
(07.05.16) Note : After last edit, a more appropriate title for this message would be: "How to animate several drawings with the same plastic mesh (e.g. arm and hand)"
Two methods : 1) Using one column, 2) Using more than one column:
1. Using one column
(1) create arm and hand

(2) create a mesh
(3) create skeleton

(4) in Plastic > Animate mode, un-check "Keep Distance" (up right) and put closer these two nodes (then check "Keep Distance" again).

(5) If necessary change SO (Tool options Bar) of this node to put the hand below the arm

(6) Animate element as usual.

2. Using more than one column:
(1) create arm and hand in the same level.
Note: If it is planned that this hand, e.g. will become one step by step animated level, then draw here the envelope of all the drawings of the future level.

(2) create a mesh of this level.
(3) create skeleton.

(4) select and Cut (Ctl-x) the hand.

(5) Paste the hand (Ctrl-V) in an empty column, and arrange columns to put the hand below the arm

(6) be sure elements are conected like this.

(7) in Plastic > Animate mode, un-check "Keep Distance" (up right) and put closer these two nodes (then check "Keep Distance" again).

(8) Animate element as usual.

More examples here, with the method explained by Toonz Harlequin 7.1. They create one sub-Xsheet with diferent levels, and then they create a mesh from this sub-Xsheet. Seems to have more sense, perhaps we can use this method when the feature "Edit sub-Xsheet in place" will be back :). https://youtu.be/T50aOjWUr6E?t=4m (4:00 to 4:11)
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Indexed here : https://workflowy.com/s/JtI13Zub8a#/d515ddc4e4da