Pen Holder with BullDog XL Foot Print

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This is a pen holder that matches the footprint of the Bulldog XL extruder perfectly. It lets you attach a pen to your printer and turn it into a drawbot, or calibrate your X and Y axes with precision. The pen, which can be up to 12mm in diameter, is held securely by a hinged mount using a 6-32 screw. An optional rubber band helps apply downward pressure if the pen's weight isn't enough. You can use this pen holder to calibrate your printer's X and Y axes with great accuracy. First, tape a piece of paper to the print bed. Next, have your printer draw a square with sides parallel to the X and Y axes. Use as big a square as you can fit on the printbed - the bigger it is, the better your calibration will be. You can do that by manually moving the extruder carriage using Pronterface or other host software, via the encoder/LCD panel if your printer has one, or via a simple G-code file that will do it automatically. Mark the X and Y directions on the paper so you don't get mixed up later. Now measure the length of the lines in the X and Y directions. Use those measured values to adjust the steps/mm settings in your firmware. The formula is: new steps/mm = target size / measured size x old steps/mm. After a few iterations, you should have accurate calibration of the steps/mm in both axes. Next, check if the X and Y axes are perpendicular to each other. Draw a square using the newly calibrated steps/mm. Measure the square from diagonal corner to diagonal corner. Did you get the same number? If not, your printer's X and Y axes aren't perpendicular to each other. You'll have to figure out how to make the necessary adjustments on your printer depending on its design. I made a spreadsheet that calculates the error/offset in the printer axis end positions based on the measurement of the long diagonal of the printed square/rectangle and the length of the sides. You can grab it here: http://mark.rehorst.com/misc/XY_square_error_calculator.ods The spreadsheet tells you how far you need to move one end of either axis to correct the error in X/Y orthogonality. I printed this pen holder with Coex3D green ABS, 0.2mm layers, 30% infill, and no support material. I used a brim to help keep the corners on the print bed when assembled. The hinge should move easily but not allow the pen to wobble. It will print tightly - you may need to use a file or sandpaper to open it enough for easy motion without wobbling.