Pi tape temperature comp

I was told to not comp temperature when using pi tape because it normalizes to the part fast enough.

In my head, however, the tape is shrinking/growing at the circumference and at that point wouldn’t give a good diameter.

Should I comp the length of the pi tape and extrapolate a diameter from there?

For reference if I’m using tape it’s usually above 40” dia and not uncommon to try and hold +/- .001”

Thanks

I think you need some gage R&R studies on your pi tape. I'd agree the temperature normalizes way faster than you can get the tape positioned, tensioned, and read. What are you doing to ensure squareness of the tape?

It’s usually on a register so I square it with the face of the part

What is the material PI tape is made from? What temperatures are the environment and measured item?

FYI a 40 inch diameter mild steel item changes circumference by 0.0008 inches for every degree change.

Most everything changes dimensions with temperature. I think you have too many variables to be able to accurately extrapolate your readings. The part you are measuring is changing dimensions with temperature as well. I think the best thing to do is apply the tape and give it time to normalize to the temperature of the part being measured.

Discrepancyform said:

I was told to not comp temperature when using pi tape because it normalizes to the part fast enough.

In my head, however, the tape is shrinking/growing at the circumference and at that point wouldn’t give a good diameter.

Should I comp the length of the pi tape and extrapolate a diameter from there?

For reference if I’m using tape it’s usually above 40” dia and not uncommon to try and hold +/- .001”

Thanks

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Have you tried just leaving it in contact with the part and checking to see if the measurement changes over time?

Being very thin, I would (disclaimer: no actual experience) expect the tape to equilibrate to the part temperature in a minute or less so long as the part and tape were not in sunlight, warm or cold drafts, or other temperature altering environmental conditions. If the measured part were aluminum, since its coefficient of expansion is twice steel, and the part were warm from machining, say, then significant temperature errors could come into play. If my calls are right, a 40” aluminum part 40 degrees F above ambient would actually measure via the pi tape .0096 larger than actual.

Denis

Denis

crossthread said:

Most everything changes dimensions with temperature. I think you have too many variables to be able to accurately extrapolate your readings. The part you are measuring is changing dimensions with temperature as well. I think the best thing to do is apply the tape and give it time to normalize to the temperature of the part being measured.

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Right but the pi tape will grow/shrink in length and is over 120” which should mean the tape in my case has shrunk ~.006 in length at 8 degrees below room temp

Discrepancyform said:

Right but the pi tape will grow/shrink in length and is over 120” which should mean the tape in my case has shrunk ~.006 in length at 8 degrees below room temp

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If your part is a ferrous alloy similar to your tape, it too has changed dimension to the same extent as your tape.
It's when you are measuring aluminum or some other material with a different CTE that you should check the expansion differential to see if some adjustment is appropriate.

Think about it a little, you are using a pi tape, not the best for persist measurements, but you are concerned about temp comp. Thats like using a starrett level to mark where to hit the wall with a slug-hammer...Phil

Phil in Montana said:

Think about it a little, you are using a pi tape, not the best for persist measurements, but you are concerned about temp comp. Thats like using a starrett level to mark where to hit the wall with a slug-hammer...Phil

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I ran a 54" bullard dynatrol. and routinely used a %$#@! Pi tape (how I hate those things)
and was to hold .002 to maybe .005 tolerances.

We had plenty of other measuring devices, calipers, custom gages set with Jo blocks, etc.

Where they are needed, is a on a thin ring, that will be heat shrunk onto something else.
A thin ring that's wiggling all over the place.
When shrunk/pressed on, it will round up nicely.

Just what size that is as you are machining it....is
the job for a %$#@! PI tape.

Let me get this straight. You are measuring parts that have a 40" diameter. And you are trying to measure that +/-0.001"? Really? For one thing that means that your π tape must be accurate to π x +/-0.001" or +/-0.00314". For a 40"diameter that π tape would need to be at least 126" long - over 10 feet. And in that 10 feet plus, the total error must be less than that 0.00314". Conventional wisdom says that your instrument should be TEN times more accurate than the measurement that you are trying to make. So we are actually at +/- 0.000314".

But lets put that aside for a minute. I just measured the thickness of the tape in a tape measure that I keep in my pocket. It is 0.005" THICK. Is that the same as your π tape? I don't know but I can't imagine that your tape is thinner than that. A 10 foot tape can't be too thin. I will go with that 0.005" number.

You have a 40" diameter part and then you are wrapping the π tape around it. So, if the π tape is 0.005" thick then the OD of the π tape is going to be 40.010". And the markings on that π tape are on that outside surface, not on the inside surface which is against your part.

Now, what are you going to use as the actual diameter that this π tape is measuring? Is it the inside surface diameter of the tape, assumedly 40.000"? Or is it the outside surface of the tape, assumedly 40.010"? Or perhaps, if the π tape has compressed in the inner 0.0025" and stretched in the outer 0.0025", is it the diameter of the mid point of the π tape which is assumedly 40.005"?

This is a range of 0.010" or a full TEN times the accuracy that you say you are getting. But shich of these diameters Is your π tape specified for? Or did the manufacturer actually provide such a specification? I strongly suspect that they DID NOT.

That is only one of the sources of error that the use of a π tape necessarily implies. Yes, the difference in the coefficient of thermal expansion would need to be taken into account. What about the amount of stretch in the tape due to the tension that must be placed on it to hold it against your part. What about the fact that there is another error introduced with whatever method is used to bring the zero mark at the start of the tape into alignment with a mark at the 10 foot plus which indicates the diameter. You must offset the wrap or overlap the two ends or both. Any method you use to bring these two marks on the π tape together to read them. Oh, and I put aside the basic accuracy of the markings on the tape. But that source of error is still there. How were they placed on the π tape? At what temperature? What printing process? Etc.

Frankly, after taking all the possible sources of error into account, I just do not see any way that you can assign a level of accuracy of +/-0.001" to any use of a π tape, much less one at a 40" diameter. I think somewhere around +/-0.010" would be VERY difficult even when all possible sources of error have been minimized.

I think that +/-0.001" on a 40" diameter would be difficult using a micrometer in that size range.

I’ve never had to measure a diameter greater than 24” so last post has me curious. If a mike won’t measure +/-0.001” on a 40’ diameter, what will?

L7

If I am not mistaken, a .005" by .5" steel tape 120 inches long will have a stiffness of 575 pounds per inch.

Or 0.57 pounds per .001" of stretch.

There is tremendous friction wrapping a rope 360 degrees around a pipe, to ensure the stretch of the tape is less than a fourth of your measurement tolerance you will need a method of tensioning the band the same way, each time.

I found a pi tape only good to the 1/64 of a inch(.015)I was lucky, I always had mics, temp is also a big deal, always left the mic on top of the work and checked it after lunch...Phil

NRDock said:

If your part is a ferrous alloy similar to your tape, it too has changed dimension to the same extent as your tape.
It's when you are measuring aluminum or some other material with a different CTE that you should check the expansion differential to see if some adjustment is appropriate.

Click to expand...

This is what I need to hear, thank you.

The rest of you are all fired

Discrepancyform said:

This is what I need to hear, thank you.

The rest of you are all fired

Click to expand...

You didn't hear it, you read it,you also are fired.

Discrepancyform said:

This is what I need to hear, thank you.

The rest of you are all fired

Click to expand...

Duly noted, everyone please place the OP on "ignore".

lucky7 said:

I’ve never had to measure a diameter greater than 24” so last post has me curious. If a mike won’t measure +/-0.001” on a 40’ diameter, what will?

L7

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I had an interesting phone call with a guy who worked for decades at Puget Sound Naval Shipyard (Bremerton) on some very interesting and close tolerance parts. He (they) commonly used Pi tapes to measure small to large round parts to .001 inch or better. That included steel parts, of course, but often aluminum, bronze, brass, and “exotic” metals. And you can be sure the Navy supplied micrometers of every description as well. Pi tapes were essential for measurement of flexible (thin wall relative to diameter) parts as the thin parts would deflect substantially due to their weight or internal stresses. That deflection made using “spanning” tools like mics all but useless. But the same part that commonly would eventually be shrink fit over a more rigid cylinder had to be carefully sized—-the Pi tape to the rescue.

He confirmed the need I mentioned in post 6 above to compensate for temperature variation in parts that were made of metal other than steel or iron. They used temperature compensation tables kept with the tapes. Many of the parts made were hydraulic valves and the like that required very close-tolerance fits.

One other thing he mentioned was, when they could, they would tape measure a “rigid” part of accurately-known-size comparable to the part to be taped. That confirmed accuracy of the measurement they were making. He also said it helped confirm that the tape was properly tensioned when measuring. I recalled prior heated discussions on this forum regarding the “uselessness” of telescoping gages and he chuckled. He said it was a similar situation with the tapes. They had guys who cussed tapes and proved they couldn’t use them. The guys that got put on the job were the ones who took the time and necessary precautions to get accurate repeating reading. And, yup, the other guys couldn’t use the tapes to get an accurate reading.


Denis

pi tape will never read down to .001, it is near impossible to get it straight and pulled up tight and get a repeat reading...That are a poor mans mic nothing else...nice to have when you dont have a 40 inch Starret in your back pocket...Phil

camscan said:

You didn't hear it, you read it,you also are fired.

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Definitely heard all the voices say it out loud