Puzzled about mirror in 2008

[EngJW]

I get two different results if I mirror a part when in part mode and when the part is mirrored in an assembly. The part is a shaft with several cam lobes in different angular positions and the rear half is supposed to be a mirror image of the front half.

1. When working as a part, the entire shaft is selected as a solid body and the rear end of the shaft is selected as the mirror plane. Everything appears correctly oriented in the mirrored half.

2. As an alternate method, I created an assembly and inserted the part (it is the only part in the assembly). Next, I did the same thing- select the part and then select the rear face as a mirror plane. The result is that the part is not mirrored at all. It is just rotated about the plane. There is an orientation option in the operation but it just rotates the part to another position, still not a mirror.

I'm baffled. Why should there be any difference?

 

[EngJW]

Never mind.

After another half hour, I found that there is a box to check that determines whether the part is mirrored or copied. It has to be checked in two places however. Not very clear, but it does work.