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How to use mitre shears for the first square corner

Updated: October 2026

The first corner opens when the fence is set to the finished angle instead of half of it, or when the plastic creases and you squeeze again. This is how to use mitre shears once the tool is already in your hand. Peter F. drew the steps from UK guides and from failure notes on the amazon.co.uk listings in our [mitre shears comparison](/hub/lawn-and-garden-equipment/mitre-shears), on 22 October 2026. A qualifying purchase can earn a commission at no extra cost to you. Beading is on [mitre shears for skirting](/hub/lawn-and-garden-equipment/mitre-shears/sub/mitre-shears-skirting-and-beading), the saw handover is [mitre shears vs a mitre box](/hub/lawn-and-garden-equipment/mitre-shears/sub/mitre-shears-vs-mitre-box), and the layout is the [mitre shears buying guide](/hub/lawn-and-garden-equipment/mitre-shears/sub/mitre-shears-buying-guide).

The first corner opens when the fence is set to the finished angle instead of half of it, or when the plastic creases and you squeeze again. This is how to use mitre shears once the tool is already in your hand. Peter F.

Our top picks

Knipex KNIPEX 94 35 215 mitre shears

Best overall

Knipex KNIPEX 94 35 215 mitre shears
9.4/10about £66.60
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Xpert Xpert GKT03104 SK5 mitre shears

Best value

Xpert Xpert GKT03104 SK5 mitre shears
9.2/10about £15.90
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O'Shine O'Shine OS003 mitre shears

Alternative

O'Shine O'Shine OS003 mitre shears
9.2/10about £35.99
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On this page
  1. 1.The angle on the fence is half the corner
  2. 2.Check the anvil before the good piece
  3. 3.Hold, squeeze once, and read a crush
  4. 4.Lock it shut and know when to stop

The angle on the fence is half the corner

The number on the fence is the angle of the cut, not the angle of the finished corner. DIY Doctor's rule is that each piece is cut at half the corner you measured. A square 90° corner is two 45° cuts. You set 45°, seat the trim, and cut both lengths so the faces close. If you set the fence to 90°, you have cut a square end. The Knipex in this set has a 45° try square and marks at 60°, 75°, and 90°, with no 135° mark. Measure the corner, then look for half of it on the fence.

Check the anvil before the good piece

The half-angle rule fails on a bowed bed, so you prove the anvil before the length you measured for the wall. Cut a cheap offcut at the marked 45° and close two pieces into a known square. A gap that grows towards the face means the fence or the anvil is not where the printing says. O'Shine reviews describe a fence that walks. Xpert reviews describe bent guide plates. If the shear arrived like that, section 22 of the Consumer Rights Act 2015 gives you 30 days to reject goods that are not of satisfactory quality. Forcing hardwood until the blade chips is damage you caused. At work, PUWER 1998 expects a suitable tool.
Knipex KNIPEX 94 35 215 mitre shears

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Knipex KNIPEX 94 35 215 mitre shears
9.4/10about £66.60
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Hold, squeeze once, and read a crush

Open the lock, set the fence, and put the trim on the anvil so it bears against the guide. Homebuilding's sequence is the short one: set the slide, insert the stock, hold it firm, squeeze, and repeat on the mating piece. One steady squeeze is the cut. A second harder squeeze, after the plastic has already gone white, buries the edge in a crushed face. Wonkee Donkee treats that crush as a blunt blade or the wrong stock. A spare is worth fitting only when it is the spare for that model. An Xpert GKT03104 review describes a blade that needed a hammer and a drill bit as a drift, with no instructions in the box.

Lock it shut and know when to stop

The turn clasp on a slider such as the GARTOL or the TOWOT is a storage lock. You finish the cut, you close the handles, and you turn the clasp. You do not cut against the clasp. Knipex tells users of the 94 35 215 to wear eye protection and to keep the shear off live circuits. Treat cable as dead before it is anywhere near the anvil. Put the shear down when the scrap crushes, when the piece is hardwood, or when the profile is too deep to sit flat. A wide skirting board and a thick architrave are saw jobs.

Common questions

What angle do I set for a square corner?
Set 45° on each piece. A square corner is 90°, and each mitre is half of that, which is the rule DIY Doctor states for marking and the case Homebuilding uses for a square panel. Setting 90° gives you a square end. Setting 135° gives you a different joint. Measure a corner that looks out of square before you assume the room is 90°.
What does the 135° mark actually do?
The 135° mark sets the fence to 135° for that cut. It is not a shortcut for a 90° corner. Use it only when half of the corner you measured lands on 135°, which is a very open return. On a normal doorway or panel you stay at 45°.
How do I know the anvil is square before I waste trim?
Cut a short offcut at the marked 45° and close two pieces against a square you trust. A gap that opens along the face means the fence moved or the bed is bent. O'Shine reviews describe a walking fence, and Xpert reviews describe bent guide plates. If the tool arrived that way, the 30-day right to reject under the Consumer Rights Act 2015 is the consumer route.
What do I do when the plastic squashes instead of cutting?
Stop. A white crease means the blade is blunt or the section is too thick or too hard, which is how Wonkee Donkee reads a crush. Another squeeze drives the damage deeper. Fit a spare only if it matches this model's fixing. If a fresh blade still crushes the same scrap, the piece belongs on a saw. The length you creased is already shorter than you measured.
Is the safety lock on while I cut?
The clasp is for storage. You release it before the cut, and you close it when the handles are shut and the shear is going back in the box. Cutting with the lock engaged stops the blade short. Lock it to carry it, and unlock it to cut. Eye protection is Knipex's instruction for its own shear, not a test of the other brands.
When should I put the shear down and use a saw?
Put it down when a scrap of the real profile crushes, when the stock is hardwood, or when the section will not sit flat on the anvil. Wide skirting, thick architrave, and anything around the 20 mm point Homebuilding treats as the stop are saw or mitre-box jobs. A shear that will not hold a square cut is the same handover.

Related pages

Cordless shears comparison

Mitre shears

Best mitre shears UK: top pick and who should skip

Mitre shears buying guide: angles, thickness, blades

Mitre shears for skirting: thin bead and quarter round

Mitre shears vs a mitre box: when you keep the saw

Sources

Public UK sources from the research digest, last checked on 23 October 2026.

  1. Homebuilding: angled cuts on moulding and trim (retrieved 23.10.2026)
  2. Wonkee Donkee: what mitre shears can cut (retrieved 23.10.2026)
  3. Wonkee Donkee: parts of mitre shears (retrieved 23.10.2026)
  4. Wonkee Donkee: what mitre shears are (retrieved 23.10.2026)
  5. DIY Doctor: angles (retrieved 23.10.2026)
  6. Knipex datasheet, 94 35 215 (retrieved 23.10.2026)
  7. HSE: PUWER overview (retrieved 23.10.2026)
  8. Consumer Rights Act 2015, section 22 (retrieved 23.10.2026)