Drill Press
A Drill Press in the Pappalardo Laboratory
Labeled Photograph
Description
Using a Center Finder
Drilling a Hole
Deburring a Hole
Reaming a Hole
Thread Standards
Tapping a Hole
A drill press is preferable to a hand drill when the location and
orientation of the hole must be controlled accurately. A drill press is
composed of a base that supports a column, the column in turn supports a
table. Work can be supported on the table with a vise or hold down
clamps, or the table can be swiveled out of the way to allow tall work
to be supported directly on the base. Height of the table can be
adjusted with a table lift crank than locked in place with a table lock.
The column also supports a head containing a motro. The motor turns the
spindle at a speed controlled by a variable speed control dial. The
spindle holds a drill chuck to hold the cutting tools (drill bits,
center drills, deburring tools, etc.).
The quill is moved up and down with a lever on the side as shown here.
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A center finder is useful for setting the spindle of a drill press or
mill accurately over a known point. A center finder is made of two
searate pieces spring loaded together.
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The center finder is used by installing it in the chuck, placing the
pointed end into a centerpunch mark, and pressing down lightly on
the quill. At first, the two parts will probably not be concentric.
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The bed of the drill or mill can then be adjusted to until the two
halves are concentric. Check for concentriccity by running your fingers
up and down the center finder. You should detect no steps. You should
be able to locate the desired position to within .001 inches using this
technique. Now the hole is ready to be drilled.
First, a center drill should be used. A center drill has a thick shaft
and very short flutes. It is therefore very stiff and won't walk as the
hole is getting started. It doesn't cut as easily as a drill bit, so
you should use cutting fliud.
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Now the hole can be drilled. If the hole is large, its a good idea to
drill a smaller pilot hole before drilling the final one. Your hole
will be more accurately positioned, rounder, and the bits will last
longer. If the hole is deeper than it is wide. use coolant and back
off occaisionally to clear the chips. As you step up in drill size, you
will need to reduce the spindle speed. If drilling a through hole,
ensure that the bit will not drill the table after moving through your
work. To set a desired depth of hole, there is a depth stop on the
quill.
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Usually, the top edge of the hole will be fairly clean, but the bottom
edge will have substantial burrs. To remove them, insert a deburring
tool into the hole and run the tool about the edge of the hole with
moderate pressure.
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A drilled hole will be accurate to about two thousandths of an inch in
diameter. If greataer precision is required for slip fits or
interference fits, a reamer must be used. The straight flutes of a
reamer cannat drill a hole. You must drill a hole slightly udersize to
start. Be sure to drive the reamer down with a constant, slow speed.
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The threads cut by taps and dies conform to a standard for the shape of
the threads. Often it is the American National Standard in National
Coarse (NC) or National Fine (NF). NC has fewer threads per inch than
NF. NC is most common while NF is favored in prcision assemblies. Also
in common use is the Unified Thread System with UNC (coarse) and UNF
(fine). The onlt difference between National and Unified threads is the
shape of the root and crest. The threads in the two different standards
will mate. Fasteners are designated by their diameter, number of
threads per inch, and shape. For example, 1/4-20NC means 1/4 inch
diameter, 20 threads per inch, and National Standard. Threads smaller
than 1/4 inch are designated by number from 0 (smallest) to 12
(largest). To convert number to diameter multiply by 0.013 and add 0.060.
Thread Standards
If you want to cut threads in a hole, use a tap. The picture below
shows what a tap looks like. It has cutting edges to cut the threads
and straight flutes to allow chips to be expelled. Note that the end is
tapered slightly to help the tap get started. Taps and dies are hard
and brittle so you should be careful working with them (try not to drop
them or force them into a hole when stuck). Be sure that the hole you
drilled is the correct size for the tap you're using or it may break
inside your part. Refer to a chart or machinist's handbook. The die in
the picture is for cutting external threads on a shaft.
Two Taps and a Die
In the clip below, the hole is tapped by hand. Note that the tap did
not go in quite straight. It is better to use the drill press to help
maintain alignment as you tap the hole. First, use the centerfinder to
place the spindle directly above the hole. Then install a tapered guide
into the chuck. Put the tap in place and apply moderate pressure with
the quill as you turn the tap. It's good peactice to back the tap up a
bit for every quarter turn of thread you cut.
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On to the lathe.
Return to the machine shop.
Return to the Mechanical Engineering Department.
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