Sunday, May 1, 2016

Tutorial: sculpting a modular bot

As a complement to the post discussing some aspects regarding the modeling of regular parts, this tutorial illustrates the modelling of the Bot designed for Painting Crusade X. It does not cover everything, but the most relevant parts regarding my tricks for sculpting regular parts are shown.

As often, I start from a core that will stiffen up everything and ease the initial setting of the putty on the armature. Its shape and size are determined directly from the concept drawing (more explanations here). To ease the final shaping, I keep the shape of the core similar (but smaller) to that of the final volume.

The armature consists of a metal wire wound up around a thick barbecue stick. The putty is set upon this armature to create the core. Once shaped roughly, I scratch its surface with a sharp blade while the putty is still crude. The stick is used both for holding the part without a direct contact (no finger prints...) AND for materializing the axis of symmetry of the part, which will be useful for the shaping and the visual checks. Ideally, I also try to assemble the wire and the stick in such a way that the stick will be easy to remove or break once the job is done, still making sure that the contact is firm. Once scratched, the core goes for a first baking.

Then I flatten a foil a putty down to the thickness needed, and I cut it into pieces with an appropriate shape: neither too large (difficult to force the adhesion without folds or air bubbles) nor too small (time loss, many joints to smooth, etc.). Again, the process of setting up the putty foils on the core and smooth the joints in between them is shown here.

As the core and the foil are both regular, their assembly gives something close to the final shape. The fine shaping is then just a matter of progressive work.

In this step, I use my fingers and my palms a lot in the beginning; then, I use knifes/blades to adjust the countours, and cylinders of different shapes and sizes that I literally roll over the part while applying a slight pressure (handle of a modeling knife, ferrule of a clay shaper,...), sometimes also using my fingers to apply some friction forces at the same time. I also let the part roll over a flat surface, taking advantage of its rotational symmetry. Then, I refine the shape of the edges with knifes and spatulas, followed by a last round of rolling with cylinders to enforce a continuity. At this stage, I avoid using solvents, because they would make the work on the details more difficult.

The body of the Bot needs pockets ("holes") to welcome the eye and the joints of the limbs. The first question is: should they be created in crude or cured putty? In this case, I've chosen to do them with the putty still being crude for the following reasons:
  • the diameter of the eye pocket is very large as compared to usual mini tooling (hand drill...), and large as compared to the size of the part itself, making it difficult and risky to be done in cured putty.
  • I don't have any tool for producing a hemispherical hole in cured putty
  • both for the eye and joints pockets, I wanted to achieve a clean, well-finished, precise result as the Bot was meant to be cast as a multi-part kit for PCX. This means that a work in cured putty would have quite certainly meant coming back with green stuff, milliput or even sculpey to improve the finish and repair potential damages (scratches, chipping,...). So I'd rather work in crude putty once and for all, instead of doing the job twice...
In other cases, the decisions can be the exact opposite. It really depends on the situation.

I start with the eye. For this, I've made a cylindrical caliber out of sculpey, by rolling it over a flat surface.  In fact, I've made it at the same time as the core and as some other parts not shown yet (planification...) to cure as many parts/tools/calibers as possible within a single baking. After the cylinder is cured, I've cut away one of its ends; I've ensured a good planarity of the cross-section, and a good symmetry of revolution of the outer surface thanks to sanding/filing. Finally, I've drawn a cross-hair with a pencil, that will be helpful for centering the caliber over the position of the eye in the next steps.

I draw a very slight cross-hair on the Bot's body, centered on the desired location for the eye. Then, the centering of the caliber for the eye only requires the alignment of both cross-hairs (that of the body and that of the caliber). I gently press the caliber against the body. The goal here is not to create a hole (the resulting displacement of putty would deform the body quite much), but to set the footprint of the caliber over the body, as seen in the third picture. Then I use a needle and a thin, elongated, sharp blade to cut along the contour of the future hole. Then, I keep on cutting to create smaller pieces that will be easier to remove, and I start removing them to create a first iteration of the hole.

To ensure a more or less flat and continuous back face for the eye pocket, I put a little bit of putty back into the hole. I apply some vaseline gel on both the cylindrical and the flat faces of the caliber: it is used both for its lubricant and slight solvent actions. I carefully introduce it into the hole and press it against the bottom of the pocket to flatten it. Then I start rotating it to enforce a clean cylindrical shape of the pocket, with sharp edges. By nature, this action will produce a hole slightly larger than the actual cylinder, but this is actually an advantage regarding the geometrical tolerances for the modeling of the eye itself (see below).

At the very beginning, in parallel with the core of the body and the cylindrical caliber, I've also shaped a couple of cylinders that will be used for the upper arms and the thighs. I've rolled them over a flat surface until I obtained cylinders with the right diameter, with lengths suitable for cutting out the parts needed. I've used Super Sculpey to benefit from its low stiffness, facilitating the shaping.

At the same time, I've also shaped spheres out of a mix of Super Sculpey and Sculpey Firm, by rolling the putty with my palms and my fingers. In this case, I wanted more stiffness to avoid ovalization that may be favored by the absence of calibers for the shaping. To obtain volumes that are close from one sphere to another, here's how I've proceeded:
  • I've rolled a cylinder with the mix of putty;
  • I've cut a portion of it, keeping track of its length;
  • I've shaped it and checked if the size corresponded to the concept;
  • If it was not right, repeat the same process with a portion of a different length;
  • Once I've found the right length, I cut other portions of the cylinder of the same length, and I shape them into spheres.
 Then it all went to the oven for baking, along with the body's core, the cylindrical caliber and other parts that I'll discuss below.

To determine the length I need for the cylinders, I help myself with the drawing of the Bot. To make things simpler, the 4 joints have the same length. I've drawn visual marks on the paper to identify more easily the cutting line; I draw another line on the cylinder where I need to cut it. Then I cut it patiently by rolling and pushing gently a sharp blade over the cylinder. I repeat the same operation to obtain the 4 of them.

I prepare the spheres by first flattening the area that will be in contact with the cylinder. The main goal is to make the joint between them as invisible as possible, and to also ease the operation of drilling a (blind) hole into the sphere. A (passthrough) hole is also drilled through the cylinder. The aim is to make way for a metallic wire to strengthen the assembly. Then everything is assembled with a little bit of glue.

Then, I'm going for the pockets that will implement the interfaces for the double-ball joints that are parts of the limbs. The process is mostly identical to that employed for the eye; the only exception is that the ball joints are directly used as calibers.

It's now time to go for the forearms. I start from a cylinder that I've prepared at the same time as the other cores. It will serve both as the core for the forearm, and as a means of holding the part while working on it and baking it. As baked Sculpey is quite easy to cut, separating the forearm from the rest of the cylinder at the end will be a formality.

I start by defining the length needed for the core, and I scratch the corresponding surface to facilitate the adhesion of crude putty. Then I start shaping it by adding different layers, while keeping a hollow volume for welcoming the ball joint of the arm. To make this step easier, check the geometry, etc... I frequently use one of the joints, covered with a very thin layer of vaseline gel, to reshape the hole as the work progresses. I obtain a raw shape that can now be refined.

At this stage, I would like to increase the height of the hollow part that hols the "knee" ball joint. Going for a progressive addition of putty, followed by a smoothing for integrating each addition to the leg. Considering the thinness of the cylindrical wall, this does not seem like a good idea. At all. So I took another option, that consisted in trimming the top of the wall to obtain a flat interface surface, with a sufficient area for a good, firm contact. Then I prepare a thick strip of putty that will constitute the new cylindrical wall. I "glue" it on the leg using vaseline gel, and I start the integration work. It consists mainly in smooting the transition between the leg and the wall, and then progressively shaping the strip with a sharp blade to obtain a continuous shape with the appropriate thickness. Then a last smoothing provides the required finish.

After shaping the "outer" part of the female joint, it's now time to improve its "inner" part. No big deal here: I'm simply adding tiny amounts of putty where needed. Then I use the male part of the joint, coated with vaseline gel, as a shaping tool, by pushing it inside gently with a rotation motion. This creates a slight deformation of the top part of the leg as it tends to "open" itself due to the ball. This can again be easily corrected with a sharp blade followed by a smoothing.

Then I want to add a strip around the bottom end of the leg. For this, my favorite method is to create an actual strip of putty with the good thickness, to cut it to the appropriate dimension, and to position it on the part. A small coat of vaseline gel can help again, by acting like a glue. I've followed roughly the same steps for the larger arm. They are ready to be cured in the oven. After the curing, it's all about files and sandpaper to adjust the potential remaining shape flaws. It's also very easy to get rid of the cylindrical support with a sharp blade, to finally obtain a single part, ready to be integrated in the kit.

By now, I guess you start to understand the logic. After the curing of the different parts in the oven, it's them time for shape adjustments. Files of different shapes, sizes and roughness are my main tools. For larger deviations, I also scratch with a blade to initiate the job more efficiently. Spatulas and blades can also be used to adjust the shapes of the holes.

I hope this long tutorial proves useful!


I've started writing this tutorial a few weeks before sculpting the 2nd bot for the last Pit Fighter Challenge: here and here. It was a good way to test it again in real conditions, and all in all I think the principles are efficient to do a clean and fast job. Here's a picture of the first step, showing the strategy in the choice of the initial parts and cores. You've got all the pieces of thepuzzle to guess of the rest of the job went.

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