Rotating Bones Shrinks and (Incorrectly) Deforms Mesh

Hey all,

I’m making a simple robotic arm sort of thing that’s meant to lift what will be a piece of machinery out of a box:

I have the entire arm set (excluding the foot) of the mesh included in a vertex group called “MovableSpace,” the upper arm and everything it touches in a vertex group called “Upper,” and two more vertex groups for stuff I’ll do later.

I named the bottom bone after “MovableSpace” and the third bone “Upper” (I left the middle bone as a default) and tried rotating it. I then used Ctrl-P and “Armature Deform” to parent them. The bottom bone works perfectly, but when I move the “Upper” bone, I get some weird issues as shown below:

Before Rotating

While Rotating

As you can see, when I rotate “Upper,” the “Upper” vertex group doesn’t rotate all the way and starts scaling down. The tip of the upper arm isn’t attached to any vertices of the lower arm or the middle cylinder thing, but is just floating inside. Also, I’m positive that all the vertices of “Upper” are weighted completely.

I get a really strange feeling that this is an easy fix that will be answered in two sentences. I had this problem a long time ago with a different model, so I must be doing something wrong.

Thanks, y’all!

It looks like a weighting issue to me, but you say you’ve eliminated that possibility. Can you share your file so we don’t have to guess?

Sure thing, and thanks in advance.

I know that everything is weighted completely because I went into Weight Paint mode and the entire “Upper” section was as red as vertices get. I’m pretty green, though, so I could be missing something there.

ScannerBase.blend (492 KB)

I’m gonna guess anyway…

it seems like you may have more than one modifier.

OR you have the meshes parented to the individual bones AND an armature modifier affecting them… this can cause similar unusuallnessness

In your vertex groups, remove the vertices that are in the ‘Upper’ vertex group from vertices that are in the ‘Movable Space’ vertex group
Also remove the top armature modifier in the modifier stack


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Whoah, thanks man! Very nice of you to upload a demo of the solution. It worked like a charm.

Excellent advice. It saved much time. Thank you!