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

  1. #121
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    Default Re: 3D Probe - Tri-balls type accuracy & DIY

    Quote Originally Posted by Mecanix View Post
    No thank you very much!

    See excessive thermal expansion between 20dC~30dC (10dC diff.). The worst materials you can possibly use to engineer a probe, or any precision measurement tools, are composites.
    Errr, isn't FR4 a composite? Perhaps I misread but it seems you intend to use your PCB to mount the balls. Could be why Renishaw use a (presumably high grade) plastic for their internals. You might look at high stability plastics rather than crappy Delrin if you believe normal plastic isn't up to scratch.



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

    Quote Originally Posted by RCaffin View Post
    Yes, plastics have a significant thermal expansion, but it would be symmetric around the central axis. I don't see (yet) why a plastic centre should affect the accuracy much?

    Cheers
    Roger
    I don't know, I haven't tested it in a real-life setting so I can't confirm Roger. Just kept failing the initial sim validations here so I didn't bother and omitted the plastic all together. In fact I still have those sims on hdd, let me see if I can convert all of those 1D objects (RB2/3, spiders, nodes, etc) to a readable 3D mesh format so folks can make sense of them and I'll post them up for you guys to understand what's going on with those plastic parts in there (pom,abs,poly,pvc,pp,pe, etc all the same really). I also have the spring/damping/force simulations data coupled with a motion sim, which I'm sure you'll be interested to see.

    Quote Originally Posted by Muzzer View Post
    Errr, isn't FR4 a composite? Perhaps I misread but it seems you intend to use your PCB to mount the balls. Could be why Renishaw use a (presumably high grade) plastic for their internals. You might look at high stability plastics rather than crappy Delrin if you believe normal plastic isn't up to scratch.
    FR4 (glass) is indeed a composite, per definition. By far one with the lowest thermal expansion coefficient also, hence the main reason it is used widely for micro-size circuit/traces (and holding probe balls). Well documented everywhere, I'm not inventing any of that stuff and very much likely this is going nowhere within +/-10dC ;-). And I do not know about Renishaw's tech and/or their materials, so I can't comment. I'm sure they know what they're doing and have engineered the correct mat comps/formulas for their particular application.

    Last edited by Mecanix; 09-11-2019 at 09:48 AM.


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

    I don't think you should be measuring anything on a space with 10dC temperature variations..



  4. #124
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    Default Re: 3D Probe - Tri-balls type accuracy & DIY

    Quote Originally Posted by jkkmobile View Post
    I don't think you should be measuring anything on a space with 10dC temperature variations..
    lmao... very true! Unfortunately my super duper non-high-tech shop here is not all that environmentally controlled. And this probe is prepping to launch for a 24/7 digitizing journey into point cloud space! Some parts might actually take a few days to scan (i.e. reverse-engineered). PS: Among those parts is a full size car dash and console to scan down to +/- 0.02~0.01mm (to put things into perspective).

    We do get pretty bumpy morning/afternoon and night/day temp variations in the autumn here... so no excuses to not have it designed half-decently from the start ;-) Just can't see myself re-calibrating this probe 3x per day.



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

    @Mecanix

    What matters most is any asymmetric temperature distribution. That is, if one side is hotter than the other side. This should not apply to your probe.
    I predict that a 10 C change in temperature will have negligible effect on the X&Y calibrations, and very little (a couple of microns?) on the Z.
    In fact, the temperature coefficients of the aluminium body and the stylus shaft will be far more significant for the Z axis.

    My worst-case was a PE mounting jig for 10 small aluminium parts spread along 220 mm. The jig was made in summer, with 20.00 mm spacing between parts. It was off by a measurable amount in winter, but the temperature shift would have been about 30 C! No problem: I simply adjusted the between-unit spacing from 20.00 mm to about 19.8 or 19.9 mm in the definition file, and kept going.

    Cheers
    Roger



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

    Quote Originally Posted by RCaffin View Post
    @Mecanix

    What matters most is any asymmetric temperature distribution. That is, if one side is hotter than the other side.
    Was told by all those professors, engineers, mat scientists & techs, that what matters most is a selection/combination of thermally stable materials having low or equal CTEs (Coeff of Thermal Exp.) for a given application/requirement. Plastics-vs-metals assembly matting seems pretty extreme in that context lolll Confused now Roger, just don't know who to believe anymore ;-) But that doesn't matter anymore cuz its done already! Not going to go all too crazy as I can't source nor engineer stable mats or nano-particles anyway. Did my half-best with what I have access too, commercially.

    What matters most right now is that they are holding onto my PCB/parcel longer than expected. I should have received this today as I have plans out of town for tomorrow and Saturday and won't be able to play with my balls!



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

    Quote Originally Posted by Mecanix View Post
    FR4 (glass) is indeed a composite, per definition. By far one with the lowest thermal expansion coefficient ....
    The CTE for FR4 is nothing special at all - and almost identical to Delrin for what it's worth (about 1E-4 per degree C). Even standard glass filled Nylon 6.6 isn't much different / worse.

    Where have you seen it used to hold probe balls before?



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

    My manufacturer's FR-4 is rated 14ppm/k, while acetals(delrin?) are over 100ppm/k. By all means use delrin if this is what you think works best for you, I'll be just fine with what I've selected and signed-off on already (validated).

    Attaching a research paper extract of various engineered plastics' CTEs, those common ones that are commercially available to us. Chose wisely!

    Attached Thumbnails Attached Thumbnails 3D Probe - Tri-balls type accuracy & DIY-cte-jpg  
    Last edited by Mecanix; 09-12-2019 at 03:41 PM.


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

    Tada! Voila, we can finally quit that CTE, continue building-up the electronics and writing that C code for this AVR!
    Quite relieved to find out that my pcb mfg didn't sabotage my 10P too much, turned up quite nicely considering the size. In perspective, one pin is only 0.16mm in width, a pad is 0.2mm wide so not all that bad (pass).
    I'm off work and out of town tomorrow so the plan is to work those late this weekend.

    All good fun ahead! As always, here goes the visuals...

    Attached Thumbnails Attached Thumbnails 3D Probe - Tri-balls type accuracy & DIY-3dprobe_v1-03_10ppads-jpg   3D Probe - Tri-balls type accuracy & DIY-3dprobe_v1-03_pcb-jpg  


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

    Was told by all those professors, engineers, mat scientists & techs, that what matters most is a selection/combination of thermally stable materials having low or equal CTEs (Coeff of Thermal Exp.) for a given application/requirement.
    Academic approach, a shade divorced from real life engineering. (Btw, I am an academic!) Lacking matched CTEs, go for symmetry of design.

    What happens if your PCB expands uniformally by, say, 5%? That will not alter the alignment of the rods on the balls, will it?
    The same Q for the core of a symmetrical spider: the spider will not twist around and misalign, will it?

    Er - I had better hedge my bets there: if the plastic used has not been tempered and thermally aged, then twisting IS possible. I had some acetal bar from two different vendors. I drilled and reamed some 8.00 mm holes in this plastic. The holes stayed at that size in the material from one vendor over a year - as tested with calibrated gauge rods. The holes in the other brand grew from 8.00 mm to about 8.15 mm ID. This stuffed up the O-ring shaft seals in the bore, which was unfortunate as the O-rings were handling propane gas and liquid at high pressure. Leaks are 'not good' with that stuff.

    It turned out the second lot had not been cured properly, and the inside of the bars was subject to tension stress. When I drilled the holes, that stress was relieved by the holes growing in size. This is a known problem with some plastics, but it has zero to do with the CTE. A domestic oven at 80 C for a few hours works fine for fixing this.

    Cheers
    Roger



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

    In fact FR4 has significantly different CTEs depending on whether you are talking in plane or perpendicular to plane. The "in plane" CTE is good due to the layers of glass fibre and very little to do with the epoxy. The Z axis CTE is usually at least an order of magnitude worse - almost entirely because of the epoxy. But it's of little practical consequence - I only mentioned it because you claimed it was somehow critical to the design.....yet now it isn't. In the same way that getting the balls absolutely flat was somehow critical....yet in fact it isn't.

    The force on the tip will vary according to the direction of contact. This is something to do with the fact that you have the legs at 120 degrees yet the axes are orthogonal. It really isn't an issue.

    As I understood it, "validation" is proving the result by passing the tests with the finished product. I don't think we've got there yet?

    I'm almost getting high on the rich, self-fertilising aroma here but in many ways enjoying the experience!



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

    Saturday's plans scrap'ed due to bad weather forecast so... what's better to do with that free time. You've guessed right, boyz machines and toys!
    All up and running at this end; mcu, connector, clearances, cabling, anodized shaft, programming... all seems to work flawlessly.
    I am seriously impressed with how sensitive this probe is... slight touch and it triggers. Plenty satisfied with the result (until now anyway!).
    All it need next is mostly cosmetic apart from a spring really. Got the enclosure and spindle mounting shaft done already, so I guess we are now moving to the spring tension/damping work!

    As always... here goes so more visuals.

    Attached Thumbnails Attached Thumbnails 3D Probe - Tri-balls type accuracy & DIY-3dprobe_v1-03_anodizedshaft-jpg   3D Probe - Tri-balls type accuracy & DIY-3dprobe_v1-03_isp-jpg   3D Probe - Tri-balls type accuracy & DIY-3dprobe_v1-03_pcbback-jpg   3D Probe - Tri-balls type accuracy & DIY-3dprobe_v1-03_pcbtop-jpg  

    Last edited by Mecanix; 09-13-2019 at 07:44 PM.


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