Does anyone out there use Lightwave? I used to have it on my system but no longer have the room to install it. If memory serves: save mesh as lightwave object (.lwo) - open in modelspace - set surfaces to smooth - select the polygons- use remove points command. I think this removed the problem in lightwave but if the model is saved back into Rhino it returns. I’m not swearing to this as I can’t check it.
A heads up, I created some different lighting scheme, in order to maybe achieve a view similar to Solidworks, shown here by Stratosfear. I end up to an image, which I believe is quite better than the original Rhino scheme, more pleasant and almost acceptable 
The logic behind my approach - version is that light enters a room and a then it reflects to all the walls. In accordance , I have put the strongest light coming from below, with the other parts of a room following in, in addition to an imaginable external one coming in an angle.
A last proposition, I created a cube while working on your file and it took some ages to mesh. Well, I checked the mesh properties and you have set "Maximum edge lenght’ to something like 0.1. This is causing the messer to compose each and everyone one planar face of the cube out of bazillions of small meshes. Hence, I suggest put it on 0 (disable) and instead put “maximum angle” to e.g. 1.0 (one). As a consequence, planar surfaces will be made of less triangles and curved ones will be made of more.
greets
P.S. Oops, as you can see in the picture below, I made a mistake, because the light from below should actually be 0,0,1 and not 0,0,-1 pardon me
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In my world, we refer to this as oil can effect… To preclude it we always finish the flat with a slight crown in the flat inside the lines of tangentcy. This should be modeled with this crown, defined in a detail and a note included to observe the area and assure that the crown is achieved. as the part undergoes welding and grinding. The flat may be modeled perfectly tangent, and it is, but the illusion is that the surface is sunken.
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Hello Tangofm
Do you ever find yourself caught up in the echoes of other conversations in this place? The only person who answered my question was @menno. If you are reading this menno I didn’t thank you– my apologies. I have seen the effect you are talking about Tangofm on some Christmas-tree decorations of all things but I would still argue that Rhino’s solution is visually different. Be that as it may, I was just after a ‘fudge’ such as Lightwave uses (and numerous other poly-modeller/renderers I would guess) to cover up this problem. I believe modelling something accurately is one thing but rendering it is a whole other thing. Or to paraphrase a well known press adage, ‘Never let reality get in the way of a good image’. These are my last words on this topic. Amen.
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is this your first time on the internet?
The real question should’ve been:
“Is there a way to stop my eyes from seeing it like this?”
Rhinos renderer is not doing anything wrong, so stopping it from doing it doesn’t make sense. Remodelling is the only way to propperly avoid it. Or photoshop it.
Here you can see that the renderer renderers the surface as a flat surface and it is a flat surface, so it does what you should expect.
By the way every one here has spent their time to put you on the right track and to give you the needed know-how to be able to avoid situations like this in the future, so you should be grateful and polite to all who contributed.
@TANGOFM even gave you a real life scenario, that kind of info is always a real treat.
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I am always grateful for the help I receive on this site even though my sometimes clumsy use of words may make it seem otherwise. If my words have caused offence I sincerely apologise and I mean it. When I say ‘Menno was the only one who answered my question’ I merely meant that I was looking for a visual (rendering) solution in Rhino not a modelling one, though looking at my original posts I realise I used the word modeller when I should have said renderer. That is not to say that those who provided modelling solutions under this topic or discussed the visuals were of any less value or interest to me.
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There’s actually an interesting tool in Rhino that can analyse what’s going on here. Because there’s a single light in the image, the graph of the value of the color across mostly flat areas will closely resemble the actual geometry. So I loaded up the image into the Texture Palette, and opened the “Graph” section. By fiddling with the V section value, I can get a value graph through the image.
As you can see, there are no dips in in the graph where the dips appear on the image. The flat section in the middle corresponds with the flat part of the model. The dips you’re seeing in the rendering are actually the way the eye reacts to sharp changes in the curvature.
So…it is, as many people have already said, an optical illusion.
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Hello,
I beg to differ here, concerning remodelling. I had a look regarding the matter we are discussing and I concluded that it is impossible to have other than tagent continuity if the pipes have circular profile and the middle surface must be flat. To repeat it for clarity, if circular profiles and flat middle surface are desired, only tangent continuity is possilbe. That means that all solutions going for curvature continuity while maintening all of the above, are wrong, because all of them all they are doing is pushing the tangent effect to a small area, normally near the center, so as to be invisible.
greets
I do not think suggestions for G2 solutions used a planar area in the middle.
-Pascal
All right, but when we ditch the middle part from being planar, we also no longer have profiles of circular arcs in the blend surfaces between the pipes… In order to make it more clear, I attach a model, where I emphasize the transition surfaces, or collars as I was writing in one of my earlier posts, in order to have G2 continuity. It does not mean it has to be G2, it can be G3 as well, or more, I only went for G2 for now
y pipe G2 small.3dm (138.1 KB)
greets
I do not see any reason to assume, from the original file posted, that the solution needs to use the full arc from the input pipes in the transition area - I don’t think you can if you want a nice looking blend. Using about 60 degrees of the 90 degree arc from each side (half pipes here, not full rounds) seems to be a pretty good starting point. The top 30 degrees fades to some non-arc profile.
-Pascal
Ok, I can accept that; I only insist to make clear that there is not such thing such as circular profiles with a planar middle part, being of G2 of each other. I do so in case someone would think that it is possible to do so by something such as a “correct blend”
greets
reality is, a part where it’s acceptable to have the flat fill, I imagine would just be made via intersecting the three tubes… if the look is important enough to spend more time/$ on the blend, then they might as well make it with the better type of blends as seen in the thread.
A simple intersection may not be acceptable for a part with internal flow, or a part with significant structural loads. In those cases a smoother blending may be needed. However for such a part a flat fill including the “depressed” appearance may be acceptable. Depending on the manufacturing method circular sections may be significantly easier to make than other shapes.
(i get it that we’re pretty far off topic but the thread was nearly finished anyway so…)
maybe… either way (flat or G2), that Y is a custom piece which the 3 pipes must then be welded/screwed to… as in, you can’t just cut the pipes some certain way then add the flat triangle and weld it in there… (well, i suppose you could… it would be some very tricky cutting&bending though)…
the easiest/fastest is just intersect them… otherwise, the flat top doesn’t seem any easier than G2… in fact, it’s probably harder to manufacture since it’s asking to have at least 5 individual finicky parts cut and welded together**… i think it’d be easier to make the G2 style piece…
i can’t really think of anything i’ve seen which was fabricated in the flat style… i have a splitter (for air hoses) which is flat at the joint but it looks more like this:
** assuming we’re talking about something like piping or smallish scale… funnily enough, i’ve done some large scale Y joints and the way i did it was not too dissimilar as the flat curved triangle thing with tangent blends… 
[EDIT]
another way i’ve seen a Y joint done irl is by intersecting 2 pipes with 90º bends. (not necessarily 90º but…) it’s 2 cuts and one weld.
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I was thinking of a junction fabricated using a pieces of bent pipe and plate, or a junction cast with the pattern made by milling/routing with a cove cutter. Probably demonstrates our different backgrounds and experience.
probably right… with me being on the not so experienced side… i’ve never designed for nor built anything in the machine shop realm… (edit… actually, i have… forgot about a couple of things… not important though /edit)
i’m basically just thinking i don’t remember seeing anything (common) fabricated in the style as originally shown in the thread… but i’m fully expecting someone to post a picture “uh… what about this?” and me saying “oh yeah… duh” 
Hope this can help :
Real world tangent behaviour 
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