UniversalPenTouchProbe

Universal Pen Holder and Touch Probe Tool Head

Table of Contents

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Introduction

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 avoid 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 was actually a probe and could 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.

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 30mm (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 :-

!!!!Piccy of parts here.!!!!!

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 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.

!!!!Piccy here of post it note on coupler.!!!!

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.

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 pre punch or drill guide holes where they cross the drilling guide.

!!!!Piccy of Adapter and conduit.!!!!

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.

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 degrees 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.

The end results should look something like this:-

!!!! Piccy of ready to fit bits.!!!!

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

!!!!Piccy of dry fitted parts.!!!!

At the oposite 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 fully 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.

The finished result should look something like this :-

!!!! Piccy of Pen holder without spring and disk !!!!

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

!!!! Piccy of finished Pen Holder Assembly. !!!!

Refinements

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 realy needing the drive electronics to be present.

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)

Acknowledgments

-- Main.AndyKirby - 29 Aug 2008