Does anyone know of a method of filleting that forces the created tangent edges to be aligned? Here's an example of the sort of misalignment I'm trying to avoid:



- Peer

Hi Peer,

Is the attached file what u want? or u also need the curved corners? (sawtooth-shaped face close to the red arrow u used in your image)

-Psygorn

Hi Peer, that kind of mismatching is normal for Mechanical CAD rolling ball type fillets.

It's because the width of the fillet depends on the angle between 2 surfaces.

If the angle is shallow between 2 surfaces, the fillet will be smaller in width, like this:



As the angle between the surfaces increases, the fillet, using the same radius, will be wider like this:



The thing that is equal between them is that they are both pieces of the same radius circle, they just have different lengths along the circle:





So in the case that you show, the angles between these pairs of surfaces are different:





Since the angles are different, the fillet surfaces' width will be different and so if they are directly connected with each other they will have the result you show.

Usually the way to get the fillets to align nicely is to have all the edges that share a common point to be filleted instead of only some of them. That then allows for juncture patches to be made where the fillets come together instead of having fillet surfaces directly running into each other:



- Michael

Thanks for taking a crack at it Psygorn! I’m traveling now and so can’t view your .3dm, but am looking forward to it when I return.

Thanks Michael for confirming that there’s not a general method for assuring alignment of tangent edges. This came up because I got a part model made in Solidworks that had a tiny mismatch despite having a corner patch. The mismatch caused me grief while modifying the model. I guess that making a patch that matches the tangents up might be difficult in some cases.

- Peer

Hi Peer, maybe I have misunderstood what you are actually asking about since you are referring to tangents and not just the widths of the fillets.

But in the case you have here it's also normal that the fillet pieces are not tangent to each other because you are keeping this edge of the model to be sharp:



Fillet surfaces will not be tangent where they touch each other across a sharp edge like that, that's just a normal result for fillets. They will be both not tangent to each other and not the same width as well.

- Michael

Hi Psygorn,

Yes the 3dm you posted is what I want. How did you do that?

Michael:

There are a few different ways of defining fillet "width" like radius, chord length, and arc length. I think what I'm asking about is something like defining the fillet width as the distance between the theoretical sharp corner and the tangent edge of the fillet:


- Peer

Hi Peer,

re:
> I think what I'm asking about is something like defining the fillet width as the distance between the
> theoretical sharp corner and the tangent edge of the fillet:

Unfortunately I don't know of any CAD programs or libraries that define it in such a way.

The most standard definition in CAD is that the fillet size is controlled by a radius value, and the radius value is expressed as an offset distance of each surface, then the offset surfaces are extended and intersected to form the fillet centerline rail.

That gives the so-called "rolling ball" type fillet shape.

It's not really feasible to experiment with replacing just one aspect of filleting with a different definition because it's an overall complex mechanism with interconnected dependencies on handling corner juncture pieces.

The closest thing that could be experimented with is if your alternate width definition could be expressed in terms of surface offsets at any given point and then that might be formulated as a variable radius fillet.

- Michael

Hi Michael,

Thanks for confirming that there aren't any CAD programs that define fillet width in this way. That's sort of what I figured and was not suggesting that MoI be changed to do it like this. Just thought someone here on the forum might have a trick to avoid these troublesome mismatches when making appearance models. For most stuff I have to stick with constant radius fillets since anything involving variable radii is troublesome to machine.

Also, thanks for the peak into how filleting works. That's pretty interesting about how the rail is made.

- Peer

Hi Peer.

I do it this way...

Hi WN,
Your method is interesting. what tool do you use to make your video?

Hi Zooen.

"what tool do you use to make your video?"

https://getsharex.com/

Thanks WN! That's a clever method you came up with.

- Peer

Very tricky!