How Do I Measure the Thickness of Acrylic?

I found a magna-mike 8500 that will do the job. Only caveat is it is $5,000.00. I am looking further. Will research the digital ones. Thanks for your input. Sure like the magna-mike tho.

Colpitt Oscillator is the answer. May cost you max $10

<removed email address> will give you the Kit/know how.

Hint: Earth Point will be the Copper foil BELOW tracing the points (of probing for thickness )with 2 foils along top surface : that is C1,C2 of the Colpitts Oscillator

mm

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The first thing to mind is an ultrasonic gauge: Ultrasonic Thickness Gauge

They may be too expensive, I don't know.

I suppose sacrificing one after you form it would be another way. That may be subject to piece to piece variation, too.

[Edit} looks like Mikerho may have a more practical solution.

I wonder if there's an optical way to do it. Somehow measure the refraction difference as it relates to thickness?

Yes, build a measuring jig.

"throat and daylight, utilizing a ball tipped mandrel below and a dial gauge above"

GA

That's the way to do it, a bit of work and low, low cost!

You beat me to it again!!!!!

Destructive testing would give you an answer on thickness and material distribution... Perhaps it's best to rent an ultrasonic gauge for a one of or small production. Remember to calibrate for your material, speed of sound is specific to the material being tested.

Stick something small and opaque on each side, so that they line up when viewed perpendicular to the surface. Then look at them from an angle. Refine viewing method as necessary for accuracy, with reference to known thicknesses to reduce complications over refraction, etc. The inner sticker might include graduation marks.

How about a making a rectangle of 74" x 20" inside dimensions. In the center of one 74" side, drill a hole so that a dowel can pass through to touch the midpoint of the opposite side. Use a depth gauge to measure how far the dowel extends from the hole. Insert the dome, measure again, and subtract.

How much precision/accuracy do you need? You can buy an inexpensive micrometer (Harbor Freight <$15 USD) and remove the calibrated barrel, saw a "U" shaped frame out of 3/4" plywood with a 4 foot deep and 20" wide throat. Mount the micrometer barrel on one leg and threaded 1/4 inch rod on the other leg. Sharpen the end of the rod to a point and adjust the threaded rod to just touch the micrometer barrel anvil with the micrometer set to read zero. You now have a custom-made micrometer to make your measurements to at least a 0.001 inch accuracy. When you are finished you can put the micrometer and original frame back together and use it as a standard one inch micrometer.

Hi,

If the acylic is clear, then a laser distance measurer could be used by reflecting the beam back and then halving the result.

An ultrasonic T gage with a 2.25 Mhz dual element transducer.

Ron

More information might be helpful.

First, this is going to be done by an employee.

Second, there are several items that I can included in the quality testing that this device will measure.

Third, over the life of the device, thousands of tests will be made; so accessibility and ease of use and transportability will be an issue. A reasonable cost is acceptable as it will be amortized over its useful life; like thousands of tests.

That rules out a stationary jig where items will have to be taken to be tested. I am not into building anything that I can purchase( I am not proficient in welding and the like. I prefer to leave that to people who are really good in that area. (I can do it, but you would not want to trust the result!). By the time I pay for someone to make a jig, I will have payed for a hand held transportable unit, imho.

With that in mind, Mikerho gets my GA. It is now a matter of choosing the proper Ultrasonic device for the application. Before my query I did not know such a device existed.

From a non engineer type, I thank you all very much for your input.

Factoring the new information, I gather this is a widespread and continuing Quality Assurance scenario rather than say R&D data for one component.

In that case you should factor into your choice, the ability to produce a digitally interpolatable record of each measurement set, complete with a real time environmental data record.

I.e. a complete image of the profile, as and when taken - rather than a series of spot measurements at nominal points noted on a clipboard, as this approach will not tell you if the 'spots' were in fact 'on target', or the 'device' properly used, aligned, calibrated, etc.

If you list the 'other' items you wish to measure - you may also get better help in choosing appropriate technology across all, or best for each.

Your assumptions are correct. There will be other polymers as well as some fiberglass components. And yes this for a production environment and not a singular event.

I have given thought to recording all the data on a continuing basis, that is all part of quality assurance; a subject I am familiar with. I am just not at that point in this project yet.

I have received help with my project from the members of CR4 many times and I appreciate it very much. Once this business has launched, I have plans on going back and getting my chemical engineering degree. Fascinating stuff.

Optical always works.

You build a simple setup, allowing to look thru the surface at, say, 60 degrees with the help of two knife edges, to be consistent. The other side needs a reflective surface, so stick one on it. On the first surface have a fine scale attached to the setup.

Calibrate with flat pieces of acrylic. Write a pass / no pass calibration table.

The full reflection will allow you to read the scale precisely.

It will take you an hour or two to build. Done.

Yes optical is what I was thinking. It has been a long time, but I seem to recall some coating measurement devices that used the refractive index to shoot a laser at a surface and measure the reflective difference across the surface. Of course I am not aware of the precision, but is was for a clear coating surface, so I am guessing it can measure at relatively small thicknesses.

Put a sticker on the inside at top of dome. Put another on the out side, just next to the inner one. Measure the distance between the two dots is what I mean. Use one of these and measure away, I think. They come in handy all of the time for other things.

Not sure what your tolerances are but this should be quiet precise. I didn't take the time to go through all posts so sorry if this is plagiarism or repeat.

Good luck, Ky.

Acrylic, dome, 6 foot, I clearly thought this was a transparent object.

I withdraw my approach and chose a different tool. Magnetic is the better choice, otherwise same steps apply. You have to subtract the thickness of the top dot though. To lazy to do a sketch but you should get it by now.

With out a nemesis (friction) in CR4 it would be a swamp with no help at all. Take the stick (offered for pulling out) or leave it. 34point5, just the name of the contributor indicates measuring to and after a point. Be glad that he is after precision and not waffle.

Good luck, Ky.

Ahhh!

The latest revelation, that the measuring technique is meant not for acrylic alone, but all kind of plastics, most of them opaque, puts the question into an entirely different light.

First, what is out then:

Optical is obviously, capacitive - due product variability, ultrasound same

What is in:

Mechanical is always in. For small pieces obvious, for large ones quite awkward. But eminently doable.

Magnetic is a perfect fit. All plastics have the same magnetic permeability as air: 1. The material does not influence the measurements at all. It would take quite a bit of development I am used to do. Do not expect it to be a single instrument for all situations. This remark was aimed at measuring in any situation. For go/nogo control Hall effect sensors are robust, and usually adequate. If you have an ABS in your car, the rotation of the wheels is measured with one.

Eureka,

Note: this method assumes that the thickness of the acrylic only varies with the distance out from the centre of the dome.

Find someone with a pool, place a weight into the centre of the dome and place it in the pool. Mark the water line on the inside of the dome with permenant marker. Fill the dome gradually with known quantities of water, marking the inside water level and the outside for each increment.

Some calculation which I have not put my wits to yet involving the volume of a sphere and buoyancy will be able to give you the thickness for all points relative to your first measurement. You obtain your zero point by physically measuring the thickness at the edge.

This method is both time consuming and slightly inaccurate, however it is free!

I hope that this helps.