Concentric Circles: Reasonable combinations of speed and laser repetition rate as a function of circle diameter

I ran a test for 16 concentric circles cut by the 4 inch focal length laser at 30%. The objective was to find how the curvature would dictate the best set of laser rep rates in kHz and laser head sweep speeds. By "best" I mean achieving the smoothest edge surface for a easy pen trace while stenciling to avoid skips and unevenness in the trace.

Speeds varied from 10 mm/sec for the smaller diameters to 30 mm/sec for the largest diameters.

Laser rep rates varied from 10 kHz to 140 kHz; the larger the diameter the higher the rep rate.

Diameters varied from 0.5 inches to 8 inches.

I centered the 10 by 12 inch sheet right in the middle of the laser chamber bed for optimal and even airflow to remove plume debris during operation.

This album has photos from the test.

The reasoning here is that the tight curves of smaller circle need a slower head speed to give the 30% power a chance to cut through and lower rep rate as the circumference is small and high rep rate laser firings would just ablate on top of each other and create a sloppier edge. As the diameter increases, the circumference also goes up and you can run a faster head to avoid being wall clock time delayed but a higher rep rate to be sure you cut through and cover all of the circumference.

This time I also affixed painter's tape to the back side of the extruded plexiglass sheet to minimize or eliminate the occurrence of burn/char marks and surface and edge chipping. No burn/char marks were found on any of the 16 cuts and less than 5 chips over all 16 circles of a small variety were observed. They were almost really like a micro-cracks that will not interfere with pen tracing.

Here is video of the laser cutting a few of the circles.

Click to see a tabulation of the results. By "better" I mean the edges had a glossier finish while "acceptable" has cloudy finish.




 

Practical Usage Notes about the Boss Lasers

1. Consider 1/8th inch Birch for prototyping your designs; a 5 foot square block is about $14.

2. The kHz repetition rate range of the lasers are 4 to 150. Use the smaller rep rates for smooth edges and the high rep rates for cutting.

3. Output Direct setting is the only time that the min and max values being different matters. Otherwise the laser only cares about the maximum value for  power objectives

4. Frequent and heavy duty users can consider buying their own lens kit.

5. Purposely de-focusing the laser by moving it by one or 2 mm away from the autofocus can create wider/bolder lines for artistic expression.

6. Orient the wood grain perpendicular to the X-Swing direction for best results.

7. High density polyethylene (HDPE) tends to melt.

8. After engraving wood with the laser, you can clear coat it for preservation but do not use any liquid wood stains.

9. After working with plastic substrates, keep the laser lid closed after job completion to allow the fumes to be fully evacuated.

10. The 4 inch focal length laser is generally better at making clean cut throughs, especially for thicker materials. The schematic below shows a somewhat exaggerated view for how the 2 inch focal length is somewhat trickier to autofocus for cutting purposes.