Tangent arcs to edges equals difficult time for me

right, the little arc needs to be drawn touching the correct edge/segment of the big arc, the one that it will still be on after you set the target r, in the first place.

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true, and thats the hard part is finding the correct line segment that fits the R i need. Not hard perse, just takes a few tries until you find the right place, but it takes up time i could be using on other parts of the model.

Post your file, let’s have a look.

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fyi, i model large and then scale down. So everything is in meters at the moment. Also note that i come from a pro-e, solidworks, and other parametric modelrs, though i have been using su pro for about 2 years now, and i LOVE IT!!! but never really needed the arc tool for precise stuff until now haha
gear box.skp (351.7 KB)

Here is how I draw arcs that are tangent to an edge (straight line) on one end and “co-linear” aligned with another arc on the other end. The following drawing shows the steps I take. The starting point is the gray semi-quadrant of the circle as seen in step 1, and we end up with the gray shape in step 8 where the curved sides are formed of two arcs which mate perfectly with each other. I use one extension, Circle Intersect from @slbaumgartner (I think).

I used a hexagon as a working surface; it can otherwise be ignored for the purposes of drawing the shapes within.

The circle is 12" radius. The goal is to a) draw a perfect curved corner of 2" radius in the upper peak of the gray shape (as seen in step 8), and b) to have the 12" radius portion of the gray shape’s perimeter start and end exactly.

  1. In the initial step 1 shape, the top-most segment of the circular arc’s six segments forming the upper-left perimeter of the shape has been truncated by the vertical line that forms the right edge of the shape. This truncation is ugly and will be fixed. The gray shape in the upper-left portion of the initial circle is deliberately not an exact quadrant of the 12" circle. The right (vertical) edge of the gray shape is offset to the left of circle center just for variety. The distance of the offset is irrelevant to the technique; the purpose of the offset is to illustrate that centerlines need not form edges of the final shape.
  2. In this example, I will draw a smooth inner corner of 2" radius at the top of the gray shape. The question to be solved is: where is the exact center point of this 2" corner radius? In step 2, I have drawn a guideline that is 2" to the left of the left edge of the gray shape. The corner-radius center point will be somewhere along this edge, by definition.
  3. In step 3 I have drawn an arbitrary short vertical edge along the guideline, and also drawn a short arc that is 2" less in radius of the outer perimeter of the gray shape - that is, 10" radius. The angular span of this arc is irrelevant. This short arc and short edge are temporary.
  4. In step 4, the Circle Intersect extension has been used to locate the exact two points where the temporary arc and edge intersect. The center of the desired corner-radius is exactly at the upper of these two points. The vertical guide has been deleted, it is no longer needed.
  5. Step 5 cleans up the drawing, deleting the temporary short arc and short edge, and the lower unnecessary guide point that was create by Circle Intersect.
  6. The curved portion of the gray shape’s original perimeter has been deleted, in preparation of re-drawing it. A new diagonal guide is created between the center of the big circle and the upper guide point that was created by Circle Intersect.
  7. The replacement 12" radius portion of the shape is drawn, using the Arc tool (Command-J on Mac). The center of this arc is at the original center of the large circle. The starting (lower) point of the arc is 12" out to the left of the circle center (which is exactly where the lower end of the original 12" arc was located). The ending point of the arc is exactly along the diagonal guide line that was added in step 6.
  8. Finally, the corner 2" radius arc is drawn, using the Arc tool (Command-J). The center of this corner radius is at the guide point created by Circle Intersect. One end of the 2" corner radius falls on top of the upper end point of the 12" arc that was added in step 7. The other end of the 2" corner radius is tangent to the vertical edge that forms the right side of the gray shape. SketchUp will show a magenta inference line when the arc’s second end point falls exactly on the tangent point.
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Yes. Available free on SketchUcation’s plugin store.

Looks like the second facet away from the spoke will fit a 3m radius fillet.

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thank you for this process. Im going to save this in my notes… as well as everything else i have learned in this thread.

I will get the plugin and try it out. I like plugins. My friends feel jealous that i have plugins and they dont since they are using the shop version of SU on the web. One plug in that was worth the money is artisan tools. Being able to sculpt an object like clay is so worth it for some of the stuff i do

ok, so its finding that right line segment that will create the radii close enough to what i need

Yes. This is quickly done with a couple of test arcs and observing the VCB to get a number close to the desired radius.

I think part of your problem may be that you have enable length snapping checked, and set to 1m. This is known to introduce errors and is best left off at all times. At first glance to many it may appear like this creates an invisible grid that will keep your model square but it does not work like this and in fact can often cause snapping that is out of alignment.

Go to Model Info>Units and turn length snapping off.

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Hahahaha! Someone else pm’d me and said to turn of line snapping as well, i hav had a couple in the last hour say it to me haha. Thats funny

Looks like its time to turn it off now:)

Thanks a million for helping me out. Most my parts i draw are for large or 3d printed parts so im not to worried but when it comes to milling and machining, it needed to be more accurate nd within tolerance.

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Yeah, it’s a frustrating setting, even more frustrating that it’s “on” by default. That’s why there is a whole thread about getting rid of it. Certainly best to make a custom blank template for yourself with it off.

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Hi It72884

Click in sequence on the Scenes tabs of this SU file for ideas.

Face plate.skp (177.4 KB)

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it wont let me open it. im on 2020 but wont let me open your file

He saved as 2021. Here it is downsaved for 2020

Face plate.skp (256.1 KB)

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great use of geometry theorems. I really liked that scene show. thats how i have done it with paper or on autocad. I forgot that a line from the center of a circle that passes through a point of tangecy will create a second point of tangent.
i also like the uses of scenes to illustrate something for others to follow

I am really lazy. Two easy ways to do this:

TIG’s 2D Tools and just work in 2d flat using then use the Fillet tool to make your fillets and then push pull the part up.


Draw everything without fillets and then use FredoCorner to add the fillets when you are done with the primary geometry…

Both will give you a good model with minimal effort. One working from 2D to 3D and the other has you working only in 3D.

The asymmetry of the part is definitely odd but the tools above won’t really care.

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Its supposed to be symmetric but skethcup arc and line snapping was causing major issues. I have figured it out now. Fredos round corner works on some applications but was not playing nice on this particular part or day haha

Check extension Round edges corners 2. I think it would be help full


Very clever! Looks useful.