Touch probe

thingiverse

When using a CNC machine, it's only a matter of time before you'll need a touch probe to detect movement in any direction on the top of the stylus. This device can be used to find the edges of stock, locate the center of holes, and even determine the top surface of the material (Z-probe). Renishaw touch probes are considered the gold standard, but they come with a hefty price tag of £1,000-£2,000. Fortunately, if you have 3D printing capabilities and some mechanical skills, you can build a basic touch probe for under £20. I've designed a prototype to explore the mechanical challenges of this device, and it's been working well after assembly. The design is optimized for CNC milling, which ensures better accuracy and rigidity. The stylus can be made from a screw or created using a lathe, but it's worth investing in high-quality styli with ruby tips. I've found a reliable supplier that offers excellent products. In my design, I used the TH M3 030 04 031 model, which worked well. However, there are some areas for improvement. The use of 6mm balls and 12x6 rolls from bearings was not ideal, as smaller diameters would require softer springs to achieve reliable galvanic contact. Ideally, the spring should be conical in type, with a wider base and narrower tip where it touches the movable plate. Additionally, if parts are made from rigid materials, extreme precision is required to prevent misalignments that can cause the probe to stay open. The balls and cylinders used in my design are from bearings, which are precise and hardened. However, I've found that with time, oxidation and corrosion can increase resistance. To mitigate this, it's recommended to use stainless steel balls and cylinders if possible. To connect the balls electrically, M2.5 screws and wires were used. If the moving part is 3D printed or milled from plastic, all balls can be connected in series. However, if the part is milled from aluminium, each ball should have its own wire to the electronic board. Detecting a single ball disconnection can be done using various methods, including resistor bridges and comparators. I've found an interesting project called Penny probe, which uses a 1p coin as a sensor plate. This design shows supreme precision and reliability in vertical directions but is less reliable in horizontal directions. Ideally, the central shaft should always rest in the same position to ensure reliable operation in all directions.