Tangent arcs to edges equals difficult time for me

Ok, so far, this has to be the most difficult drawing i have done. I mean, i have modeled some crazy stuff like engines, steam engines, aircraft parts, 3d printer parts etc, but this gear box face plate is driving me NUTS!!!

The reason it is so difficult is because there are a lot of radius’s that i need to draw into the face p[late so that when it is milled, or machined, its not alot of sharp corners. Here is a pic of the actual gear box i made with a manual lather and manual mill. FYI, manual stuff is scary at times haha.

Ok, with the attached photo of my sketchup pro piece, you can see that i did ok with the cut outs, but they are NOT accurate at all.

My question is, see the lines circled in blue? anyway to just select those 2 lines, hit arc and have it draw an automatic tangent arc with a specific radii? In other software i have used, thats how i would accomplish this, but this is the first time i have had to use the arc tool to make accurate cuts. There are alot of places, since circles in SU are segments, that it is “tangent” but still not accurate, if that makes sense. Oh, and i will make sure its just a flat drawing when i do this. I push pulled for just kicks and giggles to get an idea of what it looks like

I could do the math, but its time consuming


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Get the Magenta inference, click, then move back along the edge until you get the Magenta again, let go and type the radius you want and hit enter.


now thats interesting.
thanks Box

i can not get it to work. It makes the radius but does not keep tangency, and if it does, it jacks the drawing up by pulling the circle with it once it adjust the radius:)


hopefully last reply. though the tip you showed me works on some applications but not all. IE, in the gif i post, i use a 3m circle and move it till its tangent with the other lines. the point of tangency is extremely important. If i start further down the verticle line and make a tangent arc of 3m, its no where near the same place nor is it 3m, even when i type it in.

When i do type in a number, i do see the arc change, but its never to the correct value according to entity info.


I’m not really following, too busy just now.
Perhaps you should look at Tig’s True Tangents.

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Ill find better way to explain later. Tha k you for your help so far

Assuming you are following the directions Box gave you should be able to draw tangent arcs of any radius that fit between two straight lines. I’ll note again that you have to pull an arc until the magenta color indicates tangent, click, then move back along the edge until the arc turns magenta for a second time, then do not click, let go of the mouse leaving the arc magenta, enter a number and press return.

I’ll also note that this will only work with two straight lines. When you pull a magenta tangent arc between a straight line like one of your spokes and another arc like the outer gearbox case you are really getting a tangent to the one small facet or line of the arc that you are inferencing. SketchUp will not recalculate tangent along other connected facets of an arc. If you enter an arc radius that would not allow the arc you are drawing to connect to the single facet you started from it will misfire because the arc isn’t possible. So, this trick will work in that instance as long as the arc radius you are entering is close enough to where you started to stay on the same facet of the circle. Check the VCB to see what the arc is pick something close. Here I make an arc of 25’ which clearly is too big to stay on the single facet of the outer circle, so it forces the arc staying tangent to the line but overshoots the upper point, then I try again from the same points and as the VCB says 13’ 8" and change I go for 14’ even which stays on the same facet and works fine.


Thanks for that explanation. thats where i was trying to go last night but couldnt form the words. i was tired haha. I have been trying all morning and can not get a 3 unit radius where i need it to be. I type in 3, but like you say, it forces it to another value. You can see in the gif provided how i change the radii from 3 to 5, but yet its really 11.18.

It does need to be 3 units. S0 im trying to figure out the best way to make it 3 units. I can always use a half circle of 3 and place it the best i can haha

Just select the arc and use Entity Info to change the r. It will probably jump off the other arc when you do so, like in Endless Fix’s example, because it is one or two edges too far from the correct edge for the target r - you will need to place it on the correct edge in the first place, which is whichever edge it doesn’t jump off from when you fix the r to 5.

lol, i tried that last night and it jacks the drawing up. it moves the arc and the other lines its attached to making it a jagged mess of stuff. Im beggining to think that im going to just have to use a half circle and place it in the best place possible to get the exact dimension i need

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