Flat surface not flat in assembly, cannot use mate constraint

[Anonymous]

I am trying to use mate constrain on part that I created by projecting a line to a cylindrical surface and using a sweep. Part of the solid has flat surfaces but I think Inventor thinks it is curved because it is only allowing me to use a tangent constraint in assembly mode. Did I create the part incorrectly or is there something else going on here? See attached ipt.

[karthur1 · Accepted solution]

It looks to me like its flat and should work. Has to have something to do with the 3D Sweep. Maybe someone else con explain.

I modified your part a little so the constraint would work.

Kirk

[SBix26]

It looks to me as if Kirk's method is the simplest way to guarantee flat surfaces.  But the best control would probably involve other methods such as guide curves or perhaps creating a series of surfaces and thickening them.  The difficulty, of course is the corners, where you are transitioning from a straight extrusion to a simple revolution.  

Perhaps create the extrusion and the revolve, then use a loft with tangent conditions to create the corner (then mirror the solid).  I've attached one I created like this.

Also, just for general use: the easiest way I've found to know for sure if a surface is flat is to simply select it.  If the "New Sketch" icon appears (see below), it's flat.  If the icon doesn't appear, it isn't flat.

Sam B

Inventor Professional 2016 Update 2
Vault Basic 2016
Windows 7 Enterprise 64-bit, SP1