Recently I have had some repeatability issues regarding focus. This is noticeable in inconsistent kerf size, as well as apparent varying laser intensity.
When trying to find the cause of this issue, I discovered that my 6mm sacrificial plywood board was slightly bowed. To eliminate this I experimented with using a 2.5mm glass sheet from a picture frame on top of the usual card or ply sacrificial material to get a flatter surface.
It should be noted that adding anything potentially reflective to the lasers path can cause unpredictable reflections, and should only be done with great care.
From these experiments I found that spacing a sheet of glass approximately 8mm above a sacrificial board creates a very reliable surface to cut on. So far, the glass has not shown any signs of wear, even after a test directly onto the glass running five passes around a 20mm circle, with a speed of 100mm/min and a power of 100%.
This also extends the life of the sacrificial material, as the laser is more dispersed when it reaches the board, and the surface is no longer critical.
These experiments also revealed a number of interesting results, which may prove useful for other more specific applications. These are as follows:
- Using glass directly on top of sacrificial card, I was able to engrave the underside of the glass with low speed and multiple passes. (100mm/min, 100%, 3+ passes)
- The glass has little effect on the lasers power. As such hazardous materials could potentially be cut in a glass enclosure, with fume extraction, protecting the user & machine.
- Focusing directly onto the glass, the lasers path can be seen traveling through the sheet, making it much easier to refine the beam width.
- With opaque materials, the ideal speed and intensity can be tuned by taking note of the amount of light that passes through the stock being cut, and the glass sheet.
Further experiments will hopefully refine the minimum spacing between glass and sacrificial board, to minimise the impact of this method on the available material thickness.
I may also trial clear acrylic on the same setup.