How to specify an industrial machine knife

Industrial machine knives, the guillotine blades, slitters, granulator knives and shear blades that cut everything from coil steel to plastic regrind, are ordered by the thousand across UK industry and almost never by part number. The machine is often decades old, the OEM may be gone and the drawing, if one exists, is a photocopy in a maintenance office. So the knife is ordered as a description, and the difference between a usable enquiry and a week of clarification emails is whether that description carries six things.

Three dimensions, in the trade's order

The convention is length, then width, then thickness. A guillotine knife written as 820mm, 100 x 12mm is 820 long, 100 wide, 12 thick, and the leading standalone dimension before the knife type, as in an 820mm guillotine blade or a 100mm slitting knife, is always the length or the diameter. The trap for a reader, human or software, is that the length often arrives separated from the section: the 820 in the subject line, the 100 x 12 three sentences later, and a bore or a hole pattern in between waiting to be mistaken for one of them.

The steel and the hardness

The grade names are a small, stable vocabulary: D2 for most general blanking and shearing work, M2 high speed steel where the edge must hold at temperature, O1 for easier grinding, 52100 and 440C in particular niches, carbide tipped and powder metallurgy grades at the hard end. The grade alone is half a statement; the working spec is grade plus hardness, written as a range, 58 to 60 HRC being the common shape. Two knives in the same D2 at different hardness behave as different products: harder holds an edge longer and chips more readily, softer forgives abuse and dulls sooner. A supplier quoting without the hardness is guessing on the customer's behalf.

The edge is its own specification

The edge carries three decisions: the geometry, single bevel or double bevel, the finish, straight ground or hollow ground, and any tooth form, serrated or scalloped, for fibrous materials. The bevel angle in degrees completes it. These are not cosmetic: a single bevel knife cuts against a fixed anvil, a double bevel meets a second knife, and swapping them does not work. It is also the part of the spec most often missing from an enquiry, because the person reordering can see the length and measure the thickness, but the original bevel angle ground onto the knife is invisible without a gauge.

Replace is only half the trade

A knife is a consumable with a service loop. Most run a regrind cycle, several sharpenings before the section falls below its working minimum, and the words resharpen and regrind in an enquiry signal a servicing job with its own price, turnaround and collection logistics rather than a supply line. A well run knife account is really a rotation: one set in the machine, one set at the grinder, one on the shelf, and the supplier who tracks the rotation owns the account.