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dynamic Radius Bend


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I draw ventilation ductwork all day at work, just recently stumbled across dynamic blocks and could do with a little help with one. Attached is what i have done so far, the straight section works perfectly, just having some trouble with the radius bend. What I need is for the arc to stretch with the angle of the bend. :/ I appreciate any help, and feel free to use my working block.




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You have, discovered one of the biggest shortcomings of dynamic blocks, which is the inability to "stretch" arcs while maintaining their original centerpoint.


Here are a few suggestions;


1) Divide the arc into 2 segments, and rotate one of them so it slides behind the other. Of course this only allows you to reduce your fitting angle in half before the arc being rotated starts to peek out the other side.


2) Parametric constraints. Adding a couple of Coincident constraints, a radial constraint, and a fixed constraint can "force" the arc into submission (not sure if this is available in AutoCAD2009)


3) "Fake" the arc with an angular dimension that has the arrowheads, extension lines, and text supressed. see demo below


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Glad you found it useful, however I found several ways to "break" the block. I think it can be corrected by adding/omitting some of the action sets to the move/rotate selection sets. I don't have time to work on it right now, but if you need any help, just let us know.

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  • 1 month later...

Still having trouble with this one...


this time with scaling, attached is the block and what i want it to do. is there any way of doing this from one parameter?


90 bend.dwg


any help would be appreciated

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  • 9 years later...

I resume this thread cause i need a alternative to the n.3 way cause the area is non fillable with a hatch (due to the angular dimension).

I'm Actually using a block with solution n.2 (see and edit the attachment if you want) ... the block has 1unit dimension so scale it to have the right size.
Very often i need to keep fixed the internal radius and in this case it simply cannot be used because scaling all the block scales (obviously) also the internal radius.
I can keep straight the internal lines (just like 90Bend block of Aron) but need to fill it with the hatch and keep the insert point in the middle of the horizontal straight line.

This problem is making me crazy... how to fix it?


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I used to work as a drafter for a sheet metal company that fabricated ductwork back in 2006.


But they used an autocad add-on called CADDUCT.

I'm not sure if it's still around or not but the idea was kind of similar to how you have a dynamic block library to pull from.


It would run the estimations as you went, ensuring everything was SMACNA compliant and your gauges are sufficient for the flow.


The block library was basically a menu with dynamic 3D blocks that could be customized to fit any possible scenario.  It would automatically size one piece to connect with a subsequent section based on its size and shape.


Once you had the project created, we had an export feature that would create DXF files for every piece of ductwork.

We could then have ductwork get fabricated manually or we could send the DXF's to the plasma cutter.


We had a standard template for wiring and equipment and we would do all of the automated ventilation wiring drawings manually for each


CADDUCT also had a built in feature where you could draw a line and it would create a 2D cross-section of the ventilation plan wherever you wanted.

No idea if it's still around.

One option might be to check out the Autodesk app store at least to see what is available for ductwork, specifically.,





Edited by ammobake
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On 2/6/2021 at 12:03 AM, BIGAL said:

If radius is a constraint then why is R2 not = L1+R1, I dont play enough with dynamic blocks so just a suggestion.

i've tried but summing up the value of a parameter and a const. i got an error.

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  • 2 months later...

Hi there, I'm having similar type of issue with bend.

I've attached the block for reference, i'd be glad if anyone could help me out.


The dynamic block had 3 visibility for Uninsulated ductwork, internally insulated & externally insulated.

it usually work fine until the radius of the bend is 50 (R=50) and the external insulation arc (EINS)=50, then the lines are not perpendicular and the insulation arch length is higher than it should.


any idea on this issue is much appreciated.


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I can't even make these edits without removing the constraints. How are you changing the properties?   


You are ending up with the smaller radius of the external insulation = zero, and both radii of the internal insulation being the same.  Just can't have all that stuff being equal. 


The small radius of the pipe bend MUST be larger than the thickness of the insulation.


 It is not physically possible for AutoCAD to make a radius =< zero even if math will do it.  Well, it ain't just AutoCAD. 


The math doesn't know it is dealing with a radius.



Edited by Dana W
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