Newsgroups: rec.crafts.metalworking
Subject: Re: drilled array
From: frisbie@flying-disk.com (Alan Frisbie)
Date: 26 Jan 95 10:02:40 PST

In article <3g0c7e$q46@moe.cc.emory.edu>, 
dwagn02@curly.cc.emory.edu (Richard L Wagner) writes:

> I need to drill some arrays of small holes, .021 in diameter, 1 mm apart.
> I'm using a driller with an old vertical mill. Although the holes are 
> going ok, the pattern of 4 x 14 holes looks bad since there are apparent 
> errors in the table positioning. I'm always working from the same 
> direction on the table screws so I imagine it's basic inaccuracies 
> involved. What kind of positioner can I use to get accurate hole placements.?

I suspect that your problem is not positioning accuracy, but
your small drill bits "wandering" as they contact the work.
For accurate hole locating, you should first "spot" the holes
with a spotting (centering) drill.   This is similar in concept
to a center drill (typically used in lathes), but is designed
for use in mills and drilling machines.

Spotting drills are very short and rigid, and are made with
angles of 90, 118, and 120 degrees.   They have a very narrow
chisel edge and a very small web to ensure accurate starting.
90 degree spotting drills are recommended for use with drill
bits up to 118 degrees, but some people prefer 118 degrees.

In use, you will set up the machine's depth stop so that the
spotting drill will penetrate just enough to leave a hole of
the desired final diameter.   If you use a 90 degree spotting
drill, this is very easy to calculate (diameter is twice the
depth).   Also, if you go just a bit deeper, you can chamfer
or countersink the hole at the same time.

When you then follow this with the desired drill bit, the
outside (cutting) edges of the drill will contact the work
first instead of the web.   This will ensure that the drill
starts exactly in the desired position.   In a sense, this
is just an expansion on the use of a centerpunch.

MSC carries spotting drills from 1/8" to over 1".   A 1/8",
90 degree, "made in USA" drill is listed on page 152 of the
1994/95 catalog for $2.56 (Order number 71332048).

If you are going to be making a lot of these parts, you might
also want to consider making a drill jig.   Then you can avoid
the first pass with the spotting drill, because the jig will
keep the drill bits from wandering.

--  Alan E. Frisbie               Frisbie@Flying-Disk.Com
--  Flying Disk Systems, Inc.
--  4759 Round Top Drive          (213) 256-2575 (voice)
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