Worked example
A Ø10 mm end mill cuts with a radial width of 1 mm (10 % of the diameter) and the tool maker's feed recommendation is 0.05 mm per tooth at full engagement. The factor is 10 / (2 × √(1 × 9)) = 10 / 6 = 1.667, so program 0.05 × 1.667 = 0.0833 mm per tooth. At 0.05 mm per tooth the real chip would be only 0.03 mm thick and the edge would rub instead of cut.
Why chips get thinner
In milling the tooth cuts a comma-shaped chip that is thickest where the tooth leaves the cut. With a wide cut the chip reaches the full feed per tooth; with a narrow cut the tooth leaves the material earlier, at a shallow angle, so the thickest point is less than the feed per tooth. Rubbing instead of cutting heats the edge and wears it quickly, which is why high-speed machining strategies with small radial engagement raise the feed above the catalogue value.
What to keep in mind
- Raising the feed raises the cutting force per tooth back to the intended level but also the feed rate; check that the machine and the setup can use it.
- The feed rate follows from the result with the RPM and feed calculator; the material removed per minute with the metal removal rate calculator.
Frequently asked questions
What is radial chip thinning?
When the radial width of cut is less than half the cutter diameter, each tooth leaves the cut at a shallow angle and the chip is thinner than the programmed feed per tooth. The feed must be raised to restore the intended chip thickness.
How do I calculate chip thinning compensation?
factor = D / (2 × √(ae × (D − ae))), and the programmed feed per tooth is the wanted chip thickness times that factor. For a Ø10 cutter at ae = 1 the factor is 1.667, so 0.05 mm per tooth becomes 0.083.
When does chip thinning stop?
At a radial width of cut of half the diameter. From there on the chip reaches the full feed per tooth and the factor is 1.
Why does rubbing matter?
A chip that is too thin makes the edge rub instead of cut, which heats and wears it quickly; that is why narrow, fast cuts need a higher feed.
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.
- Drill point depth – Length of a drill's point cone from diameter and point angle, and the total depth needed for a full-diameter hole to depth.
- 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.
- 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.