop in a
three-phase with a VFD to allow me to change speeds more easily (just a
twist of a knob, unless I need a really big change, as when going from
drilling a 1/16" hole to a 1" hole), so it will be done more often.
Obviously, with the really big changes, I will still need to change
belt settings.
Assume that most of my drilling is in low carbon steel of 1/4 inch to
5/8 inch thickness.
With other materials, of course, the SFM changes (and the
resulting RPM), but nothing else does.
Enjoy,
DoN.
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