If you want to laser engrave the bottom of a bowl, or vase, or goblet or whatever, and you want the engraving centered, here’s a way I do that at the TechShop,http://www.techshop.ws.
Take note: The events described in this Instructable took place on an Epilog 60 watt Laser Engraver on an olive wood bowl. Power settings and speeds will probably be different for your situation, so practice on test pieces before committing something you value.
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Start by measuring the bottom diameter.
Use this measurement to make a simple pattern in Corel Draw.Create a circle of the same diameter as the bowl bottom and add some tabs for taping the pattern in place. In the center of the circle create a small, about 1/16” dia., circle to use as a target. Set the “pen size” of this small circle to larger than “hairline”. Use the laser to print the target circle and to cut the pattern out of a sheet of paper.
Tape the paper pattern to the bowl bottom and place the bowl on the laser bed and level it as necessary.
Disable the X-Y drive and using the “red dot” set “home” on the small target circle on the pattern. Focus the laser on the pattern. Reset to exit focus mode.
In Corel Draw compose the text you want to engrave on the bowl bottom. In set-up choose “raster” and set the power low; start at about 2%, and set the speed to 50%. Close the laser lid and hit “run”.
If you are satisfied with the results, carefully lift the tape and remove your pattern, without moving the bowl.
Reset the laser power level to about 40% while keeping the speed at 50% and engrave your text, centered, on the bottom of your bowl.