Now when we move one of the clusters, the joints follow along and stretch as they do it too. (If you've ever seen Jason Schleifer's DVD on Integrating a Creature Animation Rig Within a Production Pipeline you can also use the stretchy spline IK trick for this step, which happens to be where I pirated the idea from. If you don't know what I'm talking about, you should follow the link and buy it from Alias, it's got TONS of cool setup stuff. End shameless plug.) The only problem is that the bones don't point at the next joint down the chain to give a nice smooth deformation. We'll do that with aim constraints. The key to making a useful aim constraint is to properly define not only what it should be looking at, but also it's up vector to keep it from flipping out of control, like your boss when you're reading internet tutorials on company time. Since we already went through the trouble of orienting all of the joints in the same direction (remember back in step2?) we should take advantage of that.

Create a locator to serve as the up vector for the Top right lip joints.

Rename it TRup and point snap it to the root joint of the top right joint chain.

Orient the locator to the root by using an orient constraint. (Make sure that if you're using Maya 6 you turn off "maintain offset" or the dang thing won't align).

Change your move mode to 'object' and move the locator along its up axis somewhere out of the way towards the top of your characters' head.

Fig 7. Up Vector Locator Placement


Figure 8. Aim Constraint Option Box

Aim constrain (without maintain offset) the first joint to target the cluster on top of its child joint.

Set the aim vector to whatever you used as the axis going along the bone (Y in my case) and the up vector to whatever the axis is that you oriented upwards earlier (Z in my case).

Set the world up type to Object Up' and type in RTup in the object dialogue box to make it point at the up vector locator you just created.

You only need to restrict the rotation of the joint to two axes, the one along the bone (Y) should be free to rotate so you can use it to roll the lips in and out. Highly useful for making puckers and kissy faces. You can see if you did things correctly by checking the rotation values for the constrained joint: they should all be at zero. If they're off by a couple of degrees that's fine, but if one axis is out by 180 degrees you should check your aim constraint settings.