Can You Sharpen Convex Shears on a Twice As Sharp Machine?

If you own a Twice As Sharp machine or are considering buying one, an important question is whether it can be used to sharpen professional convex hair shears.

The answer requires understanding the difference between simply making an edge sharp and correctly restoring the geometry of a professional shear.

A Twice As Sharp, commonly called a TAS machine, is primarily designed for bevel edge shear sharpening. A true convex or Japanese-style shear has very different outside edge geometry, which is why professional convex sharpening is commonly associated with a Flat Hone sharpening system.

For someone learning professional scissor sharpening, understanding why these systems are different is more important than simply learning how to operate either machine.

What Is a Twice As Sharp Machine Designed to Sharpen?

The Twice As Sharp is a professional scissor sharpening machine that uses rotating sharpening wheels together with a clamping system.

That system works particularly well with traditional bevel edge scissors and shears.

A bevel edge has a defined flat surface leading toward the cutting edge. The sharpener can establish an appropriate angle and restore that bevel using the sharpening wheel.

This makes the TAS useful for many traditional scissors, barber shears and other cutting tools with beveled edges.

The wheels themselves are an important part of learning the system. Our Sharpening Wheels for Twice As Sharp Training Module explains the different abrasives and wheels used with TAS equipment.

What Makes a Convex Shear Different?

A true convex shear does not have the same flat outside bevel found on a traditional beveled scissor.

Instead, the outside surface transitions through a curved radius toward the cutting edge. This geometry is commonly associated with Japanese-style professional salon shears and the Hamaguri or convex edge.

That distinction matters because sharpening should restore the geometry of the blade rather than simply create a new sharp edge.

Our Convex Edge / Japanese Edge / Hamaguri Theory Module goes deeper into how this geometry works and why it matters during sharpening.

Can a Twice As Sharp Physically Sharpen a Convex Shear?

A sharpening machine can remove steel from a convex shear. But that is not the same question as whether the process correctly preserves the shear's original convex geometry.

This distinction is critical.

If a true convex edge is treated as though it were an ordinary bevel edge, the outside geometry of the blade can be changed. You may create a cutting edge, but you may no longer have the same convex geometry you started with.

A professional sharpener should therefore ask:

Am I restoring the manufacturer's intended geometry, or am I simply creating an edge that feels sharp?

Those are not necessarily the same thing.

Why Is a Flat Hone Used for Convex Shear Sharpening?

A Flat Hone scissor sharpening machine gives a trained sharpener a flat abrasive surface on which the outside of the shear can be worked and polished.

This allows the sharpener to control the progression toward the cutting edge while working with the curved geometry of a convex blade.

The process still requires skill. Owning a Flat Hone does not automatically mean someone knows how to sharpen a convex shear.

The sharpener needs to understand the geometry before touching the blade to an abrasive.

Our Sharpening Pads for Flat Hone Training Module covers the abrasives used during the Flat Hone sharpening and polishing process.

Bevel Edge vs. Convex Edge Sharpening

The easiest way to understand why machine selection matters is to compare the two basic geometries.

Bevel edge: A defined flat surface leads toward the cutting edge. This geometry works naturally with the controlled angle and wheel system of a Twice As Sharp machine.

Convex edge: The outside blade surface transitions through a radius toward the cutting edge. Preserving that curved geometry requires a different sharpening approach.

This is why edge identification should come before machine selection.

If you cannot confidently determine which edge is in front of you, you should not be deciding how to sharpen it based solely on the machine you happen to own.

Our Edge Types Training Module covers convex, semi-convex, beveled and serrated edges and the differences a professional sharpener needs to recognize.

Can You Turn a Convex Edge Into a Bevel Edge?

It is physically possible to remove the convex geometry and create a flatter bevel on a shear. The more important question is whether that is what should be done.

If a professional shear was manufactured with a convex edge, changing it to a traditional bevel changes the blade geometry from its original design.

Professional shear sharpening should generally be approached as a restoration process: identify what the manufacturer created, understand how it functions and preserve that geometry whenever appropriate.

Don't Choose the Sharpening Method Based Only on the Machine You Own

This is an important lesson for anyone starting a scissor sharpening business.

If the only machine you own is a Twice As Sharp, it can be tempting to treat every shear as a TAS job.

If the only machine you own is a Flat Hone, the same problem can occur in reverse.

The shear should determine the process.

Identify the shear. Identify the edge. Choose the appropriate machine and technique.

Twice As Sharp vs. Flat Hone

The difference between these machines makes more sense once you understand the edge geometry each system is intended to service.

The Twice As Sharp is particularly suited to traditional bevel edge sharpening. Flat Hone systems provide the control needed for professional convex and Japanese-style shear sharpening.

If you're researching which scissor sharpening machine you need, read our Flat Hone vs. Twice As Sharp comparison for a broader explanation of both systems.

Learn the Machine and the Edge

Professional scissor sharpening training should teach more than which button to push or which wheel to use.

A sharpener should understand edge geometry, abrasives, ride lines, tension, set, alignment and how both blades interact after sharpening.

The Shear Fanatic Sharpening Machines Training Module explains the differences between Flat Hone and Twice As Sharp systems and where each fits into professional shear sharpening.

Explore the Complete Professional Scissor Sharpening Training Program →

---

Frequently Asked Questions

Can you sharpen convex shears with a Twice As Sharp machine?

A Twice As Sharp can remove material from a shear, but true convex shears have curved outside geometry that should be preserved during sharpening. TAS equipment is primarily suited to traditional bevel edge sharpening, while Flat Hone systems are commonly used for professional convex shear sharpening.

What is a Twice As Sharp machine best used for?

The Twice As Sharp, or TAS, is particularly well suited for traditional bevel edge scissors and shears. Its wheel and clamping system allows the sharpener to work with a controlled bevel.

What machine should be used for convex hair shears?

Professional convex and Japanese-style hair shears are commonly serviced using Flat Hone sharpening systems, which allow a trained sharpener to work with and preserve the curved outside geometry of the blade.

What is the difference between a convex edge and a bevel edge?

A bevel edge has a defined flat surface leading to the cutting edge. A convex edge transitions through a curved outside surface toward the cutting edge. That geometric difference affects the sharpening process and equipment used.

Do I need scissor sharpening training to sharpen convex shears?

Convex sharpening requires understanding the blade geometry, abrasives, ride lines, tension, set, alignment and finishing process. Learning to operate a machine alone does not provide all of the knowledge required to properly service professional convex shears.