What Causes a Tapered Kerf in Laser Cutting?

If you have ever laser cut a sheet of material and wondered why the edge is not perpendicular, then you have suffered from a tapered kerf in laser cutting. The usual reason for this effect is that the laser beam is not fully perpendicular to the surface of the sheet. Read on to find out what can cause this issue and some tips on how to minimise it’s effect.

What is a Kerf with Respect to a Laser Beam?

The dictionary definition of a Kerf is a slit made by cutting with a saw. In laser cutting, we are replacing the saw blade with a laser beam. So a laser Kerf is the slit caused by the laser cutting into the material. The width of the kerf will vary with respect to:

• The lens used: short focal length lenses have narrower kerfs
• The focal position: out of focus laser beams will give wider kerfs
• The power used: higher power generally results in wider kerfs as the Heat affected Zone (HAZ) burns away additional material.
• The speed of the cut: Faster cutting speeds normally result in thinner kerfs as the HAZ is minimised.

What can affect the Perpendicularity of the laser beam?

The simple answer for this is that the laser beam is not firing through the centre of the lens.

When the laser beam is off centre to the lens, the refraction of the beam on the top and bottom surfaces will cause the beam to exit at a slight angle. In some cases, you can be perfectly vertical when cutting in the x-axis, but have a kerf taper when cutting in the Y-axis and vice versa. However, there are a number of factors that can affect position of the beam with relation to the lens.

Things to check:

Before carrying out the following checks, I believe it would be advantageous to first check and if necessary level your laser bed. If you are unsure on how to level your bed with respect to the laser nozzle, then I would suggest you check out this video: Setting the Work Table Level by Russ Sadler.

Your Checklist

1. The lens is seated correctly in the lens tube.
   • Disassemble, check and reassemble.
2. The laser head is not perpendicular to the work surface.
   • I’ve seen some laser heads fitted so poorly to the guide rail that it’s almost impossible to get the beam through the nozzle.
   • Use a set square to check the alignment of the laser head with the bed. Left to right and back to front.
   • If the laser head is not perpendicular, loosen the fixings, adjust it’s position and retighten the fixings. NB: I’ve had to use shims on some laser heads to get them to align correctly.
3. The laser beam needs to be is travelling perpendicular to the work surface from mirror 3. Luckily it’s easy to check if the beam is perfectly vertical in the Z-axis.
   • make a pulse mark on some material about 10mm from the end of the nozzle
   • lower the bed by 50 to100mm and pulse again.
   • The marks should overlap. If they don’t you need to realign the Z-axis. Check out this video: Perfect Laser Beam Alignment Procedure for further guidance.
4. The laser beam needs to be passing through the centre of the lens tube, not just through the centre of the nozzle hole.
   • A quick way of checking this is to place a soft material onto the work surface under the laser head. You could use plasticine, putty, or even some damp kitchen paper towel.
   • Remove the nozzle from the lens tube, and raise the bed until the lens tube comes in contact with the material and leaves a circular mark in the material.
   • Lower the bed by 5~10mm and make a low powered pulse.
   • Check the pulse is in the exact centre of the circle. If it is not centred and you’ve already carried out items 2 & 3 above, then the laser head needs to repositioned.
     • If the error is in the X-axis, then the head needs to be raised or lowered.
     • If the error is in the Y-axis then the head needs to be adjusted forwards or backwards (in the Y-axis). NB: most laser heads do not have forward or backward adjustment, so you may have to reposition and align mirror 2 instead.
5. Once items 1~4 have been completed then you can be fairly confident that your laser beam is travelling through the centre of the lens and will travel through the lens with zero deflection.

Can Anything Else Cause a Tapered Kerf in Laser Cutting?

Many people believe that the process of the laser beam focussing down to it’s smallest diameter can cause a tapered Kerf in laser cutting. However, unless your laser beam is

• significantly out of focus,
• your laser tube has a blunt gaussian intensity, or
• you are just cutting too slowly and causing an excessive heat affected zone (HAZ) around the cut.

It’s unlikely to be a major factor affecting the angle of the kerf.

Out of Focus Causing Tapered Kerf in Laser Cutting?

Your laser beam should in most cases be focussed on the surface of the material. This will result in the smallest kerf and highest laser beam intensity. When the laser beam is significantly out of focus, you will get a larger kerf and an increased HAZ. A larger kerf means a lower beam intensity and reduced cutting performance. Most people then compensate by lowering the speed, which just increases the HAZ even further. The HAZ then has the effect of burning away at the top section of the kerf to give a “V” shaped profile.

When laser cutting medium to thick materials, particularly organic materials, you should ideally be cutting at your maximum safe power output and the highest speed you can consistently cut through the material. This has the benefit of increased throughput and reduced HAZ. Use MAX/MIN power settings to reduce HAZ at corners.

There are some who would suggest that you focus 1/3 of the depth into thick material. So on a 9mm thick sheet of material, you would set the focal length to be 3mm below the surface. However, this rarely gives any significant improvement.

Laser Tube has a Blunt Gaussian Intensity

Basically, this means that the laser tube possesses a “blunt” or low intensity laser beam. It can measure the same average power output as another tube, but give significantly worse cutting performance. In fact, it’s possible for a 50W laser with a “Sharp” beam profile to out perform a “blunt” 80W or 100W laser tube.

This is why it is so important to measure both the average power output of a laser tube using a laser power meter and to carry out a “Mode Burn Test” to determine the intensity profile of the laser beam. This is the only way to determine if you have a decent, high performance laser tube.

So, what is a Laser Beam Mode Burn Test? Here is a quick overview:

• Take a piece of clear acrylic, ideally at least 10mm thick.
• Place the acrylic in the path of the unfocussed laser beam.
• Set the laser power to the safe maximum level and the pulse to continuous.
• Pulse the laser for 5~10 seconds, depending on your power of your laser tube.
• It is advisable to have the air assist blowing across the top of the burn location (but not directed into the hole) in order to remove the smoke before the laser beam ignites it.
• If you have a sharp beam, you should get a profile similar to the “spike” on the left. If you have a blunt beam, you will get a low rounded hill similar to the profile on the left.

CAUTION!!! – Only carry this out with sufficient PPE and if you are both competent and confident in your ability to carry out this type of laser operation. There is significant personal risk in carrying out a Laser Mode Burn Test

Cutting Too Slowly

Laser cutting too slowly will result in the HAZ becoming larger and causing charring on the surface of organic materials. Excessive charring will also result in the kerf becoming “V” shaped. Sometimes it’s better to run two passes than one slow pass to cut thick material.

Conclusion

Assuming your laser tube is of good quality and provides a “Sharp” beam intensity, you should be able to minimise or remove any tapered kerf in your laser cutting in a relatively short time.

Some challenges you may encounter are limited adjustments parameters relating to the positioning of your laser head. I quickly swapped out my original laser head for one of Russ Sadler’s lightweight laser heads and adjustable mirror assemblies. It made adjusting the laser head and aligning the laser beam quick and easy.

FAQ’s

What is a Kerf in laser Cutting? When a laser beam cuts through a material, it burns away or vaporises a section of the material. This is commonly referred as the laser kerf. For a CO₂ laser source this typically ranges from 0.1mm to 0.4mm but is highly dependent on the material being cut. It’s not uncommon for foams to have a laser kerf of 1mm or more.

How to calculate kerf width of a laser cut. Fortunately, this is fairly easy to do. Laser cut a shape from the test material, a 25mm square is ideal. Measure the size of the hole (A) in the material, then measure the size of the square part (B). Subtract the part dimension from the hole dimension and divide by 2. (Kerf = (A-B)/2)

E.g. A=25.2 and B=24.8.

Kerf = (25.2-24.8)/2 = 0.2mm

What is laser cutter kerf compensation? Most Laser cutting software packages will cut along the centre of the path. As shown in the kerf width calculation above, this will result in the hole being larger and the part being smaller that the specified dimension. Kerf compensation allows you correct these differences for either the hole or the part.

For example, if you need a part to be exactly 25mm in size, you would add a kerf compensation of 0.2mm (meaning the hole would be even bigger). If you needed a hole to be exactly 25mm in size, you would need to subtract a kerf compensation of 0.2mm (meaning the part would be even smaller).

What can you expect the value of laser cutting kerf acrylic to be? A lot of this will depend on the focal length of the lens used. Thicker acrylic usually requires lenses with a longer focal length. Typically the kerf for acrylic can range between 0.15mm to 0.4mm when using lenses with a focal length of 1.5″ to 4″.

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Last updated April 25, 2024

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