Probe Assembly (left), micrograph of etched tip for resin deposition (right)

Overview

The Probe Tip is a fine delicate wire held in a 3D printed Probe Arm. The Arm is a substrate for the delicate Probe Tip, allowing solid physical and electrical connections. Tips may be used for subtractive machining, manipulation, depositing resins and other fluids, and scanning or sensing.

The process was developed to use unrestricted, common materials, and require minimal skill from the maker. The method described may look complicated, but a Probe Tip can be etched in the time it takes to make a cup of tea - while simultaneously making the tea.

The Probe Assembly parts are 3D printed from PLA, and an etched Probe Tip (or a temporary improvisation thereof). The Tip is then attached to the Arm using a jig, and integrated with the Probe Assembly for affixing to the Z Axis Driver.

Generally, the probe is conductive, and is typically connected to the RepRapMicron's CNC Controller "Z Touch" input if required for sensing or scanning.

The process described here is intended for resin deposition and general purpose probes. Much finer probes - for μRepRap and other purposes - may be made by modifying some GCODE or using this manual technique.

3D Printed Components

Ensure you have printed the STL file from Pika Probe, which is part of the Probe-Z-Axis-Construction where the construction of the rest of the Probe Assembly is described. These parts may be printed in standard PLA and need no support material.

Simple Probe Fabrication

Probe Arm with Tip made from a 0.5mm diameter hypodermic needle

For initial experimentation, it is suggested that a crude, pre-made expendable probe tip is used. These can be found in the form of a 0.5mm hypodermic needle, or a 24-28 gauge acupuncture needle. If one is broken while familiarity is being achieved with the μRepRap, little effort has been expended. These are not suitable for resin deposition but will leave clear marks in a layer of marker pen ink.

If removing the needle from a hypodermic, note that they are generally held by a very strong but brittle adhesive. Cut through the radius of the plastic socket on the needle in several places with robust wire cutters. Hold the needle firmly near the socket using needle-nosed pliers or a haemostat - careful! Then crush the plastic socket with heavy pliers, and rotate the needle while pulling it out from the socket's remains. Crush any remaining adhesive lumps, and scrape off residue with a blade. Then proceed to Probe Tip Assembly.

Probe Etching Tools & Materials

Using the μRepRap Z Axis to automate precise probe etching

For a finer probe, a wire is electrochemically etched to a point with the required dimensions and properties. A fine, fragile, spire-like point may be required for biological manipulation or microelectronics. A short, robust conical tip for etching, and a ogival point with a roughened body for depositing UV-curing resin.

To make a tip an electrode is put in a container of electrolyte, the wire is clamped at the appropriate depth in the electrolyte, and a current is passed through the wire. This causes the wire to dissolve rapidly in a controlled manner. The geometry of the probe tip is determined by the depth to which the probe is immersed in the electrolyte, which may be a multi-stage process. The surface finish is determined by the type of electrolyte used.

Equipment

The following basic items are needed to etch the probe tip. Substitutions can be made i.e. lab bench PSU for the battery pack etc.:

• RepRapMicron
• Z Axis Driver with Probe Dipper
• Sacrificial stainless steel teaspoon
• Hammer and vise
• Good side-cutters
• Acid-proof non-conductive container approx 50ml-100ml to fit bowl of teaspoon
• 0.3mm diameter stainless or Nichrome wire
• 2x Croc leads
• AAA (alkaline) battery pack with switch
• Aluminium foil
• Plain table salt (iodised is OK)
• Hydrochloric acid ("Spirit of Salt")
• Acetone or isopropyl alcohol
• Pipette or similar to measure acid
• Tweezers
• Paper tissues

The following may be useful:

• Large bench magnifier and light
• Ammeter
• Voltmeter
• Clamps
• Hairdryer/blower
• Blocks for cribbing
• Rigid foam block
• Absorbent rag for spillages
• Another caffeinated beverage

An optical microscope of at least 100x is most useful for checking the quality of the finished probe tip. Common USB microscopes do not generally have sufficient resolution and their quoted magnification is meaningless.

Wire Selection

0.3mm or 28ga 316L Stainless Wire from a vape shop

The wire used may vary. The majority of development probes have been made from 0.3mm diameter (28ga) 316L Stainless Steel or Nichrome 80. These and similar suitable materials are obtainable from electrical components suppliers, vape shops, and food packaging hardware suppliers, or scavenged from braided electric fencing, heavy-duty marine fishing line, stainless acupuncture needles, or thin wire cables etc.

Note that online suppliers will frequently misrepresent plated copper wire (particularly from jewellery stores) as stainless steel, and a similar problem is encountered when purchasing Nichrome when suppliers substitute resistance wire. These materials are not compatible with the etching process.

Electrolyte Etchant

For Stainless Steel the etching electrolyte is a 5% by weight solution of plain table salt (sodium chloride) in water. To this 3% by volume of 28% hydrochloric acid is added. Hydrochloric acid is a very corrosive material that emits irritating fumes that should not be breathed in. It is generally sold as "Spirit of Salt" at hardware stores for removal of concrete splashes. Needless to say, read and obey the precautions on the container.

Etching can be carried out without acid, but interfering by-products will build up rapidly and the salt solution will need to be frequently changed. Once diluted in the electrolyte, the acid is relatively benign.

During development 5% nitric acid was optionally used to roughen the surface of the probe's shaft so that the maximum amount of resin could adhere to deposition probes. Unfortunately nitric acid is a restricted material in many localities due to its use in the manufacture of clandestine explosives.

Electrolyte Container

A 60ml urine sample pot (unused) was found to be most useful, though a laboratory 100ml beaker could certainly be used. The pot is acid-resistant, is sufficiently rough to attach a croc clip to, and has a lid allowing shaking and easy storage without having to decant the electrolyte. The container should allow for a depth of approximately 40mm of electrolyte.

Spoon-based Electrode

Bent and beaten stainless steel teaspoon used as an electrode

The electrode used here is a generic stainless steel spoon obtained from a charity shop. The spoon is bent, beaten, and crushed using a hammer and vise. The result should be a wide, flat electrode that sits on the bottom of the pot. It must be stable in that position. With the plastic pot, the cathode lead's croc clip was used to clip the spoon's handle to the lip of the container to stabilize it.

Do not leave the electrode in the electrolyte longer than necessary. It will be slowly degraded by the hydrochloric acid, and then there is no spoon. After removing the spoon, dry it with a disposable cloth and clean under running water with an abrasive pad before storing.

Power Source

Power pack with three AAA alkaline cells and a switch

The power source used here is 3 AAA alkaline batteries with a switch. Be aware that using other power sources that theoretically have the same voltage will not produce the same results. Battery voltage drops under load, and batteries have an internal resistance that affects the current in the electrolysis cell.

The switch is almost essential for reliability and timing purposes.

Etching Software

GCODE to run an etching sequence for an ogival tip can be found here. This gcode can be sent to the Z Axis Driver by the CNC control program (in this case cnc-js) to dip the probe in a sequence that controls Probe Tip etching.

Z Axis Driver

Remove the Probe Assembly from the Z Axis Driver, and dismount it from the RepRapMicron. The Axis must stand firmly on the bench, stabilize it if necessary, but bear in mind it may have to be lifted. Leave the stepper motor connected to the CNC controller. Set the Z Axis Driver so that it is at the mid-point of its travel.

Determining which way up to use the Probe Dipper

The Probe Dipper can be used either way up. Determine which way round will allow you to best fit your Electrolyte Container below the Axis Driver. Insert an M3 nut into the lower nut cavity and screw it to the underside of the Axis Driver in place of the Probe Assembly.

Probe Etching Process

Early probes were made by hand in an unreliable and intuitive manner with frequent failures. While this provided many insights, a reliable and repeatable method was needed. The following process was used to make a set of very similar probes:

Prepare Wire

The wire must be untarnished, straight, grease-free, and cut cleanly with good side-cutters.

Pull the wire through a tissue moistened with isopropyl alcohol to straighten and clean, and cut off approximately 70mm. Bend the last 15mm at right angles. This bend will make positioning the wire easier and denote which end can be freely handled.

Marking a cut wire

Note: The 20 second roughening was originally found to be effective at causing the probe to hold on to more resin when it was done with 5% nitric acid. Its usefulness when you only have sodium chloride etchant available is questionable, and input is sought from anyone who experiments with it. If you choose not to roughen, continue as below but do not immerse the probe 15mm and do not do the 20 second etch.

If doing the roughening etch anyway, make a mark 15 mm from the straight end to denote the area that needs to be electrochemically roughened.

Prepare Anode Croc lead

The anode (+ve) croc lead needs to have one jaw wrapped in several layers of aluminium foil. This provides a compliant, conductive surface that can hold the Probe Tip wire securely on the Probe Dipper.

With the battery pack turned off, connect the anode and cathode croc leads to the terminals of the battery pack. Insert the spoon's bowl into the bottom of the electrolyte pot. Connect the cathode (-ve) croc lead to the handle of the spoon, gripping it against the side of the pot.

Position the pot under the Probe Dipper, making sure the spoon and croc clips do not contact the Axis Driver.

Immersing wire to 15mm in preparation for roughening etch

Using the bent end as a convenient handle for manual manipulation, clip the probe wire to the Probe Dipper so that it is immersed vertically in the electrolyte up to the 15mm mark using the aluminium covered croc clip.

Turn on the power for 20 seconds. Within seconds a brown streak should be seen descending from the wire, and bubbles will appear at the cathode. This is material being removed from the probe wire walls. Turn the battery pack off.

Securing anode lead to prevent unwanted motion during automated dipping

Temporarily secure a loose loop of the anode lead to the frame of the Axis Driver to prevent unwanted movement.

The surface of the wire is now roughened, and the cut end somewhat rounded off.

Zero The probe

Appearance of Probe Tip when it contacts the surface (meniscus)

Reposition the wire so that it is less than a millimetre above the electrolyte's surface. Move the probe wire down using the Z Axis Driver in 50μm increments until the probe touches the surface. Move back up 50μm and set this as the probe Z=0. Back the probe up using the controller until the probe no longer contacts the electrolyte. Move the probe back to Z=0 and down 10μm increments until it touches the electrolyte. Back up 10­μm and set this as the new Z=0. Raise the probe above the electrolyte surface again.

Run The Etching Sequence

Heat a cup of water in the microwave or kettle.

Turn the battery pack back on. Load the etching GCODE into the CNC controller and run it. The probe should etch the first pass automatically. Turn off the battery pack. Now repeat the zeroing process. This time get to within 1μm of the surface.

Turn the battery pack on and run the etching GCODE again to do the finishing pass.

Turn off the battery pack and raise the probe from the surface. Remove the etched probe wire.

Probe Cleaning

As soon as the Probe Tip has been etched it must be washed in the very hot - preferably boiling - water to remove all electrolyte before corrosion sets in. Flick off excess water. Do not wipe the tip.

Note: Rather than air drying and risking contamination from dissolved material in the water, it is preferable to rinse the still wet Probe Tip with acetone or isopropyl alcohol and then blow-dry. Do this carefully or the probe may be blown away or bent by the airstream.

To prevent accidental damage or contamination, straighten the etched wires and set them blunt end first in a foam block. Cover for protection.

Probe Tip Quality And Selection

At this point you may choose to examine the probe tip and/or make several other spare Probe Tips. Your actual results will vary according to the type of wire, electrode distance, contaminants from the metals in the electrode and so forth. There are two important requirements:

1. Your Probe Tips are consistent.

2. The actual point of the probe should be smooth and the cone should have an overall ~60 degree angle.

Two different Probe Tips with 300μm shafts

The Tips above were deliberately over and under etched on the first etch pass. Outwardly they appear substantially different. Either would be acceptable, but the overall outline of the upper probe is preferred for resin deposition despite the aggressive pitting.

Closeup showing similar shape of Probe Tips

If we look at the last 100μm of the very tips, we see that they are smooth, well-formed, and of very similar angles. Remember that deposited resin layers are only a few microns thick. However, using a mix of different probes will not produce consistent deposition work. Determine what you can produce consistently. Use the others for destructive experimentation and subtractive micromachining etc.

Probe Arm Assembly

The Probe Arm stabilizes and retains the Probe Tip. It has a screw protruding which electrically and physically connects the Probe Arm to the Probe Beam while allowing exchange of the Probe.

Hardware Required

The following is needed to fit the Probe Tip to the Arm:

• Probe Arm
• Probe Tip
• Probe Assembly Jig
• Adhesive paper tape
• Cyanoacrylate adhesive
• Pointed tool
• Small flat-bladed screwdriver
• Tweezers
• Side-cutters
• M3 nut
• M3 12mm screw
• Screwdriver for above screw
• Multimeter

Hypodermic Tip Preparation

If using a hypodermic needle as the probe, pliers or haemostats suitable for bending the needle will be needed.

Place the needle in the Probe Arm such that its base just reaches the hole and the tip overhangs the thin end of the Arm with the hole in the tip uppermost.

Mark the location where the tip overhangs the Arm. Remove the needle and bend the tip down approximately 60 degrees.

Place the Arm in the Probe Assembly Jig, put the needle in the Arm's groove point down and continue at the glueing step below.

Probe Tip Attachment

Fixing Probe Tip into Arm, held in Probe Jig

Place the Probe Arm in the Probe Assembly Jig recess, groove uppermost. Lay the etched wire Probe Tip In the groove across the top of the Arm and Jig so that it lays flat in the groove, with the etched tip level with the transverse mark on the Jig. It may be necessary to straighten, kink, cajole, and generally modify the blunt end of the wire lying on the Jig to make the wire in the Probe Arm lay flat. When correctly located, fix the blunt end of the wire to the flat part of the Jig using adhesive paper tape.

Apply liquid cyanoacrylate adhesive to the centre of the groove in the Probe Arm. Use a fine pointed instrument to carefully work the adhesive around and under the probe wire. Ideally the wire from the centre up to the small end of the Probe Arm should be completely encased in adhesive. Add more adhesive if needed, but do not allow adhesive to go onto the overhanging Probe Tip or underneath the Probe Arm. As the adhesive has been applied so liberally, it may take tens of minutes to set solid.

Starting the bend in the Probe Tip

With a small flat screwdriver or similar, bend the overhanging Probe Tip wire at its base through approximately 60 degrees. Ensure the bend is as close to the Probe Arm's thin end as possible. Make sure the Probe Tip does not collide with the sides of the vertical slot in the end of the Probe Jig. Do not touch or bend the actual pointy end of the tip.

While holding the Probe Arm in place, remove the adhesive paper tape. Replace it over the Probe Arm leaving the Arm's nut hole exposed. If you have correctly bent the Probe Tip down, it will not even come close to contacting the paper tape.

Trim the excess wire, leaving enough to form a half-loop around the nut hole. Form said half-loop with tweezers and tuck it into the nut hole.

Inserting M3 screw with nut into Probe Arm

Take an M3 x 12mm screw and insert it into the hole in the Probe Arm via the underside of the Probe Jig. There is a convenient notch in the Jig that allows you to hold it in place with a fingertip. Thread on an M3 nut and screw it down.

Tightening nut and screw on Probe Arm

Align the nut with the nut cavity in the top of the Probe Arm and tighten firmly with a screwdriver, trapping the wire underneath. The Probe Jig may safely be used as a spanner if you press the Arm into the Jig with your thumb. Tighten it very firmly.

Check with a multimeter that there is electrical continuity between the screw and the probe's wire.

Integrating Probe Arm with Probe Assembly

At this point, the Probe Arm may be integrated with the Probe Assembly by securing it to the underside of the Probe Shuttle with an M3 washer and M3 nut. This provides something to hang on to while finishing the shape of the Probe Tip.

Final shaping of Probe Tip

Very carefully use tweezers to bend the last couple of millimetres of the probe tip by approximately 30 degrees. The objective is to cause the point to touch the slide perpendicularly when the Probe is in use.

The Probe Assembly may be returned and taped to the Probe Jig for temporary protective storage.