# Bizarre Addition and Subtraction

This has been a frustrating day. This file epitomizes the day I have had. I have two close surfaces:

If I do Boolean Union on the red and Blue I get a boolean difference (Blue - Red).

if I do a boolean difference (Blue - Red) I get a boolean union.

If I do (Red- Blue) I appear to get (Red & Blue)
Problem Add.3dm (78.5 KB)

Hi Jim - take a look at the purple object- especially the small surface at the front - that thing is somewhat of a mess. I’m not sure, in view of the thread on how to build this shape the other day, why you insist on making it this way - as has been pointed out, making surfaces that have adjacent edges that are tangent is generally not a good idea,

but it is also a much harder way to build this than to look for the underlying shape, which is essentially a Sweep1, and trim it off.

Problem Add_PG.3dm (259.4 KB)

-Pascal

Actually, I followed your advise and did it differently. The curved surface on the bottom is a sweep2 using the U’s as the sweep paths. The front and top are planar surfaces and the remaining one is an edge surface. So there should be no adjacent tangent edges.

Actually, As far as I can tell, you almost never follow any good advice. The better the advice the more likely you are to do the opposite. Did you even read the message you are responding to and the file that Pascal sent?

You have a real knack for making things tangent when its a bad idea to do so and not making things tangent when it would be good technique to do so.

It is a bad idea to make the rail curves tangent to the profile curves when making Sweep surfaces.
You should try to avoid making surfaces that have adjacent edges that are tangent. They will cause problems. Rhino really should identify these as bad objects.

If you want good quality surfaces using sweep it is best to have the profiles perpendicular to the rails.
That is the primary reason the technique Pascal is showing you is far superior to your technique.