Subject: Step drilling/counterboring

From: [email-masked] (James Kirkpatrick)
Date: 16 Jan 95 23:04:05 MST

I know it's common to "step drill" when drilling a large hole.  That is,
drill a small hole, then slightly larger, going up in steps until it's
the proper diameter.  I've always done this "by feel"; are there any
actual rules (of thumb or otherwise) on the step sizes?

The real question, though, is whether this also applies to counterboring.
I have a piece of steel with a 5/16" hole, and I need to counterbore it
to either 1/2 or 5/8 (not sure which yet), and would like a flat-bottom
hole to a depth of 1.75 inches (of 3.5 total).  That's a fair amount of
metal to remove, and I was planning on using a piloted counterbore to do
this to ensure good concentricity with the existing 5/16 hole.  Would it
be better to first use a smaller counterbore, then a larger one?  I can't
see how to arrange the pilots for a second operation, though, so maybe
just from that I'm stuck.  I plan on using a milling machine to do this
so horsepower should not be a problem.

Another approach would be a careful setup and an end mill but this may
not yield the same concentricity.

Jim

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From: [email-masked] (Mike Rehmus)
Date: 17 Jan 1995 18:40:51 GMT

In <[email-masked]> [email-masked] (James 

>not yield the same concentricity.

According to the instructional staff at the DeAnza College Machine 
Technology center, the proper way to step out a hole is to make only two 
passes. (Plus the center drill, of course)

1. Select the first pass drill to create a hole that is just slightly 
larger than the web of the drill used to cut the final diameter.  Drill 
the hole through with this "starter" drill.

2. Go directly to the final diameter drill (assuming you have the 
horsepower in your press, mill, etc.  

Operate both drills at the proper speed and feed with coolant.

Speeds

RPM = (Cutting speed X 4) / Diameter of the drill

Cutting speed is:
Low carbon steel	90
Aluminum		300
Cast Iron		70
Alloy Steels		50
Brass & Bronze		120

Feeds
Under 1/8"		0.001 to 0.002 in/rev
1/8" to 1/4"		0.002 to 0.004 in/rev
1/4" to 1/2"		0.004 to 0.007 in/rev
1/2" to 1"		0.007 to 0.15  in/rev
over 1"			0.015 and over in/rev

Source of table data:  Richard R. Kibbe, "Machine Tool Practices", Wiley 
& Sons

The general rule of thumb is 0.001" of feed for every 1/16" of drill 
diameter.

Turns out that drilling at these speeds is a brutal act and the chips 
fly and the drilling machine gets a workout.  But it does work and the 
holes end up having a good finish.  I think this "maximum" drilling 
effort is where the machine tool sellers come up with maximum drilling 
capacity specifications.  Probably holes of a certain size at the proper 
feed and speed in mild steel.  Ultimately the limit is the power of the 
machine tool.

I think you should only make one counterbore pass, Jim.

Best,

Mike

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From: [email-masked] (Greg Lewis)
Date: Tue, 17 Jan 1995 21:17:08 GMT

Mike's answer is a good one. I've heard it translated to a rule of
thumb: use a pilot drill 1/4 the diameter of the intended final size.

___________________________________________________________________
[email-masked]
Photojournalism sequence, Department of Mass Communication and Journalism
CSU Fresno, Fresno, CA 93740-0010

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From: [email-masked] (Charles Lamont)
Date: Tue, 24 Jan 1995 00:23:33 +0000

> not yield the same concentricity.

In the sizes you are talking about, the number and size of steps does not
matter a damn, unless you are talking about a production job.  Just do what
feels right.  If you want accuracy and a properly flat bottom (to the hole,
I mean :) you should finish the job by boring anyway, and if it is too big
to swing in the lathe that means using a boring head ideally.
 
-- 
Charles Lamont