Print-in-Place Stress Test: the Robotic Hand Interview
27 September 2026 · Charlie Williams
The Job
It was the third year of my Engineering Degree. I needed a job.
Winter becomes an unlikely stressful period for an engineering student. To be awarded your degree, by the end of it, you must have completed 800 hours of practical work experience. Practically speaking, that meant competing against hundreds of other students every July for interviews at the same set of companies for summer internships.
After failing the previous year to land an internship at my dream company, I needed a way to impress my interviewers. I had just been given my first 3D printer. I got scheming, and landed on the most ambitious project I could think of: a print-in-place robotic hand. This was my first real exploration into what I could achieve with additive manufacuring.
What’s Print-in-Place?
3D printing is a type of additive manufacturing. Because additive manufacturing builds objects layer-by-layer, rather than carving them out of material like a statute from stone, you can hide features inside your designs. “Print-in-place” objects have moving parts that all come off the print bed as one piece. This is usually made possible by hidden joints, printed inside the parts, connecting them together.
A common print-in-place technology is the hinge. This is what I wanted to use in the knuckle joints of each finger.
The hinge on the right was the basis for my hand’s fingers. It can be printed in the orientation you see, with the pin floating. The blue and yellow piece are printed together: no assembly needed! The pin is able rotate inside the cavity once the part is printed, so the two pieces act like a hinge.
But why does that pin (see the cross section in the bottom image) not sag and fuse to the other part during printing? If you’re careful about how big you make your features, limiting the size of “overhangs” and “bridges”, you can actually make parts float and not fuse. This hinge was printed relatively small. If the print head is cooling the filament enough, the pin is able to span its distance without sagging too much.
Starting small: a wiggly finger
The Joint
It was a good thing I’d started early, because it took ages to perfect the design of the finger. I needed a good hinge, firstly. Each joint needed to:
- Rotate smoothly;
- Not stick; and
- Not over-extend.
I had to go through tens of hinges until I got all the parameters just right. 3D printing is a manufacturing method where the final part is never exactly the dimensions you gave. A large gap in CAD software can become much smaller in reality (and varies printer-to-printer!) because extruded plastic tends to expand to greater widths than dimensioned. I developed iteration after iteration until what I wanted aligned with what the printer was giving me.
So the finger could wiggle! But how could I control this wiggliness?
The Tendons
Every finger we have is actuated by two sets of tendons: flexor tendons (for bending) and extensor tendons (for straightening).
Being the tight-budgeted uni-student I was, I opted for common nylon fishing line as my tendon. I picked a gauge with suitable strength and extruded holes through the top and bottom of each finger to thread the nylon through, tying knots at the finger tip. Pulling the top nylon bent the finger; pulling the bottom nylon extended it.
On an individual test-finger, I designed a housing at one end to attach a servo to. I tied each nylon string in different directions around a pulley attached to the servo and secured them with grub screws. Whichever way the servo turned, one string extended while the other was reeled in; depending on the direction, the finger either contracted or relaxed. Proof of concept!
Bringing it all together
Combining fingers of different lengths together, and finding an appropriate angle for the thumb (which was much harder than I’d bargained for), I had an entire hand.
The beauty of this design was that instead of needing 30+ assembled parts, which would have taken hours of pain-staking assembly, the whole thing could be printed as one part. The hand popped right off the print bed of my Ender-3-Pro, with each finger working and wiggling straight away.
Returning to the story: suddenly, I was in a rush. My interview was the next day, but I hadn’t got everything working. I tied the free ends of the nylon around the 3D-printed pulleys, attached these to some servos I had lying around, found a spare arduino and wrote some demo code to make the hand perform a few basic gestures. Somehow, amazingly, it seemed to do what I wanted: the fingers moved. I watched with unreal satisfaction as my piece of plastic showed me a shaka, a thumbs up, a peace sign. Later than I should have, I finally managed to find some sleep.
The Interview
I was nervous. I had dropped the frankenstien-looking contraption with a mass of wires coming out of the wrist on the interview table after walking in the room. My interviewers kept glancing at it throughout the interview, intrigued looks in their eyes. I knew that if this thing didn’t work it was no better than an art piece. Unfortunately, in an interview for an engineering internship, an art piece wouldn’t have counted for much.
At some point they became bored of going through the usual interrogation-style questioning and turned to regard the hand in the room. They asked me what it did.
I explained the technology, pointing out the print-in-place assembly. Then I pressed a button on my breadboard. Nothing happened. My heart jumped into my mouth. I frantically tried to diagnose the issue. It turned out that I hadn’t actually plugged the thing in!
I gave a small laugh, then tried again.
The servos jittered. The pinky twitched. The hand burst into life. It worked!
I left the interview on top of the world, having forgotten all of the small blunders in my answers that I usually dwell on after an interview. On the way home, I received an email from someone at the company.
I’d got the job.
Talk to Us
If you think you project could benefit from print-in-place and similar AM technologies, please get in touch!