Handyman
A mechanical cube for relearning everyday turns after a hand burn — graded resistance from friction alone.
- Year
- 2025–26
- Reading
- 05 min
- Figures
- 11
Handyman is a small mechanical cube for fine motor practice after a hand burn: one hand holds the cube, the other pinches and turns an everyday-shaped knob, and the resistance rises as the hand gets stronger. Version 1 grades it by which of four holes a knob sits in; Version 2 replaces the holes with a screw-tightened split clamp for continuous adjustment. It grew out of a collaboration between the USC Creative Media & Behavioral Health Center and the Burn Unit at Los Angeles General Medical Center.
- Team
- Yiming “Danny” Huang
- Violet Wong
- Caitlyn Guo
- Ash Kim
- Yiqi “Kiwi” Li
- Marientina Gotsis
- Disciplines
- Product design
- Mechanical design
- Rehabilitation
- 3D printing
- CAD
- Tools
- SolidWorks / FDM 3D printing / Hand sketching
Most hand exercises ask you to squeeze. A ball, putty or a grip trainer builds closing force. But many daily tasks ask the fingers to pinch a small shape and rotate it while the wrist stays still: a stove knob, a faucet, a bottle cap, a door lock, the dial on an appliance.
After a hand burn, that rotation has to be rebuilt in graded steps. Handyman is a small mechanical cube for exactly that practice — hold it in one hand, turn a knob with the other, and raise the resistance as the hand gets stronger.
Difficulty belongs to the device
Turning tasks are hard to grade. A household object is either turnable that day or it is not. Putty has no knob, and a generic exerciser does not feel like the object the person is trying to get back to.
Handyman isolates the rotation. The cube is the stable body; the knob is the object. Because difficulty is a property of the device, the same motion can start easy and become harder without switching tools. The knob shapes are ordinary on purpose: time spent turning them is time spent on the shapes of real controls.
There is no motor, sensor, battery, or software. Resistance comes from friction inside the cube.”
Resistance by fit
Version 1 is a rounded cube, 95 mm on each side, with four holes of different diameters. Each hole is a resistance level: a knob pressed into a larger hole turns more freely, and the same knob in a smaller hole meets more friction. Moving a knob from one face to another is the whole adjustment.
The three knobs resemble controls people already know — a stove-style pointer, a smaller, taller grip and a broad, lobed knob. Three knobs and four holes give up to twelve pairings: a familiar shape combined with a tighter or looser fit, in one object that can be reprinted. More than one knob can be installed at once.
A hole is one size once it is printed
Version 1 is simple, and that simplicity has a limit. A fit that comes out slightly tight or slightly loose can only be corrected with a different hole, a different knob or a new print.
The first printed cube was ready on November 18, 2025, and on December 9 it went back to the outpatient occupational therapists for a second conversation. Work on Version 2 began the next day: a screw, so the same cube could be eased or firmed in small steps while you hold it.
Resistance by clamp
Version 2 keeps the cube and the everyday knob, and moves the resistance into an adjustable clamp. A shaft runs through a 105 mm case; the knob mounts on its outer end. Inside, the shaft passes through a split-clamp bushing — a sleeve with a slit down one side, in the same family as a collet — and a screw on the opposite face pushes that sleeve closed.
With the screw backed off, the shaft turns with little drag. Turning the screw in closes the slit, the sleeve grips the shaft more evenly, and the knob takes more torque to rotate. The knob does not change. The setting does.
The torque at the fingers is friction torque: it depends on how hard the sleeve presses, how much surface is in contact, and the shaft’s radius. The screw only sets the pressure, so one device covers a continuous range from an easy turn to a firm one.
Fig. 07Concept sketch: a swappable knob on a shaft, a split-clamp bushing, and a screw labelled as the resistance adjustment.
Fig. 08Assembled in SolidWorks: star knob on the front, knurled adjustment screw and two lid screws on the side.
Fig. 09Exploded: knob, shaft, case, lid, split clamp, adjustment screw and lid screws.
0
motors, sensors, batteries or software
12
knob-and-hole pairings in Version 1
0.05 mm
between the two printed clamp bores, 6.43 and 6.48 mm
2
rounds of feedback from occupational therapists
Designed for the printer
- 01
Graded by fit (V1)
Four holes, one per side face, each a different diameter around the knob stems. The fit between a printed stem and a printed hole is the whole mechanism — no moving parts beyond the knob.
- 02
A collet-style brake (V2)
A 25 mm shaft runs through a split-clamp bushing; a knurled 6.4 mm screw on the opposite face closes the slit. Adjustable in the hand, with the lid on, and turnable without a driver between sets.
- 03
Two bores, 0.05 mm apart
Printed holes rarely come out at the modelled size, so the clamp is exported at 6.43 and 6.48 mm. Pick the one that turns freely when the screw is loose and firms up when it is tightened.
- 04
Print guidance is design
Rigid filament, so the clamp springs back. Print the shaft so its axis stays round — an oval shaft makes the drag uneven. Keep the clamp’s slit open, or the screw has nothing to close.
- 05
Knobs like real objects
Instead of a generic grip, the knobs follow controls people need to get back to. In Version 2 the star knob is its own part, so the grip can follow the task without redesigning the clamp.
- 06
Open hardware
Editable SolidWorks parts, assembly and drawings, print-ready STLs for both versions and assembly steps, published under CC BY-NC 4.0. No electronics or firmware needed to build one.
Two versions, two jobs
Version 1 is the better demonstration of “this knob feels like that object.” Version 2 is the better training device, because the load can be changed in the hand, in small amounts, and then left there for the next session.
Handyman was catalysed by a collaboration between the USC Creative Media & Behavioral Health Center and the Burn Unit at Los Angeles General Medical Center (LAGMC), through a NIDILRR grant whose principal investigator is Dr. Haig Yenikomshian. Occupational therapists Karin Blen, Vivian Duprey Avalos, Joanna Madrid and Joann Chun of the LAGMC outpatient occupational therapy unit were consulted on burn-recovery protocols for fine motor skill and gave feedback as the prototype developed.
Contributors: Yiming “Danny” Huang, Violet Wong, Caitlyn Guo, Ash Kim, Yiqi “Kiwi” Li and Marientina Gotsis. The design files and documentation are released under CC BY-NC 4.0.
