A probing question

My new/second-hand CNC mill came with a touch probe. Basically the usual simple design with three pins at 120deg resting in pairs of balls making electical contact. Any movement and at least one contact will be broken. The probe (after breaking the original) is a 5mm ball turned on the end of a 4mm or so stem, screwed into the block holding the pins. There is a fairly strong spring keeping the pins in contact with the balls. When I first had it, there was poor contact between the pins and the contacts but all has been scrubbed with a glass fibre scratch brush and the contact is now reliable. Not exactly Renishaw but at least if I break a probe I can afford to replace it (as I can make it myself). There are quite a few similar designs around on the Internet.

I've just been trying to recalibrate with my latest replacement probe. The plastic plate holding the contacts rests on three small screws which allows the probe tip to be centred (with quite a lot of effort - it's a fiddly job). However, the best I've been able to achieve is a repeatabiity of around 0.04mm or so. I'm happy to describe how I calibrated it, but my real question is whether anyone having a similar device could comment on repeatability? I have a feeling that the spring is a bit too strong but before I play with that I wanted to compare notes. Thoughts?

Hi Brian,

I was just wondering if a wiggler probe could be used as part of a touch probe (after cutting off the ball that goes into the holder) and had a look at my set. I had never noticed that it was Imperial. So I had a look on t'interweb and they seem to be all the same. I suppose the size of your balls is not too relevant to the function of your wiggler. The shaft of the probe is 3mm, but the ball end is 1/4".

I can't comment on your repeatability problem, but a few years ago I thought about using QTC (Quantum Tunnelling Compound), which is a sponge like material that has a high electrical resistance that goes to near zero when compressed. It was withdrawn from the market as it was not reliable re the point when it went into conduction, so my simple probe design would not have worked accurately enough to be of any use.

Cheers,

Rob

I struggled with ball size for a while! The original probe was 3mm, made in one piece with the shank.So I tried to replicate it by fixing a 3mm ball to the end of the probe. Epoxy and LocTite 603 "retaining compound" failed to reliably retain it (603 used because I happened to have some). Then I realised that 3mm was a bit arbitrary so I turned a ball on the end of a bit of mild steel (knew I would find a use for my ball-turning tool eventually). Ended up about 4.78mm or something - but working to an exact size doesn't really matter as it is just a parameter set in the probing routines. It is also very difficult to adjust the thing as almost the smallest perceptible tweak of the adjusting screws shifts the probe quite a lot. However, while repeatability on a single axis is fairly good, if I rotate the thing I get different results. I suspect that it's an issue with spring strength as well as the issue that the force needed to break the contact depends on the geometry of the pins. I'll have one more go at it and then probably accept the fact that it is cheap and cheerful and for most of my purposes, adequate.

I did do a search for this info and found a long thread on CNCzone started by someone who was building a probe to measure to better than 0.02mm. He was concerned about things like thermal expansion of the materials involved and did a lot of simulation as his workshop temperature could vary a fair bit. Then he said that, for example, he needed to probe a car dashboard which might take a day or two (hence concern about temperature variation) but there was no mention of compensating for expansion of the thing being measured with respect to the rest of the kit. When you are searching for real precision, you really have to look at every detail which might be why a Renishaw probe could cost thousands.

On the subject of wigglers - I still use one on my manual mill. And very good they are too - although thinking about it, about as repeatable as my touch probe! Mind you, I am on my second wiggler after starting the mill to touch off a piece of work and not realising that it was set to 2K RPM. Probably did about 3 revs before the probe swung out, hit the work, and the probe snapped where it met the pivot ball. Not sure the probe didn't ping off the other side of the workshop... I now have a sliding-disc type rather than the ball-on-stick type.

The repeatability of your probe can be affected by the spring pressure, the length of stylus and the orientation of the probe when the reading was taken. The spring pressure can be low enough to allow a reading without a false trigger - that means anything on the machine that can cause a vibration likely to dislodge the seated stylus without the stylus hitting a correctly targeted surface. If you have a mechanically quiet machine there is nothing to stop you using a much lighter spring pressure like those in a CMM.

A probe with a kinematic location, the three point contact system you describe, will have a different lateral spring pressure according the direction of probing. That said, if the stylus is stiff enough then the repeatability should be slightly reduced at the high points of the force compared to the low points around the 360 degrees of the X-Y plane. In the Z direction the repeatability will be improved because the stylus is acting as a column, and consequently much stiffer than in the X-Y plane. The spring pressure in Z is approximately four times higher in Z than the X-Y plane - the spring is acting only in compression, and not being tilted sideways as the stylus pivots on its fulcrum as in an X-Y move.

The key to accuracy of measurement is a repeatable starting point. In a machining centre the probe is usually anchored by a spindle orientation system. That means that if you use the same contact points in measuring a surface that were used when the probe was calibrated the the readings will not vary by the effects of lobing from the kinematic location - so keep things constant. If you have a cable-attached, manually loaded probe then always use the same position of the probe in the spindle and your readings will become more consistent which is improving repeatability. I haven't seen a probe that uses an arm that stops the probe in one orientation - but an arm like a tapping head or a boring and facing head uses would do that. That would leave spindle run out as a an error - I don't think high speed spindle (routers?) have spindle orientation methods.

What's left then to cope with is the error of centre line of the spindle to the nominal centre line of the probe tip. So, if you bore a hole in a piece of material you will have the X-Y co-ordinates of the hole. Without moving the machine, lift up the Z axis and swap the boring tool for the probe. If the probing routine can tell you position as well as size, then run the routine and see what the errors are between the true spindle centre-line and the probe's version of that centre line. Those values are the X-Y displacement of the probe stylus. If you can store those you can then use them to compensate for positional readings when setting work co-ordinates from the probe values.

You could try lubricating the contacts instead of abrading them. I don't know what with though - the contacts must pass current when the probe is seated. The oil should prevent the contacts from arcing.

Cropwell: I have some QTC 'tablets' somewhere - for exactly the same thing. But I was concerned with hysteresis on re-seat - basically would the stylus go back to the same point after triggering. I've no plans to go back to that project though.

Neale: I can't see why 0.020 mm can't be achieved - but that is accuracy, and a probe can only be repeatable - it has no accuracy as such of its own. So the 0.020 mm is a function of the performance of the machine and CNC system. As for thermal stability over time you are, in my view, spot on. In that case it doesn't matter how good your probe is, thermal drift will affect the readings.

Good luck

If you make the ball bigger you can use the same calibration settings with a longer shaft, it's just about angular deflection for a given movement, some geometry should reveal the ideal length.