Laser Goniometer

CAUTION: Lasers are not toys Do not point any laser toward a person’s eye or a pet’s eye. Any laser has a very real risk of causing eye damage within a very short time (possibly as short as a blink reflex). Staring into such a beam for several seconds is likely to cause damage to the eye. Do not point any laser toward civil or military aviation. This can cause temporary blindness in the aviators, and is illegal in most locations. Do not point a laser toward a person’s body. Pointing a laser at a person’s body could be perceived by that person as coming from a weapon’s laser sighting system. That person may be armed and fire their weapon at you.

Overview

Jan Švancara's document on building a Laser Goniometer outlines a great way to measure bevel angles with a simple laser device.

This device makes those measurements very precise. And his use of kids toys (Erector Set) to make it lends to a bit of fun.

3D-printed laser goniometer

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Bottom of laser goniometer

Alternate Version

I've developed a 3D-printed design using a laser easily sourced from Amazon (and possibly available from other sources as well).

The jig consists of 3 pieces:

1. Scale: used for reading the angles (the white part in the picture to the right).
   • There is a hole built into the scale for hanging it. This is shown in the lower right part of the first picture.
2. Laser: a strong, rechargeable laser (the red part in the picture to the right; the selected one is actually coloured black).
   • A thumb screw holds the laser in place on the scale.
   • The laser has a rechargeable lithium ion battery, and uses a USB charger to recharge the laser’s battery. Such a charger is not included.
3. Magnet Tool Holder: a part which uses magnets to hold the knife being measured (the blue part in the picture).
   • The magnets are mounted so that they and the retaining screws’ heads are below the surface of the bracket, helping to reduce the chances for scratching the knife’s surface.
   • Two thumb screws hold the tool holder in position against the scale, and allow it to be realigned as necessary.

The lower picture to the right shows the bottom side of this tool. The two black thumb screws are used to hold the bracket (blue part) against the scale (white part). The bracket can be adjusted to better align the knife’s edge relative to the laser which is held in a fixed position in the scale.

Usage Notes

Aligning the Knife Blade with the Laser

Double-bevel Blade			Single-bevel Blade
Properly Aligned	Improperly Aligned		Properly Aligned	Improperly Aligned
Centered: Light beams equi-distant from the 0° mark	Rotated clockwise: Light beams unequal distance from the 0° mark		Properly aligned: Left light beam at the 0° mark.	Rotated counter-clockwise: Left light beam away from the 0° mark.	Rotated clockwise: Right light beam too far due to over rotation.

When aligning the knife in the scale, you need to rotate the knife in the scale until you have proper alignment of the reflected laser beams.

The pictures to the right show blade alignments which are both correct and incorrect.

Aligning Single-Bevel Blades

This is especially key when measuring a single-bevel knife. With this style of blade, you rotate until the flat side’s dot is on 0° and then read the other side’s dot. If you continue to rotate the blade, the left side will stay at 0° but the right side will increase, giving an incorrect reading.

Aligning the Laser

Rotating the laser

You may find that you need to rotate the laser within the hole on the scale (the white part in the picture to the left). It may not be perfectly aligned with 0 on the scale when no blade is being measured.

The rotation of the laser is shown in the picture to the left with blue arrows.

How it Works

The laser light reflects off the blade’s sharpened edge, and shows the blade’s edge angle on the scale. There are two laser reflections shown, one on each side of the scale. Those represent the edge angle (β) for that side. The sum of the edge angles is the included angle (α).

When using this, you will probably also see extra laser dots around 0°. These are the result of the laser’s beam being wider than the knife’s blade. Please ignore those.

Double-Bevel Knife Edge Notes

Double bevel knife edge

For knives which have a double-bevel edge, there is a real probability that the blade may be held in a manner where it is not aligned correctly relative to the laser’s light beam. If this is the case, one side may reflect a different measured angle than the other side.

This not a problem.

Example Calculations

If the left side measures 12° and the right side measures 16°. We can average these two angles to determine the actual bevel angles (β) for the knife.

These calculations are: \begin{align*} \tag{1} β &= \frac {12° + 16°} {2}\\ &= 14°\\   \\ \tag{2} α &= 2 \times β\\ &= 28°\\ \end{align*}

As a side note, there could be a knife where the two bevel angles are not equal. In that case, you may need to be more dilligent in measuring the separate bevel angles.

Single-Bevel Knife Edge Notes

Single bevel knife edge

For knives which only have one beveled edge, the angle measurement is different. Japanese marking knives are like this, as are the Felco/Swiss Army gardening knives.

As you would expect, there are two separate bevel angles. These are denoted as β₁ and β₂ for this discussion.

As expected with knife blades like this, one side should be 0°, and no averaging of the angles is required. However, in the case where one angle is not 0°, you will need add the two measured angles to determine the bevel angle.

Example Calculations

If the left side measures 12° and the right side measures 2°, the bevel angle (β) is determined by adding the two measured angles. \begin{align*} \tag{3} β_1 &= 12° + 2°\\ &= 14°\\   \\ \tag{4} β_2 &= 0°\\   \\ \tag{5} α &= β_1\\ &= 14°\\ \end{align*}

Sources for Buying One

You can purchase one of the 3D-printed versions from Colvin Tools.

Notes on Making One

The jig I made is comprised of two 3D-printed pieces, with a laser and a few other parts attached. The Fusion 360 file (f3d) is also supplied in case you would like to modify this design for different sizes or styles of lasers.

Right-click on the links below to save the noted files.

The following parts are also needed. For the Amazon-sourced parts, links are provided to the currently-available parts.

The following tools are needed to tap the holes in the scale (white part) for the thumb screws.

1. Tap, M6 x 1 mm thread
2. Tap wrench to hold the tap