Fusion 360 chamferpart of edge

Edit 2 (last one I swear): Definitely use the bottom chain, otherwise the toolpath attempts to avoid the corner material (because it is model-aware, unlike every other 2D toolpath):

Fusion 360 chamferall edges

Chamfer Clearance is how much you want the tool to avoid collisions with the model, which is why it's usually easier to use 2D Contour with a modeled chamfer instead of 2D Chamfer.

Use an open chain to mill one segment of a contour, here's a screencast showing how to alter your chain (not your part, but this should apply to any model):

Fusion 360 chamferreddit

So, why is there a "2D chamfer" if a contour with "chamfer" checked does the job?  Is it specifically for the case where there is no chamfer in the model??

Fusion 360 chamfernot working

Since you have the top of the chamfer selected as your geometry, you'll want to set your Chamfer Width to zero, and set the tip offset to at least the vertical depth of the chamfer. The handier selection is to use the bottom and set a marginal tip offset so you aren't cutting with the very end of the tool.

Well normally you can just select the top of the curve, set your Chamfer Width to 0, and set your tip offset to .075 or so (like I have in the screenshot) but since there is no arc, the 2D Contour toolpath specifically avoids the bottom of the edge when using the contour around the square. Seems like a bug to me. What say you @jeff.walters @dave.anderson ? Turning on Pass Extension does not fix this.

The 2D Chamfer toolpath is only half-2D. It is model-aware and will avoid collisions with other areas of your model, making deburring much more painless and quick.

I tried using 2D chamfer on a square corner, and it works fine.  But for reasons that are not evident, I CANNOT figure out how to use it to cut a chamfer that is in the model.

Also, it appears that the operation looks for a closed loop. What if I only want to chamfer one side of a square. It that possible?

Fusion 360 chamfermultiple depths

Fusion 360 chamfertoolpath

I found the "chamfer" check box in the Countour tool dialog.  If I choose the BOTTOM edge of the chamfer, and check the box, it comes out perfect.  I found reference to the bottom edge in the chamfer checkbox pop-up help.

Fusion 360 chamfertutorial

It should be noted that this option only shows in the toolpath with an allowed tool selected such as a chamfer mill, so maybe that's when OP is missing the option.

Further, in the example you generously gave above, you chose the UPPER edge of the chamfer for 2D contour, and the help specifically calls for the BOTTOM edge to be selected.  As I said, it worked flawlessly when I did that.  And I would never have found it without your pointer.

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To get the hole to chamfer you will have to play with the Lead settings otherwise it won't clear the opposite side and it'll tell you it cut passes because they would cause a collision.

The handier selection is to use the bottom and set a marginal tip offset so you aren't cutting with the very end of the tool.

Fusion 360 chamferEmpty toolpath

- In such case, the question I originally came to ask: "how do I model a 45d eg chamfer cut with a 1/8" mill" is no longer valid, is that so?

Also if I answered your question, please select that as a solution so if others search for this issue they can find it more easily.

Now if you use 2D Contour (Chamfer box checked on the Passes tab) it will round the corners off unless you turn Outer Corner Mode to Keep Sharp Corners or Keep Sharp Corners with Loop:

Yes, you can certainly model the chamfers on. If using an actual chamfer tool, you would select the bottom edge of the chamfer, set the chamfer value to zero, and whatever small value you want for the point to go past the bottom.