This is a pen holder that has the same foot print as the BullDog XL extruder.  It allows you to mount a pen on your printer to use it as a drawbot or to calibrate your printer's X and Y axes.  The pen, up to 12mm diameter, is secured in the hinged mount by a 6-32 screw.  An optional rubber band can help apply downward pressure if the weight of the pen is insufficient.  The hinge uses a 6-32 screw also.

This pen holder be used to calibrate your printer's X and Y axes very accurately.  First, tape a piece of paper to the print bed.  Next, you're going to have the printer draw a square with sides parallel to the X and Y axes.  Use as large a square as you can on your printbed- the bigger it is, the better your calibration will be.  You can do that by manually moving the extruder carriage via pronterface or other host software, via the encoder/LCD panel if your printer has one, or via a simple gcode 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 the measured values to correct the steps/mm settings in your firmware. The formula is: new steps/mm = target size / measured size X old steps/mm. After a couple iterations of that process, you should have accurate calibration of the steps/mm in both axes.

Next, check the orthogonality of the X and Y axes.  Draw a square using the freshly 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 are not perpendicular to each other.  You'll have to figure out how to make the necessary adjustments on your printer depending upon how it is built.

I made a spreadsheet that will calculate 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 have to move one end of either axis to correct the error in X/Y orthogonality.

I printed with Coex3D green ABS, 0.2mm layers, 30% infill, no support material.  I used a brim to help keep the corners on the print bed.  When assembled, the hinge should move easily, but should not allow the pen to wobble.  It will print tight- you may need to use a file or a little sand paper to open it just enough for easy motion without it wobbling.