"Richard Ferguson" wrote in message
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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?

Yes, in fact, quite accurate. You'll get good performance within those
speeds for mild steel. Do keep an eye on the sharpness of your drills,
however. Dull ones will cut quite hot, whereas sharp ones will cut cool.

One tip-----the color of chips coming off your drill are a great indicator
of a desirable speed. If you chips come off steel colored, you're likely
running too slow. Look for chips to come off ever so slightly yellow when
you drill dry. Anything more than that, you're going too fast. By the
time chips are coming off blue, you've likely already damaged the cutting
edge from speed and heat.. When your chips come off pale yellow when
drilled dry, the drill should do quite well when you lubricate it with
cutting oil.

Pilot drilling:

If you expect a ¼" drill to cut size, it's good shop practice to open the
hole using a smaller drill first. Generally one chooses the next size down
when using fractional drills, so use the same rule of thumb for number or
letter drills, meaning you'd choose a drill approximately .015" smaller.
Not only will the finish drill cut size that way, but the hole will
generally be a much nicer one with fewer irregularities. Needless to say,
lubricate the drill when you're drilling. Even an acid brush dipped and
wiped will be quite beneficial as opposed to nothing. Flood cooling works
even better, but it's messy and not many home shops have a drill press so
equipped.

Harold






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.

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? If so, what is
the rule of thumb for pilot drill sizes? Do I need to change RPM
between the pilot hole and the final drill size?

Assume that most of my drilling is in low carbon steel of 1/4 inch to
5/8 inch thickness.

Thanks in advance.

Richard