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Laser Focus
What is Focusing the Laser
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The laser tube itself produces a wide inconsistent laser beam which is not dense or powerful enough to perform effective engraving or cutting.
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It is bounced off of several mirrors and ultimately sent through a convex lens (focal lens) in the laser nozzle assembly.
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The focal lens forces the wide inconsistent beam to converge into one fine, consistent, dense and powerful laser beam at a particular distance (focal length) from the focal lens.
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The catch is, beyond the focal length the beam begins to diverge or scatter back into a wide and inconsistent beam.
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Think of the laser beam like an "X". The center of the "X" is the consistent, dense and powerful portion of the laser we want to utilize to perform engraving or cutting. The distance from the focal lens to the center of the X is known as the focal length.
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Ideally we want the focal length (Center of the "X") to be right at or just below the surface of our material.
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While the focal length does not change, the material thickness with which we are working does change. This means we have to adjust the Z height (laser bed) up or down set the correct focal length.
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Adjusting the distance of the material (Z height) to the laser nozzle in order to achieve the correct focal length is known as focusing the laser.
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As shown in the graphic below the wide inconsistent laser beam passes through the focal lens, converges and similarly diverges forming the shape of an "X". The "focal point" (center of the "X") is located at the distance known as the "focal length" from the "focal lens"
Determining the Focal Length - Ramp Test
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There are a variety of different lens options. OMTech lasers come with a single lens which has a pre-determined focal length that can be found on your machines webpage (see snip below). However, due to tolerances in manufacturing and other variables, including the distance the lens is from the tip of the nozzle, the stated focal length is not always accurate or easy to measure, although it should be close.
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The best method for determining the correct focal length is called a "Ramp Test"
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To perform a ramp test
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Locate a piece of flat material (most commonly wood) anywhere from 12" up to the same length as your bed is wide. I recommend using 1/2" or thicker to prevent the wood from bowing in the middle.
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Place the piece of material (most commonly wood) on the laser bed with a another piece of wood approximately 2" to 2.5" thick under one side (we will use the left side for this example). This creates a ramp with the high side being on the left and the low side being on the right.
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Move the laser nozzle to the left edge of the ramp
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Raise the bed to so that the material nearly touches the laser nozzle
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In Lightburn draw a line from left to right, assign a layer and appropriate power and speed settings to that line. For my 60w I usually use 200 speed and 25 power. Be sure to check you line length so it fits on your material.
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Close the lid and run the program.
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Once the program is complete, open the lid and DO NOT MOVE THE MATERIAL.
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Inspect the line that was lasered. You should see the line start out wide but eventually get skinnier until it gets to a point where it begins to get wide again. Think of it like an "X". The center of the "X" or the skinniest part of the line is where the test achieved the ideal focal distance. This is the location we want to measure the focal length.
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If the line does not go from wide to skinny to wide again (X or hourglass shape) try adjusting the height of your ramp up or down and run the program again
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Mark the location with a pencil or pen
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Using the control pad, move the laser nozzle over top to this location.
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Accurately measure the distance from the top of the material at the location marked to the bottom of the laser nozzle.
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I found it is easiest to get an accurate measurement by using a triangular style step gauge which you can make yourself on your laser. I have created a file which can be downloaded here.
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This distance measured will for all intents and purposes be your focal length.
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It is not the actual focal length (like what is shown as the focal length on the OMTech webpage above) since the focal lens is inside the nozzle at a constant, but to us, undetermined distance.
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Remember this distance as it will be used to focus your laser
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How to Focus the Laser
Manual Focus
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Lay the material flat on the laser bed under the laser nozzle.
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If necessary use magnets (found here) or some other method to hold the material flat.
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Set/hold the gauge mentioned above on the top surface of the material.
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Manually turn the knob to adjust the laser bed height up or down until the focal distance found during the ramp test is reached
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This will need to be done each time a new material thickness is used.
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Auto Focus
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Lay the material flat on the laser bed under the laser nozzle and more importantly the autofocus probe.
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If material is not under the autofocus probe, when the autofocus is run, the probe may crash into the laser bed and/or the nozzle may crash into the material.
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If necessary use magnets (found here) or some other method to hold the material flat.
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Being certain you have material under the autofocus probe, run the auto focus program by pressing the "Focus" button then the "Ent" button on your machine controller.
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This method requires a setup of the controller via Lightburn only after a ramp test is performed.
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This will need to be done if a ramp test is performed and a different focal distance is found
- Video instructions can be found below or if your prefer, step by step instructions can be found below the embedded video
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- Step by Step Instruction
- Being certain you have material under the autofocus probe, run the auto focus program by pressing the "Focus" button then the "Ent" button on your machine controller.
- Accurately measure the distance between the material and the probe nozzle. (similar to the focal distance found during the ramp test)
- Determine the difference between the focal length found during the ramp test and the offset distance measured in step 2.
- Connect the computer to the laser controller
- Open Lightburn
- Go to "Edit" --> "Machine Settings"
- Click Read
- Click Save
- This creates a backup file of the original settings. In the event sometime this inadvertently or incorrectly changed and original setting need to be restored this file can be used to perform that restore.
- Scroll down to the bottom and expand the "Vendor Settings" dropdown.
- If you receive a warning click "Yes"
- Scroll down to "U-Axis Settings"
- Adjust the value in "Home Offset (mm)". (Note that the value is in mm so you may need to convert the value found in step 2 above if you used imperial measurements)
- If the focal distance found during the ramp test is greater than the offset distance found in step 2 above, add the difference found in step 3 above to the current value in "Home Offset (mm)"
- If the focal distance found during the ramp test is less than the offset distance found in step 2 above, subtract the difference found in step 3 above to the current value in "Home Offset (mm)"
- Click "Write"
- Repeat steps 1 thru 3
- If the focal length found during the ramp test and the new offset distance found in step 2 equals 0 you have completed the auto focus controller setup. If the difference is anything other than 0 repeat steps 4 thru 6 and 9 thru 11 (skipping steps 6 and 7 so as to not save over the original saved settings). Repeat as needed
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