Measuring Glass Thickness

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ClohKr

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I work for a Stained Glass manufacturer. I have been assigned a project seeking to be able to measure the thickness of the glass, flowing down a 300ft lehr, closer to our Hotend. Our glass flows from a furnace, down a forehearth, through a set of forming rolls, and down the lehr. Our glass thickness ranges from 0.500" down to 0.075". The hotend lehr zones are typically 990 deg F. Right now the way we measure the thickness of the glass is we wait for the glass to come out of the lehr, at the coldend, and cut a piece out and measure using a micrometer (the glass being around 30" wide and 300ft long, continous flow). If the glass is not in the target range then the coldend operators call it up to the hotend and they adjust. It takes around 15 min for the glass to get from the hotend to the coldend any adjustments made, that may or may not have been correct, will not be seen for another 15 min. Being able to measure the thickness of the glass closer to the hotend will greatly improve productivity and save the company time and money. Contact sensors don't seem to be the logical solution since roll wraps in our forming rolls or last glass from an end of a run will typically lose the flatness and could damage a contact sensor. I have been looking at displacement lasers to see if this is the avenue to go down. My concern is the temperture and reflectivity of the environment. I am asking if anyone out there has had any information I could use to find the best solution. Maybe some companies I could call to ask what they are using or if you know of a company marketing a device that I might wanna try. Thanks.
 

SAC

Senior Member
Location
Massachusetts
There are various contact and non-contact methods for measuring the thickness of a product. For example, ultrasound - try a google of "non-contact ultrasonic thickness gauge" to get an idea of what is out there.
 

ClohKr

Member
What kind of resolution?
I need to be able to measure the thickness of the glass while the glass is moving down a 300ft heated lehr at about 3ft-14ft per min. Our glass is anywhere from clear to solid black. We manufacture all colors, clear and solid.
 

GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
Occupation
Retired
several good ideas already ... steel and paper web thickness is often measured with radiation devices ... commonly called "beta gauge" or "gamma gauge". In the case of steel, temperatures are in the range of yours; in the case of paper, it is room temperature.
 

jeremysterling

Senior Member
Location
Austin, TX
What kind of resolution?

I need to be able to measure the thickness of the glass while the glass is moving down a 300ft heated lehr at about 3ft-14ft per min. Our glass is anywhere from clear to solid black. We manufacture all colors, clear and solid.

By resolution, I think he is asking how accurate does the thickness measurement need to be: microns, thousandths of an inch, hundredths, etc.

e.g. plus or minus 3% .001 inch
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
By resolution, I think he is asking how accurate does the thickness measurement need to be: microns, thousandths of an inch, hundredths, etc.

e.g. plus or minus 3% .001 inch
Exactly. For example Sick Sensors has an ultrasonic unit that is accurate to 0.025 mm, +- .15% on a 12 bit analog output. But for some people that is not good enough. But the operating temperature is 70 degrees C max., so you may need to put it far enough down the line to be that cool. That's likely to be the case with most measurement devices by the way, electronics and heat don't mix well.

Sick UM30

If the temperature issue is what you are looking to solve, there are some fiber optic based sensor systems that use high temperature glass fibers and you can locate the sensor unit itself in another area. Lasers can be used for clear glass detection but I'm not sure about measuring accuracy, you'd have to work with a manufacturer to discuss all this. I like Sick, every time I give them a problem someone bird dogs it until they come up with a solution, even if it's going to have to come from someone else.
 

DavidA

Member
Location
Fresno, CA
An instrumentation guy I talked to yesterday also recommended a fiber optic solution with remote amplifier. You would just have to develop a few calibration presets based up the color of the product you're running. Gave a ballpark of $250 for the instruments.
 

ClohKr

Member
Can you measure the forming roll instead of the glass?
The speed of the forming rolls and the e-section and lehr are running at different speeds. Typically the forming rolls run slower than the e-section (what we call the 10 feet before the closed lehr; the e-section has removable box covers to enable open top) by about 3-4 fpm. This causes the glass to "stretch" before solidifying. Measuring at the forming rolls would not get us the correct thickness of the actual product being "stretched". We need a measuring device somewhere a little further down the line.
 

ClohKr

Member
An instrumentation guy I talked to yesterday also recommended a fiber optic solution with remote amplifier. You would just have to develop a few calibration presets based up the color of the product you're running. Gave a ballpark of $250 for the instruments.
Can I get some further info regarding the fiber optics? I have talked to a rep from Keyence about the type of fiber optics they had available and he said the only applications he had would only show me if the glass was present, and not be able to tell me the thickness of the glass. I did like the fiber optics option though, being able to withstand highter temps and the flexibility to place them where other devices would could not fit. If you could get me in touch with the instrumentation guy that would greatly help me, thanks.
 
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