Standard End Mill Sizes Chart

Shank diameter and tolerance

The shank is what the collet actually grips, so its tolerance governs runout before the cutter ever touches the part. Standard practice for both inch and metric carbide end mills is a ground shank to h6; the cutting diameter is normally toleranced on the minus side only, so a nominal cutter never comes in oversize.

• h6 shank — the default on quality carbide tooling; matches a standard collet bore and gives a repeatable grip.
• Cutting diameter — typically 0 to −0.002″ on inch sizes, or the h6/h7 equivalent on metric, checked over the full flute length.
• Reduced shank — a 20 mm cutter on a 16 mm shank, or a 1/2″ cutter on a 3/8″ shank. Cheaper to make and lets one collet size cover more cutters, but it moves the weakest section closer to the cut.
• Runout — measure at the cutting edge, not the shank. 10 µm at the edge is what a good holder plus a good shank delivers; anything above 20 µm will show in the finish and in edge life.

What actually changes the price of an end mill

Two 1/2″ end mills can differ by a factor of four in price. The spec items that move it are, in rough order of effect:

• Carbide grade and grain size — submicron and ultra-fine grain substrates cost more than standard grain, and they are what let a small cutter survive in hardened or high-temp alloys.
• Coating — uncoated, TiAlN or AlCrN is a modest step; diamond or a specialised multilayer is a large one.
• Geometry — a plain square corner is the baseline; variable helix, unequal index and a wiper facet each add grinding operations.
• Corner radius or ball nose — ground radius needs a separate grinding pass and is normally priced per radius.
• Length of cut — long-reach and reduced-neck cutters use more carbide and more grinding time, and reject more in inspection.
• Tolerance class — a cutter held to a tight diameter and runout band costs more than a general-purpose one of identical size.

The end mill cost build-up walks through these line by line, and the price index lists observed factory price bands by size.

Common mistakes to avoid

• Treating 1/2″ and 12 mm as interchangeable — the difference is 0.7 mm, which is 0.7 mm of cut width you did not plan for.
• Assuming a metric cutter will substitute for an inch one in a collet. A 6 mm shank in a 1/4″ collet is not a fit.
• Buying the longest flute available "to be safe" — length of cut is the single biggest cause of chatter and deflection.
• Ordering an odd diameter to avoid a radius change when an 8 mm cutter plus a tweak to the toolpath would have cost less.
• Comparing quotes by diameter only. The grade, coating and geometry are what you are actually buying.

Frequently asked questions