3D Probe - Tri-balls type accuracy & DIY

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    Member Mecanix's Avatar
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    Default 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!

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    Member dharmic's Avatar
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    Default 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.



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    Default 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



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    Default 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!



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    Default 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...



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    Default 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.



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    Default 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



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    Default 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...



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    Default 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



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    Default 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.



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    Default 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.



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    Default 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...

    Attached Thumbnails Attached Thumbnails 3D Probe - Tri-balls type accuracy & DIY-3dprobe_pbc_v1-jpg  
    Last edited by Mecanix; 08-20-2019 at 08:16 PM.


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

    Um ... having been down that path several times - just how do you intend to attach the balls to the PCB holes? You will need to use very hard balls: brass need not apply imho.

    Cheers
    Roger



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

    Quote Originally Posted by Mecanix View Post
    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.
    If you have trouble getting the accuracy you need then you could buy this probe https://www.kurokesu.com/shop/TPA2 which has a 2um repeatability

    Mactec54


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

    Quote Originally Posted by RCaffin View Post
    Um ... having been down that path several times - just how do you intend to attach the balls to the PCB holes? You will need to use very hard balls: brass need not apply imho.
    That's an excellent question. The whole idea of using Vias on the PCB is to have the pcb manufacturer to tin the wall of those 6 holes. I guess the plan is to secure those precision R2 copper balls with solder paste from under the PCB. That way I can hot-air gun the kit and precisely adjust those balls on the flat granite table. A friend recommended me to drill the 6 balls (?!) and attach them to the body of the probe with a set screw. I doubt I'll go this/his route as thermal exp would ruin this sort of set-up overtime. If you have any suggestions by all means, now or soon could be a really good time to share your ideas... I'm just about to send over the gerber to my pcb mfg and run these solids in the lathe ;-)

    Understand this is not a final design; more or less a proptotype to validate the data I've pulled out from several thermal, linear & non-linear analysis. Once I have this proto in hands, and validate how it behaves I'll re-do it with fatter this & shorter that, etc...

    @mactec54 - I'm doing the smallest 3D Probe in-the-World! Will crunch out 1um or less when done I appreciate the link up, unfortunately the advertised probe is way too big for some of the parts I'll have to angle & scan on my table.

    ps. quick preview of what came out from the cad side so far. I'll resume this tomorrow and hopefully turn a prototype this coming weekend - if the weather is bad and worth hiding in the shop lol

    Attached Thumbnails Attached Thumbnails 3D Probe - Tri-balls type accuracy & DIY-3dprobe_protov1-jpg  


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

    Quote Originally Posted by RCaffin View Post
    Um ... having been down that path several times - just how do you intend to attach the balls to the PCB holes? You will need to use very hard balls: brass need not apply imho.

    Cheers
    Roger
    To attach the balls, apply the solder past to the pads on the PCB, place the parts, then preheat the balls to just above the reflow temperature of the solder paste. Place the balls on the pads. They should melt the solder. Then put the whole thing in the oven and let the rest of the solder reflow.

    Do a normal cool down.

    Done.

    BTW, you want to make sure that your balls are completely clean before you heat them.



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

    Are they still vias when they're that big, or just plated through holes? Not that it makes any difference. I used stainless bearing balls. No solder, rather, a 3d printed "cage" which holds the balls in place (with the PCB about 1/3 of the way up the ball when seated) and springs up to the back of the enclosure cavity to push it down onto the balls.



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

    Quote Originally Posted by dharmic View Post
    Are they still vias when they're that big, or just plated through holes? Not that it makes any difference. I used stainless bearing balls. No solder, rather, a 3d printed "cage" which holds the balls in place (with the PCB about 1/3 of the way up the ball when seated) and springs up to the back of the enclosure cavity to push it down onto the balls.
    You got me there. They come out as vias on gerber therefore I would assume the tin'ing process being the same regardless of the hole size (3.8mm dia in this case). Drilling is the same process though, I'm guessing. Really don't know how they manufacture those... as long as they are tin'ed I'm happy. The method @maxspongebob best/better described is all what matters e.g. reflow and let cool on the flat granite surface. Let's see...



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

    Quote Originally Posted by Mecanix View Post
    That's an excellent question. The whole idea of using Vias on the PCB is to have the pcb manufacturer to tin the wall of those 6 holes. I guess the plan is to secure those precision R2 copper balls with solder paste from under the PCB. That way I can hot-air gun the kit and precisely adjust those balls on the flat granite table. A friend recommended me to drill the 6 balls (?!) and attach them to the body of the probe with a set screw. I doubt I'll go this/his route as thermal exp would ruin this sort of set-up overtime. If you have any suggestions by all means, now or soon could be a really good time to share your ideas... I'm just about to send over the gerber to my pcb mfg and run these solids in the lathe ;-)

    Understand this is not a final design; more or less a proptotype to validate the data I've pulled out from several thermal, linear & non-linear analysis. Once I have this proto in hands, and validate how it behaves I'll re-do it with fatter this & shorter that, etc...

    @mactec54 - I'm doing the smallest 3D Probe in-the-World! Will crunch out 1um or less when done I appreciate the link up, unfortunately the advertised probe is way too big for some of the parts I'll have to angle & scan on my table.

    ps. quick preview of what came out from the cad side so far. I'll resume this tomorrow and hopefully turn a prototype this coming weekend - if the weather is bad and worth hiding in the shop lol
    30mm dia is not even close to the smallest in the world Renishaw TP6 basic model is 25mm dia and they make smaller than this

    1um yes anything is possible in the right environment

    Mactec54


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

    Quote Originally Posted by mactec54 View Post
    30mm dia is not even close to the smallest in the world Renishaw TP6 basic model is 25mm dia and they make smaller than this

    1um yes anything is possible in the right environment
    I was hoping you'd noticed the blink at the far end of my sentence

    Yea, I know about that TP6... received a quote 2 or 3 weeks ago from the local re-seller here and nearly had a heart-attack while falling off my chair. Still traumatized today... so so so much money for something so dumb that will eventually(most likely) smash in the side wall of your vise.

    Wait! we'll do the Tri-Compact model in the OD24.5mm with a 16bits 'something' not long after the one I'm prototyping now (I'm in the mood LOL)



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