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IndustryArena Forum > GENERAL MANUFACTURING PROCESSES > CHUCKING and MEASURING > 3D Probe - Tri-balls type accuracy & DIY
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  1. #1

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    Aug 2019
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    3D Probe - Tri-balls type accuracy & DIY

    Good morning all,

    I'll have to reverse engineer quite a few non-prismatic parts this autumn (3D contours, radius, etc) and gearing up for a cloud point system.
    I do have a budget for this, however, and judging from the extremely HIGH cost of some relevant equipment, I find it hard to justify the expense for what I need to scan.
    Another problem is the constraints I'll have to work within... e.g. probe's OD can't exceed more than 31mm on a critical part and most (if not all) of the probes within my price range are rather let's say bulky, some literally over-sized and heavy, others [no comments!].
    That leads me no choice but to DIY my own 3D measuring probe (/end of short-story-made-long lol)

    My question is: what sort of accuracy are you getting with the classic DIY 3 balls/3 poles type probe. I'm aiming at 0.01mm~0.02mm (which is by far the maximum my milling machine can cope with when up to temp).

    Realistic? Share your numbers!

  2. #2
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    Re: 3D Probe - Tri-balls type accuracy & DIY

    Mine's rough but repeatable to about 0.05mm. If I built another one now there's a few things I'd pay much more attention to:
    - Material selection: I used a carbon fibre tip and I suspect it's a lot more elastic, so it deflects more before it trips;
    - Spring selection: I used whatever is handy and I think it's a little stiff - there's a sweet spot where it's just stiff enough to ensure re-engagement but any more and it takes too much force to trip (hence deflection in the probe);
    - Concentricity. I can dial in the tip but that puts the probe shaft on a bit of an angle so I can only test at the tip. Not what you need on some side engagement tests;
    - Size of triangle. Wider = more accurate because levers; Mine's about a Ø45mm circle because I had Ø60mm delrin stock to turn the enclosure for the case. It seems about right, I'd be leery of getting smaller.

    I think your 31mm constraint might be your biggest drama but care in your build and using a commercial probe shank and tip should easily get you in that range of accuracy.

  3. #3
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    Re: 3D Probe - Tri-balls type accuracy & DIY

    My CNC has a basic single-step resolution of 0.98 microns.
    With a Z-axis probe I can see that in a profile of ground stock.
    With a 3D probe the resolution is a bit larger - maybe a few microns.
    I made both the 3D units and the Z-axis units (several of each).

    Do you need a full 3D probe, or would the XY abilities of your CNC combine adequately with a far more simple Z-axis probe?
    You might also like to read my thread on MYOG stylii, at MYOG and Repair Touch Probe Styli . Some of my stylii use ruby balls, while other use CZ balls or even balls from a race. I had to drill them myself.

    Cheers
    Roger

  4. #4

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    Aug 2019
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    193

    Re: 3D Probe - Tri-balls type accuracy & DIY

    @dharmic - way (way) much more useful information than asked for, I appreciate the time you took to help out with this. Thanks man!
    I can sorta live with a huge max of 0.02, which is, well, what my machine can cope with on a very good day. I was afraid to get 0.05 which would not work for me though...
    I'll try to make two types; a lightweight micro-size one (OD30mm) with a light comp spring and, a heavy-duty one with weight and see what comes out winning. Only way to find out in-the-real-world (CAE aside) is to make them both :/ what a pita lol
    For the stylus; you've convinced me to go for the ceramic shank and SS mating screw type, so I'll see how this performs first. Thanks for the heads up

    @RCaffin - I had a look at your thread and found it quite good and informative, thanks! I will certainly give this a try for when I'm fed-up of breaking my tips due to my often (recurrent) over-travelling mistakes ;-)
    In my case I need the full 3D capability unfortunately. I'll try to document the built process... I'll shoot you a note when I do so you can keep me on track!

  5. #5
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    Re: 3D Probe - Tri-balls type accuracy & DIY

    Is what you are probing electrically conductive?

    On small machines (benchtop sized) what's sometimes done is to ground the part, and detect when the probe tip gets grounded out... the method has it's drawbacks, but it is compact...

    Not that it helps you... but... something that I've thought about on occasion is not using a switch closure, but detecting stylus deflection using a more 'analog' approach... perhaps capacitively, similar to how digital calipers work; inductively with something similar to a LVDT; or optically by a photo-interrupter sort of arrangement. All of these would require creating a custom PCB and doing some micro-controller programming...

  6. #6
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    Re: 3D Probe - Tri-balls type accuracy & DIY

    I was going to mention the strain gauge style probes which measure force applied to the shaft in terms of much, much smaller deflections than the three pairs of balls and give better accuracy too, but (like the cap sense and others) they add orders of magnitude more complexity to the probe which would make me baulk from attempting a home build - and I'm an electronics engineer by training. If the simple ball switch arrangement will yield the required accuracy and I believe it will, I'd keep it simple.

    Good point on the electrical side though __Britt - a simple conductive touch probe would make things a lot more compact (and simple), even if you were to drop a spring and rocker plate in there to save tips in case of overruns.

  7. #7
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    Re: 3D Probe - Tri-balls type accuracy & DIY

    A while back I ran across this probe on the web and, if I was buying one, I would really like to try this one.
    The videos on the site are very informative and I learned a lot about probing. The price is higher then the average 3 switch probe, but no where near the cost of some.

    https://hallmarkdesign.co.nz/probe

  8. #8

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    Re: 3D Probe - Tri-balls type accuracy & DIY

    @Britt - a whole lot will be composites and anodized surfaces. I actually wanted to bring-in a conductive shank of a known diameter once... to touch off work offsets (vise's clamp?) but that idea of a divorce with my Haimer/3Dtaster crunched that project real quick ;-)

    As you and @dharmic mentioned, there are far better tech avail but its all down to time-vs-money isn't it. Decoupling/filtering those noisy environments aka 10/12/16bits resolution ADCs, C code for the SPI, or whatever bytes those new ICs needs to be interfaced with... I can see a whole lot of R&D work which would then justify getting the $5k Renishaw solution and just call it a weekend.

    You guys just reminded me... that is a NC switch those tri-balls circuit :/ Looks as if I'll have no choice to throw in a custom pcb to invert the signal anyway (got a tool setter on the machine that I'd like to keep). arggghh...

  9. #9
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    Re: 3D Probe - Tri-balls type accuracy & DIY

    Renishaw use strain gauges these days, which explains both their sensitivity and their price.

    I did look at the idea (and I do have the strain gauges and the necessary electronics workshop), but eventually I found better things to do. In fact, 99% of the time I use a wobbler of known diameter for zero alignment and rely on the balls screws for the rest.

    BUT: any good ideas????

    Cheers
    Roger

  10. #10

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    Re: 3D Probe - Tri-balls type accuracy & DIY

    The wobbler is by far the winner in this category. Properly adjusted and spun at the right rpm its just always spot on. Simplicity often makes perfect! Too bad he doesn't do Z or I'd have mine back.

    It terms of ideas; the capacitive sensor would be the cheapest and easiest to implement. Although would require semi-serious mechanical & electrical engineering and most likely precision machining and processes also, which would then, again, shoot you up close to the Renishaw price range once completed (i.e. time). Strain gauges in those sizes and orientation aren't all exactly cheap to my knowledge (requires custom-made, ideally). In all cases we are talking about very low voltage (µV) so... lots of work at the interfacing part also :/

    As for myself/my req; I'm sticking to the tri-balls for now. I'll crack on with a OD30mm along with what appears to require a custom PCB now, and see how that one goes.

  11. #11
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    Re: 3D Probe - Tri-balls type accuracy & DIY

    Could also use an inductive or even hall effect sensor maybe, to measure tiny deflection.

    As far as the NC/NO is concerned Mecanix- I just used a couple resistors and a cheap FET (2N7000?) inline on the cable from the probe socket in the electronics enclosure back to the BOB.

  12. #12

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    Re: 3D Probe - Tri-balls type accuracy & DIY

    Quote Originally Posted by dharmic View Post
    Could also use an inductive or even hall effect sensor maybe, to measure tiny deflection.

    As far as the NC/NO is concerned Mecanix- I just used a couple resistors and a cheap FET (2N7000?) inline on the cable from the probe socket in the electronics enclosure back to the BOB.
    Good call on the hall sensor. I'd need to dbl-check this but I'm sure I've seen new ICs that detect linear motion and rotation (magnetic).

    How wonderful! and its just now you tell me I could have simply used a fast(enough) switching fet to invert the signal LOL Just joking, just upset I had to spend 3+ hours finalizing the PCB an hour ago (not bigger than a coin, OD26mm). Had to throw in a TH Led (for boot/live testlight mostly) and a 4mm MCU in the kit for calibration and sensitivity adjustments (I plan on measuring the tri-balls independently, e.g one set of balls = one mcu pin). Decoupled but no room left for a ferrite bead I'm afraid. I'll deal with noise at the control I guess...

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