Determination of diameter and depth of bottom hole and chamfer of hole before tapping thread
(1) The determination of the diameter of the bottom hole is that the cutting edge is mainly cutting metal in the process of tapping thread, but it also has the function of squeezing metal, which causes the metal to bulge and flow to the tooth tip. Therefore, before tapping the thread, the hole diameter (i.e. the bottom hole) drilled should be larger than the thread inner diameter.
The diameter of the bottom hole can be calculated by referring to the manual or according to the following empirical formula:
Brittle materials (cast iron, bronze, etc.): diameter of drilling hole d0=d (thread outer diameter) -1.1p (pitch)
Plastic materials (steel, copper, etc.): diameter of drilling hole d0=d (thread outer diameter) -p (pitch)
(2) When the depth of drilling is determined, when tapping the thread of blind hole (no hole), the depth of hole should be greater than the length of thread because the tap can not be tapped to the bottom.
The depth of blind hole can be calculated according to the following formula:
Depth of hole = depth of required thread +0.7d
The diameter of common thread bottom hole can be calculated simply as follows
The size of the thread to be tapped is multiplied by 0.85
For example: m3--2.4mm
M4--3.1mm
M5--4.2m
M6--5.1mm
M8--6.8mm
Calculation method of metric thread:
Bottom diameter = large diameter-1.0825* pitch
Calculation method of British thread:
Bottom diameter = large diameter-1.28* pitch
Diameter of brittle material drilling d = D (thread outer diameter) -1.1p (pitch)
Plastic material drilling diameter d = D (thread outer diameter) -p (pitch)
In addition to the above empirical formula, the tolerance grade of thread should be considered
External diameter pitch for common metric thread.
Metric thread (mm tooth)
Tooth depth = 0.6495* pitch P
(tooth angle 60 degrees)
Internal tooth aperture=
Nominal diameter-1.0825 * P
M20x2.5-6h/7g (right hand) - (single head thread) - (metric coarse tooth)
(nominal diameter: 20mm) (tooth pitch: 2.5mm)
(internal thread matching grade 6h)
(external thread matching grade 7G)
Left - double head-m20x1.5 (left hand) - (double thread) - (metric fine tooth)
(nominal diameter: 20mm) (tooth pitch: 1.5mm)
American thread
(uniform standard thread)
Tooth depth = 0.6495* (25.4/ tooth per inch)
(tooth angle 60 degrees)
3/4-10UNC-2A
(UNC coarse teeth) (UNF fine teeth)
(1a 2A 3A tolerance fit class of external teeth)
(1b 2B 3B tolerance fit class of internal teeth)
UNC United States standard coarse thread
3 / 4 inch OD, 10 teeth per inch
Tolerance fit of external teeth Level 2
Pipe thread (PT in British)
Tooth depth = 0.6403* (25.4/ tooth per inch)
(tooth angle 55 degrees)
Pt 3/4-14 (taper pipe thread) taper pipe thread, taper ratio 1/16
3 / 4 inch, 14 teeth per inch
Pipe thread
(PS straight thread) (PF fine thread)
Tooth depth = 0.6403* (25.4/ tooth per inch)
(tooth angle 55 degrees)
PS 3/4-14 (straight pipe thread)
Pf1 1 / 8-16 (straight pipe thread)
(fine teeth)
Straight pipe thread
3 / 4 inch, 14 teeth per inch
1 / 8 inch, 16 teeth per inch
Pipe thread (US NPT)
(tooth angle 60 degrees)
NPT 3/4-14 (taper pipe thread) taper pipe thread, taper ratio 1/16
3 / 4 inch, 14 teeth per inch
Trapezoid thread
(30 degree Metric)
Tm40 * 6 nominal diameter 40mm tooth pitch 6.0mm
Trapezoid thread
(29 degrees acme thread)
Tw26 * 5 outer diameter 26mm, 5 teeth per inch
Square thread
Calculation of the teeth
Considering the formula of condition calculation
Metric tooth to British tooth conversion thread number per inch n = 25.4 / pitch P
Pitch P = 25.4 / thread per inch n
Because the rotational speed n determined by the workpiece material and tool is n = (1000 cycle speed V) / (cycle rate P * diameter d)
Because of the speed determined by the structure of the machine
The influence of rapid movement of cutter seat on the maximum speed of teeth n = 4000/ p
The influence of the rapid movement of the cutter base on acceleration and deceleration
Calculation of the cutting point and the withdrawing point
(calculation of incomplete teeth) minimum distance L1
L1 = (pitch P) * (spindle speed s) / 500
The distance of the tool back is L2
L2 = (pitch P) * (spindle speed s) / 2000
The depth of teeth and the diameter of the base d, the depth of teeth H = 0.6495 * P
Diameter of tooth base d = nominal outer diameter D - 2 * h
Example: 3 / 4 "-10unc 20mm long of external teeth made by car
Metric tooth to English tooth pitch P = 25.4 / (inch thread n)
P = 25.4 / 10 = 2.54mm
Because the outer diameter of the speed determined by the workpiece material and the tool d = 3 / 4 inch = 25.4 * (3/4) =19.05mm
Speed n = (1000 cycle speed V) / (cycle rate P * diameter d)
N = 1000V / pD = 1000 * 120 / (3.1416*19.05)
=2005 RPM (RPM)
Because of the speed determined by the structure of the machine
The influence of rapid movement of cutter seat on the maximum speed of teeth n = 4000 / P
N = 4000/2.54 = 1575 rpm
Speed determined by the comprehensive workpiece material tool and mechanical structure n = 1575 RPM = 2005 rpm
The two speed selection is lower, namely 1575 rpm
The influence of the rapid movement of the cutter base on acceleration and deceleration
Calculation of the cutting point and the withdrawing point
(calculation of incomplete teeth) minimum distance L1
L1 = (pitch P) * (spindle speed s) / 500
L1 = 2.54*1575/500=8.00mm
Minimum distance L2
L2 = (pitch P) * (spindle speed s) / 2000
L2 = 2.54*1575/2000=2.00mm
Tooth depth and bottom diameter d = nominal outer diameter D-2 * H = 19.05-2 * 1.65 = 15.75mm
