Blade shapes and edges explained

Most people choose a blade by what fits the handle they own. That works, but it skips the question that decides how the cut comes out: what the edge was ground to do. A blade that tears foam will slice card cleanly, and a blade that carves balsa beautifully will skate off styrene. The shapes below are the whole vocabulary, and there are only five of them.

Straight edge

The general-purpose shape, and the one most people picture when they think of a craft knife. A straight edge cuts on the draw, along a rule or freehand, and suits anything you want a clean line through: card, paper, thin plastic sheet, leather, masking film, mount board.

It is also the shape that most rewards a fresh blade. A straight edge that has gone slightly dull stops parting card and starts crushing it, and the give-away is a furred edge on the offcut rather than any change in how the knife feels.

In 4D's range this is the Straight Edge Blade #2 for medium and heavy work, and the #11 Double Honed Blade for fine work, which is the blade most people mean when they say "a scalpel blade".

Chisel edge

Ground square across the tip rather than to a point, so the cutting happens on the flat rather than at the tip. That makes it a removing and paring tool rather than a slicing one. It pushes under things: dried adhesive, a decal, a lump of filler, a mould line standing proud of a plastic part.

The chisel edge is the shape model makers reach for most after the point, and the reason is that it cannot dig in. Held flat against a surface, it takes material off in a controlled shaving instead of cutting a groove, which is what you want for cleaning up a joint without leaving a scar in the part.

4D stocks the #17 Small Chisel Blade for detail work and the #18 Large Chisel Blade where there is more to remove. The #5 Angled Chisel Blade does the same job at an angle, which helps when the surface you are paring is recessed.

Curved edge

A curve lets you cut long and sweeping without the tip catching. The contact point rolls along the edge as you draw the knife, so the cut stays continuous where a straight blade would want to be lifted and reset.

It comes into its own on anything you are cutting to a shape rather than to a line: trimming vinyl around a curve, cutting an opening in mount board, stripping back a covering film, or scraping rust and residue off a surface where a point would gouge.

The #22 Curved Edge Blade is the standard one. The Contoured Blade #25 carries the curve further, and the #10 Curved Edge Blade and #12 Mini Curved Blade are the light-duty versions for smaller work.

Saw and serrated edge

A serrated edge cuts on the stroke rather than on pressure, which matters for two opposite kinds of material. Tough, fibrous things like wood, rope and plaster need teeth to get started at all. Soft, springy things like rubber and foam get crushed flat by a downward cut and need an edge that works sideways instead.

That second case is the one people miss. If your foam is coming out with a compressed, wavy edge, the blade is not blunt, it is the wrong shape.

4D carries the #13 Fine Saw Blade for delicate work, the #15 Narrow Saw Blade for getting into tight cuts, and the Saw Blade #27 where there is real material to get through.

Double honed

Not a shape but a grind. A double honed blade is shaped, then refined again on each side, which gives a keener edge that holds up longer under repeated cutting. It is the difference between a blade that is sharp and a blade that stays sharp through a session.

The #11 and #2 are both double honed. If you cut a lot in one sitting, this is where the money goes furthest.

Two facets or three

Facets are the number of grinds that make up the cutting edge, and they are the least visible thing on a blade that changes how it behaves.

A two-facet blade has a primary bevel with a secondary micro-bevel along the edge. The micro-bevel is slightly blunter than the primary grind, which sounds like a loss and is not: it supports the very edge, so it survives repeated cuts instead of rolling over. This is the sensible default for almost everything.

A three-facet blade adds a second micro-bevel, producing a noticeably finer edge. It is the grind used on surgical blades, and it is what you want when the cut itself is the finished surface and nothing will be sanded afterwards. It is sharper and it is more fragile, which is the trade.

Which to pick

Before you buy

Shape decides what the blade does. The handle decides whether you can fit it at all, and the two are unrelated: a light-duty blade will not sit in a heavy-duty handle, whatever its shape. Excel group their handles into families and each family takes its own blades.

The compatibility chart on our Excel page lists every replacement blade against the handles it fits, so you can check before ordering rather than after.