Im designing sheet metal roll forming equipement and need to apply an equal constraint not to the radius of an arc but to its length.
This is so I can virtually animate the development of the product as it passes through the machine.
I have the finished product sketch off to one side of the sketch and the part sketch over the origin is back developed to various stages in the machine, and each arc length in the part sketch should be equalled to its relevant arc in the finished product sketch and driven by an angular dim. Then the rolls are projected off the contour flange.
Im currently using a lobsterback technique where I dimension the length of the individual lines running around the inside of an arc but this is clunky and fails a lot. I can get a lobster to work on simple products but starts to waste a lot of time on complex sketches.
I right click during an arc dimension comand and get only radius / diameter options.
Any clues ?
Dan

Inventor 2013 will do Arc Length. You may have to upgrade.

There is a way if you use construction geometry but I hope you like Math.

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There is a fairly easy way to dimension arc length prior to 2013 (the formula for arc length is not complicated), but I'm having trouble envisioning the described workflow. Can you attach an example of your current workflow?

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JD Im doing an incremantal flat pattern in reverse.
I start off with a flat piece of High Tensile sheet a given width and step by step
folding it into the shape of the finished product that has the same unfolded
length as what I started with.
Each station can do a 20 to 30 degree bend that has a 3 to 5 degree springback.
So I have to be able to drive the angle of the bend which reduces its radius but the arc length
stays the same throughout the process.
So if I did that with a formula it would still be pretty complex.
Boss has no qualms about upgrading from 2009 to 2013.

ok just installed 2013 and went looking for equal constraint for arc length and couldnt find it.
any quick tips ?
Dan

You will need to dimension both and equate the values via the parameters, the Equal constraint is still equal length linear segments or equal radius for arcs/circles.

you dont get an equal constraint that fixes the arc length, rather its the dimension
go to dimension the arc, but right click during the radius command and you get the arc length option.
You used to only get this in the dwg for dimensioning but now its also in sketch and you
can use it to drive an arc length or to fix it.
just what I needed, now I can power my reverse flat patterns without having to use lobsterbacks. 8^)

For anyone who doesn't have access to 2013 - I put this post together on measuring and driving Arc Length:
http://cadsetterout.com/inventor-tutori ... -sketches/

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Heres a few screenshots of a 2013 sketch showing the animation of one of the bends in a simple piece. Get these 2 jpg's out and toggle through them.
Ive made a few of them and they dont crash as bad as the lobsterback ones I use to have.

the other one

Heres the 2013 ipt

Great stuff Ripperton!

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Thanks
forgot to mention the arc length dim goes on the construction line arc that is 0.396mm offset from the inner line arc across the thickness of the sheet.
0.396mm is 44% of 0.9mm which as far as I know is the "K" factor so the "K" factor is actually adjustable.
This is also a solid not a sheet metal ipt but I have a sheet ipt of the same part and they both flat pattern out to the same width.
Here is how I project the rolls off the sheet part still in an ipt.