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Dimensioning curves by arc length?

OTE_TheMissile

Alibre Super User
The guy next door to our machine shop builds custom countertops and just ran up to me with a picture on his phone of a (pretty poorly dimensioned) print of a curved counter that needed to be made. The print called out (twice) that the counter be 8" deep (my D1), the radius of the inside curve (D4), the radius of the outside curve (fortunately it was indeed 8" larger than the inside radius), and the arc lengths of both the inside and outside curves.

Our neighbor was trying to quote the job and didn't know if he could get two of these things out of a single sheet of his countertop stock. I quickly recreated the shape in Alibre with the inside and outside radius, but when it came time to set the overall "width" of this shape, all I had was a pair of arc lengths. When you click on a curve in Alibre, you can either get a radius or a diameter. If I tried dimensioning off the ends of the shape, all I get is an angle.

I could swear I stumbled into a way to get Alibre to use arc length dimensions once, years ago, but for the life of me couldn't remember how and dude needed a quick answer. I ended up holding a protractor to his phone and we both agreed that the ends looked 90° to each other, but I really wish I'd been able to use the arc lengths I'd been given just so I could give him a 100% confident answer.

TL;DR, is there a way in Alibre to dimension curves in Sketches by arc length? Change my A1 dimension from an angle to the linear length of either the inside or outside curve?
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OTE_TheMissile

Alibre Super User
Yeah I know how to figure it out, but that 90° was just an assumption on our (well, my) part. It might not be correct. I'm talking about dimensioning the length of the arc in the Sketch itself to be used as a driving dimension.
 

idslk

Alibre Super User
Hello Mike,
which values do you have and which do you want?
You have inner diameter, outer diameter and an arc length and you want to get the angle? Or something different?
Regards
Stefan
 

idslk

Alibre Super User
Hello Mike,
maybe like this
(positionangle is only for a fully constrained sketch...)
If you input the arclength to the Equation editor the arc angle will be calculated and the sketch will be driven with this value.
upload_2020-2-28_22-57-35.png


or use sheetmetal...

Regards
Stefan
 

Oldbelt

Alibre Super User
what is your arc length? to one of the radi .
the angle = arclenght * 360/ ( 2*radi * Pi)
 
Yes Mike I agree that a properly noted "Length of Curve" dimension is needed. I say this even though I have only needed such a thing a dozen or so times over the past (call it) 52 years. -- Lew
 

DavidJ

Alibre Super User
Staff member
Just to note: Since Alibre works internally using Radians for angles - the calculations can be simpler.

ArcLength/Radius = Angle

No need for those 360s and Pi. Try it if you aren't convinced.

EDIT - in this case, to get (almost) the desired outcome - use dimension tool for the angle, if desired arc length is 70" then type in
70"/D4
,(based on Inside R)
or 70"/(D4+D1)
, (based on Outside R).
It is important to include the length units when entering, so that the calculated angle is a dimensionless ratio value.
 
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NateLiqGrav

Alibre Super User
Arc length and arc angle dimension NEEDS to be added. The measurement tool can tell you the dimension - you can easily calculate it - but you can't dimension it in the sketch or drawing? Something so basic to CAD shouldn't take (10+ years since I suggested it) to be added.
 

sz0k30

Senior Member
I asked for "Arc length" dimensions years ago. Very simple since the computer already knows what it is! And I am tired of people telling us how to figure it out. I know the formula. I know how to figure it out. I don't want to figure it out. I want Alibre to figure it out and just give me the darn dimension!
 

oldfox

Alibre Super User
I don't want to figure it out. I want Alibre to figure it out and just give me the darn dimension!

Alibre won't just give it to you, you have to ask for it by using the Measure (Ctrl+M) tool. Then just copy and past that value
wherever you need it. No math required.
 

idslk

Alibre Super User
Hello Lew,
i know this kind of dimension style, but i never have used it. For what do you use it? Do you have a real world example?
Regards
Stefan
 
i know this kind of dimension style, but i never have used it. For what do you use it? Do you have a real world example?
Hi Stefan -- The primary usage in my experience is establishing "section lengths" as part of "Master Gage" and "Master Model" components -- though I have used is (less commonly) in "Aircraft Tooling" for "Round Section" Assembly Jigs. It is an RPITA to prove in any case. -- Lew
 

NateLiqGrav

Alibre Super User
At my work we bend tubes and prefer to use the full sizes that we keep in stock instead of cutting them. That requires knowing (and sometimes controlling) the center line length of the arcs.
 

lamar

Senior Member
At work we bend a lot of tubes on a CNC tube former and an arch length is an absolute must on the drawings to calculate the center line length.
If we could have a arch length measurement on the drawings it would alto populate when the model is changed. it would take some of the human error out because of the draftsman forgetting to update the drawing manually.
Max are you listening?? could this be added to the drawings??
 
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