What Do End Mill Dimensions Mean? The D1, D2, L1, L2, L, R and Z Codes

At first glance the tables in cutting tool catalogues look like code: D1, D2, L1, L2, L, R, Z. These are not arbitrary abbreviations but the shorthand that the industry has settled on for the dimensions defined in the ISO 13399 tool data standard. This guide explains each of them and shows which one deserves attention at the ordering stage.

Short answer: the column glossary

D1: Cutting diameter

This is the diameter of the circle described by the cutting edges of the tool. It is the width of the slot the cutter will produce, and it is also the D value used in the cutting speed calculation:

n = (Vc × 1000) / (π × D1)

If the catalogue says Ø8, the tool cuts a slot 8 mm wide. Tolerances vary with the manufacturer and the grade; in precision work the tolerance line in the catalogue has to be examined (typically h10 or tighter).

D2: Shank diameter

This is the diameter of the cylindrical section that enters the holder. It is independent of the cutting diameter.

The practical consequence of this distinction is the following: on small-diameter cutters the shank is usually thicker. An end mill with a Ø1 mm cutting diameter is mostly produced with a Ø4 mm or Ø6 mm shank, because gripping a Ø1 mm shank safely in a collet is not possible and vibration would be inevitable.

Critical point: if a Ø6 collet is in use, the row with D2 = Ø6 is the one to take. A row with D1 = Ø6 may have a D2 = Ø8 shank, and in that case the collet will not grip the tool. The two columns in the table have to be read separately.

L1: Cutting (flute) length

This is the length of the cutting edges. It sets the maximum axial depth (ap) that can be taken in a single pass.

• For a pocket 18 mm deep, L1 ≥ 18 mm is required.
• A long L1 is not always an advantage: as the cutting length grows the tool becomes more flexible, and deflection and vibration increase. As much L1 as is needed is preferred, and no more.

Rule of thumb: once the L1 / D1 ratio exceeds 3, the cutting parameters have to be reduced.

L2: Neck / reach length

This is not present on every tool. It is the non-cutting but reduced-diameter neck section that follows the cutting portion. In deep pockets and slots it prevents the body from rubbing against the edge of the workpiece.

If there is no L2 column in the table, that product family has no neck relief, which is normal. ATH Tools tables do not show empty columns; only the dimensions actually used in that family are listed.

L: Overall length

This is the end-to-end length of the tool. It does not directly determine cutting performance, but it does determine two things:

1. Overhang: the further the tool stands out of the collet, the greater the tendency to vibrate. Deflection grows in proportion to the cube of the overhang: doubling the overhang increases deflection eightfold.
2. Reach: where a deep mould cavity has to be entered, sufficient L is required.

Rule of thumb: the shortest tool that can do the job is preferred.

R: Corner radius

This is the rounding at the corner of the cutting edge.

The corner is the weakest point of the cutting edge: stress concentrates there. Even a small radius spreads that concentration and extends tool life noticeably. For this reason a corner radius tool is preferred in roughing wherever possible.

Z: Number of flutes

This is the number of cutting edges (teeth) on the tool. It enters the feed calculation directly:

vf = fz × Z × n

At the same spindle speed and the same feed per tooth, doubling Z doubles the table feed. That is why "more flutes = faster" appears to hold. But there is a limit: as the number of flutes rises, the volume of the chip gullet falls.

Five checks before ordering

1. Is D2 compatible with the collet in use? (The most common mistake.)
2. Is L1 greater than the pocket depth?
3. Is L longer than necessary? (A short tool = less vibration.)
4. Is Z suitable for the operation to be carried out? (2–3 in full slotting.)
5. Is R correct for the intended geometry? (R = 0 where a square corner is required.)

From dimensions to tool

In ATH Tools product tables each row is a separate order code; with the columns above in view, the required dimension can be added straight to the basket. Typing a dimension into the search box above the table (8 or Ø8, for example) narrows the rows down.

Once the dimension has been settled, the cutting parameter calculator can be used to work with the right parameters; entering the cutting diameter and the number of flutes gives the spindle speed and the feed.