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Line tangent to two circles?

I'm laying out a simple design to be cut out of plate aluminum and need some help. I can manage to get a line from a point to the tangent of a circle, but I can not figure out how to get a line tangent to two circles.

In the above image I need to get the red line to be tangent to the large circles and the smaller circles near the edges. If I start the line on one circle, then attach it to the tangent of the other, it always changes the size or position of one of the circles.

The way I need to cut this with the tools I have it won't really matter because I can scribe these lines with a ruler, but if I ever sent it out for manufacture I would need to have everything complete and cleaned up so I want to learn how this needs to be done.
4 REPLIES

Re: Line tangent to two circles?

Make sure you have Maintain Relationships and Relationship Handles enabled.
If the two circles are there and you start the line command,
you can click on the first circle. Pull off the circle at an angle that is
approximate to the tangent angle.
you can do this anywhere around the circle.
The line will remain tangens and follow around the circle.
Move the mouse over the second circle close to where the line will be
tangent to the circle.
You will see the tangent alignment indicator. Click and the line will be
tangent to both lines.
If you cannot figure out how to get dynamic placement to work,
just place a line connected to the two circles. Add tangent relationships
between the line and the circles.
Tangent is in the Relate command group.
Regards,
Rick B.

Re: Line tangent to two circles?

I tried placing the line on the circles and using the relate tangent button, but it would resize one of the circles or move it to where the tangent would be on the second circle. That makes me think I need to turn off one or more of the other relate features to keep the sizes and positions where I need them. It is highly likely that I have something set up wrong to make this fail to work as described, I'm still just kind of bashing my way through things. The more I use it the more I like it though.
If I upload the .dft file would you be able to tell me what I'm doing wrong so I can learn to do it the correct way? I'm sure you'll find a lot of ugly mistakes since I've never been trained in any drafting or CAD systems, but it gets my jobs done so ugly seems to be fine for me.

Re: Line tangent to two circles?

For whatever reason the tangent relate function is working today, maybe I didn't have enough extra past the circles for it to work last night.

Re: Line tangent to two circles?

The more you constrain geometry, the less it will move on you.
When you add the tangent constraint, the diameter of one of the circles
changed.
Put a diameter dimension on the circle to hold the diameter.
Rick B.