# Is it possible to make a perfect tangent between two circles?

Hi everybody.

I’ve tried to make the following shape according to these specific dimensions:

I got this result:

My question is that I’m not sure if I made perfect tangents to join the circles. In the model the circles have 48 segments. Is there any way to make sure they are perfect?

I made an example with 24 segments next to to the final shape to illustrate what I mean.
Are they any way to make sure the red lines are tangent to the circumferences?

This is the file: solid07.skp (110.4 KB)

You might find TIG’s True Tangents useful. See Sketchucation for it.

This looks like a case in which it might make sense to use more than 24 sides for a circle. The exact number depends on the angle of the tangent lines.

TIG published a True Tangents extension that can calculate true mathematical tangent points, but…

Because SketchUp always represents circles as inset regular polygons, most of the time a true tangent line won’t touch the SketchUp representation! By increasing the number of sides you can get close enough to meet any practical requirement, but it will never be perfect.

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Close enough?

I used Common Tangents from the True Tangents extension to find the angle and then the closest number of sides for the circle. 50, in this case.

It’s a nice, quick exercise to draw.

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Thank you so much guys I have to leave to work now, I’ll try it when I get home

@DaveR and @slbaumgartner that extension worked awesome.

But just one doubt, it seems 49,50 and 51 are appropiate number of segments for the tangent of those circles, I don’t see any difference between using them. Why did you choose 50? it’s because it’s a pair number?

PS: to delete the guidelines I can use the eraser tool or edit > delete guides, is there any way more appropieate to remove them?

EDIT: why is the shape not filled?

Untitled.skp (45.8 KB)

Thank you.

When I measured the angle between the perpendicular to the tangent and the diamter line parallel to the red axis, I got ~7.2 degrees. 360/7.2 is 50 so I used that. It’s incredibly rare that I would use an odd number for the number of sides because that usually creates problems.

As for deleting Guidelines, I set up a keyboard shortcut for Edit>Delete Guides. It’s very frequently used.

You’re right.

Besides, if it looks good for 49-50-51, I think the one in the middle is the more appropiate.

I did it with 50 segments but I wasn’t able to get the shape filled with a middle line. I had to do this way and then delete the lines, is it correct?

Sorry, I add one question more. In the finished draw I did yesterday I had this problem when I push/pull.

is it normal it happens? I fixted it yesterday drawing a new line to close the form and then deleting it.

It doesn’t look right to me. The nearly horizontal line across the lower circle doesn’t seem to be in the right location. You shouldn’t need those lines anyway. When I drew it, I drew two circles with the default 24 sides. Ran True Tangents and figured out the correct number of sides to use. Then I drew tangent lines between the circles and the face filled in between the circles.

I did as you have said (I will repeat the process), I started with 24, I changed it to 48, and then I tried 49,50 and 51. Don’t need to answer this time I bet I will get by

A circle can only have a whole number for its segments.
e.g. 24…48, 49, 50, 51 etc
So choose the nearest to suit…
Often it’s best if it’s divisible by 4 [for various unconnected reasons] - but in your case 50 should be fine ??

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TIG makes a good point. Most of the time I use a number that has factors of 2, 3, and 4. Not in this situation, though.

Thank you both

EDIT: Forget what I was going to say, but it seems it’s not perfect if you zoom in, the only thing it seems you can do is look for the number of segments that better suits as TIG said.

Which is what I did by calculating the number of sides based on the angle.

You’re awesome DaveR.
My shape didn’t get filled at first because as you have shown on your video, when I hover the mouse it has to say endpoint not guide point.
What program did you use to record the video?
I use OBS (open broadcaster softwarare), but the only way it seems to capture SketchUP interface is capturing the whole screen.
Good trick to cut the outline before extruding the object and then paste it in place.

You help a lot, can I make something for you in return?
I’d like to make a modest donation for your support.

Thank you.

SketchUp is pretty good about telling you when something isn’t right. If you have a series of edges that you think form a loop but you don’t get the face you expected, the issue is either a gap as you had or the edges are not co-planar. You just have to figure out which it is. Of course it could be both.

As for the video capture, I use Camtasia for that. Excellent resolution and a very robust video editor. Not free but well worth the small investment if you are doing videos longer than those appropriate for animated GIFs.

I appreciate your offer of a donation. Is there a local non-euthanizing animal shelter? Donate something to them. Go walk a dog a two and give them some love.

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heartwarming Dave!

@DaveR

You told me:
“I appreciate your offer of a donation. Is there a local non-euthanizing animal shelter? Donate something to them. Go walk a dog a two and give them some love.”

So I did this:

It’s not exactly what you proposed to me but it’s a little donation to an association that protects cats and dogs.
There site is:

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