§1The profile is the tool
An ordinary single-point tool traces a profile by moving along a path; a form tool is the profile, and reproduces it by feeding straight in. The shape of the part is ground into the tool once and cut every time.
This inverts the usual relationship between tool and path. Where a shaped part would otherwise need a controlled tool path — a template, a copying attachment or CNC — a form tool needs only a single radial feed, and every part comes out identical because the shape lives in the tool, not the motion. That makes form tools superb for high-volume production of small profiled parts on simple machines. The costs are three, and the rest of the page addresses them: the tool is expensive to make, its profile must be corrected before grinding (§3), and it cuts along its whole width at once, so the force is high (§4).
Contents§2Flat and circular form tools
Two constructions dominate, differing in how the tool is sharpened and how much re-grinding it survives.
A flat (dovetail) form tool is a block with the profile on its end face, held in a dovetailed holder; it is simple and cheap but, once its face wears, re-grinding the top face slowly consumes the tool. A circular form tool is a disc with the profile ground around its rim (the hero); mounted to rotate slightly for sharpening, it is re-ground on a radial face, so a great many sharpenings are possible before the disc is used up — the whole circumference is, in effect, tool life in reserve. The circular form tool is therefore the choice for long production runs, the flat form tool for shorter ones or simpler profiles. Both carry the same corrected profile; only the body differs.
Contents§3Profile correction for rake
A subtle but essential point: the profile ground on a form tool is not the same as the profile wanted on the part, because the tool cuts with rake and its edge sits above centre. The tool profile must be corrected.
If a form tool had zero rake and its cutting edge lay exactly on the work centreline, the depths on the tool and the part would match one-for-one — a radial step of, say, 5 mm on the work would need a 5 mm step on the tool. But a form tool is normally given top rake for a clean cut, which tilts the cutting edge relative to the radial direction, so a given depth on the part corresponds to a smaller depth measured on the tool face. Every diameter in the profile must therefore be recomputed through the rake and clearance geometry before the tool is ground — the “depth of form” correction. Skip it and the part comes out with the wrong step heights. The correction is pure trigonometry, done once per profile, and it is why a form-tool drawing shows tool dimensions that differ from the part dimensions they produce.
Contents§4Full-width cutting force
Because the whole profiled edge cuts simultaneously, a form tool engages a wide swathe of metal at once, and the cutting force is correspondingly large — the property that most limits how form tools are used.
Cutting force scales with the width engaged and the feed. A 20 mm-wide form tool taking a feed of 0.05 mm/rev in steel (specific cutting force ≈ 2000 N/mm²) sees a tangential force of about 2000 × 20 × 0.05 = 2000 N — and the radial component pushing tool and work apart is larger still. Narrow the tool to 10 mm but feed a little harder at 0.08 mm/rev and the force is 2000 × 10 × 0.08 = 1600 N. These are heavy loads for a small turned part, which is why form tools demand rigid machines, firm workholding, generous support and light feeds — and why very wide forms are split into stages rather than cut in one impossibly-loaded pass.
§5Sharpening without losing the form
The whole value of a form tool is that its profile stays true; so it must be sharpened in a way that renews the edge without altering the shape.
This is exactly why the flat and circular constructions are built as they are. A flat form tool is sharpened only on its top (rake) face, and a circular one only on a radial face at its notch — in both cases a surface chosen so that grinding it back exposes fresh edge with the same profile behind it. Grinding any other surface would eat into the form and change the part. The rule is absolute: a form tool is re-ground only on its designated sharpening face, never on the profile itself, and the corrected profile of §3 is what guarantees the renewed edge still cuts the right shape.
Contents§6Where form tools win
Form tools are a production choice, justified by volume and repeatability rather than flexibility.
They come into their own where many identical profiled parts are wanted — turned fittings, knobs, spindles, valve components — on automatic lathes and screw machines that plunge the tool and part off in seconds. Every part is identical because the shape is in the tool, the cycle is short because there is no path to trace, and simple machines suffice because no contouring is needed. Against that, a form tool is worth making only if the run is long enough to repay its cost, the machine is stiff enough for the force, and the profile is fixed — change the part and the tool is scrap. Where those hold, nothing beats a form tool for fast, repeatable profiled turning; where they do not, a single-point tool or CNC contour is the better answer.
Contents§7Quick reference
The working core of the page on one card rack.
Principle
profile ground into tool
cut in one radial plunge
Two types
flat (dovetail) · circular
circular → more re-grinds
Correction
tool profile ≠ part profile
corrected for rake
Force
F ≈ k·width·feed
high → rigid setup, light feed
Best for
long runs, fixed profile