Precision Conduit Bending

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TwoBlocked

TwoBlocked

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Bradford County, PA
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Industrial Electrician
Got a test coming up at my new job at a manufacturing plant. I LIKE conduit work, but don't say I am the best, especially with hand benders. I am more used to rigid using a machine. Anyhoo, been practicing and always seem to end up with the piece running a bit long using the exact measurements in the tables. I suspect why this happens and would like some feedback on my ideas, if you fine folks are interested. (I did some searching, but didn't find anything specific...)

The gain for a 90 degree bend is given as the radius multiplied by 0.43, yet when a piece is bent, the gain is more. I think there are two reasons. The radius is calculated at the midline of the conduit, and the midline stretches some. It's not like the inner part of the bend shrinks as much as the outer part stretches, leaving the midline the same length. So when bending a sample 90 deg and then measuring, don't we usually measure the outside bend dimensions? I do, and that is the way a stub is measured and bent - from the outside of the unbent part to the end of the bent part. So there is additional gain, half the diameter of the conduit, and any midline stretch.

Wouldn't there also be some gain due to midline stretch on 30 deg offsets? I know according to the tables it is negligible for 3/4" conduit , like 1/16" per bend on the center line. But with the midline stretching, and not merely "cutting the corner", it might be significant, maybe 1/8" per bend?

So if any are curious, one part of the test is to cut and thread 1/2" RMC to length and then hand bend a 90 deg stub and also an offset. The dimensions will be given at the beginning of the test. The tolerances are +/- 2 deg and +/- 1/4".

Not really worried about the test, although I want to do my best. So any tips would be appreciated. :)
 
In case anyone wonders, one dimension on the conduit bending test ended up 1/8"' off. I think my thoughts on the midline stretch are correct.
 
TwoBlocked said:
practicing and always seem to end up with the piece running a bit long using the exact measurements in the tables.
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One reason might be related to the mfg of the bender vs the table mfg. Found each bender mfg has their own criteria and measure for shrinkage on bends of comparable size conduit. There are even difference within the same mfg between hand benders (even between old/new), and power benders (even between models). Not sure exactly why given the radius of bend should be the same for a given conduit size.

Bending IMO is an art and subject to slight variation between individuals. Could be as simple as your perspective as you line up your hand bender to your marks. If your bends are consistently "off" by the same amount you could adjust your bends to accommodate the variance from the chart values.
 
Reminds me of a story my Dad told me. He was a tool and die maker, and they used these small 6" metal rulers a lot. I remember him always coming home with one in his shirt pocket.

He said they had one guy in his department that kept making everything 1/16" too small.

Finally, one day he realized at some point he had cut 1/16" off the end of his ruler without realizing it.

But going back to the original question - I'd say 1/8" doesn't seem like much to worry about for a piece of conduit, but if it bothers you, then yes, add (or subtract) a correction factor if you are consistently off.
 
steve66 said:
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But going back to the original question - I'd say 1/8" doesn't seem like much to worry about for a piece of conduit, but if it bothers you, then yes, add (or subtract) a correction factor if you are consistently off.
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No, 1/8" doesn't bother me a bit. In fact it would have been move if I hadn't used a fudge factor ;)

But something else happened with a different part of the test. Was given the HP of 3 motors and told to determine the minimum conductor sizes and minimum size conduit at 97F and all wires in same conduit. Length was not given, nor important. Was called in to discuss why I had a wrong answer. Turns out they had been teaching incorrectly and had the wrong answer on the master test. They did not start with the required ampacity at 125% of FLC ... They gave me a 110%, as if it would matter ;)
 
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