Touch Probe
Assembled Touch Probe positioned over a PIKA Stage
The Touch Probe is a very sensitive flexure-based touch indicator that is attached to the PIKA Z Axis, and moves when its attached probe contacts the surface of the Stage. This allows the user to determine the Probe height during use, and to level the Stage during calibration.
Technically it is a Scott-Russell flexure with the driving end attached to a flag. This flag terminates near the probe point so that it can be easily seen at the same time as the Probe’s point during use.
One drawback of this Touch Probe is that it is no longer possible to view from directly above the Probe with a microscope.
The Touch Probe can be assembled with minimal hardware and effort.
Basic tools and components for assembling a Touch Probe
The flexure is a single piece print. It requires a good 3D printer and good PLA or PETG filament to be fabricated. Source code is in gitgub as pika_touch_probe.scad and prebuilt STL files are found here on Printables.
Examples have been successfully printed on a Prusa Mk4 and a Prusa XL. If the print does not successfully deposit the first layer but subsequent layers do connect the flexures, the part can still be used. Remove excess “hair” from the print, but do not attempt to remove any wayward material from the thin flexures.
The Touch Probe is intended to be used with a standard etched μRepRap Probe Tip, though it can be used without for levelling Stages. Fabrication of the Probe Tip requires an assembled PIKA Z Axis and is described here.
The probe tip can be as fine as one needed to deposit a 4μm resin droplet, though a conical or ogival point is required. Suggested wire diameter is ~0.3mm
It is highly advantageous if the wire the probe is formed from is made perfectly straight.
You will require:
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A clean, flat work surface
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Fine tweezers
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A tool with a fine point
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Cyanoacrylate adhesive (or thin epoxy resin)
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Low-tack adhesive paper tape (“Blue Tape”)
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Sharp side-cutters
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M3 x 12mm screw
A large bench magnifier and good lighting are highly desirable.
Place the printed probe flexure flat on the work surface. It may be held still with a piece of tape if desired.
Positioning the Probe Wire in the Flexure
Tear off a convenient length of tape and keep it to hand. Put the Probe wire in the right-angled groove in the printed part, with approximately 1.5mm-2mm of the tip overhanging the pointed end of the printed part.
Ensure the wire is as flat as possible, and pushed up against the inside angle of the right angled groove. Secure the probe wire at the blunt end with the tape.
Applying adhesive to the taped-down assembly
The probe wire invariably moves a little at this point. Adjust it back into position and if necessary use more tape, but allow ~30mm of untaped probe at the pointy end.
Working adhesive around the probe wire with a pointed tool
Place a drop or two of adhesive 15mm back from the probe tip. With your pointed tool, lead the adhesive under, over, and around the probe wire to secure at least 20mm of wire. Do not come within 5mm of the probe tip, as the adhesive will creep, and cyanoacrylate glue vapours may condense on it. Add extra adhesive as necessary, which may be done in stages if desired.
Breathe on the adhesive to ensure there is trace moisture, which will initiate the curing of the adhesive under dry conditions. Due to the volume of adhesive used, it will take some time to cure and it is recommended that the adhesive be left for an hour to fully cure.
Remove the tape. If any tape is trapped in the adhesive, leave it there.
Cutting the excess wire from the probe
If your experimentation requires an electrical connection, cut it 10mm or so away from the adhesive (this use case is not discussed further). Normally though it is sufficient to cut it closer to the adhesive.
Attaching Touch Probe to Z Axis Probe Arm
Insert an M3 x 12mm screw into the assembly’s 3mm hole, inserting from the pointy end. This screw is used to connect the assembly to the Z Axis Probe Shuttle.
Assembled Touch Probe positioned over a PIKA Stage
Orient the assembly and Probe Shuttle so that the point is above the centre of the Stage, and so that you will be able to view the movement of the flag arm as it moves towards the wire probe.
The Touch Probe’s flag arm will move when the Probe Tip contacts the surface. The flag’s motion caused by overshoot is magnified by at least a factor of 10. When trying to observe contact it is essential to move the probe in increments of at least 5μm, preferably 10μm. This allows the probe to accelerate, and a sudden movement of the flag arm is more easily perceived by the human eye than gradual movement. Simply put, the flag arm twangs when you thump it.
Home the PIKA axes. Manually position the probe to within 1mm of the Stage surface (or rather the well-cleaned slide placed on it). Viewing through the microscope, lower the Probe Tip as close as possible to the surface while being able to determine that it is not in contact.
Lower the probe in steps of 10μm until movement is seen on the assembly’s flag arm (it is not necessary to have the tip in sight at this point).
Back up 15μm. Lower the probe in a 10μm increment again, watching the flag arm for any movement. If the flag moves, back up 5μm. The Probe Tip is now within 5μm of contact.
Note: Depending on the quality of the 3D print, it is possible that the Probe Tip may in fact contact the surface a few microns before the flag can be seen to move. If you determine this e.g. by observing marks on an ink-coated slide, note the discrepancy and take this offset into account.
Essentially, more accurate location of contact is determined by raising the probe 10μm, raising or lowering it by 1μm, and then moving it down in a 10μm increment again until the difference between contact and not-contact can be observed.
Depending on the accuracy of your specific hardware, this may be taken below 1μm. It is informative to know what level of precision can be achieved with your particular hardware
Positioning can be validated by seeing if it is possible to deposit a resin droplet or cause a scratch on an ink-covered glass slide at distances above and below what you believe to be zero.
The Stage is supported at three points by adjustable screws. Start by ensuring that the Stage is evenly separated from the base’s Stage Pillar by 3mm on each side (M3 screw shafts are an acceptable spacing guide). Wind each adjustment screw up and down several times during this process to bed the screws in. This should level the Stage to 100μm-200μm.
It is strongly recommended that only the pair of adjustment screws on the left side of the Stage be used for levelling, and the right-hand adjustment screw only altered as a very last resort.
The process described here levels a 4000μm square area. Once you have found the operational envelope of your particular μRepRap the principle can be applied to larger areas.
As a first pass, we will only level the Stage to the nearest 50μm. With a glass slide in place and the axes Homed, probe (4000,2000) – that’s microns - and use this as our initial zero Z height.
Raise the probe by 300μm and move to (0,0). Determine the Z height. Raise the probe 300μm and use an appropriate screwdriver to either drive the left-hand near Stage screw down, or raise it, depending on the Z height. One full turn will cause the centre of the Stage to move approximately 200μm.
Move to (4000,0), probe the Z height, raise the probe, and adjust the far left-hand screw. You may wish to note how much you turned the screws and their deviation from zero.
Go back to (4000,2000) and determine Z=0. It will have moved slightly. Raise the probe, determine adjustments to (0,0) and (4000,0).
Keep repeating this until you have achieved the desired accuracy of level. You will find yourself making smaller and smaller adjustments to the Z levelling with each repetition. If you can get to within 4μm (approximately 1/64 turn) then each square mm will now be level to within 1 micron.
Any wire probe can be used for deposition, but long, thin probe points will flex excessively when used for etching. The Touch Probe has an advantage in that contact with the Stage causes it to rise vertically slightly, rather than immediately skidding sideways as angled Probe Tips are wont to do.
A Touch Probe can be used in exactly the same way as a standard Probe Tip. However it is extremely hard to see what is happening at the point of contact as the microscope cannot be placed immediately over the probe’s tip. It is recommended that some experimentation is made with an ordinary Probe Tip, and that initially it the Touch Probe is only used for levelling the Stage. Once some experience with the μRepRap’s behaviour has been had, then move to using the Touch Probe.