Boolean difference when objects feather approach here?

see attached 3dm file.

The dk blue object between A and C sticks out a little and needs trimming flush to light blue and silver objects.
I dont in fact need the dkblue aft of centre of the radiused curve of the lightBlue item, silver and light blue are the shape required there anyway.
Thinking of Boolean difference it away using light blue and silver items, I can see failure looming as it feathers at A and C and at D.

Can someone show best way with this file attached to cut that portion away. Light blue and silver are the correct shape and to remain unaltered. I then plan on boolean union all three colours together.

There are times in life a strong intersection isnt reality. The ‘cutter’ gradually leaves the object to be cut,i.e.feathers away. I seem to find my shapes are often involving such.

What is best practice when such as here occurs ?

The resulting edges will receive a varying fillet radius. Making one object stick out to give a step cut not feather is introducing a step on what should be a perfect smooth edge, It might mess up fillet later on. Initially I had a cutting solid that booleaned fine but then I noticed it had left such a minute step at area C, I corrected it to a perfect flush fit, but then it failed ! so used trim and surface patching to make good instead.

I had wondered about slicing dkblue away with a vertical surface sheet running through left to right,A down to D, D being at where dkblue meets light blue at the radiused underside, but finding that exact spot at D and A to get a perfect joint also troubles me.

…and anyway, this illustrates a need for ||Boolean Difference, removing the dkblue with ltblue and silver items is job done, its what it needs.

best way boolean difference.3dm (282.6 KB)


Hi Steve, there is a bit of mis-alignment creating naked edges in here, needs a bit of tidying up.

Thanks, so I spent ages tidied all up, one minor area has knock on effect all over,
now it has no naked edges, top silver intersects at edges the turquoise blue.

boolean difference the silver and blue from the dk blue, still fails…why now ? :cry:

I hate booleans when they do this. more often than not.

best way boolean difference tidied up still fails.3dm (371.4 KB)


Hi Steve - Boolean operations will tend to fail when there are coincident surfaces that are not simple planes. If there are coincident edges or surfaces coincident with edges as well, even worse. The best tool for understanding why any one Boolean operation fails is to Intersect the two objects and look at the resulting curves - if those could be used to trim or split, then the Booloan probably should work. In this case, not…

red= curves of intersection:

Better to trim and split here by hand I would say.


If I were to fill in the area between left and right light blue and silver objects with a solid and union it to them, would that then have worked ?

To trim by hand , is that creation of trim curves or trim surfaces and using trim command ?
make sure I understand procedure.

Somewhere in the sands of time I recall someone saying dont use trim, , was that to trim surfaces or solids ?

…maybe surfaces, as I was told that what you think is gone forever is in fact still there, it sort of hides it from view, comes back to bite you later the poster said. Now solids being joined surfaces, are they ok to trim from being told that ?

Its easy to reach for trim , but that lesson haunts me.


Hi Steve,

I wouldn’t be afraid of trim and split, you’ll be lost without, I had a look at the model and there are a few places where the geometry let’s you down, as well as the coincident surfaces that Pascal mentioned. I think the quickest approach here is to redo the whole thing using dupedge to start with and get the curves all clean and matching nicely.

This looks like a job for wirecut. Start with a block and cut-away what you don’t need.

you wouldnt believe it but I dupedges the whole thing and resnapped everything, then reskinned everything,.
how I missed those is beyond me, spent ages as well…

WireCut, sounds interesting, not aware of that at all.

will explore and tidyup even further, though Pascals comments would point to boolean a waste of time here.

I am forever finding my objects feather apart, due to the nature of the work, and booleans dont like such.


Tidied up and now using a solid right the way across, to get round the situation of shared surfaces in the inner faces.
Boolean difference fails, so its not just the centre surfaces at fault.

Plan B, create a cutting surface extending beyond the solid. fails also.

Now why does that red surface fail in boolean split, its created from an intersect, then extended above and a perp line from base of curve downward to a hrz line , using line perp from curve, to see if the curves there differ,(found as expected that they dont)…, so as to tell it where to cut, as a feathered area confuses boolean !

still it fails. :cry: :sob:

when two radiused items as that occur and meet, thats how it is on the real item, If i push one downards then I get a thin sliver remain on top, and an incorrect shape below.

They wouldnt union, move brown upwards 0.1, boolean union succeeds, but the items have to be flush., I cant have minute steps in surface to allow a tool to succeed.
boolean split fail despite cutting surface extend beyond.3dm (299.6 KB)

Hi Steve
Boolean for Steve.3dm (1.7 MB)

There are still coincident surfaces and an area where the top surfaces don’t meet, have a look at the model attached, the steps are laid out in the layers ( jings, not a video) and the steps show one way of fixing things up. Hopefully resulting in a happy smiley…

Thanks BrianM, :smiley:
I will study this carefully.
To justify the effort gone into this, :astonished:
Lot to learn in this still.
Training or sheer experience ?