How to draw a rectangle with one side as chord of a circle?

[Carkl3000]

Hi. I’ve just begun working on my first sketchup model. I’m trying to design a small Newtonian telescope. The “upper tube assembly” of the telescope will be composed of two rings (about 5" diameter) joined together with three or four rectangular struts (4" long) to create an open cylindrical frame.

I have no problem constructing the two rings, but I’m having trouble with the struts. I want to make rectangles of a defined dimension (1" x 0.5") with one side of each as a chord of the inner circle of the ring. Then I’d be able to pull those rectangles down 4" to create the struts. I just can’t figure out a good way to draw those rectangles on chords of the circle. Is there any good way to do it?

Thanks!

[JimD]

Hi,

It would be nice to see where things currently stand in your model. So if it’s at all possible, please post it up so we can know what you have without having to guess.

Otherwise, starting off from scratch… If I were to model this object, I think I’d do a bit of design work for how the layout of the varios cross-sectional areas are suppose to look. Once decided upon I’d group that geometry together so that it can be used as a template/jig for constructing the main model.

*[ If you don’t already know about how and why objects should be Grouped or, turned into Components (because as you say you’re relatively new to SU)… then make sure that you go through one of the tutorials so that you see how groups/components are suppose to work.]

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Circles within SketchUp are made up of flat segments. . . and you can control how many segments, or Sides, your circles have. Use this fact to your advantage… And take a look at the Entity Info’ window, so that you can make changes to the side counts of circles that have already been drawn should that be necessary.

You can (and probably should) use a value which is evenly divided so that your chords fall into the right positions. And its often a nice strategy to reduce the side count value down so that you can easily identify certain alignment points within a circle… (e.g., you can have a 3 or 4 sided circle, just long enough to set up some guidelines for (triangular, 120º; or square 90º reference points,. . . and then reset the side count back up to whatever you wanted it to be once the guides are in place)… better still just draw in a secondary circle to use as an alignment aid/jig with the side count set to whatever you need.

When snapping your rectangles onto a Circles Edge Segment. . . make sure that you’re locking onto the End Points or Mid-Points in a consistent manner, so that you have even symmetry built into the model. Zoom in if necessary so that this is all fairly clear.

[. . . this is the main reason I’d be doing a little cross-sectional study just to work out how I’d like things to be placed.]

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Rotating a rectangle into alignment with a circles chord isn’t too tough, but it might be if you’re not too familiar with how the rotate tool works… so I’m not really sure where you’re having problems from a modeling standpoint. I’m not sure, but again it’d be nice to see your model if possible.

[filibis]

Are you talking about these kind of beam? And struggling how to ‘union’ them with cylindar where they intersect?

http://wwwcdn.skyandtelescope.com/wp-content/uploads/collimation_secondarycard_m.jpg

[Carkl3000]

Thanks for the replies! Here’s a jpg showing where I am so far. Should give you an idea of what I’m trying to do.

Thanks for tips about changing circle resolution. I changed that from 24 segments to 240. That helps some. I’m aware of groups and components and I know what these features are supposed to do in theory. In practice, I’m still working out how to implement them effectively.

Filibis, I’m hoping my picture here will make my question a little more clear. I’m trying to design a truss tube newtonian. It’ll consist of an upper tube assembly (UTA) that will hold the focuser, spider, and secondary mirror. The UTA will be connected by struts or trusses to the primary mirror box (housing the primary mirror and alignment hardware.). I’m trying to design it so that the UTA will nest inside the mirror box so it can be packed up small for travel by plane. I can see already that adding the spider and secondary to the UTA will be a challenge. Well… everything is a challenge at this point, but I’m enjoying learning. It’s a powerful piece of software.

Thanks for the advice so far.

[Carkl3000]

I think I’m staring to get somewhere. The three struts (one wider to hold the 2" focuser) and the two rings are individual groups and I can move them all around if I need to without any unwanted attachments.