# Drawing a tangent arc (with a specific radius) between a circle and a straight line

#1

Hi everybody.

I already asked about circunstances in which I need to draw tangents between circles or drawing arcs for a corner.
But in this case I’m trying to replicate a model in which I have to draw an arc with 38mm of radius between a circle and a straight line.

I read what you suggested me in another occasions in which it worked, I tried with Tangent-Tools and wich TIG 2D Tools, I also tried to draw and arc and then input the radius but I think that all doesn’t cover this situation.

It would be ok if the program would show me a second snap point (in this case I drew a black circle trying to illustrate what I mean).

It would be possible to me to do it approximately, but I’d like to know if it’s possible to make it perfect (I know 24 segments for a circle in this case it’s probably not enough, so feel free to propose another number of segments).

This is my file.

EjerBas02.skp (42.5 KB)

EDIT: The exercise belongs to this site. https://autocadparatodos.blogspot.com.es/2013/10/EjerDesBas-02.html

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#2

A circle isn’t really appropriate here. Use a 2-point arc. And use more sides for the circular large end.

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#3

but it will be an stimation. no way to make the program create the correct endpoints?

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#4

What makes you think it will be an estimation? You can specify the radius of the arc. You can also make sure the ends of the arc are on the edges of the shape. That will be very difficult with a circle. In fact with the circle you’ll be making an estimation.

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#5

I used the Arc tool after identifying the center of the 38 radius.

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#6

You’re so fast, I was reinstalling, I will edit the post with my result.
If you changed the radius after drawing the arc then it’s probable not correct.
but in your case it looks well.
Give me 5 minutes to upload it.

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#7

I did not change the radius after drawing the arc. The radius is more accurate than you’d need for something this size.

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#8

I’ve been fighting against some bug splats that’s the reason of my delay.
But I have no idea about how to figure out where the center of the 38 radius circle is.

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#9

The center of that radius is 38 from the straight edge and 68 from the center of the large circle. I used the Arc tool to draw an arc to intersect with the guideline. Then from the end of that arc, i drew the 38 radius intersected the straight edge and the circle.

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#10

O_O Cool, it’s like magic hehe.

I guess they’re basic maths, but … why 68?

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#11

30 + 38 = 68

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#12

I thought there was some sinus or cosine function involved, now I feel dumb, haha.
I experimented a bunch of bug splats during this process, I think I would be able to reproduce them.
Has the scale something to do? or the number of segments?
I worked with 96s for the external ones and 48s for the inner ones, and drawing it in milimeters.
openGL is 4x multisample anti-aliasing and use fast feedback is enable.

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#13

48 segments

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#14

I suppose I could have used some trig but I wasn’t feeling that energetic.

Hard to tell. It could be something related to an extension you have installed. Someone from the SketchUp team might look at your Bug Splats and give you an idea.

I would use the same number inside and out for the circles.

You could try working with Fast Feedback turned off and see if that changes anything.

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#15

That gets it pretty close.

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closed #16

This topic was automatically closed 91 days after the last reply. New replies are no longer allowed.

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