Measuring a diameter without a caliper

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RSG
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Measuring a diameter without a caliper

Post by RSG » Tue Apr 09, 2019 6:13 am

Quick question. Does anyone have any suggestions on how to measure a dia. of a part accurately without a caliper?

I have to get a measurement of my mill column for the riser that is around 11.65" Dia and I don't have a 12" caliper. I have an idea of how I could do it but would like to hear other options.

Thanks
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John Hasler
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Re: Measuring a diameter without a caliper

Post by John Hasler » Tue Apr 09, 2019 9:10 am

Box it in with squares, measure the sides, average.

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mklotz
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Re: Measuring a diameter without a caliper

Post by mklotz » Tue Apr 09, 2019 9:12 am

I've read that one can obtain accuracies of thousandths with a pi tape. They're expensive to buy so try to borrow one if possible. Alternatively, measure circumference with accurate linear tape measure (commercial or homemade) and divide by pi = 3.14159265...
Regards, Marv

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earlgo
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Re: Measuring a diameter without a caliper

Post by earlgo » Tue Apr 09, 2019 10:13 am

If you had a 12 in or larger height gage, you might be able to emulate a caliper by clamping a flat bar to the height gage base and use it like a caliper.
Alternatively you could take a length of fine wire and wrap it around the column a number of times, measure the length where it overlaps, divide by the number of wraps, and then divide by PI. Marv's suggestion but enhanced a bit. You'd have to compensate for the wire diameter, though.
--earlgo
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RSG
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Re: Measuring a diameter without a caliper

Post by RSG » Tue Apr 09, 2019 1:15 pm

Thanks guys!

Marvs idea was what I was thinking too. I was going to try Mylar which has a low stretch rate. Short of that I don't have a height gage :cry:
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f350ca
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Re: Measuring a diameter without a caliper

Post by f350ca » Tue Apr 09, 2019 10:31 pm

Marv is right, I have a PI tape that uses a vernier to read to 3 decimals. But it assumes the part is round and gives the average dia. The tape measures that crank into a housing are flat, ie no form to keep them straight so they will wrap around a cylinder. Think they're only divided in 1/8ths though but you should be able to guess to a 1/16. Your 11.65 dia would be 36.599 circumference add a 1/16 and the dia is 11.67. What tolerance do you need.

Greg

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liveaboard
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Re: Measuring a diameter without a caliper

Post by liveaboard » Wed Apr 10, 2019 2:04 am

Make a caliper; a C or D of steel with a screw on one end to adjust.
of course, after that you'll still only be able to measure the gap with whatever measuring tool you have.
Or you could take your home made caliper to a shop that has a big caliper.

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Re: Measuring a diameter without a caliper

Post by John Hasler » Wed Apr 10, 2019 8:31 am

f350ca wrote:
Tue Apr 09, 2019 10:31 pm
Marv is right, I have a PI tape that uses a vernier to read to 3 decimals. But it assumes the part is round and gives the average dia. The tape measures that crank into a housing are flat, ie no form to keep them straight so they will wrap around a cylinder. Think they're only divided in 1/8ths though but you should be able to guess to a 1/16. Your 11.65 dia would be 36.599 circumference add a 1/16 and the dia is 11.67. What tolerance do you need.

Greg
Choose one of the eighth marks as a starting point. Mark the finish with a scribe. Measure to the nearest eighth with calipers (that's assuming you trust the marks).

If the shaft has (or you can create) a line or mark on it parallel to its axis wrap the tape around as many times as will fit with one of the marks on the line. Mark and measure as above. Divide by the number of turns.

Don't forget to allow for tape thickness. For hyperprecision allow for helicity.

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Rich_Carlstedt
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Re: Measuring a diameter without a caliper

Post by Rich_Carlstedt » Wed Apr 10, 2019 1:28 pm

When you say "without a caliper" I assume you mean that you do not have one big enough ?

But, if you have a 6 inch caliper, you can do it.
Take the caliper and open it all the way, and then place the beam against the column and close the jaws while
holding the beam tightly ( and LEVEL !) against the column.
Record your measurement. Now measure the "length of the jaws and that is your "depth "
What you have is a Chord on a diameter. Now you can calculate very accurately your column diameter.
Rich
https://www.easycalculation.com/formula ... ength.html

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Rich_Carlstedt
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Re: Measuring a diameter without a caliper

Post by Rich_Carlstedt » Wed Apr 10, 2019 2:53 pm


whateg0
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Re: Measuring a diameter without a caliper

Post by whateg0 » Wed Apr 10, 2019 3:57 pm

Rich,
Your instructions made no sense to me.

RSG,
Can you weld up a C-frame, doesn't have to be deep unless you can't get between the ram and the base. Then use calipers or adjustable parallels to measure or take up the gap. Use 1-2-3 blocks or such to leave a small enough length you can measure it with what you have. Kinda the same thing as John suggested, but in case you don't have squares.

Dave

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mklotz
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Re: Measuring a diameter without a caliper

Post by mklotz » Wed Apr 10, 2019 3:59 pm

Rich_Carlstedt wrote:
Wed Apr 10, 2019 2:53 pm
Sorry, wrong link
https://sciencing.com/radius-circle-chord-5929711.html
Or you could just give him the formula

r = (c^2 + 4 * s^2)/(8*s)

where:

c= chord length
s = sagitta (perpendicular distance from chord to circumference) = jaw length from tip to inner surface of caliper beam
r = radius of circle

and, of course, the diameter is twice the radius.
Regards, Marv

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