Laser Engraving Slate ~ first test in new shed/workshop

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user 43593

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10 Dec 2023
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Having moved some of my gubbins into the new shed I was keen to see what might needed adjusting with the use of my 4.2 watt JTech 445nm blue diode laser. Because of a delay in moving from one shed to another, I have not used my CNC machine mounted laser since the beginning of August 2023. I was anxious to test how much accuracy it had lost in the machine movement and whether the belt tension had withstood a period of remaining stationary without harming the belts by causing flat spots due to compression through weight.

I thought that an engraving on a piece of slate 300 x 200 x 5mm would be a reasonable first test. I loaded an image into Lightburn and the only processing I did in Affinity Photo, before placing the image in LightBurn, was to remove the background by changing it to an alpha channel (making it transparent) and saving the image as a .png file which supports the use of alpha channels.

Several images illustrate the test process used.

1. duke 900SS untouched ~ this is the original image file

2. centre ~ demonstrates a 3M painter's tape cross and pencil marks made at precisely the half distance from both of the X & Y edges. The laser beam is illuminated at 0.25% strength which is just enough to permit correct alignment of the beam on the centre of the item to be engraved. The laser beam may well move to the edge of the workpiece and begin the engraving because the image dimensions are known because of entering them into the LightBurn laser engraving software.

3. 3mm gauge ~ This shows the 3mm thick piece of wood I use to rest the plastic shroud upon. When I feel a slight resistance to the movement of the wood under the shroud, I know that the laser beam will be sharply in focus. When I first obtained the laser, I had set its focal point to be exactly 3mm from the base of the plastic shroud and it has not lost its focus in nearly 3 years.

4. focus ~ shows the distance between the base of the plastic shroud and the workpiece. I double checked and made sure to move the screws in the clamps on the left side further from the workpiece. I wanted to ensure that the shroud would not bump into the cap head bolt heads, which can be seen just proud of the clamps. With very little machine vibration when laser engraving, it is frequently possible to engrave a workpiece without clamp fixation. When I clamp a workpiece, I know that any deviation or error in the engraved path is not due to me forgetting to hold the workpiece securely.

5. finished ~ this shows the conclusion of the test. The image is accurately drawn for size and content with a fair amount of detail displayed. I am not satisfied that this is the best which I can do. I will improve what can be seen by increasing the laser power from 80 to 100% and possibly changing the dwell time from its current 500mm per second. A change in detail would permit a dwell time of 1000mm per second and a reduction in engraving time.

In my own experience, when slate is burnt correctly, the image takes on a golden hue that is very pleasing to the eye. It is clear to me that the detail in this image was too much for a better representation. The image was set to reproduce at 304 dpi and that has merged some of the finer details into each other. I will see what I can do with the dpi reduced by around 100 and then I expect to see a much clearer subject definition.

As usual, any comments or questions are welcome.


  • duke 900SS untouched.png
    duke 900SS untouched.png
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  • centre.jpg
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  • 3mm gauge.jpg
    3mm gauge.jpg
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  • focus.jpg
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  • finished.jpg
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