Arc joining question

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teegee
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Arc joining question

Post by teegee » Sun Apr 09, 2017 6:44 am

I'm having some trouble joining two arcs of a specified diameter smoothly. For example, in the attached drawing, there's a requirement to continue the line from B upwards with a R25 arc, which smoothly joins to a R50 arc, which terminates at point A.

In the top drawing, I did this with AT (arc, tangent, point, radius 25) from the line at B, and pick a random endpoint somewhere between B and A. Then I continued from that point with AD (arc, 2 points, radius 50), endpoint at A.

The problem with this method is that the mid-point has to be found by trial and error. in #3 for example, the transition is not smooth, #2 may be too shallow, etc.

Is there a better way to do this? I tried polyline arcs as well, but the first arc from B is always at 90 degrees..

Cheers!

Rob
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arcs.dxf
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Husky
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Re: Arc joining question

Post by Husky » Sun Apr 09, 2017 7:53 am

Hi Rob,

good news first:

I would also start with the AT tool - except the "random endpoint". It needs to be the top endpoint of the white line in B.
Then I would use the CTR tool - two tangents + radius for the 50 diameter transition.
Husky-2017.04.08-01.png
Husky-2017.04.08-01.png (25.46 KiB) Viewed 6196 times
Now the bad news:

I think with this given layout it can't mathematically work. You have to change the distance of point A to B or the radian.
arcs-qcad_mod.dxf
(116.29 KiB) Downloaded 441 times
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Matfie
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Re: Arc joining question

Post by Matfie » Sun Apr 09, 2017 9:47 am

is there actually a requirement that the line terminating at point A is at any particular angle?

(ie Husky has made it tangential with the blue line forming the point of A, but you didn't explicitly state that as a requirement..)

If you don't have a requirement to terminate at any angle, then I would use Tangentially Connected (AN) to draw an R25 curve from the start line but further than you need to go, then use Tangent, Point, Radius (AT) to draw an R50 tangent to that curve, ending at point A. Then just trim the R25 curve excess part.

EDIT: a picture showing two arcs, one intersecting the point A and one intersecting the line. you can see the difference
arcs.png
arcs.png (19.22 KiB) Viewed 6193 times
PS Husky used D25/D50 not R25/R50 for his curves, which is why his intersection is much closer.. In fairness, your drawing had D25/D50 shown on it; seems a failure of specifications here :)

teegee
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Re: Arc joining question

Post by teegee » Sun Apr 09, 2017 3:09 pm

Thanks for your replies! Matfie is right, termination at A is at any angle. And yep, I screwed up the specification -- I scaled things up 50 times to make an easier testcase.

AN + AT seems to result in a perfect solution!

Could this be done with polyline arcs as well, so that I can tweak the center node later?

Cheers,

Rob

Matfie
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Re: Arc joining question

Post by Matfie » Mon Apr 10, 2017 8:01 am

I don't think you can draw it directly with a polyline, since you would have to guess the middle point (guessing is bad) but you could draw it like that and convert to a polyline.. however, moving the points will not necessarily do what you want, since the attribute "tangentially connected" will not be maintained. You can convert to a spline, but then the attribute "arcs" will not be preserved when dragging the control points.

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