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Weird Concentric circles TP result

constadin
Hello all,

I am using a TIGO TP20 setup (PCDMIS 2017 R2) and I am measuring a small cylindrical part (somehow resembling a very short tube). The outer Diameter being 9mm and inner 6 mm. I have it placed vertically on my fixture and trying to true position control the inner circle as to the outer. The problem is that the face I am having the part on Z+ has no rotational datum and as such I only level on the top plane and probing from Z+ the 2 circles. There is no room (part being 4 mm short) for cylinders.

In any case, this is a simple TP callout of 2 circles. The issue is that while without moving the part on the fixture I am getting repeatable results at a few microns in 2 axis resulting in a TP of less than 0.01mm, when I turn the part 180 degs I am getting 0.03 shift in Y axis. Not only that, but rotating the part continuously by 180 Degs, I am getting consistent repeatable results over and over with the same offset...

The circle even being small I am probing as least squares, 360degs full circles and 23 and 17 hits accordingly in order to have a better approach.

Nevertheless, the distance between the 2 circles should always be the same no matter the orientation / rotation which is left unconstrained (actually was constrained externally with a plane of the fixture). The call out is only requesting TP from Datum A (outer dia).

Maybe it is the leveling plane curling (flatness was 0.015-0.020 mm) due to the fixture grip (not really stressing it much tbh), but still... why have consistency between placements?

For me, this is the first time seeing this happening and I am thinking it has to do with the rotation that's left "unconstrained" and this should not be happening imo. Any ideas maybe?
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  • There's a lot going on here.
    I would look into a few fundamental thoughts before red-tagging it, especially if it's a sample set of a larger lot or batch.
    1: Can you affirm the OOS condition using another means? I know 0.0008" / 0.020mm is too tight to determine with a caliper, but i'm sure if you throw it on a surface plate with a v-block and indicate the ID while spinning the part, you'd be able to affirm concentricity to the OD with ease.
    Note: if both cylinders are tri-lobed this could potentially be invisible in a v-block, so look at your form error plots of the circles for patterns/lobing.

    2: ​How is it manufactured?
    3: What kind of screwup during manufacture, could have caused this condition?
    4: How could it have been manufactured and brought to you as presumably acceptable?

    Another possible contributor is your probe itself. Since it's such a small diameter, I'm guessing you are using a smaller probe? 1mm? the Probe diameter-to-shank clearance for a 1mm probe is miniscule (the 1mm probe with TC shank gives you just 0.010" between the shank and the edge of the sphere, when it's made perfectly) and something like a glob of glue at the sphere, or a slightly bent tip, no matter the calibration results of that probe can interact when you start probing at a depth into the part.

    If you don't assign a rotational datum, the demon simply refers to the rotation as being square to the machine's volume, about your local level plane. The floating tetrahedron animation in the graphic view is a tool to make sure you clearly isolate all 6DOF to the part's constraints. If there's no rotational datum on the print, technically, you don't need to establish one in the alignment, unless you are trying to lock down a possible issue with the part or method.

    Thanks for the answer!

    1. Well the offset seems to be around 10 microns which is difficult to measure with any other mean. Even if I place it manually on a v block as you proposed 10 microns are out of limit for any other equipment unfortunately :S

    2. No idea truly. These are prototypes and fabricated a while ago and given to me to test my program. Who made them and how it requires a lot of digging and I am not good at that (mainly because I am working in a French speaking factory and my French are not that great :P )

    3-4. Not really difficult for such a small part to have deformation I guess. They are supposed to use high end lathes and have experience on these kind of parts... Who knows :?

    I used 1mm initially because that's what logic dictates. I later used a 2mm one that seems to be performing just fine (similar results). There is definitely no shanking happening.
    Finally, the strategy we follow programing parts here is creating an external alignment on the fixture that we recall on the main program and re calibrating ofc on the part DCC. When we recall the external alignment all the 6 dof are constrained and the tetrahedron is by no means floating when creating new alignments. To the best of my knowledge, after my leveling and 3 translations, the rotation is maintained based on the fixture external alignment (considering that the leveling is not the same vector as the rotation).

    I agree with you, no need to lock rotation in a circular part that has no rotational element. I need to measure only 2 dias (outer, inner) and the position of one to another. For me, after loosing all hope yesterday, I accepted that 10 microns of distance offset is possible in a slightly bended part (I had flatness and circularity around 10-20 microns). The issue is with TP callouts this offset translates to 2x the radial distance result meaning 10 microns TP and 30 micros TP. The operators gonna ask me... well what is it then? 10 or 30? Difficult to explain that to them :P
Reply
  • There's a lot going on here.
    I would look into a few fundamental thoughts before red-tagging it, especially if it's a sample set of a larger lot or batch.
    1: Can you affirm the OOS condition using another means? I know 0.0008" / 0.020mm is too tight to determine with a caliper, but i'm sure if you throw it on a surface plate with a v-block and indicate the ID while spinning the part, you'd be able to affirm concentricity to the OD with ease.
    Note: if both cylinders are tri-lobed this could potentially be invisible in a v-block, so look at your form error plots of the circles for patterns/lobing.

    2: ​How is it manufactured?
    3: What kind of screwup during manufacture, could have caused this condition?
    4: How could it have been manufactured and brought to you as presumably acceptable?

    Another possible contributor is your probe itself. Since it's such a small diameter, I'm guessing you are using a smaller probe? 1mm? the Probe diameter-to-shank clearance for a 1mm probe is miniscule (the 1mm probe with TC shank gives you just 0.010" between the shank and the edge of the sphere, when it's made perfectly) and something like a glob of glue at the sphere, or a slightly bent tip, no matter the calibration results of that probe can interact when you start probing at a depth into the part.

    If you don't assign a rotational datum, the demon simply refers to the rotation as being square to the machine's volume, about your local level plane. The floating tetrahedron animation in the graphic view is a tool to make sure you clearly isolate all 6DOF to the part's constraints. If there's no rotational datum on the print, technically, you don't need to establish one in the alignment, unless you are trying to lock down a possible issue with the part or method.

    Thanks for the answer!

    1. Well the offset seems to be around 10 microns which is difficult to measure with any other mean. Even if I place it manually on a v block as you proposed 10 microns are out of limit for any other equipment unfortunately :S

    2. No idea truly. These are prototypes and fabricated a while ago and given to me to test my program. Who made them and how it requires a lot of digging and I am not good at that (mainly because I am working in a French speaking factory and my French are not that great :P )

    3-4. Not really difficult for such a small part to have deformation I guess. They are supposed to use high end lathes and have experience on these kind of parts... Who knows :?

    I used 1mm initially because that's what logic dictates. I later used a 2mm one that seems to be performing just fine (similar results). There is definitely no shanking happening.
    Finally, the strategy we follow programing parts here is creating an external alignment on the fixture that we recall on the main program and re calibrating ofc on the part DCC. When we recall the external alignment all the 6 dof are constrained and the tetrahedron is by no means floating when creating new alignments. To the best of my knowledge, after my leveling and 3 translations, the rotation is maintained based on the fixture external alignment (considering that the leveling is not the same vector as the rotation).

    I agree with you, no need to lock rotation in a circular part that has no rotational element. I need to measure only 2 dias (outer, inner) and the position of one to another. For me, after loosing all hope yesterday, I accepted that 10 microns of distance offset is possible in a slightly bended part (I had flatness and circularity around 10-20 microns). The issue is with TP callouts this offset translates to 2x the radial distance result meaning 10 microns TP and 30 micros TP. The operators gonna ask me... well what is it then? 10 or 30? Difficult to explain that to them :P
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