Worked example
A Ø10 mm drill with a 118° point. Half the angle is 59°; tan 59° = 1.6643. The cone is 5 / 1.6643 = 3.00 mm long. To drill a blind hole with 20 mm at full diameter, the tip must go 20 + 3.00 = 23.00 mm deep. A 90° point on the same drill would give exactly 5 mm, because tan 45° = 1. As a rule of thumb, a 118° point is about 0.3 × D long.
Through holes and blind holes
- Through holes: the tip must come out of the far side before the hole is full diameter at the exit. Add the cone length h to the material thickness, plus a small allowance for run-out and burr.
- Blind holes: the hole bottom is a cone, so the programmed depth is the full-diameter depth plus h. A drawing that gives the depth to the full diameter is different from one that gives the depth to the tip; check which.
- Flat-bottom holes need a second tool (an end mill or a flat drill) after the drill; leave the cone length as extra depth for the drill so the second tool has stock to clean up.
- Drill depth in CNC cycles: many drilling cycles let you give the depth to the tip or to the full diameter; some work with the point angle and compute h for you. The result here is the same number.
Frequently asked questions
How long is the point of a 118° drill?
About 0.3 × the diameter: h = (D/2) / tan(59°) = 0.3 × D. A Ø10 mm drill has a 3.00 mm point.
How do I calculate drill depth for a blind hole?
Add the point length to the depth you need at full diameter: total depth = full depth + (D/2) / tan(point angle / 2).
What is the difference between 118° and 135° points?
118° is the general-purpose point; 135° (often split-point) is flatter, so the cone is shorter – 0.21 × D instead of 0.30 × D – and it centres itself better in harder material.
Is the point angle the full angle or half?
The full, included angle between the two cutting lips. The calculation uses half of it, measured from the drill axis.
More free machinist calculators
- Inch to mm – Convert inches to millimetres and back, fractions like 1 3/8 included. Exact factor 25.4, with a chart of every 1/64 inch.
- RPM and feed – Calculate spindle RPM from surface speed and diameter, surface speed from RPM, and feed rate from chip load – metric and imperial, with formulas.
- Taper – Taper ratio, half angle and taper per foot from the large and small diameter and the length, or the small diameter from the angle.
- Bolt circle – X and Y coordinates of evenly spaced holes on a bolt circle, with the hole spacing, start angle and centre offset. Copy the table into your program.
- Sine bar – Gauge block height for a sine bar angle, or the angle from a known stack, for 5 in, 10 in, 100 mm, 200 mm or any other bar length.
- Metal removal rate – Metal removal rate in cm³/min or in³/min from depth, width and feed – milling and turning, metric and imperial, with the formulas.
- Right triangle – Hypotenuse and angles from two legs, or the legs from the hypotenuse and an angle – for chamfers, dovetails, tapers and hole positions.
- Machining time – Cutting time from length, feed and spindle speed – turning with feed per revolution, milling with feed rate and number of passes.
- Decimal to fraction – Turn a decimal inch or a millimetre value into the nearest fraction – 8ths to 128ths – with the error in millimetres.
- Degrees to DMS – Convert decimal degrees to degrees, minutes and seconds and back, with the angle in radians.
- SFM to m/min – Convert surface feet per minute to metres per minute and back, and inches per minute to mm per minute, with the exact factors.
- TPI to pitch – Convert a thread pitch in millimetres to threads per inch (TPI) and back, for matching metric and inch threads.
- Cpk calculator – Paste measurements from Excel, enter USL and LSL and get Cp, Cpk, Pp and Ppk. Says plainly when 25 values are too few for an index.
- Tolerance stack-up – Add up the tolerances of a chain of parts: closing gap, worst case and statistical RSS for up to eight dimensions. Free, in your browser.
- Fit clearance – Enter hole and shaft limits and get minimum and maximum clearance and the kind of fit – clearance, transition or interference.
- Chip thinning – Feed per tooth to program when the radial width of cut is below half the cutter diameter, so the real chip thickness stays right.
- Ball-nose stepover – Scallop height left by a ball-nose cutter from the stepover, or the stepover for a wanted scallop height, on a flat surface.
- Surface finish – Theoretical Rt and estimated Ra of a turned surface from the feed per revolution and the tool nose radius.
- Torque and power – Torque from power and speed in newton-metres or pound-feet, and power from torque and speed. The formulas with worked examples.
- Spur gear – Pitch and outside diameter, centre distance, ratio and circular pitch of a pair of external spur gears from the module and the tooth numbers.
Making parts for a customer who wants paperwork?
Automotive and aerospace customers ask for proof that the parts match the drawing. See first article inspection (FAI), PPAP explained and the true position calculator.