untitled 176 i TECHNICAL INFORMATION – TAPS Tap Drill Size Drill Size Formulae The correct size of drill to give the desired percentage of thread can be calculated by using the following formula Whitw.
Trang 1Drill Size Formulae
The correct size of drill to give the desired percentage of
thread can be calculated by using the following formula:
Whitworth Form Threads (inch calculation)
1.28 Drill Size = Nom Tap Dia – ––––– x % of thread depth
TPI eg: 1/4 BSW 75% thread required:
Drill Size = 250 – –––– x –––– = 250 – 048
Therefore Drill Size = 202
Nearest Standard Drill = 5.1mm = 2007 inch
Unified Form Threads (inch)
1.30 Drill Size = Nom Tap Dia – ––––– x % of thread depth
TPI
Metric (ISO) (mm)
Drill Size = Nom Tap Dia in mm – Pitch
eg M6 x 1 = 5.00mm drill
The percentage of height of thread being tapped is
important in its relation to efficient and economical
tapping
Too great a percentage of thread serves no useful
purpose, while throwing an unnecessary strain on the
teeth of the tap itself, additional power to tap is
required because more metal must be removed in a
given time This often results in tap breakage and
difficulty in holding size
Taps are different from other cutting tools as few
operating variables are possible The feed per
revolution is fixed by the lead or pitch of the tap
The rate of metal removal per tooth is thus governed
by effective chamfer length, number of flutes,
revolutions per minute, and minor diameter of the
product In addition, application of the cutting fluid is
difficult as chips can not always be removed from the
cutting zone and the cross-sectional area of the tap is
often small compared to the load imposed Since
freedom of choice as to relief, rake, and shear angles
is frequently limited, and the tap must usually stop
and reverse in the cut, it should be realised that every
reasonable precaution should be taken to favour the
tap
Tests have shown that tapping torque increases when
the minor diameter decreases Two principal reasons
for this should be emphasised
On a basic thread form a 50% thread height represents the removal of only 31.25% of the basic area, while an increase to 75% thread height increases the area to be removed to 60.9%, or practically double the first amount With the tapped hole increased to maximum pitch diameter, the figures increase to approximately 40% and 72% of the area The second cause of increase in torque is, (except in the case of spiral point or spiral flute taps) when a tap enters a drilled hole, it starts cutting chips which will usually remain in the flutes as the tap advances If stringy chips result, they roll over and over between the minor diameter of the hole and the bottom of the flutes, causing considerable friction and compaction
As the drilled holes becomes smaller, the amount of chips to be removed becomes so great that the friction generated may require as much power as does the actual cutting
It is, therefore, an advantage to tap users to keep the minor diameter as large as possible If tapping difficulties continue, the Fine Thread Series should be considered, as the volume of chips is much smaller and the strength of the internal thread is practically the same
Warning: When drilling holes in materials which tend to work harden, care is needed to ensure the drills are sharp otherwise tap life is decreased
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(Coarse)
Tap Pitch Tapping
M2.5 0.45 2.05
M4.5 0.75 3.7
M12 1.75 10.2
METRIC
(Fine)
Tap Pitch Tapping
M12 1.25 10.8
M16* 1.5 14.5
M20* 1.5 18.5
METRIC CONDUIT
Tap Pitch Tapping
SPARK PLUG
Tap Pitch Tapping
M12 1.25 10.8 M14 1.25 12.8
NPT–NPTF
(Taper) DRYSEAL
Tap T.P.I Tapping
1" 11-1/2 29.0 1-1/4 11-1/2 37.5
UNC
Unified National Coarse
Tap T.P.I. Tapping
3(.099) 48 2.0 4(.112) 40 2.25 5(.125) 40 2.6 6(.138) 32 2.75 8(.164) 32 3.4 10(.190) 24 3.8 12(.216) 24 4.4
1" 8 22.0
*METRIC CONDUIT
Taper pipe threads of improved quality are obtained when taper is pre-formed using Sutton Taper Pipe Reamers.
Always wear eye protection when using cutting tools.
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NPSF
(Straight) DRYSEAL
Tap T.P.I Tapping
G–BSPF
ISO G Parallel Series
Tap T.P.I Tapping
G 1" 11 30.5 G1-1/4 11 39.0 G1-1/2 11 45.0 G1-3/4 11 51.0
G 2" 11 57.0
UNF
Unified National Fine
Tap T.P.I Tapping
3(.099) 56 2.1
4(.112) 48 2.35
5(.125) 44 2.65
6(.138) 40 2.9
8(.164) 36 3.5
10(.190) 32 4.1
12(.216) 28 4.6
1" 12 23.5
*1" 14 24.0
1-1/8 12 26.5
1-1/4 12 29.5
1-1/2 12 36.0
BSW
British Standard Whitworth
Tap T.P.I Tapping
1" 8 22.0
2" 4-1/2 45.0
BSF
British Standard Fine
Tap T.P.I Tapping
1" 10 22.5
*UNS
*WHIT FORM
Rc–BSPT
ISO Rc Taper Series
Tap T.P.I Tapping
Rc1/4 19 10.8 Rc3/8 19 14.5 Rc1/2 14 17.5 Rc3/4 14 23.0
Rc 1" 11 29.5 Rc1-1/4 11 38.0 Rc1-1/2 11 43.5
Rc 2" 11 55.0
Always wear eye protection when using cutting tools.
Taper pipe threads of improved quality are obtained when
taper is pre-formed using Sutton Taper Pipe Reamers.
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Metric Fraction Gauge mm Drill mm T.P.I Drill mm T.P.I Drill mm