Originally I was wondering how topside edges/outside fillets were actually machined so I could understand what I am seeing in a drawing and how to properly model it. I stumbled on this question before asking which was very informative and confirm some of what I already expected. For wood work I would call it a quarter round router bit. However with metal it appears to be call a radius cutter or a corner rounding end mill.

So I have been struggling with this part and parts like if for over a week now with various degrees of success. The blueprint is from a 1943 so standards may have changed a little since then.

enter image description here

In the front view we can see that the fillets on the part the sticks out to the front have a radius of 0.21. In the TRUE VIEW OF BASE ALONG LINE 'B', we can see the partial fillet along the length and the arc of the termination cut. With no radius given on the arc I am assuming it either covered in the spec which I do not have or there was a standard set of tools at the time.

At first I thought I would just make a section of the tool and revolve it at the point partially along the edge where I needed the fillet to terminate. Because of the acute angle of the faces, I would need to do this twice, once with the cutters axis parallel to each face. However in doing so I noted that there are to problems:

  1. The remaining material edge arc is convex instead of concave
  2. Since the to outside faces are are an acute angel, the remaining material did not blend into the cut as anticipated.

Failed router cut

So that got me thinking and why I did a bit of research on how fillets are made. My next thought was to make a circle the diameter of an end mill bit and have the quadrant of the circle follow the arc of the fillet. The principal seemed sound and would give the right arc shape to match the drawing, but when it came tome to perform the sweep I kept getting errors.

The is the circle and path selection (note the arc is mainly blue instead of dashed yellow).
End Mill Selection

And here is the error message I receive. I actually just discovered that when I click on the final error message it produces the green circular section that I had previously not seen and confuse me as I had the behaviour set to keep the circle section in the same orientation:

End Mill failure


How does one model this type of fillet termination in inventor?

I am using Inventor LT 2021

On a side note, is that a normal way to dimension the length of the fillet?

I have also tried rails selecting the fillet radius as the inside path and an offset matching fillet arc for the rail guide. It did not work.

I tried creating a cylinder and sweeping the cylinder. It did not work.

I tried creating two circles on the different plane offset from each other and applied an edge rail. It failed.

  • $\begingroup$ Based on some trial an error for different arc angles on the path, it seems that it blows up when the flat surface of the section is tangent to the path. so assuming the arc starts perpendicular to the circles surface, the sweep fails at angles of 90 degrees and greater. At least that is my current observations $\endgroup$
    – Forward Ed
    Commented Jun 18, 2023 at 19:48

1 Answer 1


I did this with solidworks, but I used the revolved cut feature which should be available in inventor too.

Basically the steps are:

You make a fillet

enter image description here

You revolve cut the end corners

enter image description here

In your case the corners are not exactly 90 degrees, in which case you can modify the revolving sketch as follows:

For acute angles

enter image description here

For obtuse angles

enter image description here

Remember to keep the revolution axis normal to the upper face.

In the case of very obtuse angles, like in the picture above, you won't exactly get the circular line perpendicular to the external profile, due to geometry:

enter image description here

but as long as acute angles are nearly 90° this isn't very noticeable. I think you can see this on your blueprint too, albeit very slightly.


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