[Maya] Newb: Cannot add expressions

[chap]

Hi Everyone,

I just downloaded the Maya PLE and some online tutorials. Sorry if my terminology is off.

This particular tutorial had me creating a bouncing ball. All's fine until I get to the squashing of the ball.

scaleX and scaleZ are to be transformed by two expressions in relation to scaleY (which was done with Keys).

When I enter the expression, I get :
Error: Attribute already controlled by an expression, keyframe, or other connection: what_ever_variable_im_trying_to_change

These are the formulas I've tried:
sx = 1/sy
sz = 1/sy
ballScale.scaleX=1/ballscale.scaleY
ballScale.scalez=1/ballscale.scaleY

Gist of the tutorial:
make a prim sphere, radius 2 (ball)
make a prim plane, 25x25, 1x1 subdivisions
make new group for shpere (ballTranslate)
adjust ballTranslate's pivot to base of ball
move ball to edge of plane
with 60 frame, add a key to 1, 20, 40, 60 (ballTranslate)
move ballTranslate to a higher Y position for keys at frame 10, 30, 50
Adjust graph for more natural bouncing
create new group between ballTranslate and ball, called ballScale
in Channel Control window, move everything except scaleY
scaleY to a flattened horizonally squash shape at frame 1, 20, 40, 60 (ballScale)
scaleY to a flattened vertically stretched shape at frame 10, 30, 50(ballScale)
open up the Expression window, enter expression 'sx = 1/sy' name scaleX

That's when I get the error.
Please help, thanks!

Chap

[chap]

Ok, after some experimenting, I notice that you can't use expression on a node if you've done any transformation on it what so ever. You gotta apply the expressions BEFORE you do anything to the node.

So, I ungroup the ballScale, and created a new group called ballScale. I enter the expressions (SUCCESS!), THEN I scaled the Y.

And now it works.

But, is there a way to add an expression to a node after you've done something to the node, like some kind of transformation?

Thanks!