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.
TECHNICAL INFORMATION – TAPS Tap Drill Size The percentage of thread in tapped holes i 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 As the thread height increases, the width of the tap and amount of material removed increases rapidly 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 Drill 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: 1.28 75 Drill Size = 250 – –––– x –––– 20 100 inch = 250 – 048 Therefore Drill Size = 202 Nearest Standard Drill = 5.1mm = 2007 inch Unified Form Threads (inch) Drill Size = Nom Tap Dia – 1.30 ––––– TPI x % of thread depth Metric (ISO) (mm) Drill Size = Nom Tap Dia in mm – Pitch eg M6 x = 5.00mm drill 176 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 TECHNICAL INFORMATION – TAPS Tapping Drill Size Chart ALL SIZES ARE “SUGGESTED SIZES” ONLY AND MAY BE VARIED TO SUIT INDIVIDUAL REQUIREMENTS METRIC UNC (Fine) (Coarse) Unified National Coarse Tap Size Pitch mm Tapping Drill mm Tap Size Pitch mm Tapping Drill mm Tap Size T.P.I Tapping Drill mm M2 M2.5 M3 M3.5 M4 0.4 0.45 0.5 0.6 0.7 1.6 2.05 2.5 2.9 3.3 2.0 2.25 2.6 2.75 3.4 3.7 4.2 5.0 6.0 6.8 3.5 4.5 5.3 7.0 8.8 10.8 10.5 12.5 48 40 40 32 32 0.75 0.8 1.0 1.0 1.25 0.5 0.5 0.75 1.0 1.25 1.25 1.5 1.5 3(.099) 4(.112) 5(.125) 6(.138) 8(.164) M4.5 M5 M6 M7 M8 M4 M5 M6 M8 M10 M12 M12 M14 7.8 8.5 9.5 10.2 12.0 14.5 16.5 18.5 20.5 22.0 3.8 4.4 5.1 6.6 8.0 1.25 1.5 1.5 1.75 2.0 1.5 1.5 1.5 1.5 2.0 24 24 20 18 16 M9 M10 M11 M12 M14 M16* M18 M20* M22 M24 10(.190) 12(.216) 1/4 5/16 3/8 7/16 1/2 9/16 5/8 3/4 14 13 12 11 10 9.4 10.8 12.2 13.5 16.5 M16 M18 M20 M22 M24 2.0 2.5 2.5 2.5 3.0 14.0 15.5 17.5 19.5 21.0 7/8 1" 1-1/8 1-1/4 1-1/2 7 19.5 22.0 25.0 28.0 34.0 M27 M30 M33 M36 M39 3.0 3.5 3.5 4.0 4.0 24.0 26.5 29.5 32.0 35.0 M42 M45 M48 M52 4.5 4.5 5.0 5.0 37.5 40.5 43.0 47.0 *METRIC CONDUIT SPARK PLUG Tap Size M10 M12 M14 M18 Pitch mm Tapping Drill mm 1.0 1.25 1.25 1.5 9.0 10.8 12.8 16.5 METRIC CONDUIT Tap Size Pitch mm Tapping Drill mm M16 M20 M25 M32 M40 M50 1.5 1.5 1.5 1.5 1.5 1.5 14.5 18.5 23.5 30.5 38.5 48.5 i ISO METRIC NPT–NPTF (Taper) DRYSEAL Tap Size T.P.I Tapping Drill mm 1/8 1/4 3/8 1/2 3/4 27 18 18 14 14 8.4 11.0 14.5 17.5 23.0 11-1/2 11-1/2 29.0 37.5 1" 1-1/4 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 177 TECHNICAL INFORMATION – TAPS Tapping Drill Size Chart ALL SIZES ARE “SUGGESTED SIZES” ONLY AND MAY BE VARIED TO SUIT INDIVIDUAL REQUIREMENTS UNF Unified National Fine BSW British Standard Whitworth BSF British Standard Fine T.P.I Tapping Drill mm Tap Size T.P.I Tapping Drill mm Tap Size T.P.I Tapping Drill mm 3(.099) 4(.112) 5(.125) 6(.138) 8(.164) 56 48 44 40 36 2.1 2.35 2.65 2.9 3.5 *3/32 1/8 5/32 * 3/16 *7/32 48 40 32 24 24 1.9 2.55 3.2 3.7 4.5 3/16 1/4 5/16 3/8 7/16 32 26 22 20 18 4.0 5.4 6.8 8.3 9.8 10(.190) 12(.216) *3/16 1/4 5/16 32 28 32 28 24 4.1 4.6 4.0 5.5 6.9 1/4 5/16 3/8 7/16 1/2 20 18 16 14 12 5.1 6.5 8.0 9.3 10.5 1/2 9/16 5/8 3/4 7/8 16 16 14 12 11 11.0 12.7 14.0 16.5 19.5 3/8 7/16 1/2 9/16 5/8 24 20 20 18 18 8.5 9.8 11.5 12.8 14.5 9/16 5/8 3/4 7/8 1" 12 11 10 12.2 13.5 16.5 19.5 22.0 1" 1-1/8 1-1/4 1-1/2 1-3/4 10 9 22.5 25.5 29.0 34.5 40.5 3/4 7/8 1" *1" 1-1/8 16 14 12 14 12 17.5 20.5 23.5 24.0 26.5 1-1/8 1-1/4 1-1/2 1-3/4 2" 7 4-1/2 25.0 28.0 34.0 39.0 45.0 1-1/4 1-1/2 12 12 29.5 36.0 i i Tap Size Always wear eye protection when using cutting tools *WHIT FORM *UNS Taper pipe threads of improved quality are obtained when taper is pre-formed using Sutton Taper Pipe Reamers NPSF Rc–BSPT (Straight) DRYSEAL 178 ISO Rc Taper Series G–BSPF ISO G Parallel Series Tap Size T.P.I Tapping Drill mm Tap Size T.P.I Tapping Drill mm Tap Size T.P.I Tapping Drill mm 1/8 1/4 3/8 1/2 27 18 18 14 8.5 11.0 14.5 18.0 Rc1/8 Rc1/4 Rc3/8 Rc1/2 Rc3/4 28 19 19 14 14 8.2 10.8 14.5 17.5 23.0 G1/8 G1/4 G3/8 G1/2 G5/8 28 19 19 14 14 8.8 11.8 15.0 19.0 21.0 Rc 1" Rc1-1/4 Rc1-1/2 Rc 2" 11 11 11 11 29.5 38.0 43.5 55.0 G3/4 G7/8 G 1" G1-1/4 G1-1/2 14 14 11 11 11 24.5 28.0 30.5 39.0 45.0 G1-3/4 G 2" 11 11 51.0 57.0 TECHNICAL INFORMATION – TAPS Recommended Thredflo Tapping Drill Size Chart (Roll Forming Taps) “Thredflo” (Fluteless) Roll Taps: Tapping Drill Sizes for Roll Formed Threads in ductile materials only ISO COARSE Pitch mm Tapping Drill mm M1.0 0.25 0.75 M1.1 0.25 0.85 M1.2 0.25 0.95 M1.4 0.3 1.10 M1.6 0.35 1.25 M1.7 0.35 1.35 M1.8 0.35 1.45 M2.0 0.4 1.60 M2.2 0.45 1.75 M2.3 0.4 1.90 M2.5 0.45 2.05 M2.6 0.45 2.15 M3.0 0.5 2.75 Metric Fraction M/C Screw Gauge UNC T.P.I BSW Tapping Drill mm 1/8 M3.5 0.6 32 2.90 32 3.60 24 4.30 3.70 #8 32 3.80 3/16 #10 24 4.30 1/4 20 5.80 20 5.80 5/16 18 7.30 18 7.30 16 8.80 16 8.80 M5 0.8 4.60 M6 1.0 5.50 M8 1.25 7.40 3/8 M10 40 3.10 5/32 0.7 Tapping Drill mm 3.20 #6 M4 T.P.I i THREAD SIZE 1.50 9.30 179 ... improved quality are obtained when taper is pre-formed using Sutton Taper Pipe Reamers NPSF Rc–BSPT (Straight) DRYSEAL 178 ISO Rc Taper Series G–BSPF ISO G Parallel Series Tap Size T.P.I Tapping Drill