Offset Srf requires surfaces extended but no go without explode

I have a whole load of surfaces all joinEdge’d together to ensure no leaks.
I now need to offset 2mm inwards but from experience I know the resulting exterior edges if they curve inwards, which they do, will also move inwards so the item in ortho views shrinks a little, so I extend surface beforehand then offset then trim to original overall extents to regain true overall length height etc.

However extendSrf wont work, not without exploding all my careful work. I have joined into the object an object from a forum poster that wouldnt extendSrf, not sure if thats caused the problem, appears not. I dont recall such need to explode surfaces so as to extend them on a previous task.

Any other suggestions ?

It causes a problem, I only want the external edges extending, but imagine an L shaped surface made from two surface abutting, forming the letter L, extending the shorter exterior edge of the cross part of the L upwards also sees that edge get extended beyond the abutt. i.e. abuttment is lost so I no longer have one unified skin to offset. The L is a compound curve so all sorts of unwarranted internal offsets result.


Try the Shell command

Shell works with solids…
My cowling would require back front and underside sealing up to create a solid, to satisfy step 1, then I get to delete the ‘caps’ then it shells it but then I have an inner and outer and unable to delete the outer. I say that because I was unable to select anything at step 1 so I tried an experiment.

Have extruded a circle, capped it, then was able to run shell, remove the ends, but cannot separate out the inner from the outer.

I just need a surface 2mm inset from the original.


You didn’t supply a model so how was I to know?

Why won’t OffsetSrf work? In V5 it supports surfaces and polysurfaces.

Hi John,
I am not able to provide this model as I would have to password protect it, it is highly valuable and confidential.

It is all made from sweep2, then they are JoinEdge to ensure leakproof. Nothing more.

OffsetSrf will work on it.
What happens is with offset, you can see the white arrows pointing inwards, thats good, but the edges curve inwards at front and along its base and so the resulting shape will not retain its original length or height. What I do to compensate is extend the surfaces a little beforehand, and thats where the problem lies, I need to explode my object to enable extendSrf to work.

As such I will offset inwards the original shape. Then ditto a clone of the original which has been exploded and only had the edges extended, Then do some cut and join !

I also have a problem with the offset in that I have a planar object in this surface with 90deg angles from it to the adjoining flowing curves, and offset doesnt tackle that sharp change in direction at all well !, it wrecked it and lost a surface ! and created a small sphere there !

I will have to put a flowing surface in its place, then afterwards cut that out and put the flat surface back in.


That’s how “Offset” is defined. It creates the new surface along the “Normal” directions along the surface. What your describing is more of a copy or an extrude.

The polysurface to the left was offset while the one to the right was extruded.

Hi John,
As all of its sides have to head inwards along the normals, it must be offset, so nothing for it but to add the lossage back in afterwards.

Offset though cant cope with my planar surface sitting down in the shape, like a lake sitting in the bottom of a quarry, latter with flowing curves. All quarry sides were joinedge to the lake , lake edge was a rail for sweeps. It just can’t handle the angle there between lake sides and hill !


Maybe having used JoinEdge is causing the problem. Hard to tell without a model.