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measuring wall thickness
2

measuring wall thickness

measuring wall thickness

(OP)
hi,

i need to measure the wall thickness of a certain product , the product is made of ceramics and has an egg like shape which makes it very difficult to measure, in addition the wall thickness is not constant and varies along the profile . the average wall thickness is about 8.5 mm.
i tried using standard mechanical measuring tools (all sorts of big calibers and similar methods) , an ultrasonic gauge and i even found a kind of magnet based gauge but they all have the same problem- not accurate enough, i need an accuracy of at least +- 0.01 mm , if anyone has an idea on how to do it id love to hear it.

thanks

RE: measuring wall thickness

You have to be so damned careful using stuff like that at 0.01, just because you double the chance for operator error to ruin the reading.

There are external calipers that look similar to what MintJulep posted, that have digital readouts and read thicknesses directly. We have some that read to 0.005mm (and are therefore probably good to about 0.01 perhaps). Get one with the right tips, or even get one and grind the tips to the right shape for your application, calibrate, and go?

RE: measuring wall thickness

CMM ? but that'll probably only give you the outer shape.

average thickness is 8.5mm and you need an accuracy of 0.01mm ? boy you must have defined a lot of (three?) deciminal places in your dimensions.

how did the manufacturer inspect it ? (to verify that it was to drawing)

is it a complete "egg" shape, or open ?

are you prepared to sacrifice a sample ?

another day in paradise, or is paradise one day closer ?

RE: measuring wall thickness

A good CMM will get the inner profile as well, if you can provide access for it (presumably you have a hole somewhere in order to have tried using calipers). You may need a whole set of different probe tools to reach around corners, etc.

Another trick would be to cast (using eg. replicast or a low expansion wax) both i.d. and o.d. surfaces, with a tooling insert cast into both surfaces so that the distances from a datum can be measured and the two profiles reconstructed. The problem is that you need to be able to extract both castings, which means no re-entrant surfaces, so you are limited to only parts of the egg shape in any given casting.

RE: measuring wall thickness

That would only give the average wall thickness over the whole egg. Any local place or many local places could be thicker or thinner than the average.

RE: measuring wall thickness

ultrasound ? will measure skin thickness, but the accuracy demanded will be challenging for any instrument ... precisely measuring the thickness will depend on determining the normal ... i guess you could sweep through a bunch of angles and get the minimum.

displacement (archimedes) if you can get the external volume (easy) and the internal volume (accessable ?) and as pointed out above will give the average thickness only.

another day in paradise, or is paradise one day closer ?

RE: measuring wall thickness

(OP)
the client uses an XYZ machine to inspect it , the shape is opened so its not a problem to insert a measuring device inside , it looks like a an egg that was cut in half

RE: measuring wall thickness

(OP)
cant use what ajack1 suggested or mintjulep because the part is very long (about 400 mm ) and i need to measure the thickness threw all the length and this plier shaped devices are not accurate enough when they are long because they bend

RE: measuring wall thickness

an egg shape, or an open (half) egg shape imples something spherical, equal dim'ns . now one dim'n is very long ... so it's more like a channel. can't you measure it section by section (rather than looking for something that will encase the part ? it sounds like you need some laser device that'll scan both surfaces ? i'd've thought a CMM could do this ?

another day in paradise, or is paradise one day closer ?

RE: measuring wall thickness

Then you need to make a "non-bending" specialized micrometer that will give you results to the accuracy at ALL wall angles the tips will encounter at all temperatures you are going to use the non-bending micrometer in.

Expensive, but you are going to have to spend it -> if you really, really really need that accuracy.

RE: measuring wall thickness

"Expensive, but you are going to have to spend it -> if you really, really really need that accuracy."
My thoughts also. And the laser / non-contact approach, too.

Define your project parameters better. I'm still trying to discern the size of the object you want to measure. I see 8.5 mm thick (variable) and 400 mm long. Give more specifics if you want good answers (W x D x H, material, surface roughness, etc.). AND what is your budget? This can be done, but I suspect it will be a custom gage. Do you have a budget for that? And what throughput do you require for measuring parts? One or two, or twenty per shift?

If you could relax the +/- 0.01 mm resolution requirement, then I would propose a robotic system. Could be done with a used robot.
1. Use the CAD model to develop measurement scanning paths on the top/bottom surfaces. Each path point has a normal surface vector for perpendicularity. Easy with CAD tools available today (<wink> of which I sell).
2. Use a high resolution laser displacement sensor (Keyence is my favorite, but there are many others) to measure top/bottom.
3. Do the 6DOF arithmetic to generate thickness values.
4. The challenge to this is the best robots on the market today have repeatabilities of +/-0.020 mm or so. Darn good for a robot, but won't meet your +/-0.01 mm requirement.

Option2: You could continue with the laser displacement sensor concept, but mount it on a CMM or an articulated arm CMM (if the repeatability is suitable). Fixturing your part repeatably would be critical.

Option3: Look at the Keyence applications guides, they will give ideas of mounting TWO sensors through which a product is passed to determine thickness. Many types of laser displacement sensors available: 1D with a tiny laser spot, 2D with a scanning line, 3D (magic) to provide depth. Contact Keyence Applications, maybe they have a solution already.

Option4: go the ol' skool hard tooled gage route. Can be done, but $$.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering
www.bluetechnik.com

RE: measuring wall thickness

Yah know, we never asked if he only has one of these eggs - as if for a university or research single-use gadget, or if he expects to make a bunch of them - each of which is going to vary of course by some additional margin.

I didn't know robots could get a positioner within 0.02 mm accuracy in 3D! Nor that a radar/laser/ir sensor could repeat a 0.02 mm reading without hitting the surface.

RE: measuring wall thickness

I wonder what the usage is that is sufficiently sensitive to thickness.

Not for this particular application, but if the use is sensitive to a characteristic, that suggests the usage itself is a means of measuring it.

For example, no one makes AFAIK, a CMM that can track 1/10th wave over a 2 meter mirror as is required for space telescopes, but the optics used in the telescope certainly can - hence that is the method used to measure the precision of the optics rather than mechanical measures.

OTOH translucency is a related characteristic that is very sensitive to thickness. Maybe that's an option, though the OP is looking for a 0.1% or less variation.

RE: measuring wall thickness

I'm not sure how you'd even define the thickness, where it is variable and on an irregular surface.

RE: measuring wall thickness

See, if this is not a one-of-a-kind part - or is the first of many that must be built - then you drill numerous holes in enough test pieces to establish (to whatever level of statistical process control you need) that the process is building good parts.

At each of 200 or 300 holes in 3 - 10 - 30 - 40 eggs, you measure the wall thickness at the hole.

RE: measuring wall thickness

(OP)
how do i make a "non-bending" specialized micrometer that is at least 300 mm long and still light enough so a man can hold it while he is measuring?

RE: measuring wall thickness

do you have a cross sectional or any drawing you could attach?
the 0.01mm would be dificult even if a accesable with most micrometers

RE: measuring wall thickness

"I wonder what the usage is that is sufficiently sensitive to thickness." ... i was wonderering the same thing ... 0.01mm accuracy over 8.5mm ... it's possible (just, maybe) that this is a feature of (bad?) design ... that this is trapped between two well controlled surfaces so excess thickness might cause something (the mechanism?) to bind.

in that case i'd've looked for something that would mould in place.

another day in paradise, or is paradise one day closer ?

RE: measuring wall thickness

With a part having a 400mm length you might be able to probe the inner surface from each end to the middle using a CMM with an extended probe. Or you could also cut the dimensional qual specimen into several short lengths.

Just out of curiosity, what process did you use to manufacture a 400mm long tubular ceramic component having a non-circular radial profile and a wall thickness tolerance of +/-0.01mm? Sounds like a pretty impressive piece of manufacturing work.

RE: measuring wall thickness

Quote (tbuelna)


Just out of curiosity, what process did you use to manufacture a 400mm long tubular ceramic component having a non-circular radial profile and a wall thickness tolerance of +/-0.01mm? Sounds like a pretty impressive piece of manufacturing work.

A very very large chicken.

RE: measuring wall thickness

(OP)
sorry, cant say what its for but the accuracy demand is necessary

RE: measuring wall thickness

"how do i make a "non-bending" specialized micrometer that is at least 300 mm long and still light enough so a man can hold it while he is measuring? "

I'm picturing a dial indicator mounted to a flat surface on a "c" clamp. Zero the indicator on the opposing surface, lift the plunger to slip the C over the wall of the part, and read the result. If the C frame and surface are not too heavy it might work well.

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