Introduction: Automaton Hand

I was given the brief to design and construct an automaton for a university project. It was stipulated that the piece's mechanical parts must be constructed of laser cut acrylic. The model was to be marked on complexity, innovativeness and amount of moving parts. The materials used must be 3mm thick acrylic and the overall size used must be no more than A4 (420 x 297mm).

The brief allowed for ultimate possibilities, I had several ideas but finally decided to try and emulate a human hand. I searched through images and videos online for inspiration and ideas to make initial sketches.

First of all I built a prototype concept from 3mm plywood to confirm the mechanisms worked which can be seen in the video:

Materials used for the initial plywood concept:

Roughly A4 (420 x 297mm) of 3mm thick Plywood
Quantity 35, M3 x 12 mm button head machine screws
Quantity 25, 3mm x 6 mm long button head wood screws
Quantity 35, M3 nylock machine nuts
Quantity 100, M3 washers
Various gears and racks that can be bought from ebay
2 part epoxy resin(shown in the pictures above to glue the plastic racks to lever sliders).

Tools required for plywood concept

Dremel tool with drill bits and cutting disks
Various G-clamps
A vice
Hobby sized band saw
Sand paper (various grades)
Square file
Round file
Engineers rule
Flat Head Screw driver

The sizes used for the fingers and palm were from my own hand. The design was not finalised when I started but it evolved as I built it. Spending lots of time analysing and measuring my hand, I split each finger into parts and joined them with the m3 bolts to act as pivot points to give articulation. This was repeated through each finger which were then attached to the palm section with slots. Again, I simply drew around my hand and took measurements for sizes. The mechanisms that bring the fingers all together were cogs and rack and pinion mechanisms which I found was the best way to transfer actuation throughout the hand. This is a concept model, and even I could not reproduce it again to the same specifications but I learned a lot about gearing, ratios and levers to apply to the final concept model.

After experimenting with the initial plywood concept model I created parts on pro engineer. This software package is not necessary to create the parts or the laser cut file as they are only 2D sketches. I extruded the sketches to 3mm and assembled the parts to test the mechanisms. From the working assembly in pro engineer I created a drawing file with all the individual parts. I could then export it to DXF file format to be laser cut.

Video of working CAD assembly:

Materials for acrylic model

Roughly A4 (420 x 297mm) of 3mm thickacrylic (colour of your choice, transparent works well)
Quantity 35, M3 x 6 mm button head machine screws
Quantity 10, M3 x 6 mm countersunk machine screws
Quantity 2, M3 x 10mm button head machine screws
Quantity 1, M3 x 10mm button head machine screws
Dichloromethane plastic weld
Toothpaste (cutting compound for tapping threads to stop the acrylic shattering)

Tools used

Spirit laser cutter
M3 tap
Small flat file
Allen key (2mm)
Countersink drill bit

Each finger has 3 main parts ordered in size with 3 ligament levers. The 2 pivot bolts on the middle part of each finger are countersunk to give clearance for movement using the countersunk bolts. Clearance holes are cut at 2.8 mm diameter, holes to be tapped were cut at 2.3 mm diameter. Glueing required for each finger rack was done using Dichloromethane which is a solvent plastic weld.

The model is operated from the transparent crank rod handle and can be turned clockwise or anticlockwise. The crank handle operates a gear to the opposable thumb mechanism and also a crank to the pinky cog. The pinky cog drives a rack and pinion that transfers rotational motion to linear motion pushing and pulling the levers extending and moving the finger. This is repeated from the pinky to the index finger.

This was a long frustrating process with many iterations in development of the piece produced but the end result turned out great. Hopefully my efforts are reflected in my mark.

I have posted the DXF file used to cut the parts on the laser cutter. This was the file that was used to cut the parts for my final model. With this anyone should be able to reproduce their own acrylic automaton hand.

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