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Universal Pen Touch Probe

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documenting a generic tool/artpiece
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Universal Pen Holder and Touch Probe Tool Head


Having read and been inspired by Homebrew Touch Probe, The "One Penny" Touch Probe, the work done by Greenarrow and ongoing discussion re PCB Prototyping on the Rep Rap Forum. I was looking for a way to combine as much of this as possible into a useful, simple and most importantly cost effective Tool Head.

I wanted to make a tool head that could take a number of different pens, one at a time. That would tell me when the pen tip was dragging or touching on the surface of the work piece. Where the pen "touched down" destructively I wanted the tool head to give a little so as to reduce damage to the pen tip.

Recovering from a "touch down" the pen should ideally return as close to it's centered position as possible.

In achieving all of the above using the techniques used by touch probes it is a minor step forward to make a pen insert that is actually a probe and can be used to touch digitize (although very slowly) a three dimensional object.

Again a minor step forward is to use the touch tool head to check the Z Axis Bed alignment and it's degree of parallelism with respect to the carriage assembly.

All in all the components are readily and cheaply available, no special machine tools are needed. Total build cost should come in at around 10 UKP or less where you have the parts already to hand.

Making the Tool Head

Pen Holder

The Pen holder is made from the following parts :

  1. One plastic 20mm Conduit Male Adapter
  2. One plastic 20mm Conduit Coupling
  3. One piece of plastic 20mm Conduit 70mm Long
  4. Three M3 Pan Head Machine Screws with nuts and washers
  5. A light Spring of minimum ID 28mm
  6. A spring retaining disk ID 20mm by OD 34mm (Make the disk OD greater than your spring OD or it won't work)
  7. A Jubilee or Hose Clip which will wind down to at least 16mm and open up to at least 22mm

If like me you had problems finding a spring you can wind one using springy wire. I used a 20mm Conduit coupling for the former. The OD of these is approx 24mm. A 1.5mm stainless steel welding rod was used for the wire. Stainless Steel being what it is does not like being tightly wound around the former and expands to a useful ID of approx 28mm when you let go.

The collected parts should look something like this :

Pen Holder Parts

We don't really need all of the coupler just one half of it. Measure down inside the coupler the depth to the first edge of the shoulder. There is usualy a shoulder in the middle to stop the conduit going all the way through. Mark this depth on the outside of the coupler then make a cutting line around the coupler.

The easiest way to get a just about square (with the pipe ends that is) cut line around a pipe is to use a strip of paper with a straight edge. wind the strip around the pipe align the top edges together and with the mark you have measured and made on the pipe. This is then pretty much a straight edge. Run round it with a pen to make your cut line. I use post it notes as the gummed edge keeps it in place once I have lined it all up.

Making Cut Marks

Cut along your line and discard the half of the coupler that has the shoulder in it.

The best saw I have found for cutting plastics is a fine toothed joiners tenon saw. The teeth are big enough to run clog free and the blade stiff enough to give a straight cut. If you have it you can also use a Mitre Box to get better cuts.

Alternatively you could use a pipe cutter and save yourself the fuss of marking the full cut line as these usually cut fairly square anyway.

Mark up the conduit side of your Male Adapter with three marks spaced at 120 degrees around the circumference. You can use whatever measuring tools you prefer to do this or cut out the template and use this.

Insert one end of the short length of conduit into the Adapter and lightly score a drilling guide around the conduit using the edge of the Adapter as a ruler edge. Transfer the three 120 Degree markings on to the guide line you have just scored and punch centers or drill guide holes where they cross the drilling guide.

Drill Guides on Conduit and Adapter

Slide the half of the coupler you kept onto the conduit and up against the edge of the Adapter. Use the best edge ie the one you didn't cut. Transfer the 120 Degree marks to the edge of the half coupler from the Adapter. Some couplers have tiny ridges along the inner bore that effectively make the coupler a tight tapered fit to the conduit. You may need to sand these off with a bit of wet & dry wrapped round a drill bit before the coupler half will slide easily onto the short conduit.

Dismantle it all and proceed to cut out the holes that will hold the M3 bolt heads. My bolt heads had a diameter of 6mm so drill three 6mm holes though the short length of conduit where you have marked them ie at 120 degree intervals around it's circumference. Use a round file of approx 3mm Diameter and file in indents to both the Adapter and Half coupler edges 3mm across and 1.5mm deep. Again if you don't have a file of this size wrap some fine sand paper around a drill bit of the right size or close to it and use this.

The end results should look something like this:

Ready To Fit

Trial fit the pieces together but this time put the bolts in. It should look something like this:

Dry Fit With Bolts

At the opposite end of the short length of conduit from the Adapter measure down 20mm and mark a cut line around it's circumference. You will not be cutting around this line.

Use a saw to cut a cross through the end of the conduit you have just marked. Cut each cross cut down as far as the 20mm cut line you have just marked.

When you are happy Glue the plastic parts together using the Solvent Weld Adhesive produced specifically for the plastic conduit you are working with. When it is firmly set make sure the bolt heads are flush inside the conduit and do up the outside nuts as tight as they will go without damaging the plastic. This should pull the bolt heads flush into the plastic and embed them in the in any remaining soft plastic/adhesive.

The finished result should look something like this:

Pen Holder Glued

Finally, just add on the Spring, Spring Retainer Disk and hose clip and you are all done with the Pen Holder for now.

Pen Holder Fully Assembled


The Mounting is made from the following parts:

  1. One mounting plate cut and drilled as per supplied template.
  2. Three M5 bolts with two washers and a nut each, use over length bolts and trim to fitted length.
  3. Six M3 Pan Head bolts with three washers and a nut each, use over length bolts and trim to fitted length.

The Darwin Mounting is modeled off the [TobyBorlandOriginal PlyRap Toby Borland Original] I am currently building. It bolts straight on to the carriage assembly using the carriage assembly's existing mounting holes, no modification necessary.

Choose your material for your mounting, print out the template, tape it on to your material and mark, punch, drill and cut your way to something that looks like this:

The Finished Mounting Plate

I used some 8mm MDF I had already got lying around.

Insert the six 3mm pan head machine screws with two washers as spacers on the top side and one washer to the lower side, allow a little on the under side to take either solder or crimp tags and an additional nut and washer for making connections then mark and trim your M3's to length. The two washers on the top side space the pen holder off just enough to prevent the pen holder pin nuts from catching on the mounting plate.

Double check that the M3 Heads and washers that the pen holder pins will rest on are not touching or the touch sensing won't work.

Take your already constructed pen holder assembly remove the spring and retaining disk then pass the spring end of the pen holder through the mounting plate, rest each pin on the pen holder between a pair of bolt heads on the mounting plate.

Put the spring over the bottom and sandwich it between the mounting plate and retaining disk, fix it all in place using the screw ring on the adapter. Make alignment marks on both the mounting plate and pen holder. It is normal for the pen holder to be a little off center, the alignment marks will make sure that your adjustments don't interfere with each other, by ensuring you put everything back the correct way around each time you take it apart.

Tweaking time, gently bend the pins on the pen holder until the whole thing sits comfortably with each pin making a short circuit between each pair of bolt heads and the pen body is at exactly 90 degrees to the plain of the mounting plate.

Check your spring and retaining washer is not fouling any of the M3 bolts that pass through the mounting plate. Trim the contact bolts where necessary.

Finally now is the time to adjust your spring, prune off any unnecessary turns and open out the spring until you achieve a firm but not excessive pressure between the pen holder pins and the contact M3's. The washers are just larger than the heads of the bolts, you may need to file flat's on the faces of either or both the washers and bolt heads that are closest to get the clearance necessary. I didn't need to do this but they are very close.

The end result should look something like this:

Pen Holder and Mounting Assembled

Use a multimeter on the resistance setting to test each pair of contact bolts, make sure non of the pairs are shorted out or making a partial contact with each other. (Tip: remove the pen holder before doing this as the pen holder pins intentionally shorts the contacts together). With the pen holder in place on the mounting test each pair of contact pins to ensure that their is a circuit between each contact and that rocking the pen holder in the correct direction breaks the circuit.

Touch Sensing

The Touch Sensing Wiring is made from the following parts:

  1. Six crimp on ring terminals
  2. Two pieces of tinned copper wire
  3. A pair of wires to take your connection back to the controlling electronics.
  4. Six M3 Nuts.

The collected parts should look something like this:

The Wires Needed

I used standard small size insulated automotive ring terminals and cut off the insulation to make them less bulky.

Crimp terminals onto each one of the wire pairs and one each onto a piece of tinned copper wire. Fasten these four on to the M3 contact posts using M3 nuts. Bend your wire to shape so that it goes around the spring with plenty of clearance and mark up the free end of the wire. Crimp on the remaining two terminals to the free ends of both pieces of tinned wire using the marks as a guide for where to crimp them. Trim off any surplus. Connect the two terminals you have just crimped to the remaining terminal posts using M3 nuts.

The results should look something like this:

Wiring Complete

I have drilled a small hole through my mounting plate to take the wires out of the top side out of the way. You may choose to route your wiring differently.

Mount your completed tool head onto your Cartesian Bot. Check continuity with a multimeter. You may find that the closed circuit resistance is quite high. Mine was of the order of 40 to 80 Ohms. The contact surfaces of standard BZP Machine Screws are a touch grim but are adequate for our needs. Replacing the material with a better conductor and gold plating it would improve this.

The finished item should look something like this:

Finished Tool Head Mounted

In practice a closed circuit means that the pen/probe tip is in free air, an open circuit means that the pen/probe has bumped into something. For such a simple device it is surprisingly sensitive.

The probe output can be connected to an analogue or digital input. I recommend that a "Pull-Up" resistor is added connecting the analogue/digital input to either a voltage reference or the positive power rail.

Due to the wide range of contact resistance that defines the closed circuit state, if used with a digital input it could be beneficial to clean up the signal using a Schmitt trigger circuit or use a digital input that has Schmitt trigger capability inbuilt.

Alternatively use either a spare analogue input or your unused extruder heater/thermistor analogue input to measure the returned voltage and apply the software equivalent of thresholding or Schmitt triggering. A little bit of de-bounce wouldn't hurt either.


Here's a simple touch probe.

Touch Probe Parts

The parts are a piece of 16mm by 80mm acetal rod drilled out by 2.5mm drill to a depth of 20mm and a stiff pin which is actually a slide rail from the head assembly of a CD-ROM drive I junked a while back.

Touch Probe Assembled

Here it is assembled. Something which is dificult to see from any of the photographs is that the lower end of all the adapters has a slight taper filed around it. This is to centre the forward end of the adapter sleeve into the internal shoulder of the Male Conduit Adapter.

Touch Probe on Darwin

Here it is fitted, once clamped up the taper at the lower end ensures that the accessory doesn't wobble about in the holder. You may want to grind either a point or ball tip onto the touch pin. This will give you finer resolution, how sharp a point you grind depends on what material you are wanting perforate/not-perforate with it.

Pen Adapter Sleeves

An adapter sleeve is basically anything that you press into service so as to increase the diameter of your pen's barrel or length such that it fits the pen holder.

Tool Head With Pen in Holder

Here I have actually used the pens cap.

The objective is to remove any movement of the pen within the holder so that you can plot and digitize with acceptable precision.


  1. Insulating tape wound around the barrel of your pen top and bottom to make up the diameter.
  2. A sleeve made from plastic tubing that makes up the difference, ie Plastruct.
  3. Turn a sleeve from bar stock of the correct Outside diameter.
  4. Drill a sleeve from bar stock of the correct Outside diameter.
  5. Use balsa wood dowel or ordinary wood dowel as bar stock.

Here's one I prepared earlier

Pen Adapter Sleeve Parts

The sleeve is made from Acetal (Delrin) Rod I could buy the Acetal Rod quite cheaply over the internet 1 Metre cost approximately 2.50 UKP as each sleeve/adapter is around 80mm long this should be enough to make 12 for less than 0.25 UKP each.

Again the lower end has a small taper filed around it. The back end has a cross cut into it so that the hose clip grips everything together snugly. You will not that the hole for the pen is a little off center. My drill is not exactly a precision instrument. However this doesn't actually matter providing you don't move the pen in its holder during the print run.

Pen Adapter Sleeve with Pen on Darwin

Above you can see the Sleeve and Pen mounted securely on my Darwin Cartesian Bot.


The templates in PDF and DXF formats are here. Print them out on paper and then follow the directions on them to use them.

Before spending a lot of time on the templates check the printed test dimensions against what the templates say they should be just in case your printer and driver combo scales the image out of true.

More Pens and Accesories

Q. Do you keep stationary?
A. Yes until they get close then I go wild!!

Drawing Pen

We need to have some suggestions and thoughts about this particular class of pen.

Staedtler Lumocolour Pen

Red-Ink Lumocolor pens - permanent type.

These ink pens are available in permanent and non permanent varieties and come in a range of tip sizes from Broad through Medium and Fine to Super Fine.

Conveniently they are all the same body size so one adapter fits a whole bunch of pens. The pen body is a simple parallel tube approx 10mm in diameter. This makes them an easy starter pen to make a simple adapter sleeve for.

The permanent varieties are water proof and can be used as etch and photo resist ink pens. They are also readily available and relatively inexpensive.

HP Plotter Pen

There are many Plotter pens that can be used with a suitable adapter, including a drag knife for cutting sticky backed vinyl, labels etc.


The electrical contacts in this case were made using Bright Zinc Plated M3 Pan Head Machine Screws.

Logical improvements are:

  • Replace the bolts with a material that can be gold plated and electroplate them to ensure a longer contact life.
  • Screw an internally threaded tube over the M3 contact pins that has again been gold plated to improve the contact surface.
  • Add an LED to the tool head so the bed alignment can be checked without really needing the drive electronics to be present.
  • Scale up the assembly to be able to take larger pen and syringe types (create an RP variant)
  • Use the assembly with an ink resist pen to create a printed circuit board to replace the wires connecting the contacts together.
  • Create an RP version that is based on the Darwin interchangeable tool head designs.
  • Modify the contact assembly to work with an R,2R resistor ladder so it can resolve the direction of impact upon the touch sensor using A to D techniques.

Uses for the Tool Head

  • CNC Etch a Sketch.
  • Pen Plotting.
  • Checking the Cartesian Bot's Z Axis Alignment.
  • Digitizing your own hand made components for printing.
  • Photo plotting (You need to add a light pen, light source and shutter or switch to do this).
  • Simple PCB prototyping through direct application of etch or photo resists.
  • Pick and Place (You will need a pick and place pen together with vacuum source and on/off control).
  • Solder Paste Dispenser (You will need a syringe and needle dispenser combo together with a compressed air source and controller).
  • Drawing templates directly onto the material they will be cut from.


-- Main.AndyKirby - 29 Aug 2008