This tutorial will show you how to make an articulated figure for 3D printing using a combination of Blender and Tinkercad.
The first step in making an articulated figure for 3D printing is to figure out what range of motion you are trying to get out of the figure. For the figure in this tutorial, you will use a pivot joint, and a combination pivot/hinge joint.
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Step 1: Model the Figure
Once you know what you are going to be making, you can model your figure.
Model your body so that the arms and legs are connected to the body, and the hands and feet (you can leave the head separate).
Step 2: Select the Arm
The next step is to sever the appendages, but it is necessary to connect them first. To separate the limbs from the body, begin by selecting the limb, and the immediate area like in the picture.
Step 3: Bisect the Shoulder
Then use the option shown here in the tool panel to make a cut at the shoulder like this.
Step 4: Separate the Arm
Immediately after bisecting, select every vertex of the arm after the line of bisection and then press P to separate the selected area from the rest of the model, and move the other pieces away from each other so that there's space between them.
Step 5: Keep Separating
Repeat this process for every joint that you want to articulate.
Step 6: Fill the Faces
Next, select the perimeter of all of the open faces and press F to fill the face. Once you have filled every open face, use CTRL+J to join the separated pieces again so you can edit them together.
Step 7: Knife Tool
Press K to use the knife tool and draw lines between the cardinal vertices of the face. Keep using the knife tool to divide the face, making sure to do it along points that are easy to remember.
Step 8: Knife and Deform
Next, use the knife on the face opposite the one you just knifed in the same exact pattern, but mirrored. Select the central vertex of both faces and pull until you have one end concave and the other convex like this.
Once you have done this for every part that requires a hinge joint, export each part individually as an STL and then import them into Tinkercad (you can copy and paste between Tinkercad files).
Step 9: Make the Joint
In Tinkercad, make any joint that you will need to use in your model. In this case, I used a combination hinge and pivot joint, which can easily be turned into just a hinge or just a pivot joint in Tinkercad.
Step 10: Insert the Joint
Duplicate this joint as many times as you need and move it into position inside of the piece you are articulating.
Once you're satisfied with the position, change the joint into a hole and the body part into a solid and merge the two. This will make a hole the shape of your joint in the body part.
Step 11: Make More Joints
Repeat this process for the remainder of the body parts, being sure to position the joint within each part in such a way that you will be able to put the two parts together when it is printed and still have a joint between them. I recommend leaving an ample amount of space between each part just to be sure.
The hands and feet don't need to have a socket cut into them, so at this point you can just ungroup one of the joints and attach just the pivot joint part to the hand.
Once you're done with every joint, save the file from Tinkercad as an STL and import it back into Blender to finish.
Step 12: Halve the Body Parts
When the file is in Blender, use the bisect tool to cut each body part in half in such a way that every socket used is also cut in half.
Step 13: Fill and Finish
With the pieces bisected again, the last step is to close every face so that once printed and assembled, creates the desired socket.
Take some time after you finish to go over the model and make sure the mesh isn't strange in some places, and then you're ready to print.