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PostPosted: Fri Jan 25, 2008 4:57 pm 
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Joined: Fri Jan 25, 2008 4:17 pm
Posts: 2
Hi all!

I've bought a few #5 TwistTies that I plan on building with an outer radius of 100cm (40"). What I'm trying to figure out is how one calculates the inner radius.

Any input appreciated!

/Ulf


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PostPosted: Fri Jan 25, 2008 7:25 pm 
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Joined: Sat Aug 13, 2005 10:48 am
Posts: 365
Location: East Texas - USA
Ulf,

Calculating the diverging route curve would be very dependent on the uniformity of the curve(s), and also the use of a straight vs. a curved frog. You can however, layout the 'straight' section along your 100cm outer curve - and then marking the centerline of the resulting diverging curve.

Take a ruler and draw a straight distance of 1cm just touching the curve centerline points. At the midway point (50mm) carefully measure to the curve offset (location of the curve at that mid-point).

This is the mid-cord ordinate. The offset can then be plugged into the formula for determining the resulting curve radius:

L= Length between curve points (base)
z = Max Offset ( ~1/2 base to intersect)
r (effective radius of the curve) = (L^2) / (8z) + z/2

Thus if the offset is say 2.6mm (EXAMPLE VALUE ONLY):
r = (100^2) / ( 8 * 2.6 ) + (2.6 / 2)
r = (10,000) / ( 20.8 ) + (1.3)
r= ~ 482.07mm (or close to 19" radius)

If the offset is only 1.5mm at the mid point along a 100mm cord you'll end up with about a 33" (836.33mm) radius.

Note: That's NOT the answer - just an example - the actually measured offset would need to be put into the formula. You can see that the measured mid-cord ordinate offset will be very small and getting good accuracy in measuring is important for reliable results. :shock:

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Last edited by emccamey on Fri Jan 25, 2008 7:38 pm, edited 1 time in total.

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 Post subject: Curved radius
PostPosted: Fri Jan 25, 2008 7:36 pm 
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Joined: Tue Mar 14, 2006 9:26 pm
Posts: 25
I get a little over 18" in 3rdplanit. You can take a straight turnout and flex it to any desired radius.


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PostPosted: Sat Jan 26, 2008 3:40 am 
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Joined: Fri Jan 25, 2008 4:17 pm
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thanks guys!


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