Machinedesign 3126 Hairtrap Crop 0 0

How to Engineer a Nightmare

Oct. 23, 2008
Friends think you’re weird for your fixations with CAD, servos, and gear efficiency?

Want to show them that an engineer is someone to be reckoned with? David Hackl has some tips for you.

Hackl has been involved with the somewhat gory, somewhat scary Saw movies II through V, in which villain/savior/psycho Jigsaw pits characters against physical, mental, and emotional traps designed to teach them lessons about living better lives. Hackl was production designer for Saw II, III, and IV and directed Saw V, which opens October 24, 2008.

He gave us some insights into the mechanics of scaring moviegoers out of their wits.

MD: Where do you come up with ideas for the traps? DH: The ideas for the traps come from everywhere. By Saw II we had exhausted our research of historical torture devices. Although we still use many of the old standards as inspiration, we have had to become more creative to make traps that would serve the stories.

There is a good deal of brainstorming involved. Often a small idea will turn into something great. For Saw II director Darren Lynn Bousman came to me with an idea for a device. It was basically a jar that cuts and traps someone’s hand so the jar eventually fills with blood. I started some R&D working with camera irises which turned into four hinged triangles with razors on the inside edge. These were mounted on the bottom of a glass box and the Hand Trap was born.

From the time I was a kid, I have loved taking things apart to find out how they work. My brother and I had a place under the basement stairs the we called The Lab. We once made a mini-trebuchet that could fling bundles of fire up to 40 ft. Yes, we measured, because that’s what boys do. And I’m lucky I still have all of my fingers and eyesight.

My dad managed a machine shop where I worked summers as a teenager. We would build hotstamping machines from scratch, and I learned a great deal about pneumatics and servos from that experience.

MD: What is your process for designing and building the traps from initial inspiration to final machine? DH: Unlike in the “real” world, the inspiration-design-final machine time is incredibly compressed on a Saw film. Often the whole process takes place in a matter of two weeks.

We start with rough sketches and visual references that help establish the look and feel of the device. I always try to push for something that is visceral, something that immediately looks like it is going do something evil.

For story purposes there is a basic set of rules for a Saw trap. It must be something a person of normal intelligence and skill could create, and it must look like it could be fabricated by a single person or maybe two and moved whole or in parts to any location, although sometimes traps are locationspecific. It must look like it is made out of found objects with a few custom parts. And a trap needs to be visceral. But it must have an out; if the character performs the task required by Jigsaw, he must have a way to escape or survive.

Then we move through the practical aspects of the device. Keeping the mechanics simple is the key to a good trap. Along with meeting the story requirements, simplicity also lets us work quickly and get the trap to the shooting floor.

The stunt supervisor is involved from the beginning. The trap must look like it can hurt you, but it really can’t. Safety is the most important issue. We use many soft parts painted to look like rusty metal, along with low-torque servos, safety stops, and fail-safe devices to ensure safety for actors and those working with the trap.

For safety we employ a lot of small motors that have low torque and sometimes insert “slip” bushings or cushioned mounts that offer little resistance in case of mishap. Often we need to make traps remote-controlled, but where possible we hard-wire devices to ensure accuracy.

Jason Ehl has been our main prop builder since Saw II. He is a sort of mad scientist, industrial engineer, and artist mixed into one big cranium. Jason works closely with Tony Ianni (my former art director, now production designer) and me through the development stage. He then takes the ideas to his shop and starts to build.

Jason uses CNC machines with AutoCad to custom fabricate many of the parts for the traps. It is often better to build from scratch then to spend hours shopping for parts that may not fit the device or will have to be copied into a soft part anyway.

Every trap design works differently, and we employ almost every type of mechanism you can imagine. Pneumatics work well because they are lightweight, scalable, and still maintain precision at low psi. We have also used hydraulics, but mainly for heavier applications like the “drawing and quartering” devices in the hotel room in Saw IV.

Precision is necessary mainly to ensure safety and to get what we need on cue. We aim for precision as much as time and money will allow but this is for the movies, not a prototype destined for mass production.

We are always aware of the need to record dialogue during trap scenes. So we try to make the devices as quiet as possible. This is another place where precision comes in, but sometimes there is no getting around the noise. (You’ll see what I mean in the second-tolast trap in Saw V.)

We monitor every stage of the build to work out the bugs and modify designs as we go. These traps are basically working prototypes that have little room for error. And we’ve never had a critical mishap—so far.

Once the trap is complete, we take it to the studio for testing. This is where the scenic art team work their magic to make the trap look evil. Then it’s showtime.

MD: Do you have a favorite trap out of all of those in the series? DH: One of my favorite traps was the “angel” trap in Saw III. Agent Kerry is found hanging from the ceiling of a dungeon by chains. On her back is a mechanical device that looks like folded angel wings. Cables lead from the ends of the wings to a series of brackets that wrap from her spine to her sternum. The brackets are attached using surgical screws — one to each rib. When the wings unfold — well, you get the idea.

Designing this device was complex mainly because it had to appear as if Kerry and the device were suspended from the ceiling, and the device had to seem to be triggered remotely.

We achieved the floating effect by using clever camera work and a 24-ft-long, cantilevered steel arm hidden in the back of the set.

But articulating the angel wings was another matter. For lightness, we used 3/8-in. aircraft-grade aluminum for the wings and ribs. The arms were attached to a piece of 6 10-in. steel U-channel that housed a single pneumatic cylinder to run the wings.

The rib brackets had to break free on cue. We attached them to the back plate via a series of electromagnets controlled by a microswitch that had to be triggered just after the arms began to move. This was trouble, but it worked in the test stage.

The problem came when we had to incorporate the prosthetic elements that created Kerry’s breakaway rib cage. The latex “skin” had to be prescored to break on cue. It had to tear away in chunks with each rib, and it all had to look organic with blood and guts inside. But the weight of the prosthetic was too much for the magnets, so we had to boost their power literally overnight.

Next day was the shoot, and all seemed ready to go. New magnets were in place and holding. The actress was harnessed into the trap, the cameras rolled, and nothing happened. The combined weight of the stronger magnets and prosthetics made it impossible for the pneumatic cylinder to drive the wings open.

Jason Ehl and the prop team scrambled to make adjustments. On take two, Agent Kerry’s torso exploded with all of the beauty and gore of a classic Saw trap.

It wasn’t until later that Jason came to me and told me the wings only opened because he and a prop man had attached black stove-pipe wire to the ends of the wings and pulled really hard. But it looked great, and the actor was safe and in my business that is all that matters.

Props like this hair trap from Saw IV were inspired by torture devices throughout history. For the actors’ safety, the custommade moving parts incorporate stops, low-torque drivers, slip bushings, and even soft parts painted to resemble metal.

The angel trap in Saw III was designed to work with a pneumatic cylinder driving the “wings” (seen at the top of the photo) apart while electromagnets ripped open the “ribs” of a latex prosthetic (shown here after they were triggered). The two systems were independent but timed to look connected. In the end, a well-timed tug on hidden wires augmented the pneumatic drive.

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