Using offset - Is this an anomaly?

[watkins]

Dear All,

Try the following:

Draw a circle and pull it up into cylinder using push/pull.
Use the offset tool and create a circle on the top face with a smaller diameter.
Draw up the inner area to form a cylinder.
Draw up the outer area to the same height as the inner cylinder.
Delete the circle on the top face.

All is fine, but now try the following:

Draw a circle and pull it up into cylinder using push/pull.
Use the offset tool and create a circle on the top face with a larger diameter.
Draw up the inner area to form a cylinder.
Draw up the outer area to the same height as the inner cylinder.
Delete the circle on the top face.
When I do this I lose the base cylinder's outer surface.
Use undo a couple of times until you get to the second step, then pull up the outer area first. Then pull up the inner area and delete the circle. All is fine.

Is it just my installation of Sketchup or does this happen to you? If so, can someone tell me why this happens? Just curious.

Regards,
Bob

[Alan Fraser]

I'm not getting that on this example, Bob. Everything works fine...but I know what you mean. I have had similar things happen quite frequently, which usually require doing an Intersect with model, or in extemis, actually redrawing parts of circles or even etruded sections face by face.

[plot-paris]

well, watkins, the same happens to me.

my explanation ist the following:

if you pull up the outer face first, the lower circle (for better understanding henceforth called "rufus") is a necessary construction geometry for the inner as well as the outer cylinder.

if you pull up the inner face first, "rufus" is only necessary to define the outer face. it is however not necessary for the construction of the inner cylinder.
thus if you pull up the outer face as well, the inner and outer cylinder are seperated geometry.

if you, just to look inside the model, delete the outer face (labeled in the drawing above) and select the inner face(lower part), you will find that in the case where you drew up the outer circle first, the inner face(upper part) is not selected - meaning that lower and upper part are different faces (divided by "rufus").

if you pulled up the inner circle first you will (by clicking the inner face(lower part)) select the whole inner face - meaning that this is one single face. therefore the circle on top of the shape is necessary to define it; thus if you delete it, the whole inner face vanishes.

I hope this was understandable

[watkins]

Dear Plot-Paris,

Yes, that all makes sense. What is confusing is that after using the off-set tool one can click on individual areas and see that they all separated by circle entities. So intuitively, one assumes that on the next push-pull the circle entity will divide the cylinder, which it doesn't. Pulling the the outer annular area first solves this problem, or doing what Alan suggests, intersecting before deleting

Another mystery solved.

Many thanks,
Bob