Hey everybody :)

I'm new to Moi, and CAD for that matter, and have been giving myself some basic modeling exercises to pick up the program and the concepts of CAD.

I've been racking my brain with fillets for the last 2 days. I've read on the forum that fillets can be quite tricky and that there are workarounds when a fillet won't calculate. I also found out that different applications handle fillets differently, so one app might be able to do something that another would not.

I've been trying to model a computer stand and there is a transition going from round to square and I've tried every possible way I could think of to get this to work, the most direct being to just loft from a cylinder to a square and fillet the transition. For some reason though this doesn't seem to be possible. I've been trying it for hours, multiple different shapes, trying rounded squares, nothing seems to work. I tried getting the point count on both the circle and rounded square to be the same - still didn't work. I tried taking the file and importing it in Fusion 360 - it still wouldn't fillet.

The solution, I suppose, would be to do a manual fillet as described here:
http://k4icy.50webs.com/tutorials/Fillet_Using_Sweep_and_Blend.html

But now I'm just curious - what prevents this from filleting, not just in Moi, but in Fusion 360, I'm assuming there's just something not possible or very difficult to do, but I don't really understand what that is... My question is - why won't this work, what are good general methods of working to ensure that filets and most operations will work and where can I read up more on proper and efficient ways of working with nurbs. Basically - I'm looking for something explaining the concepts and methods one should use or not use when working in CAD.

Thank you so much!

Hi VOLEN_CK

The top round EDGE of a cylinder with no round top face, nor round bottom face,
can be BLENDED with the bottom rectangular EDGE of an extruded rectangle without top nor bottom face.
(For the Blend, do NOT use a round curve, nor a rectangular curve, use the EDGES. i.e. delete the starting circle and starting rectangle curves.)

- Brian

Post your file.

Hi Volen, it's because fillets will shrink to a very small or 0 size when 2 surfaces become smooth to each other. Situations where 2 surfaces come very close to being smooth to each other tend to make filleting difficult because the fillet becomes very small and it's difficult to use a tiny surface to intersect and trim against other nearby surfaces.

Here's a more visual demonstration of this, here are 3 different instances of surfaces meeting at different angles, and all have been filleted with the same fillet radius:






So the way a CAD fillet works is that it constructs an arc between 2 abutting surfaces that is tangent to each surface. All the arcs shown above are segments of the same radius circle, but on the sharper angle a longer length arc is needed, and you can see as the angle decreases and becomes shallower the length of the arc becomes shorter and shorter, it is still a piece of the same radius circle but using a smaller and smaller piece of that circle.

Now, imagine the surfaces are totally smooth to each other - at that point there's no length of arc left that will fit in there at all. In a situation like you've made there where surfaces come quite close to being smooth to each other it also means that the arc size becomes very short or even 0 in those places and that will tend to make it difficult to fillet. Filleting requires many complex steps of operations such as computing offset surfaces, intersecting offset surfaces, extending fillet surfaces, intersecting fillet surfaces with each other, and building corner juncture patches where fillets collide into each other. Many of these operations are difficult to calculate on a tiny sized fillet surface.


> what are good general methods of working to ensure that filets and most operations will work

Well, for filleting you will want to have surfaces that meet at a more distinct angle rather than coming very close to being smooth to each other where they meet up. For your situation you'd probably plan in advance that filleting is not going to work there, and trim away some space between the surfaces and put in a Blend instead.

Hope this helps! - Michael

That's some nice teaching material ! Thanks Michael .

I would also add to his quest to understand fillets, the "order" of particular fillets will allow success or box you in.



In this example, I believe the top fillets or blends attempting to achieve, marked in "red" , should come "AFTER" the attempted "green" blends and fillets.

But don't misconstrue. This is not something "different" than Michaels post..... Just added.

2 cents.

Some programs can just do it!

Here's how I would achieve a result in MoI:

Wow... this is incredible, you guys are absolutely amazing, thank you so much for taking the time to help me out, I didn't expect anything like this.
My problem is definitely solved... and in multiple different ways :)

@bemfarmer - thanks so much for the file, my brain would have fallen out of my head if I just saw that geometry and had to figure out how to make it. Tested it now, will definitely be exploring more of blend, seems like an incredible tool. Thank you!
@Michael Gibson - Thanks very much Michael, I get it now, reading the help file also makes more sense after seeing these, I see what I've not understood when reading about the fillet command. Thank you so much, I will definitely be considering the angle surfaces meet at now and most likely be considering a blend for this. Thank you!
@BurrMan - Thank you man, that's really awesome of you to take the time, I see your point about the order in which you fillet, I really was not considering that and thank you for the blend demo, I think that's what I'll be going for :) Could I please ask what the first application was that you were using? I couldn't see its name on the video.

Thanks again guys, I'm off to do some modeling now and hopefully end up with more than a square and circle after another 4 hours of trial and error :)
Will post some results... good or bad ;)

The first program was SpaceClaim.

SpaceClaim is not a low price program! ;)
Try DesignSpark Mechanical who is the SpaceClaim's Kid but free! :)
http://moi3d.com/forum/index.php?webtag=MOI&msg=6186.1

Thanks very much again everybody.

It appears, as it normally happens for me, that things are more complicated than they seem... I've attached the reference of what I'm trying to model and just a few quick fails I've gone through. I've been pulling my hair out over this for the last few hours again ( I have no hair...). The joint I'm trying to model has a very smooth transition from a capsule / cylinder to a rounded rectangle, but I just can't get that transition where they blend right.

I was hoping a blend might do it, I was also thinking of making a solid and boolean-ing a curve to trim for a smooth profile, but this doesn't give me the correct top view... I'm completely confused at this stage, it's my 3rd day trying to get this right :)

Off to work now, then will be back at it again after that small break :)

Thank you all again for the incredible help.

Thank you BurrMan and Pilou :)

Hello Volen_ck;
I don't have acess to moi at the moment, but think about a profile extruded in the same direction the cylinder is. Draw a circle -> extrude! add a rectangle to the circle, trim to a closed curve sequence, join and extrude, Cut the result from the side view to the angle. Make the fillets Bool all together.
Or have a look here - for a different method for a similar problem:

http://moi3d.com/forum/index.php?webtag=MOI&msg=7626.1

Have a nice day
Karsten

Hi Volen, that's a pretty complex shape you're trying to build, it's not really a good candidate
for starting out and learning things because it has a high difficulty level with shapes melting into
each other. Often times forms that have that kind of melting quality can be easier to do in a
sub-d modeling program rather than in a CAD program, it's a much more difficult type of CAD
model. CAD modeling is strongest when most of the design is defined well by 2D profile curves
which usually isn't the case with melty type forms with complex transitions.

It looks like your main problem though is that you're trying to do too much of it all in one single form
right from the start, for example this piece here would probably need to be ignored to start with and
you'll build it later by doing a boolean with a narrower block:



- Michael

Something like a sequence more like this (see attached 3DM file also):



The last steps are putting in a large radius fillet here first:



Then a smaller radius fillet here:



So you've got to kind of build things in more stages and sub components than what you've been trying, and the smaller rounded areas
have to wait until the end until the larger fillets have been done first.

But these can also be some pretty difficult fillets for MoI to calculate as well, this has one of those "surfaces meet smoothly" type
situations in it where you can see the last fillet shrinks down to a point. When it's just a point at the very end of the fillet it can be
more feasible though. You may need some assistance from another CAD program with more robust fillets in it though for
this type of multiple level filleting.

Hope this helps give you more of the idea of how things are blocked out into separate pieces to start with though.

- Michael

Hi Michael,
In your example, in the third shape, i can put the larger fillet but can't put the smaller.
In the fourth shape , i can put the smaller fillet.

@Finema
in the third shape Select the 2 half curves before to call the fillet! ;)

i do that but failed
have you tried ?

Hi Finema
Another way to solve your first problem (I think)

https://kdJonesgb.tinytake.com/sf/MTI0NjIyMl80NzE1Njkz

Best regards
Keith

I did this with loft, mirror, and fillet.

Hi Finema,

> Hi Michael,
> In your example, in the third shape, i can put the larger fillet but can't put the smaller.
> In the fourth shape , i can put the smaller fillet.

You'll have to try a variety of different radius values, some radius values will work and others will be problematic. If you examine the very ends of the fillet when putting on the large one, you'll see some radius values leave a little wiggle in the shape at the very end, if that happens then the smaller fillets won't work although they may work on one side.

Like I mentioned above, these are some pretty difficult cases for filleting and you may need to use a different CAD program to put them in.

- Michael