topic Re: Object snap not working when creating TTR in AutoCAD Forum

Object snap not working when creating TTR

Anonymous — Wed, 10 Oct 2018 18:00:40 GMT

My object snap disables itself when I'm selecting object points for ttr between two circles. Is this WAI? How can I get around it? I need specific points for where the tangent connects, not just somewhere I clicked on the circle.

https://gyazo.com/c8ddbc9807fb96032da91b756f89a118

Re: Object snap not working when creating TTR

beyoungjr — Wed, 10 Oct 2018 18:07:37 GMT

Hi,

This is normal behavior for TTR!

Your tangent position will be dictated by the selected objects and radius entered. You cannot drive or override a tangency beyond the mathematical result determined.

\Maybe your workflow needs looked at? Please post your example dwg and describe what you want connected. This will reveal your full process and not just what I saw on your screencast.

Blaine

Re: Object snap not working when creating TTR

Anonymous — Wed, 10 Oct 2018 22:35:21 GMT

So I can't even enter a first position? I understand that for tangents, for one point on an object for a specified radius there will be another single point on the other object with which it aligns, but I can't figure out how to input that first point. I tried typing coordinates but it just ignores it.

Attached file: I'm trying to tangent the big circle (R3) to the two smaller circles under it, but I need it to touch the top quadrant reference snap of the small circle on the right, then whatever point that lands on the circle on the left.

Re: Object snap not working when creating TTR

beyoungjr — Thu, 11 Oct 2018 12:05:28 GMT

Hi,

I created a screencast for you to observe (sorry no audio). At the beginning I show my running OSNAP settings so you are aware that they are useful but not interfering with my steps.

I tried to show several ways of connecting objects at points tangent. Remember that lines to/from quadrants only work between objects where the X or Y quadrant points are co-linear, as in the 2 circles at lower-left in your drawing. Also remember that those quadrant points will not continue as tangent points to the next object unless co-linear.

Fillet is always using tangent points so you will not see an indicator for OSNAP. You will need to set your radius.

TTR doesn't care which running OSNAP you may have turned on because it is only looking for tangent connections. If you enter a radius that is too small the command will fail. There is no radius too large.

TTR and TTT positioning of the resulting circle often depends upon where you pick when selecting. Of course you would finish by trimming these circles as necessary.

Typically you will not be entering coordinates for any of these because your cursor control easily identifies the connection points.

Hope this helps,

Blaine

Re: Object snap not working when creating TTR

j.palmeL29YX — Thu, 11 Oct 2018 13:09:03 GMT

@Anonymous schrieb:

Attached file: I'm trying to tangent the big circle (R3) to the two smaller circles under it, but I need it to touch the top quadrant reference snap of the small circle on the right, then whatever point that lands on the circle on the left.

This is geometrical not possible. An R3 arc which begins at the upper Quadrant doesn't reach the left cicle. The radius must be 4.257... to reach the left circle. Or greater, but than you don't have tangencies between the arc and the circles.

cadder

Re: Object snap not working when creating TTR

j.palmeL29YX — Thu, 11 Oct 2018 14:15:26 GMT

In the drawing above I created the arc using parametric constraints.
If you are not familar with this you can also draw a correct circle with the basic commands.
In the video https://autode.sk/2y9qNBS you can see why an R3 circle never can touch the one circle at the upper quadrant and touch the other circle tangent.
To solve the problem you must either rotate the big circle - then it doesn't touch the small circle at the quadrant or you scale the big circle to another radius. The correct radius you find with some construction (helping) geometrie as shown beginning at about 1:30.

cadder

Re: Object snap not working when creating TTR

j.palmeL29YX — Thu, 11 Oct 2018 15:47:31 GMT

@j.palmeL29YX schrieb:
The radius must be 4.257...

Oops, the drawing 181011-forum example.dwg which I posted above is wrong.

Here a corrected version - the radius of the circle/arc is 4.250, of course the same result as shown in the video.

Sorry for the confusion.

cadder

Re: Object snap not working when creating TTR

RobDraw — Thu, 11 Oct 2018 16:51:37 GMT

TTR is not going to work in this way. A circle that is tangent to another at a specific point (quadrant) and tangent to another circle has to have a calculated radius and can only be tangent to the 2nd circle at a resultant point. In other words, there is only one circle that can be tangent at the quadrant of a circle and tangent to another circle. The radius is a calculated value, you cannot assign a radius. This is a geometry problem. There are a couple guys over at CADTutor that really have their heads around these types of problems and will show you how to do this.

Re: Object snap not working when creating TTR

j.palmeL29YX — Thu, 11 Oct 2018 17:03:45 GMT

@RobDraw schrieb:
TTR is not going to work in this way.

Therefore I didn't neither use nor suggest the TTR method, but draw the circle withhelp of some construction lines. The radius is the result of this construction.

cadder

Re: Object snap not working when creating TTR

RobDraw — Thu, 11 Oct 2018 17:27:42 GMT

@j.palmeL29YX, sorry, I wasn't really responding to your post. I clicked the wrong button.