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How to Determine the Focal Length Measured from the Tip of the Nozzle.
Here, I describe one technique to determine the proper level of the nozzle so you can get the optimum focal length form the tip of the nozzle, not from the lens within the assembly. Trying to determine the length from the lens to the surface to be lased is very difficult because the lens is hidden within the assembly of the nozzle. This technique will allow you to determine this distance from the tip of the nozzle to the working surface.

The idea is to use a surface that is sloped. As the nozzle, while lasing, moves at a constant speed, will reveal the conical shape of the bean along the length of the lasing. The key here is to keep at a constant rate of speed, especially if you are lasing on wood. Speed is not as important if lasing on anodized aluminum since the beam of the laser must have relatively good focus on the anodized aluminum to see the mark.

Speaking of anodized aluminum, using this technique, the range of focal distance can now be determined very quickly. The distance of the optimal range can be determined by subtracting the height of the slope from the beginning of the mark and the height of the end of the mark. The range distance can then be divided by two to determine the most optimal focal length.

The aim of all this is to fashion a go, no-go gauge that can be used to set the height of the nozzle every time you put another work piece to be lased. After the determination of the optimal focal length for the lens you use, a block of material, preferably wood for it's stable characteristics over temperature differences, and form the material so it matches this distance and label the block with the lens' actual focal length so you can associate the correct block for the lens that you are using for your particular application. You could even create a block for various levels so thick materials can be lased and cut with multiple passes.

ok, so here is how to complete this task of determining the optimum focal length from the tip of the nozzle:

• Use a scrap piece of material that can be marked by the laser. The least flexible the better. Pick a length that will create a slope that will not be too steep to make it difficult to determine the optimal point, or too shallow that the optimal area is too long.
• Align this material with the x-axis, the nozzle side to side motion axis.
• Loosen the nozzle mount and slide the nozzle up at its highest point.
• Use some material to prop up one side of the marking material. The high side should be as close to the nozzle as possible just in case you have a lens with a short focal length.
• Adjust the feedrate in your control software for manual jogging at about 80 IPM, this may differ depending on the material you will be lasing.
• Put on your safety goggles and close the laser enclosure.
• Position the nozzle at one end of the sloped material.
• Turn the laser on and jog to the other end of the sloped material and then turn the laser off.
• Mark the position with the thinnest line or kerf and move the nozzle just over that position.
• Take a variable thickness item, like a book, or post-it notes and use only enough pages to fit between the optimum position and the nozzle.
• Fabricate a permanent gauge block and use it before each job.

Here is the video that explains the process of determining the optimum focal length as measured from the tip of the nozzle to the surface to be lased.

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