Three length lines, one decision
Every end mill spec sheet carries three length figures, and buyers who conflate them order the wrong tool. Length of cut (LOC) is how far the flutes extend down the tool - the axial distance over which the cutting edges exist. Overall length (OAL) is the whole tool, shank end to tip, which matters to holders and clearance. Reach is the practical figure the part cares about: how deep the tool can actually cut, which is LOC unless the holder or the part geometry interferes first. Three numbers, three different questions, and a sheet that states all three per line item is the only kind that can be quoted, ground and inspected to the digit.
What length of cut actually controls
LOC sets two things at once: the depth of a single full-flute engagement, and how much flute is available to carry chips out of a deep pass. Cut deeper than the LOC and the shank - plain, unfluted, larger than the core - rubs the material instead of cutting it, which is a stall at best and a broken tool at worst. Cut shallower and the extra flute length is dead weight you paid for in rigidity, because the section below the holder that is not cutting is free to deflect. LOC is therefore a match-the-feature decision, not a bigger-is-better one: the tool should be just long enough for the feature and no longer.
The rigidity trade
Everything in milling punishes overhang. The farther the cutting edges sit from the holder, the more the tool deflects under side load, and deflection is what turns a precise geometry into chatter, taper and dimension drift. A long-LOC tool in a shallow pocket is a self-inflicted overhang problem: the buyer bought reach the part never uses and pays for it in finish every pass. The trade runs the other way too - a stub tool forced into a deep pocket cuts deeper than its flutes or never reaches the floor. This is why the LOC family question (stub, regular, long) and the LOC value question are asked together at the desk, per line item, with the pocket depth written next to the tool line.
OAL: the holder's number
Overall length looks like the least interesting line until it bites. Holders pull the shank in to a grip depth; a collet nut, a holder body or the part itself may demand clearance above the feature; some machines have Z-travel limits that a long OAL challenges. OAL is also a packing and handling fact - long-series tools are more fragile in transit and deserve the stiffer packaging the desk specifies per program. None of this is deep engineering; it is the kind of mismatch that surfaces at the machine and costs a week. Writing OAL per line item is how it stays surfaced on paper instead.
| Line | Definition | What it decides | Common mistake |
|---|---|---|---|
| Length of cut (LOC) | Axial length of the flutes | Max full-flute depth; chip travel | Ordering longer than the feature needs |
| Overall length (OAL) | Full tool, shank end to tip | Holder fit, clearance, packing | Ignoring grip depth and Z-clearance |
| Shank length | Plain cylinder behind the flutes | What the holder grips | Assuming shank equals cutting diameter |
| Reach (practical) | How deep the tool can cut in situ | Whether the floor is reachable | Confusing reach with LOC |
How LOC interacts with the rest of the geometry
The length lines never decide alone. A long LOC in a small diameter is a flexible tool by construction, so programs compensate with lighter passes and the flute counts that suit the material - the flute count guide and the depth strategy note in depth and width of cut cover the neighbors. Chip evacuation gets harder as LOC grows, because chips travel farther before freedom, which is another reason deep-slot tools lean toward the open gullets of lower flute counts. And the holder itself is part of the system: grip length, runout and projection belong on the same page, covered by the holders reference. A tool is a system with the holder and the pass; the sheet should say so.
For OEM and custom programs
Standard catalog LOC values cover most pockets, which is why they exist. The exceptions are ordinary ground-to-order work: a LOC matched to one cavity depth, a shank length fitted to a special holder, an OAL trimmed to clear a fixture. Those are drawing lines, quoted with the per-specification quantity, sampled, and confirmed in writing - samples for ground-to-order tools typically about 15 to 25 days, bulk about 30 to 45 after approval, per the MOQ and lead time reference. What the desk will not do is guess a length from a photo; a dimension that matters goes on the drawing, and every dimension on a pocketing tool matters.
When LOC alone cannot solve reach
Some pockets defeat every flute length: deeper than a long series reaches, or walled so closely that even a necked body rubs. The standard answers are geometric, and all of them are spec-sheet lines. A neck relief behind the flutes pulls the body diameter down so the tool follows itself into the cavity. An extended shank with a relieved section changes where the holder grips without changing the cut. And the holder choice - a slim nut, a longer collet, a shorter projection - is sometimes worth more than any tool change, which is why the desk asks about holders when a reach problem arrives. All three answers change the drawing rather than the catalog, so they route through OEM grinding with the feature depth stated. What they share is the discipline this note keeps repeating: the length lines are written from the part inward, never from the shelf outward.
What to write on the spec sheet
Each line item gets the block: diameter, LOC, OAL, shank - four numbers, in that order, per item. Next to deep features, write the feature depth too; it is the fact that lets anyone check the LOC choice. If a line exists purely for reach, say "long series - pocket depth per part" so the intent survives the quote. The four numbers take thirty seconds per item and eliminate the two failures that reach causes in practice: the tool that rubs its shank, and the tool that never touches the floor. Sizes and quantities go through the quote form and return confirmed per specification in writing.