need perfect radius

Greetings.
I am making a lamp for my living room. I will use a 45 pound plate for the base and some 1/2" rebar for the lamp itself. Base will be on one wall an
d light bulb will be about 7' away. I need to bend an 11' piece of rebar i nto a perfect arc. It would be 180 degrees of a circle. I am thinking to ma ke a fixture out of plywood. Fixture would be similar to what an electrici an would use to bend conduit. I don't intend on heating rebar. Just bend on fixture. Other way would be to create a fixture using a coomplete sheet of plywood. I think that is overkill.
Any thoughts?
thanks Bob
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There's many with a lot of practical experience on here. However, none have replied - so my little thought, for what it's worth...
I don't think the template will work, because of spring-back in the bar. Steel is very elastic. Accurate template yes I believe in your skills - but it won't impose an outcome on the steel.
I suspect
- use a string and chalk to mark-off the radius on a flat floor
- find and object with a smaller radius to bend the bar a sector at a time - where it springs-back to the radius you want
- laws of the universe coming through metallurgy mean you can "unwind" a radius quite easily in the minutes after making the bend
- in metal-forming, some craftspersons over-bend, then get the corrent radius on the "unbend"
Something for you to look up also - see "yield drop" and "Luder's bands". Mantras like "Perfect Planning Prevents Poor Performance" only work in limited deterministic situations often with already-existing commodities. You will realise that even with a perfect method you might get a "jagged" outcome with local tighter bends.
Rebar is cheap (ish!) and if you don't get what you want first time, you will be usefully experienced the second time :-)
Hope this is helpful. Best wishes, Rich Smith
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"Richard Smith" wrote in message writes:

There's many with a lot of practical experience on here. However, none have replied - so my little thought, for what it's worth...
I don't think the template will work, because of spring-back in the bar. Steel is very elastic. Accurate template yes I believe in your skills - but it won't impose an outcome on the steel.
I suspect
- use a string and chalk to mark-off the radius on a flat floor
- find and object with a smaller radius to bend the bar a sector at a time - where it springs-back to the radius you want
- laws of the universe coming through metallurgy mean you can "unwind" a radius quite easily in the minutes after making the bend
- in metal-forming, some craftspersons over-bend, then get the corrent radius on the "unbend"
Something for you to look up also - see "yield drop" and "Luder's bands". Mantras like "Perfect Planning Prevents Poor Performance" only work in limited deterministic situations often with already-existing commodities. You will realise that even with a perfect method you might get a "jagged" outcome with local tighter bends.
Rebar is cheap (ish!) and if you don't get what you want first time, you will be usefully experienced the second time :-)
Hope this is helpful. Best wishes, Rich Smith
******************* I tend to shy away from job requests the include the words perfect, precisely, exactly, just, or only. Certainly not as part of the job title without a tolerance in the description. The word "just" along with "only" has its own special hell. "make it just like" lumps it in with the other three (and their synonyms), but its also often used to denigrate the value of the job as a pre-negotiation technique. "Its just a simple job." "You only have to push a few buttons."
Anyway: I wanted to help, but I would hack and slash my way towards an acceptable tolerance. I don't have a clue how to JUST make it PERFECT. LOL. I guess my first choice would be a ring roller as it does just that. Work slowly towards the acceptable tolerance.
A fixture to bend conduit? Hmmmm. I bent a lot of conduit in 23 years as a licensed contractor and if anything I used the application itself as a "fixture." I think it might be more accurate to call it a gage than a fixture. I bent the conduit with a tool called a conduit bender. Occasionally I used a tool called a pipe bender to make slight offsets in larger conduit. Often to save time I'd get the bend or bends right and leave the ends long, then cut to fit. Especially if working with a helper. Like wire, conduit is cheaper than time. Rebar would fall in the same class I think. Rebar is cheaper than time, so get it close and then cut the ends off to fit.
********************
Rich, Since Ernie quit participating in this group there is a lot less activity than there once was. He was our "resident expert" so to speak.
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I've had fun winding-up people about the word "just" and resulting in them having to think what they are about to say, inspecting it for that word. In engineering design jobs... Tee-hee... :-)
Rich Smith
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On 11/18/2018 12:07 PM, Richard Smith wrote:
> >> ... >> title without a tolerance in the description. The word "just" along >> with "only" has its own special hell. "make it just like" lumps it in >> with the other three (and their synonyms), but its also often used to >> denigrate the value of the job as a pre-negotiation technique. "Its >> just a simple job." "You only have to push a few buttons." >> ... > > I've had fun winding-up people about the word "just" and resulting in > them having to think what they are about to say, inspecting it for > that word. > In engineering design jobs... > Tee-hee... > > Rich Smith >
As a contractor those words would "just" make me automatically increase the price of the job, because I "only" wanted to deal with that if I was going to get paid for the headache.
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wrote:

"It's a simple job, it just take you a few minutes". Every time a potential customer says that to me I know the job is really going to take much longer. When they say that to me I ask them if they are a machinist or a welder. The answer is always no. But they can tell JUST by looking the job will go fast. Eric
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writes:

I think TeeNut had posted this years ago. I don't remember exactly the wording but it went something like this:
We only do three kinds of work here. Quick Cheap and Accurate If you want it Quick and Accurate it won't be Cheap.. If you want it Cheap and Quick it won't be Accurate. If you want it Accurate and Cheap it won't be Quick. Take your pick and wait your turn.
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On Tue, 20 Nov 2018 16:06:11 -0500, "Phil Kangas"

One thing I won't do is crappy work. And not just because it rankles me. You know if you do a quick and dirty joib that looks crappy but that the customer insists is OK the customer will then blame you for the crappy job when it doesn't work or when someone sees the job and comments on how crappy the work is. Eric
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That was my view - but I would make theatrical inrushes of breath and interject with things like "Ooooo, that doesn't sound promising - this is all going to be more costly". Etc.
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On Sunday, November 18, 2018 at 11:59:55 AM UTC-5, Bob La Londe wrote:

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Thank you Bob.
I said, "Fixture would be similar to what an electrician would use to bend conduit." Yes, the fixture I am thinking to cobble up would be like I sai d, what an electrician would use to bend conduit. I was more wordy while y ou got right to the point with - a conduit bender :)
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On Saturday, November 17, 2018 at 7:50:25 AM UTC-5, Richard Smith wrote:

the base and some 1/2" rebar for the lamp itself. Base will be on one wal l and light bulb will be about 7' away. I need to bend an 11' piece of reb ar into a perfect arc. It would be 180 degrees of a circle. I am thinking t o make a fixture out of plywood. Fixture would be similar to what an elect rician would use to bend conduit. I don't intend on heating rebar. Just b end on fixture. Other way would be to create a fixture using a coomplete s heet of plywood. I think that is overkill.

Thank you Rich. I think it was answered last week. I think 'perfect lookin g' was the phrase I was after. Perfect is a strong word. I am going to us e a piece of 2'x4'x1/2" plywood to bend the rebar around. I can attach it t o the side of my workbench and just bent a small arc at a time. Eventually I will put 20 or so larger lamps down my driveway. I will have local shop do the bending for those. I will use full 20 feet lengths for driveway. Not sure if I will use rebar for that tho. While lamps will be lightweight , and rather small, I am not sure how rebar will take the wind load. Ther e will be some dancing of the lamps in the wind. Should be pretty cool with motion detectors on each lamp.
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