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TAPS ENGINEERING DATA
THREAD FORMING TAPS
THREAD FORMING TAPS DATA
DRILLS THREAD FORMING TAP ENTRY LENGTHS: Overall & Thread Lengths Entry
Entry taper length is measured on the full diameter of the p Taper
thread forming lobes and is the axial distance from the entry Length
diameter position to the theoretical intersection of tap major
diameter and entry taper angle. Entry Taper Angle
Whenever entry taper length is specified in terms of number of
END MILLS threads, this length is measured in number of pitches (p). Entry Diameter
BOTTOMING LENGTH = 1-1/2 to 2-1/2 PITCHES 2
PLUG LENGTH = 3 to 5 PITCHES
ROUTERS The chamfer on BOTTOM taps is approximately 2 threads long speeds are attained, adequate lubrication is essential for
and requires a drilled hole depth 3-4 pitches beyond the full prolonged tap life and thread quality.
THREAD MILLS thread required. When a controlled maximum chamfer shorter LUBRICATION: Since it is more important to ‘lubricate’ the
& TAPS than 2 threads is required, an additional charge will apply. We cold-forming tap than to ‘cool’ the tap, form taps should be
will not guarantee the performance of taps with the shorter used with conventional lubricating cutting oils or EP (extreme
ENGRAVERS chamfer. pressure) rated oil...soluble oils and similar coolants are not
Entry diameter, measured at the thread crest nearest the front recommended.
of the tap, is an appropriate amount smaller than the diameter PRE-TAPPED HOLE SIZE: cold forming taps require a larger
of the hole drilled for tapping. See below for tap/drill size pre-tapped hole size than conventional cutting taps. To insure
formulas, and formulas to determine maximum and minimum a properly tapped (cold formed) hole, adhere to the following:
drill hole sizes for appropriate percent of thread.
TAPPING SPEEDS: Form taps operate most efficiently
at spindle speeds 1-1/2 to 2 times faster than those
recommended for conventional cutting taps, especially in
softer materials and/or with fine pitch form taps. As higher
BORING BARS FORMULA FOR TAP / DRILL SIZES FOR DECIMAL / INCH FORM TAPS:
( )HOLE SIZE = Basic Tap O.D. -
0.0068 X % of Thread*
Threads per Inch
For example:
To determine drill size for a 1/4-20 thread forming tap at 65% of thread: 0.250 -
( )* Use whole number for % of thread...for 65%, use 65 (not 0.65).
REAMERS 0.0068 X 65 = 0.2279
20
FORMULA FOR TAP / DRILL SIZES FOR METRIC FORM TAPS:
( )HOLE SIZE (mm) = Basic Tap O.D.(mm) -
SAWS % of Thread X mm Pitch
147.06
* Use whole number for % of thread...for 65%, use 65 (not .65).
TECHNICAL There is no true method of predicting percent of thread that will be obtained when tapping with forming taps due to the many
variables involved. As a starting point, however, 55% for maximum drill size and 75% for minimum drill size can be used as a
guide. Any desired percent of thread can be approximated by using drill sizes in between. To determine theoretical maximum and
minimum drill sizes (for average operating conditions), see formulas below.
INDEX For UNIFIED INCH Threads: 3 For 60° Metric Threads:
Max. Drill Size = Basic Major Diameter - 8N
1 Max. Drill Size = Basic Major Diameter - 0.375P
Min. Drill Size = Basic Major Diameter - 2N
Min. Drill Size = Basic Major Diameter - 0.5P
N = T.P.I. (Threads per Inch)
P = Pitch
Note: For Basic Major Diameter and Pitch, use millimeter value to obtain drill
size in mm. To convert mm to inch value, divide by 25.4:
mm Value = Inch Value
25.4
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