Coil-over Pen for CNC Plotting

I made this pen holder for my DIY CNC when I was at the Detroit Maker Faire.  I plotted mazes and invited kids to solve them by using a Playstation controller to drive the machine.  This pen design allows for the machine to adjust to small variations in the paper surface and maintains a consistent force at the tip.


REQUIRED MATERIALS FOR CNC PLOTTER PEN ASSEMBLY

(1) PILOT ULTRA FINE POINT PEN OR SIMULAR (ANY ART STORE)
(1) 1/2” OD ALUMINUM TUBE (LOWES)
(1) 3/8“ RUBBER CORD HOLE FINISHER (LOWES)
(1) LIGHT LOAD SPRING BETWEEN 3/8” AND 1/2” DIAMETER (LOWES)
(1) PIECE OF ELECTRICAL TAPE


CNC PLOTTER PEN

Many times it is useful to use your CNC machine to draw a layout or to plot information onto a surface. I made this plotter pen out of things I had laying around and it worked great, so I thought it was worth passing on the information of how I built it. The major goals when I set out to make my pen was that it needed to fit into the collet of my router, be able to adjust for small variations in the height of the plotting surface, and be able to cap the tip while not in use.

I sourced the 1/2” OD aluminum tube at Lowes. They have it at Home Depot, but the interior diameter is smaller, not allowing for a nice fit for the pen I chose. If you are using a different pen, this might be an option for you as you want a very close fit, keeping your pen as steady within the holder as possible.

The pen I used is a PILOT ultra fine point felt tip. I wanted to go with a felt tip pen because of plotter tradition but also because ballpoints can produce a dead spot when the pen changes direction. The PILOT pen also has a very straight body and fits almost perfectly into the aluminum tube.

The spring is the hardest thing to get perfect. I bought a multi pack of 25 different size/shape springs at Lowes that had a spring that worked great. Basically, you want a spring that applies the amount of force you do when you are writing. Too much and you’ll crush the tip, too little and you won’t make a consistant mark.

The 3/8” cord hole finisher is an example of something I had laying around that worked great. Really, the only purpose it serves is as a stop for the spring. This could be achieved a variety of ways, but the rubber finisher is quickly removable in case you need to swap out your pen.

This is a very straight forward and simple solution to the problem using a coil-over approach to adjusting the pen to uneven terrain and allowing for some flexibility for zeroing the machine before you begin the plot.


THE ASSEMBLY

To begin building the pen assembly, cut the 1/2“ aluminum tube into two pieces, one tube 2.25” long and one .25” long. The 2.25” tube will be the sleeve that allows the pen to move up and down. The shorter tube is the stopper that stops the pen from coming out of the assembly. Because of the close fit of the pen in the tube, the tolerance might need to be increased by passing a drill bit through the inside of the 2.25” tube. The pen should be able to slide freely inside of this tube but still be tight.

The shorter piece of tube should be glued onto the end of the pen (if necessary). I was able to friction fit the piece on the end of the pen as the plastic body is flared to accept the plug at the end of the pen. Since this piece will want to move freely when it is in the chuck, the exterior diameter wants to be slightly less than the tube that will be held by the collet. I achieved this by putting the pen with the small piece of aluminum installed on the end into the chuck of my drill press. Using a piece of emery cloth, I ‘turned’ down the OD.

This slight difference can be seen in the drawings.

Once this is done, slip the pen into the aluminum sleeve, then into the spring, and last install the rubber hole finisher slightly compressing the spring. Next, apply a piece of electrical tape over the end of the aluminum and the foot of the spring. This keeps the spring from slipping out of place.

When putting the pen into the collet, don’t bottom out the pen, leave some space inside to allow for vertical movement. The collet should grab the longer tube allowing the smaller OD aluminum ring, as well as the pen, to move freely.

When you zero your machine, compress the spring slightly.


The PDF version of this instructable can be downloaded HERE
More information on the DIY CNC can be found HERE