Guillotine knife vs shear blade: what actually differs
Both are long straight blades. Both cut with a vertical stroke. Both are sold by the same suppliers and often described with the same words — and that is exactly why buyers order the wrong one. The difference is not the shape. It is the edge angle, the steel, and what the blade is being asked to survive.
One sentence version
A paper guillotine knife is thin, hard and finely angled because paper is soft but abrasive. A shear blade is thicker, tougher and bluntly angled because steel is hard and the cutting force is enormous. Swap them and both fail, in different ways.
The five differences that matter
| Paper guillotine knife | Shear blade (metal) | |
|---|---|---|
| Edge angle | 20–23° | 25–30°, steeper for thick plate |
| Hardness | HRC 60–64 | HRC 53–62, depending on the material cut |
| Steel | Tungsten or molybdenum high speed steel (W18Cr4V / T1, W6Mo5Cr4V2 / M2) | Shock-resisting tool steel (5CrW2Si, 6CrW2Si, 9CrSi), sometimes HSS for thin sheet |
| Body thickness | 9.7–13.75 mm — the clamp has to close on it | Usually 16–40 mm, set by the machine's blade holder |
| Failure mode | Slow wear, then fine chipping along the edge | Cracking, deep spalling, edge roll-over |
Why the angles are so different
Cutting paper is closer to slicing than to shearing. The blade has to enter a compressed stack and part fibres with minimal deformation, so the edge is ground thin and polished — a rough edge tears fibres instead of cutting them, and the tear shows up as a fuzzy cut on the bottom sheets of the lift.
Cutting steel is a controlled fracture. The blade is driven through material that resists with hundreds of times more force per millimetre of edge, so the wedge behind the edge has to be thick enough not to collapse. A 20° edge in a shear would fold on the first stroke; a 30° edge in a guillotine would crush paper instead of cutting it, and the clamp would not hold the lift squarely against that much downward pressure.
Why the steels are different
Paper knives are made from high speed steel because the problem is abrasion, not impact. Coated paper carries clay and latex particles that grind the edge away; T1 and M2 resist that wear at high hardness and can be re-sharpened many times.
Shear blades are made from shock-resisting steel because the problem is impact. A shear hits a plate with a clearance of a few percent of its thickness; if the clearance is wrong or the plate is harder than expected, the edge takes a hit. A steel at HRC 64 would rather crack than absorb it, so shear blades are run softer and tougher, at HRC 53–62.
Can you grind one into the other?
Usually not, and it is worth knowing why before someone offers to do it cheaply.
- Guillotine knife reground as a shear blade — the body is too thin for the holder, and the steel is too brittle for impact. It may cut a few hundred strokes and then chip.
- Shear blade reground as a guillotine knife — you can grind the angle, but the body thickness will not fit the clamp, and the steel will not hold HRC 62 reliably. If it does fit, the edge dulls quickly because the alloy was chosen for toughness, not wear resistance.
The one legitimate case is a shop that already owns the right steel and only needs a different geometry. That is a drawing question, not a catalogue question — send the dimensions and the material being cut.
How to tell which one you are holding
| Look at the bevel | Narrow and polished → paper. Wider, coarser grind → metal. |
| Measure the body | Under 14 mm → almost certainly paper. Over 16 mm → almost certainly shear. |
| Check the holes | 4 or 5 counterbored holes in a row → guillotine clamp pattern. Slots or a single large bore → shear holder. |
| Look at the failure | Even wear along the whole edge → abrasion, paper. Local chips and cracks → impact, metal. |
Frequently asked
Is a guillotine knife the same as a shear blade?
No. Both are straight blades with a vertical cutting stroke, but a guillotine knife for paper is thinner, harder and ground to 20–23°; a shear blade for metal is thicker, tougher and ground to 25–30° or more. They are not interchangeable.
Which is harder, a paper knife or a shear blade?
The paper knife, typically HRC 60–64 against HRC 53–62. Higher hardness gives wear resistance, which is what abrasive paper demands; lower hardness gives toughness, which is what impact shearing demands.
Can I use a paper guillotine knife to cut thin sheet metal?
No. The edge will chip almost immediately, and a chipped edge on a guillotine can mark or tear the stack on every following cut. Thin sheet still needs a shear blade ground for metal.
Do you make both?
Yes. Paper guillotine knives in T1 and M2 from one piece, and shearing machine blades in shock-resisting steel to your machine model or drawing.
Send your blade dimensions and what it cuts
Related reading: T1 vs M2: choosing the steel · Steel grades for machine knives · Why guillotine blades chip · All technical guides →