Draw a 3rd circle tangent to 2 circles

[dniemeye]

I can't control the location of a 3rd circle using ttr. I'm drawing two circles and then a 3rd tangent to both but I want the 3rd circle to be centered down below the two original ones and all it will do is be above them. See drawing. Any help would be greatly appreciated.

 

Dave

[dniemeye]

Sorry, I forgot to mention, I'm using AutoCad 2007.

[SLW210]

All I did was click on the bottom of the circles. You could also just Mirror the third circle with the centers of the smaller circles as the axis.

[dniemeye]

Thanks but what I want is different. See drawing, this is what I want but I had to manually place the circle to be "close" to describe my wants. Couldn't place it using TTR. Wait a minute, oooohhhhh, I was able to finally by choosing point way over to the outside of the original 2 circles and then it worked. Thanks.

 

Dave

[SLW210]

Sometimes you need to let the cursor hover over your line a second or two. It takes a little practice to get tangent to work the way you want it.

[eldon]

There are four solutions to drawing the TTR circle, and you have to give AutoCAD a slight clue by selecting the smaller circles near the required tangent point

[SLW210]

There are four solutions to drawing the TTR circle, and you have to give AutoCAD a slight clue by selecting the smaller circles near the required tangent point

 

That's what I meant by practice to get it right. You need to have some idea as to what you want. Thanks for helping on that eldon, my brain still has not quite woke up today.

[JD Mather]

I addition to picking close to the intended tangent points, pick your points in a counterclockwise direction.

[Pocket]

If you have a problem with tan circles you can always manually locate the new circle

 

In larger drawings with multiple deviants, this is not very effective but for singular use when CAD is being stubborn its handy

[Guest Alan Cullen]

There are four solutions to drawing the TTR circle, and you have to give AutoCAD a slight clue by selecting the smaller circles near the required tangent point

 

eldon,

 

There is actually unlimited solutions to this. The third circle can be any radius you wish and still comply with the tangential requirements to the other two circles. For it to be a diffinitive solution, the radius of the 3rd circle must be specified

[CarlB]

the radius of the 3rd circle must be specified

 

Of course, and that's the "R" in TTR

[Guest Alan Cullen]

Thanks, my bad, I was looking for Radius to be specified. I never use things like TTR, so it didn't dawn on me.

[eldon]

eldon,

 

There is actually unlimited solutions to this. The third circle can be any radius you wish and still comply with the tangential requirements to the other two circles. For it to be a diffinitive solution, the radius of the 3rd circle must be specified

 

Alan, I think you are only half wrong here. I found another 4 solutions with the fixed radius, but maybe someone will find some more

[khoshravan]

I addition to picking close to the intended tangent points, pick your points in a counterclockwise direction.

 

What is the benefit of obeying this rule?

[SLW210]

What is the benefit of obeying this rule?

 

Try selecting in both clockwise and counterclockwise and observe the results.

[khoshravan]

Alan, I think you are only half wrong here. I found another 4 solutions with the fixed radius, but maybe someone will find some more

 

This is an interesting geometry question:). I think in general answer is 8 circles as elodn mentioned. They are 4 pairs and the line connecting centers of each pair is in right angle to the line connecting the centers of two circles. With color indexing it is easier to understand.:wink:

 

Also special cases should be discussed. All cases depends on the radius of the three circles and distance between centers of two circles with regard to radius of third circle.

3circles tangent.dwg

Edited June 30, 2011 by khoshravan
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