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TAPS ENGINEERING DATA
(CONTINUED) TERMS DEFINITIONS
DEFINITION OF TERMS APPLYING TO TAPS (cont.)
PITCH DIAMETER ROOT DRILLS
On a straight thread, the pitch diameter is the diameter of the The bottom surface joining the sides of two adjacent threads, END MILLS
imaginary co-axial cylinder...the surface of which would pass and is identical with or immediately adjacent to the cylinder or
through the thread profiles at such points as to make the width cone from which the thread projects. ROUTERS
of the groove equal to one-half of the basic pitch. On a perfect
thread this occurs at the point where the widths of the thread SPIRAL FLUTE THREAD MILLS
and groove are equal. On a taper thread, the pitch diameter at & TAPS
a given position on the thread axis is the diameter of the pitch A flute with uniform axial lead in a spiral path around the axis of
cone at that position. a tap. ENGRAVERS
RAKE SPIRAL POINT
The angular relationship of the straight cutting face of a tooth The angular fluting in the cutting face of the land at the
with respect to a radial line through the crest of the tooth at the chamfered end; formed at an angle with respect to the tap axis
cutting edge. Positive rake means that the crest of the cutting of opposite hand to that of rotation. Its length is usually greater
face is angularly ahead of the balance of the cutting face of the than the chamfer length and its angle with respect to the tap
tooth. Negative rake means that the crest of the cutting face is axis is usually made great enough to direct the chips ahead of
angularly behind the balance of the cutting face of the tooth. the taps cutting action.
Zero rake means that the cutting face is directly on a radial line.
STRAIGHT FLUTE
RELIEF (or Thread Relief)
A flute that forms a cutting edge lying in an axial plane.
The removal of metal from behind the cutting edge to provide
clearance and reduce friction between the part being threaded TOLERANCE
and the threaded land.
In producing a tap to given specifications, tolerance is:
(a.) the total permissible variation of a size; (b.) the difference
between the limits of size.
CHAMFERS FOR THREAD CUTTING TAPS BORING BARS
The tap chamfer is the tapering of the threads to distribute PLUG TAPS REAMERS
cutting action over several teeth. The type of hole to be tapped
has much to do with the chamfer style of that tap that’s best This style, with a 3-5 thread chamfer, is most widely used in
suited. Some holes go all the way through; some, while not through holes and where there is sufficient room at the bottom
through-holes, are relatively deep; some are quite shallow (a in blind holes.
little deeper than diameter). Each of these three kinds of holes -
through, deep-bottoming blind, and shallow bottoming - has a SEMI (or Modified) BOTTOMING TAPS
tap chamfer best suited to threading requirements.
This style, with a 2 to 2.5 thread chamfer, should be used
whenever possible in difficult material applications in blind holes, SAWS
when threads are not required to the bottom of the hole.
TAPER TAPS BOTTOMING TAPS TECHNICAL
This style, with a 7-10 thread chamfer, has the longest chamfer This style, designed with a 1 to 2 thread chamfer, is made INDEX
of the three to distribute action over the maximum number of with just enough chamfer for starting in the hole; as the name
teeth; and the taper also acts as a guide in starting the cutting implies, it is designed to thread blind holes to the bottom.
action in the hole. Taper style taps start the thread square with
the workpiece. Taper taps are commonly used in through holes NOTE: Taper, plug and bottoming taps as a set, in a given size
and in materials where a tapered guide is necessary. (for example: 1/4-20 NC) are identical as to size, length and vital
measurements; the difference is in the chamfered threaded
portion at the point. As a rule, such taps when used by and are
furnished in sets of three of a given size... namely, taper, plug
and bottoming (and should be used in that order).
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