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I have tried playing with bones and various other things. I am sure this must be something others have dealt with but because I don't know how to describe it for a web search, I am not finding anything that can help.

Assume there are 4 planks, each is screwed to another at each end so all 4 are connected to make a 4 sided shape. If I rotate one side, I need the other sides to stay connected where I pinned them as if a screw or something went through the two pieces. I whipped up a simple blend file and just manually moved the parts to demonstrate what I am trying to describe.

How can I do this?

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002

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Blend file

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  • $\begingroup$ Are you perhaps describing "Parenting"? $\endgroup$
    – Ron Jensen
    Commented Nov 22, 2021 at 21:35
  • $\begingroup$ Sort of. I know I can parent board A to B and B to C and C to D but then that leaves D with no parent. I am trying to figure out a way to tie all four corners down. Imagine putting a pin through each overlapping end of the planks. I tried bones but had the same problem. I could put bones from one end of A to the other, then from one end of B to the other and so on. But that last bone couldn't be hooked back to the first one. At least I couldn't make it work. If I tried connecting that last bone to the first one, it disconnects the first bone from the 2nd. $\endgroup$ Commented Nov 22, 2021 at 21:47
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    $\begingroup$ This is one of those problems that's trivial if you are willing to settle for only one bone being able to rotate and very difficult if you want any of the four of them to. You're modeling a pantograph by the way $\endgroup$ Commented Nov 22, 2021 at 21:51
  • $\begingroup$ YES!!!! I knew it had a name but for the life of me I couldn't think of it. That is exactly the type of thing I am trying to make. I am trying to model a physical object that has joints like this. I want to move say the top bar and have the legs on the left and right move as they would in the real world. It is very difficult because the real version actually has some added geometry above the top bar in this example. If I could get this working, I think I could use the principal to work out the rest. $\endgroup$ Commented Nov 22, 2021 at 21:57
  • $\begingroup$ There are some questions here about scissor lifts that might be relevant to your question. $\endgroup$
    – Ron Jensen
    Commented Nov 22, 2021 at 22:00

2 Answers 2

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You are trying to model a pantograph. The basic method for doing this is very straight forward assuming you are willing to leave one leg pinned:

The bones

It is important that the bones overlap at the pivot points of the pantograph, but they are not connected. Instead, two constraints are use:

On the top bone, a copy location constraint is used to copy the location of the tip (tail) of the left bone to the root (head) of the top bone:

Copy location constraint

Notice that Head/Tail is set to 1.

The second constraint copies the rotation of the left bone to the right bone:

copy rotation constraint

Now, when the left bone is rotated on its Z axis, the top bone will follow but stay parallel to the X axis, and the right bone will rotate.

The problem is that for an arbitrary pantograph you need four of these chains and you need a driver or other mechanism to select which chain is driving the deforming bones.

The usual solution is to pin one end of the pantograph and use mechanisms similar to those discussed in this thread on Blender Artist The specific mechanism you would use depends on what compromises you are willing to make. The mechanism for a scissors lift, especially one with multiple crossings, can be very different than the one for a drawing pantograph, for example.

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  • $\begingroup$ The basic concept you outline works for objects where the angles of the left/right are equal. But I need them to not be equal so as one moves say 2 degrees the other might move 3 to keep the solid top bar connected. I will check out that article you are linking to. This was the most promising example so far but still failed. I've been out of 3D for many years. Use to use Lightwave a long time ago. Had the same problem there, no way to pin multiple points on an object. (having 2 origins would be awesome for physical things like this.) $\endgroup$ Commented Nov 22, 2021 at 23:09
  • $\begingroup$ It looks like this is a lot harder than I ever thought. In the real world you just run a screw through the two ends and done. In 3D everything is one direction and can't reconnect to itself. Thanks for the pointers. It gives me ideas. Nothing is quite working but the ideas are very helpful. $\endgroup$ Commented Nov 22, 2021 at 23:16
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    $\begingroup$ You need to use drivers if you want to work with rectangles that aren't parallelograms. You can also try some stunts with IK chains but they get a lot trickier pretty quickly as well. $\endgroup$ Commented Nov 22, 2021 at 23:22
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    $\begingroup$ Ok, I got it SORT OF working. I need to spend time tweaking for my particular needs. But your help got me going on the right track. Between the demo and pointing me at other discussions, I am able to move forward. The end result is partly IK to control some of the movement then drivers to control the remaining angles. It is not pretty and I have only tested it on a very simplified model so far. THANKS SO MUCH!!!! $\endgroup$ Commented Nov 22, 2021 at 23:58
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    $\begingroup$ Glad I could be a bit of help. When you do solve the problem, I'd really like to see your solution posted as an answer here, if you wouldn't mind. $\endgroup$ Commented Nov 23, 2021 at 0:29
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The previous answer pointed me in the right direction and watching a couple more videos helped. I got to the point of creating driver actions for the middle and upper connection plates in the video (see link below) but eventually just got tired of tweaking because I couldn't get it quite right. I was on the right track but fine tuning was making me crazy so I moved on.

The end result was to create bones for the arm bars on one side and let the arm bars on the other side follow using constraints. The only part that needed drivers (or manual fudging since I gave up) were the middle and upper connection plates. After setting a position, I had to gently nudge those plates to make things line back up just so as they kept drifting just enough to be annoying to me. Since this was mostly a learning project, I called it close enough. I learned a lot.

Here are some images, the tiny bone in the middle is one that needed an angle correction after posing.

lamp 1

lamp 2

lamp 3

Here is the link to the video: https://youtu.be/ca6jmr-sRIs

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