Shearing Machine Cutting Defects Bow Twist Camber

Introduction

In precision sheet metal fabrication, there is nothing more frustrating than shearing a narrow strip of metal, only to have it curl up like a corkscrew.
These defects are technically known as Twist, Bow, and Camber. While many operators immediately blame the hydraulic system or the machine frame, the issue is almost always related to physics, setup, or worn-out shear blades.

As a leading manufacturer of Hydraulic Shearing Machines, JSTMT has analyzed thousands of cuts. In this guide, we break down exactly why these defects happen and how to eliminate them.

 

Here are the 5 most common causes and how to fix them.

Defining the Defects: 

Before fixing the problem, you must identify the specific type of distortion.

• Twist: The strip curls into a corkscrew shape. This is common when cutting narrow strips.
• Bow: The strip curves upwards or downwards (vertically) at the ends, forming an arch.
• Camber: The strip curves sideways (horizontally), looking like a banana.

 

1. Cutting Strips That Are Too Narrow

The most common cause of twisting is simple physics. If you are cutting a strip that is narrower than 8x the material thickness, the metal will almost always twist.

The cause of Twisting is attempting to cut a strip that is too narrow relative to the material thickness.
When the upper shear blade comes down, it applies “Rake Force.” If the material is wide, the rest of the sheet holds the strip flat. If the strip is narrow, there is no material to hold it back, so the Rake Force twists it.

• The Rule of Thumb: To avoid twist, the width of the cut strip should be at least 10x to 15x the material thickness.
• The Fix: If you need a 10mm strip of 3mm plate, you cannot shear it perfectly flat. You must shear a wider piece and machine it down, or use a laser cutter.

 

2. Incorrect Rake Angle Setting

The rake angle is the slope of the upper blade. A higher rake angle requires less force but causes more twisting.

• High Rake Angle: Requires less hydraulic force (good for thick plate) but causes MORE twisting.
• Low Rake Angle: Requires more force but produces FLATTER parts.The Fix: If your machine has an adjustable rake angle, reduce it to the lowest setting possible for that thickness. This flattens the cut.

 

The Solution:
If your machine has an adjustable rake angle, always use the lowest angle possible that can still cut through the material.

• Example: Do not use the same angle for 2mm steel that you use for 10mm steel. Lower the angle for the 2mm sheet to eliminate the bow.

 

3. Blade Gap Clearance is Too Wide

If the gap between the upper and lower blades is too wide, the metal will not shear cleanly. Instead, it will “roll” or fold over the bottom blade before fracturing. This creates a large burr and causes the part to Bow (arch).

 

The Solution:
You must calculate the precise gap based on the tensile strength of the metal.

• Action step: You must adjust the gap based on the material thickness. Read our technical guide on [How to Adjust Shear Blade Gap Clearance] .

4. Dull or Chipped Blades

This is the most overlooked factor. A sharp blade cuts; a dull blade “smashes.”
If your shear blades are dull, they apply excessive vertical pressure to the metal sheet before the cut actually happens. This vertical stress stays inside the metal strip, causing it to bow downwards after it falls behind the machine.

 

The Solution:
Inspect your blades daily. If you see shiny, rounded edges, it is time to rotate or replace them.
Upgrade Tip: If your blades are dulling too fast (especially on Stainless Steel), your material grade is too weak. Switch to [H13 High-Carbon High-Chrome Shear Blades] to maintain a sharp edge 3x longer.

The Solution:
You must calculate the precise gap based on the tensile strength of the metal.

• The Fix: Check your blades for shiny rounded edges. If they are dull, you can try rotating them to a fresh edge. If all 4 edges are worn, you must install new [High-Precision Shear Blades]  to restore a straight cut.

 

5.Grain Direction (Material Stress)

Sometimes, the machine is perfect, but the metal is the problem.
Steel sheets have a “Grain Direction” created during the rolling process at the steel mill. This creates internal stress.

• The Test: If you cut a strip north-to-south and it twists, try cutting a strip east-to-west.
• The Fix: Rotate your sheet 45 or 90 degrees. Cutting across the grain often releases the stress differently, resulting in a straighter part

 

Summary: How to Get the Perfect Cut

To eliminate Bow, Twist, and Camber, follow this checklist:
1. Ensure the strip width is at least 10x the thickness.
2. Reduce the Rake Angle to the minimum setting.
3. Set the Blade Gap correctly .
4. Ensure your blades are sharp.

 

Need Replacement Knives?

If you have reduced the rake angle and checked the gap, but your parts are still twisting, your blades are likely the culprit. At JSTMT, we manufacture [6CrW2Si and H13 Shear Blades] designed to hold a sharp edge longer, reducing the friction that causes twisting.

Twisting is often the first sign that your blades are worn out. JSTMT manufactures precision-ground [Replacement Shear Blades] compatible with Amada, Accurshear, LVD, and JSTMT machines.