Tool Life, Wear and Cutting Economics
Taylor tool life, how speed shortens life, the cost-per-part and production-rate speeds, and when to change an edge.
Taylor’s tool-life equation
The tool life T is the time a tool cuts before it wears to a set limit (a flank wear land of 0.3 mm is the usual criterion). F. W. Taylor found in 1907 that life falls steeply as cutting speed rises, in a simple power law.
V × Tn = C
| Tool material | n (typical) | C (V for T = 1 min) | V for 60 min | V for 15 min |
|---|---|---|---|---|
| HSS | 0.125 | 50 m/min | 30 m/min | 36 m/min |
| Carbide | 0.250 | 334 m/min | 120 m/min | 170 m/min |
| Ceramic | 0.500 | 1 900 m/min | 245 m/min | 491 m/min |
The constants depend on the workpiece, the cut and the tool; the table is typical for plain steel. Use your own wear measurements where you can.
What speed does to tool life
Worked example: a 20 % speed increase
- T = (C ÷ V)1/n. For carbide (C = 334, n = 0.25) at 150 m/min: T = (334 ÷ 150)4 = 25 min. At 180 m/min (20 % faster) the life is only 12 min: 52 % shorter.
- The factor for a speed increase of 20 % is 1.2−1/n: 0.23 for HSS (n = 0.125), 0.48 for carbide and 0.69 for ceramic. The more sensitive the tool, the more a small speed change matters: HSS loses 77 % of its life.
- Slowing down pays the other way: 20 % slower multiplies carbide life by 2.4.
- For a 20 min life, carbide needs a speed of 334 ÷ 200.25 = 158 m/min.
- Speed is the most expensive way to remove more metal. Doubling the feed (with a feed exponent of 0.5) cuts life by 4.0 times, doubling the speed by 16 times: for the same removal rate, raise the depth and the feed before the speed.
Tool wear and how to read it
| Wear | Cause | Typical situation | Remedy |
|---|---|---|---|
| Flank wear | Abrasion on the clearance face behind the edge | Gradual; sets the tool life (VB = 0.3 mm in ISO 3685) | Lower the speed, use a harder grade or coating |
| Crater wear | Diffusion where the hot chip slides on the rake face | Steel with carbide at high speed | Lower the speed, use a coated or ceramic grade, coolant |
| Built-up edge | Work material welds to the edge | Low speed in soft or gummy metal | Raise the speed, sharper tool, lubrication |
| Chipping and fracture | Mechanical shock, thermal cracks | Interrupted cuts, too high a feed | Lower the feed, tougher grade, rounded edge, ramp in |
| Notch wear | Hardened skin or scale at the depth-of-cut line | Castings, forgings, work-hardened surfaces | Vary the depth of cut, cut below the skin |
| Thermal cracks | Repeated heating and cooling in milling | Flood coolant on interrupted cuts | Use dry cutting or mist with carbide in milling |
- Look at the edge under a magnifier at the end of a run: the wear pattern tells you which parameter to change.
- Flank wear that is even and slow is normal; fast flank wear means too high a speed. Chipping means the feed or the shock is too high.
- Change an edge before it fails: a worn tool makes heat and force, and takes the workpiece with it.
Cutting economics: the cheapest speed
A faster cut shortens the time per part but uses up tools sooner. The cost per part has two parts that pull in opposite directions, so it has a minimum. For a machine rate Cm ($ per minute), a tool-change time tc and a cost Ct per edge, the cost per part is
cost per part = Cm (tm + th) + (Cm tc + Ct) × tm ÷ T
Teconomic = (1 ÷ n − 1) × (tc + Ct ÷ Cm) Tmax production = (1 ÷ n − 1) × tc
Worked example: turning a 50 mm bar, 100 mm long
- Machine rate Cm = $1.00/min ($60/h), handling th = 2 min, edge change tc = 1 min, edge cost Ct = $6.00, carbide with C = 400 and n = 0.25. The cut time is tm = π × 50 × 100 ÷ (1 000 × V × 0.25 mm/rev) = 62.8 ÷ V minutes.
- Economic tool life: T = (1 ÷ 0.25 − 1) × (1 + 6.00 ÷ 1.00) = 21 min, which means a speed of V = 400 ÷ 210.25 = 187 m/min and a cost of $2.45 per part (62 parts per edge).
- Maximum production rate: T = (1 ÷ 0.25 − 1) × 1 = 3 min, at 304 m/min: 26.4 parts an hour against 25.5 (3 % more) at the economic speed, but a cost of $2.69 per part, 9 % higher; only 15 parts per edge.
- Too slow or too fast both cost: at 120 m/min the cost is $2.55 and at 400 m/min $3.26 per part. Over 5 000 parts a month the economic speed saves $1 203 against the maximum-production speed.
- Choose the economic speed in general; use the faster speed only when the machine is the bottleneck of the whole shop and the order must ship.
When to change the tool
- By count: change after a fixed number of parts (62 in the example) with a safety margin, the simplest and most reliable method.
- By measurement: flank wear of 0.3 mm for finishing and 0.6 mm for roughing, or a change in size, finish, noise or spindle load.
- By time: a cutting-time counter on the control stops the cycle at the set life.
- Index, not discard: inserts have several edges: log which one is in use.
- Never wait for a tool to break when it ruins the part: the tool is the cheapest component of the job.
Troubleshooting
| Symptom | Likely cause | Try |
|---|---|---|
| Tool life far shorter than the chart says | Speed too high, no coolant, wrong grade, rubbing | Lower the speed 15 to 20 %; check the chip load; use the right grade |
| Finish gets worse over a run | Flank wear growing | Change the edge at the preset count |
| Edges fail at random | Chipping from shock or a bad setup | Check runout, rigidity and the entry; lower the feed |
| Slowing down does not help | Rubbing: the feed is too low | Raise the chip load |
| Cost per part is high | Too slow or too fast | Find the economic speed from the Taylor constants |
| Parts size drifts | Tool wear | Offset the tool or shorten the edge life |
Related
- Speeds and Feeds: the overview of this guide.
- Tool Materials, Coatings and Geometry: the tool materials with their different n and C.
- Cutting Force, Power and Torque: the load behind the heat.
- Running a Laser Shop (machine-hour rates) and Lathe Turning Calculator.
FAQ
What is Taylor’s equation?
V T^n = C: cutting speed times tool life to a power n is a constant. A small rise in speed shortens life a great deal: about 50 % for a 20 % rise with carbide.
What is the best cutting speed?
Not the highest: the economic speed minimizes the cost per part, which corresponds to a tool life of about 15 to 60 minutes for carbide. Use the maximum-production speed only when output is critical.
How do I find n and C for my tool?
Cut with two or three speeds, measure the time to reach 0.3 mm of flank wear, and fit a straight line on log-log paper: the slope is −n.
Is it better to raise the speed or the feed?
For the same removal rate, raise the depth and the feed first; speed has the biggest effect on life.
References
- Taylor, F. W., On the Art of Cutting Metals, Transactions of the ASME 28 (1907); ISO 3685, Tool-life testing with single-point turning tools.
- Kalpakjian, S. and Schmid, S., Manufacturing Engineering and Technology (tool life and economics).
- Boothroyd, G. and Knight, W. A., Fundamentals of Machining and Machine Tools, CRC Press (economics of machining).
Last reviewed: October 2026. The Taylor constants are typical for steel: measure your own for important jobs.