# How to draw an S bend

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Hello, I am a raw newby, and my class starts next week. Can't wait.

Can anyone please advise the best way to draw an S curve AoB in the attached drawing? How do you determine the radius of the arcs AO and BO?

At point B, the curve needs to be a bit more horizontal initially, like at point A. My mistake with the sketch

Edited by PJ01

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The vertical requirement, 't', is specified; is there a similar horizontal requirement?

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Start an arc att "A" with END at "O" and DIRECTION parallel to the horizontal lines, and then do the same from "B". You can also use the kommand BLEND.

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Ah, yes. Arc - Start, End, Direction is the way to go.

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This is a double reverse curve and the formula that applies is relevantly simple you have 3 variables r1 r2 and offset for parallel lines. You can do set r1 what is r2? r1=r2 etc My cheat sheet is at work. I have it on my wish list to do one day but was looking at the more complicated non parallel lines solution. Just need a starting pt, there is also what is the rad for a offset given a length the double must fit between.

If you look at the diagram you can see the diagonal line intersects the circles.

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Thanks kindly, for all these suggestions. Tried the Arc: Start, End, Direction command and it worked perfectly

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I would do it another way with some temporary lines. First of all,draw a line between A and B. (If you have the osnap Mid between 2 points then this line is unnecessary.)

Then draw a line from X to A (X is a point somewhere along the line - use Osnap Nearest). Then start the command Arc, and when the start point is needed, give a null response (i.e. just press Enter or right click). The arc will start drawing from the end of the last line drawn, tangential to that line, i.e. will start drawing from A, tangential to the line A. Draw the arc to the mid point of the line A-B at O (or use the osnap mid between 2 points). Then repeat the arc command by pressing Enter or right click, and again give a null response to specifying the start point. The arc will draw tangentially from the last arc, and finishes at B.

Erase temporary lines and job is done.

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Use the BLEND command, Tangent for the Continuity option. (This is in plain AutoCAD)

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Use the BLEND command, Tangent for the Continuity option. (This is in plain AutoCAD)

Doesn't BLEND create a spline? Whereas the OP appeared to want two arcs.

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I would just find the centre of the arc, which is the intersection of two perpendicular lines, one perpendicular from point A the other perpendicular from the centre of line Ao

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Doesn't BLEND create a spline? Whereas the OP appeared to want two arcs.

I do like your solution, quickest and easiest. No need to draw the diagonal line, just snap using the mid point between two points modifier in the osnaps.

FWIW - The OP didn't mention arcs, just wanted an S curve.

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Some solutions

R1=R2 (off^2+dist^2)/(4*off)

(off^2=dist^2)/(2*off) - R1 = R2 where R1 is known

AB= ( 2*off*(R1+R2) )^0.5 length of sloping line.