Measuring circumference

"The shape is not round. Traditional laser measurement systems use time-of-flight techniques and record diameter. I need circumference. Has anyone in this forum heard of anything that will meet my need?"

A general ball-park estimate of the average diameter or circumference just might possible have some bearing on the equipment needed to get the figures you are asking for.

As it now stands it is NOT a very lucid inquiry.

OTOH it may generate some ingeneous WAGS!

knee jerk response: manufacture precision go-not go gages for smaller diameter. If larger I dunno.

Oh yeah, CMM.

Can you give us an idea of the diameter?

Would a 6" caliper or veneir work?

Is the shape for a casting? a machined part? a rapid prototye part?

The more info the better answer you will get!!

We're all finding this difficult with the information you've given.

It would help if you could post a drawing or photo.

I am a rookie at this website, so I'll try to give you a picture of what I am measuring.

In tube production, the welding process is accomplished by heating the open seam edes and squeezing them together. A substantial amount of material is squeezed out of the weld seam. The amount that is squeezed out is quantified by measuring the circumference change across the welding machine. This is traditionally accomplished by using tape measures. Unfortunately this methd is only accurate +-0.020 as established by gage R&R. I am looking for a device similar to a laser scanner that could measure the circumference during production, or at intervals throughout production.

I am not able in my short allotted time to upload an image. I'll try later.

Todd

Visitor.

Have you thought of a Pi Tape. It is used in the production of pipe of fittings in the plastics business. These may be out of round

What is the measurement range? 0.100 +/-.005, 10.000 +/-.005, 100.000 +/-.005, or other?

What is the material?

What is the tube wall thickness?

I have a fairly extensive background in metrology and the result you seek will be a challenge.

Systems based on Renshaw probes will be flexible and fairly accurate. A good computer system should be able to interpolate the raw data into something meaningful in terms of the circumference. But the sampling rate will be slow system cost high and data crunching time/cost noticable.

A laser probe system will be faster and more flexible and should yield a better overall result. I have not shopped this market in many years so a fresh search for vendors via google or local metrology vendors sales reps would be a place to start.

If you are trying to do on the fly measurement of a single tube diameter then a stainless cable wrap under tension with a indicator or electronic sensor would be a system that could be home built. Calibration, wear, and burr contacts would be likely problems to solve. Small balls strung on the wire would limit contact area and significantly improve reliability of the data set.

Mr. Gee

This sounds like a traditional induction welding process in a tube mill. the coil stock is rolled into an almost closed "C" then run through an induction welding stand where the C was closed and the weld formed. The resulting scarf is trimmed right after the weld occurs. I have seen this done on exhaust pipes, fence posts, structural pipe etc. If excess material was removed in the welding process, then the resulting pipe would be undersize in the finnishing stands that followed.

Now, brain storming:

Crossing techniques from other industies, we used to profile 8 ft long logs 8 to 32 inches in diameter for a veneer lathe using 16 to 32 lasers along its length. The log was rotated on centers and we were able to profile the irregular shape to allow centering in the peeling lathe to optimize yield.

Using a similar approach, you could use multiple lasers in a fixed ring around your pipe and get an accurate profile, or perhaps one or two lasers on a ring that is allowed to rotate around the pipe. The second technique would allow hundreds of profile points, and depending on the laser and standoff distance be accurate to thousandths of an inch. You could verify results with test plugs.

Sounds like an excellent way of doing it - but Costa Packet. Are there any commercially available packages? Price guide? I don't know how much Todd has in his pocket!

The laser measurement devices are in the $2,000 US each price level.

So if he has $50,000 or so, multiple lasers could be used. A rotating ring, 1 or 2 lasers computer and software I would estimate at at half that, but would need detailed information to quote accurately.

If this is in an industrial enviroment the payback could be in months.

For the log measuring system for the lathes the system is sold for about $250,000+.

The payback is relatively short in increased yield. Typical cycle time for a lathe is 10 logs per minute.

Pi tapes are fairly accurate because of the more than 3 to 1 ratio of meaurement vs the result from the calculation. If your part is not round this may be the best you can get. Have you considered using a portable Farro arm which works similar to a CIM? If you bump enough points around the circumference of the part you should get as good or better picture of the part dia. than you would geet with a Pi tape. You could also anaylize the data on a CAD system and see the max and min. diameters and just how out of round it is.

Questioner did not read and check his question, which is so open as to be completely unanswerable......he's probably 10 years old and its a homework question from his teacher!!

I must say, the question is only unanswerable if the responder thinks he all ready has all the answers. Perhaps I should have clicked the "Start a Discussion"

The pi-tape is the current method of measuring the open seam. It is limited in accuracy. CMM and other such devices are very slow. The idea of rolling something around the tube OD is fascinating. I wonder more about the stainless tube idea. What is that about? What is the technique actually being suggested?

I thought of this idea this morning. If I used an electromagnetic acoustic transducer to generate Lamb waves in the tube wall, could I use time of flight measurements to quantify the tube circumference? Could the resolution be accurate enough?

I suspect the nature of many questions asked on this sight could be unanswerable.

Thank you for your time and input.

Todd

Hello tjohnston

Your question as has been pointed out in a charitable manner above, is actually unanswerable.

While formulae of varying types can be brought in to try and answer your question, during the calculations, errors will always creep in automatically.

There is a simple reason:

All circular measure calculations require the use of an irrational number, called by the Greek discoverers "Pi".

The number "Pi" is irrational, because you can never actually get to the end of the decimal places: It never gets to a final decimal place, just an approximation.

That would give you a good reason to believe there was a designer, of Mathematics as well as everything we can perceive, who is able to laugh at mankind's attempts to accurately quantify any circular measure.

Just as an extra bonus, there are other irrational numbers, the Greeks discovered "Rho" too, upon which all living things use during their complex assembly, and for the rest of their natural life on this fine Planet Earth......

Hi,

What about 3 axes cordinate measurement machine (CMM). It use to come with software which allows for measuring just any number of points or a contour and then make the corresponding calculations to give you almost any parameter you need: Inside and outside diameters, deviations in form, concentricity, circularity, etc. and the accuracy can be in the range of 0.001 mm.

Something like a rolling measure which roll along the circumference and measure total distance traveled. Don't know if anyone make them to +/-0.005. Contact Mitutoyo and see if they have anything. Maybe you're looking at a new product.

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