低身高数控探测器，适用于ER20型号车床

Summary

This is a simple design for a Renishaw-style touch probe, that screws directly onto an ER20 spindle (taking the place of the collet), rather than chucking into the collet as most probes do - thus saving a fair amount of your limited Z-axis range. It was inspired by the work of Andy Pugh (see www.youtube.com/watch?v=2ia1_NKQJKs and files linked from there), both for the idea of having it mount directly to the spindle, and the idea of using rods instead of spheres for the fixed contacts. (Rods are a lot easier to mount at a precisely controlled location - just press-fit them into a hole. Also, this design captures the moving contacts once the rods are fully inserted, so the body of the probe can be a single piece - no fasteners required!)

The supplied STLs are configured for M25 x 1.5 spindle threads - but unfortunately that doesn't appear to be a detail that is required by the ER20 specification. Please measure your threads before printing!

I used 20% infill, with no support - all horizontal holes have a teardrop shape, and the two places where there is an actual horizontal overhang are small enough that your printer should be able to bridge them (and aren't dimensionally critical anyway).

In addition to the two printed pieces, you will need the following:

• About a foot of 1/8" solid brass rod. (The rods need to be highly conductive and solderable, so brass seems like the only reasonable choice. Hollow rods might work, but would be less dimensionally accurate I think.)
• A small compression spring - I used a 7/32" dia. x 11/16" spring from a Harbor Freight spring assortment.
• Some sort of 2-pin connector, plus the mating connector and wires to reach the probe input on your CNC controller. My intended connector was a 2-pin Molex 0.1" header, but I couldn't find one when the time came to assemble the probe, so I used a 4-pin header instead (with the two outer pins bent, so that the 2-pin socket couldn't be plugged into the wrong pins). You could solder wires directly to the probe, thus avoiding the need for any connectors, but I wouldn't advise that - you will have to turn the probe body multiple times to install or remove it, permanently connected wires would get quickly tangled up, and probably broken.
• Soldering equipment.

The first thing you're going to want to do with the printed parts is to test-fit them on your spindle - no use doing anything else if the threads don't match! You may need to turn the body back and forth to scrape off excess plastic on the threads, and this is going to be easier to do if none of the other parts are installed yet.

You will probably need to ream out the printed holes so that the rods will fit into them. A 1/8" drill bit works, but requires extreme care to not accidentally drill through the end of the hole. I then chucked the brass rod into my drill and worked it into and out of each hole.

The rod needs to be cut into ten pieces:

• Six pieces that are 25mm (1 inch) long, for the fixed contacts - a mm more or less won't matter. Grinding/filing a very small bevel on the end will make them easier to insert.
• Three pieces that are 15.5mm (5/8") long, for the moving arms - again, exact length isn't critical.
• One piece for the actual probe, perhaps 2" long (11mm of this length fits within the plastic hub). A longer probe increases the maximum depth of the cavity you can measure - but it also magnifies any runout or other imperfections in the system. You may want to grind down the end of the probe to a smaller point, so that you can detect smaller features with it - this would ideally be done on a lathe, to keep the point concentric with the rest of the rod.

Apply solder to one end of each of the six contact rods. You want to do this before inserting them into the plastic, as it's going to take a fair amount of heat to get the solder to initially stick to the rod. (Re-melting the solder later will take much less heat.)

Insert the three arm rods and the probe into the hub piece. Make sure they're in as far as they can go - the diameter of the arm ends should closely match the diameter of the main body. Insert the hub into the body, with the spring in between (I superglued the spring to the top of the hub, as it wouldn't stay in place otherwise). You should be able to push the hub at least a few mm into the body without it catching on anything, and the spring should be able to push it back out, past the bottom of the body. If you feel that more spring force is needed, you could glue some sort of spacer at the tip of the cavity that the spring goes into. What I did was drill a hole through the tip (there was a hole there already, due to my printer not doing bridging very well), and inserted a small screw to push the screw down a bit.

Once you're happy with the hub, hold it in place while inserting the six contact rods (NON-soldered end first!) into their holes in the body, one by one. This is going to take a fair amount of force - I could only get them about halfway in by hand, and had to use channel-lock pliers to squeeze them in the rest of the way. Make sure that the contacts go below the hub arms, and bottom out at the end of their holes. With the probe not deflected, there should be six points where contact rods touch arm rods, but no point where contact rods touch each other.

Now, solder your connector onto one of the pairs of nearby contact rod ends on the top of the body - the pair with a notch under it is the intended location, but it doesn't actually matter which pair you use. Then, bridge together the other two pairs, using short pieces of wire (I used cut-off resistor leads). The general idea is to pre-apply solder to the connector pins or wire ends, while nowhere near the printed plastic, and then quickly re-melt this solder with the solder previously applied to the rod ends. If you do manage to melt any of the printed plastic (you can see where I accidentally did this, in the close-up photo), make sure you scrape off any protrusions that might prevent the body from fitting into the spindle taper.

Install the probe on your spindle, connect the wires between it and your controller, configure your G-Code software for a normally-closed probe, and start probing! The probe is designed to handle at least 2mm of overtravel after contact in any direction, so you can use a relatively fast probing speed (I'm currently trying 100mm/min), although a slower speed may give you more precise/repeatable results.

This probe design has no provision for any adjustment for runout or other inaccuracies, which would have made it more complicated. The prototype turned out accurate enough for my needs as-is; if you're pickier than I am, the one thing you could do would be to use a needle file on the point of contact between rods. You would do this on the contact closest to the direction in which the probe is leaning.

I have supplied the OpenSCAD source code, in case you need to adjust anything for your needs. Some changes would be fairly minor, with few side-effects - changing the spindle thread pitch, changing the probe rod diameter, changing the spring diameter or length, slightly reducing the contact/arm rod diameters. But anything more than that, such as increasing the rod diameters, or moving to a different spindle size, is much more involved. You would need to manually tweak multiple angles/diameters/offsets to keep the contact rods properly contained within the probe body - I cannot imagine how the code could automatically calculate all of that.