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# Thread: How do I draw a circle on a cylinder?

1. ## How do I draw a circle on a cylinder?

Or any arbitrary Polyline on the surface of a cylinder? Simple projection didn't work because I want to 'attach' a circle ON the surface. Say differently, how do I 'BEND' a flat circle in a 3-D space along the surface of a cylinder?

I saw a guy who was wondering how to put 2-D Australia map on the globe in other forum. I guess my question is a little bit simpler than that because we need to bend in only single-dimension. I couldn't find any menu or command for this and now I am considering to write a VBA program which can transform the curve on cylinder mathematically but this will be a tedious work. Is there anybody to help me with possibly simpler way to do this?

FIY I got ONLY a 'regular' AutoCAD 2005.
Thanks,

fudata

2. ## Re: How do I draw a circle on a cylinder?

Extrude your circle into a cylinder that protrudes "into" the existing cylinder. Make a copy of the first cylinder and Subtract it from the cylinder made from the circle. You now have the original cylinder plus a cylinder with a bite out of it wher it meets the original. Go from there - maybe Explode the new cylinder and delete all but the curved end face.

3. ## Re: How do I draw a circle on a cylinder?

Thank you John.

But I think your approach is the one I mentioned as 'Simple Projection'. I tried it already and realized that the projected (or equivalently same as the crossing line of two cylinders) circle is not the same as a 'circle bended following a cylindrical surface'.

Maybe I didn't describe my problem correctly before.

fudata

4. ## Re: How do I draw a circle on a cylinder?

Hi fudata

Try this:
1. Create the cylinder you want to 'draw' the circle on
2. Create a cylinder representing the circle you want to imprint which intersects the first cylinder (through one side only, unless you want the circle drawn on both sides)
3. Use the IMPRINT command from the Solids Editing toolbar.

Hope this helps.

5. ## Re: How do I draw a circle on a cylinder?

I've had a similar problem with a nozzle re-inforcing pad on a cylindrical vessel. The solution still evades me. The pad physically starts of as circle cut from steel plate and then it is rolled, therefore, a projection becomes an ellipse or more precisely, a pair of ellipses (representing the thickness of the plate (which you don't have to worry about). OK. So if you take the diameter of the circle and make an arc that length on the cylinder that now gives you the minor axis of the ellipse required for the projection (the straight line distance between the 2 arc endpoints). The major axis would be the diameter of the circle. Draw this ellipse and extrude it through the cylinder. Subtract it as suggested before from a copy of the cylinder.

Back to the Re-Pad problem. I can offset the thickness of the plate from the arc to give me the shape of the second ellipse.....then what?

Any routines for developing re-pads would be much appreciated.

Thanks

6. ## Re: How do I draw a circle on a cylinder?

Originally Posted by fudata
Thank you John.

But I think your approach is the one I mentioned as 'Simple Projection'. I tried it already and realized that the projected (or equivalently same as the crossing line of two cylinders) circle is not the same as a 'circle bended following a cylindrical surface'.

Maybe I didn't describe my problem correctly before.

fudata

7. ## Re: How do I draw a circle on a cylinder?

Thanks for postings, guys.

Graeme,

Still I don't understand how a projected view of 'a circle bended in a 3-D space along a surface of a cylinder' can be an ellipse. To check your 'ellipse' method I myself drew a 2-inch diameter circle, cut it with scissors and glued it on my mug cup, which has approximately 3-inch diameter. Then closing one eye I looked carefully the circle keeping perpendicular angle on my mug. It simply does not seems to be an ellipse.

Besides, mathematically if the diameter of a circle is closer to or greater than that of the cylinder then it is clear that the projected view of the circle attached on a cylinder is not an ellipse. Say, the diameter of a mug cup is 3-inch. Then half of the circumference is 1.5*Pi=~ 4.7 Then draw and cut 4-inch diameter circle and put it on the mug. We still can see all of the circle becuase the diameter of the circle is less than 4.7. What does it look like? It looked like way more flattened ellipse, which is definetely not an ellipse.

I attached a drawing file to make my point clearer.

fudata

8. ## Re: How do I draw a circle on a cylinder?

Hmm. You're right that you can't "project" a larger cylinder onto a smaller one. It never occurred to me that anyone would try.
Shouldn't the bend relief be the same as (or greater than) the bend radius? Then projecting a larger circle onto a smaller one wouldn't be an issue. Okay, so that's an ideal.
If it helps - I draw both x-axis and y-axis cross-sections of the folded item and Extrude them to the required size then Union them. That leaves square corners. Then I draw a plan outline with bend relief which I Extrude by more than the finished object depth, place it over the first model and use the Intersect command.
But, I don't use bend relief less than the bend radius, so that still may not help you.

9. ## Re: How do I draw a circle on a cylinder?

After trying hard during last few days(actually a few months) I finally got a 'partial' solution for this problem. I say 'partial' because it is not exact solution but I think it would be practically good enough.

For exact solution we need a transformation formula from a circle to 'the projected curve of a bended circle along a cylinder'. Assume we are looking down a circle of radius Rc on XY plane, and y-direction diameter is touching a cylinder, the main axis of which is aligned to y-axis and has a negative z-coords.

The equation of a circle :

x^2+y^2=Rc^2

The equation of a curve which is a projection of bended circle along the cylinder in XY plane : (x' and y' denotes coords of this hypothetical curve)

y'^2= Rc^2 - Rd^2 arcsin^2 (x'/Rd) where Rd=radius of a cylinder

by geometry. Then we could make a VBA macro to make this hypothetical curve(x', y') from the circle(x,y) and extrude it through the cylinder. The cross section of two cylinders is the curve we want. This is exactly the same way as Graeme suggested except the hypothetical curve is not an ellipse but a rather complex form including arcsine. (Thanks, Graeme for the idea.)

Leaving somebody else to develop such code, I proceeded to the direction of rather a practical approach. During navigating all menu commands in AutoCAD2005 I found we can draw an arc using CENTER,START,LENGTH and realized SPLINE command works in 3-D space too and tried to make spline approximation of bended circle. I can't explain this method easily without a drawing so please take a look in the attached drawing if you are interested. Comments or any idea are welcome.

Thank you again guys.

fudata

10. ## Re: How do I draw a circle on a cylinder?

I found another good APPROXIMATE solution while searching Autodesk discussion forum.

Igor Mironenko suggested to draw a 3D SPHERE on a surface of a cylinder and get an INTERFERE solid. When diameter of a circle is way smaller than that of a cylinder it will be a very good (and simple) approximation.

http://discussion.autodesk.com/threa...ssageID=145907

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