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.

coated aluminum and wood etching

Today we etched a logo into the painted surface of a thermos mug and a wooden sign.

Try to use the lowest power possible and only move up in power if the paint on your aluminum surface is very thick. We want to avoid reflecting the laser beam back in the direction it came from off the metal surface, as this could potentially damage the optics and laser tube by energy redirection.

A good choice for wood is 1/8 inch Birch which can be purchased for about $14 for a 5 by 5 foot square.

The rep rate range for our lasers is between 4 and 150 kHz. Use lower rep rates for smoother edge along cut lines and depths, and higher rep rates to cut all the way through thicker (> 1/4 '') materials.

A trick Seth taught us is to purposely defocus the laser after and autofocus by backing the the Z direction by 1-2 mm (move the bed downwards). This can save you time by making your cut lines purposely not go all the way through, while at the same time broadening ("bolding") the line. This will have roughly the same effect as a medium single pass scan, but much faster.

Rotary Roller Examples

Boss Laser recommends that on some occasions when shallow etching glass, whether on flat or rounded materials, you will be better served by first attaching wet non-shiny newsprint to the glass. The challenge is that the laser chamber is dry and with air flowing briskly during operation, the engraving run must be paused periodically to take a brush to re-wet the newsprint.

Here is a video showing Seth's engraving and the use of wet newsprint

I have attempted to engrave a very detailed 5 inch bird onto a cylindrical 7 inch tall glass. I am now also engraving the main parameters of power %, scan gap interval in mm, and laser pulse frequency in kHz. This way when I engrave up to four birds on one glass, after several cycles of hand and/or machine washing, I'll know which set of parameters holds up the best.

Right now,  I am experiencing inconsistent results, even in back to back runs on the same laser with no change of tool position. I speculate that air flow consistency and possible soiling of the optical head on the relatively long 25 minute runs may cause deterioration of engraving performance. I tried to improve the former by standing attendant to the control knob on the compressor to even out the air flow manually as needed, but some flaking issues persist. I may also try wet newsprint. You can compare the two back-to-back engravings below.


First run ... relatively clean

Second run that same day, ... aborted due to excessive flaking.

Rotary Roller Tool

It is important that the laser is first completely off and powered down BEFORE disconnecting the Y-axis controller cable and replacing it with the cable to the rotary roller accessory. Failing to do so will result in the X/Y/Z limits to be incorrectly set when you have returned to linear mode after removing the Rotary Roller tool.

Materials

There is a company that sells waterproof papers

https://terraslatepaper.com/


laser engravable name bars, paperweights and card mount/holders. They also sell laser tape and Cermark tape, the latter can be used to engrave raised designs on uncoated metal.

https://www.johnsonplastics.com/


LED strips and bars for side illumination of cast acrylic etchings

https://www.superbrightleds.com/


3M company recently launches at thee CES show in January a free no account needed on-line tool to help determine what type of adhesive is optimal for joining any two substrates together. Though it is mainly geared to the medical device industry, the tool is easy and quick enough to work for any general purpose.

https://findmyadhesive.com/

Speed Calibration Tests

We suspected that our head speeds on both the 2 inch and 4 inch focal length laser were slower by about a factor of 2 versus the parameters settings indicated in software. However, after running cut and etch tests on wood using Boss Laser standardized test files, we found agreement between the Boss Laser and Build RVA timings of within several percent. Therefore, I attribute our initial concerns to a bias prompted by the very ambitious simulation times indicated in the Preview feature of RDWorks.

The standardized laser  cut and engraving test files are found here.

 Engraving on Wood

Seth also proved us some practical guidance regarding setting up parameters for working with wood.

1. In the "Cut Optimize Handle" menu, engage the check boxes for (a) ordering, (b) working inside to outside and (c) auto determine. Set your height to 600 in an up to bottom configuration.

2. You can obtain the best results by scanning/engraving perpendicular to the wood grain.

3. Good choices of wood for working with the laser are Cherry and Alder.