Hole Making Tools
Drills
are the most commonly known rotating tool, but most people are not familiar
with the variety of drills available. The
jobber length twist drill is the most popular bit available, but twist drills
come in various lengths, helix angles, tip angles, tool materials, and coatings.
Other types of drills include spade drill, insert drills, core drills,
straight flute drills, and gun drills.
Center
Drills are a unique style of drill used primarily to create an accurate
location for another drill, or to locate a lathe center.
These drills are characterized by a smaller 120º
tip, a short straight section and a larger 60º
centering angle. They are numbered
in relative size #00 thru #8, in lengths of 1-1/2”, 2-1/2”, 4” & 6”.
Of course the shorter the length used, the more rigid (and more accurate)
the process.
Reamers
are generally multiple (4, 6, or 8), straight flute (although spiral flute
versions are available) tools designed to create a very round and accurately
sized hole. They cannot remove more
than about 5% of their diameter, so they must follow a drill very close to the
finished size of the hole.
Taps
are the tools used to create the threaded hole to accommodate a bolt or
similar device. Taps are available in
2, 3 or 4 flute designs in tapered, plug, or bottoming tip styles and
(optionally) spiral points and / or spiral flutes.
Of course the are available in most of every possible combination of the
above for every diameter / thread pitch combination, inch and metric.
Countersinks
are the tools used to create the tapered
hole that allows countersunk fasteners to be flush with the surface.
The most common angle is 82º
(included), but 60º, 90º, 100º,
and 120º are also available.
Counterbores are used to create the flat
bottom hole that sometimes is required to submerge bolt heads, or other
components at, or below the surface of the part.
Boring
Bars, when used with a boring head are classified as a rotating tool.
These tools are generally used to enlarge a hole to an accurate size and
roundness at a diameter greater than that available with the reamers at hand, or
at a size not available with those reamers.
Milling Tools 
Face
Mills are generally 2” diameter or greater and designed to remove material
at the axial aspect of the tool, creating a flat face perpendicular to the axis
of rotation. (generally the X-Y plane)
Endmills
are available in sizes from 0.004” diameter to over 4” in diameter.
Despite the name, they are capable of cutting on the radial and axial
aspects of the tool. A simple
examination of a contemporary machine tool supply catalog will illustrate the
diversity of the sizes and configurations available.
Slot
Mills cut only on the radial aspect of the tool and are generally limited to
cutting slots or grooves with a high length to width ratio.
Variations include cutting tool material, width, diameter, and tooth
configuration.
Thread
Mills generate a thread profile similar to a tap except that the tool is
only in contact with the part tangently, not radially.
This makes it possible to generate all threads of a specific pitch (that
are larger than the tool diameter) with a single tool. A
version of the tool has a single 60º
“V” profile that can be helically interpolated to generate most thread
pitches within a given range.
Form
Mills produce a specific profile or form dictated by the shape of the tool.
The thread mill above, is one type of form mill. Other forms include
chamfers and radii of various sizes. Other
variations include width, diameter, material, and tooth style.
Rotating
Tool Terminology
Clearance
is the amount of space on the non-cutting pace of the cutting edge that keeps
the tool from rubbing the workpiece as it rotates and traverses. Large clearance
angles are preferable when cutting at high feed rates, but reduce the durability
and resharpening capability of the tool.
Rake
is the deviation (negative, positive, or neutral) the cutting edge is from a
theoretical line drawn between the centerline of the tool and the point where
the tool engages the work.
Flutes
are the generally helical grooves in a cutting tool between the cutting
edges. The helix angle and flute geometry are important in chip
ejection. Since the number of
flutes is equal to the number of cutting edges, it is typically used as the
numeric determiner; i.e. 2 flute tap or 4 flute endmill.
Relief
Rouging
Center cutting
High Helix
Spiral point
Spiral flute
