MYOG and Repair Touch Probe Styli


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Thread: MYOG and Repair Touch Probe Styli

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    Default MYOG and Repair Touch Probe Styli

    Background
    Many times I have read sorrowful comments about ‘a drawer full of broken touch probe styli’. I have always wondered why the owners do not repair them. But can you repair a stylus?

    Well, first of all, styli are not a gift from the Gods, nor are they made on the latest bit of dubious technology (the 3D plastic printer). They are made from simple components, and they are assembled by (poorly paid) production line workers. If those workers can put together a touch probe stylus, why can’t we do the same?

    MYOG and Repair Touch Probe Styli-7426-jpg

    Next thing to consider is just how complex is a stylus? Well, it isn’t. There are just 4 parts, going from left to right:
    1. The thread for attaching the stylus (M2 in this case)
    2. The metal base
    3. The shank (the bit which usually breaks)
    4. The (ruby) ball on the end
    OK, there is a 5th bit: some glue between the shank and the other bits. Trivial.

    So how would you go about repairing a stylus with a broken shank? Pretty simple I think: remove the glue at both ends of the broken shank, make a new shank, and reassemble the stylus with some new glue. How to remove the glue depends on what sort of glue was used, but that is known technology.

    Now that you have mentally disassembled a broken touch probe stylus, made a new shank and rebuilt it, the obvious sequitur is ‘could you make the other bits’? Could you in fact make your own styli and not have to spend $50 per stylus?

    Making a Stylus
    The thread is easy: a stainless steel grub screw can be inserted into a tapped hole in the base. M2 is a standard probe thread, but bigger threads are also used. I use M2.

    The base itself is just the sort of thing anyone with a lathe could make. I made up a batch on my CNC lathe. In this case a hex base has been machined for ease of handling, but a sideways 1 mm through-hole works as well. Mind you, the hex base does stop the stylus from rolling away! The hole up the middle is 1.6 mm (tapping size for M2), opened out to suit the rod being used (see later). It does not have to be drilled to the rod diameter: you can turn the rod down a whisker to fit, leaving a shoulder.

    The shaft can be a short length of good quality pultruded carbon fibre rod – readily available from eBay or you local kite shop in metre lengths (cheaply). I used several diameters. It can also be lots of other materials, most of which are not as stiff as CF (see below). A word of caution here: don’t make the shaft too strong. It is better that the shaft breaks than the whole touch probe is destroyed.

    The ball on the end with a hole drilled in it – well, that is harder, but not impossible. So we will spend a bit of time exploring the possibilities. Note that the ball does not have to be metrology-grade ruby: lots of people have used case-hardened steel balls just as well. Some stylus vendors offer more exotic materials. But ‘ruby’ does look the ‘thing’.

    Other materials
    Before getting onto ruby balls as the ultimate touch probe endings, let me mention a couple of other materials which can be used for a Z probe (but not a 3D probe).

    MYOG and Repair Touch Probe Styli-7553-jpg

    Ti 6Al4V welding wire. This is ‘readily’ available, very hard, cheap and can be machined. A thread and a nut on the back end will allow a solid mount. Both long and short probes are easily made this way. Polish the tip with very fine paper. Note that in this case the Ti wire also serves as the stylus shaft. Other testing for a 3D probe suggests it is pretty good there too. The top three probes below are 2.4 mm Ti ‘wire’. (It comes very straight.) You should be able to get some fine detailed XY scans with any of these.

    Most biro tips contain a tiny carbide ball in the end. If you save an empty biro and pull the tip out, you can have a very hard and fine-pointed carbide-ball probe for tracing fine detail. If you mount the biro point into the base, note that you no longer have a breakable shaft: there is no safety factor for accidents. The fourth unit down here is such a tip (from a Fine Pilot ball point pen).

    PVC welding rod (at the bottom) is not the greatest material for a stylus point and shaft, but it does have two advantages. It is very (very) cheap, and it is not strong: it bends, it breaks, and (usually) the rest of your gear survives intact. It is great for developing probing software! Note that in this case the PVC is again both the ‘ball’ on the end and the shaft.

    Case-hardened steel balls, as in ball races, can be used. You need to grind a small facet on the surface of the ball to get you through the few microns of surface case hardening so a carbide drill can do the rest. Don’t try to drill straight in: whatever drill you try to use (HSS or carbide) will skate off the surface and break – or least become immediately blunt. This process works moderately well, provided you hold the ball very securely. We will come back to this option in a while.

    OK, Ruby Balls
    These look right. You can buy drilled ruby balls from the web. Sadly, I have to report that they are rather expensive for a hobbyist: I have been quoted about $20 each in small quantities. That was for metrology grade 24 balls, with a roundness tolerance of 0.6 microns and an overall diameter tolerance of 2.4 microns. (I got some samples.) But do you really need those specifications?

    For the rest of the story, see the attached PDF file.
    (OK, sneak answer: the first photo was totally MYOG, including drilling the 3 mm ruby ball. Yes, you can!)

    Cheers
    Roger
    PS: not sure whether the PDF has uploaded
    Yes it has - it is the big black blob below.

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    Default

    Awesome article in the pdf, thanks!



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    Default Re: MYOG and Repair Touch Probe Styli

    Thank you.
    It was rather educational, learning how to drill tiny holes in Ruby, at Moh 9.
    Cheers
    Roger



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    Default Re: MYOG and Repair Touch Probe Styli

    Excellent write-up. Thanks for sharing how you jigged those small ruby, this actually might come in very handy. Good stuff, thanks!



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    Default Re: MYOG and Repair Touch Probe Styli

    Start with CZ balls for practice. Ruby balls (undrilled) are still expensive.

    Cheers
    Roger



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