(This article was first emailed on May 17th, 2021 to fans who had registered an interest in finding out more about our Tricorder.)
The tricorder is made from a mix of different materials. Replicating the original as closely as possible, machined and extruded aluminium provide key design accents as well as the basic structure of the tricorder. Die cast zinc is used for the display stand, a stitched leatherette for the strap, and four different kinds of precision-engineered injection-moulded thermoplastics combine refined function, durability and aesthetic accuracy to complete the replica.
All the injection-moulded parts necessary to make a tricorder collectively weigh only 241g (8.5oz), but require two and a half tons of tooling to make them.
Sixteen large mould bases, each weighing 80 – 250 kg (175 – 550 lbs), of pre-hardened S136H stainless steel, are carved out using intricate and highly accurate mechanical machining, electrical discharge machining (EDM) and hand-finishing processes to create the 68 cavities that are needed to produce every tricorder.
Toolmaking to produce high-quality, highly toleranced parts is a complex and time-consuming process. The tools not only need to have perfectly machined and finished cavities, but also need an intricate system of location pins, ejectors, molten plastic feed tubes (sprues, runners and gates) and a network of internal channels through which cold water is pumped to cool the mould as rapidly as possible during each injection moulding cycle.
The design of the mould tool is more than just making a perfectly shaped cavity. To make sure the inlet gates are perfectly placed, and tool temperatures and injection pressures are all optimised, powerful simulations are used to model the dynamics of the molten plastic flow to ensure that the resulting part is smooth, repeatably accurate with minimal shrinkage, and doesn’t experience extreme temperature gradients that would build stresses into the part or result in scorch marks (too hot) or weld lines (where two cooling fronts of plastic meet) during the moulding cycle.
Molten plastic is injected into the mould under enormous pressure. To stop it leaking out of the tool at the join between the two halves of the mould, and thus creating unsightly flash, the mould must close very accurately and securely. As larger parts require more pressurised plastic to fill them, an injection moulding machine’s closing pressure, measured in tons, determines the size of the part that can be produced on that machine. For the largest tricorder component, the back panel, which will be moulded in the same tool as the two doors, the moulding machine has a closing force of 160 tons.
Injection moulding tools are made in a multistage process. The tricorder mould bases were milled to give them a perfectly flat surface, after which a CNC (computerised numerical control) machining process was used to rough out the basic shapes of each mould. Eleven different milling cutters with a range of tip shapes and diameters are needed to machine out the basic tool shapes.
The tools were then heat-treated to increase their hardness before the shapes were refined by CNC machining with smaller milling cutters to get closer to the actual geometry of the parts. To complete each cavity’s form an EDM (electrical discharge machining) process is used.
Often referred to as spark erosion, the process literally erodes the surface of the tool with high voltage arcs, created between the surface of a fine wire or copper pattern and the inner face of the tool, vaporising minute chunks out of the surface of the tool as the wire or pattern is slowly pushed into it, submerged in a dielectric oil bath. EDM leaves an even, microscopically-pitted surface that, depending on the strength of the voltage applied, results in anything from a coarse granular-looking surface to a smooth satin finish.
Once the tricorder tools have completed this stage they will be tested. The complex tools are assembled and fitted to injection moulding machines so that a series of “first-shot” mouldings can be produced. The fit and finish is assessed and any issues can be addressed at this point, in a process called tool touch-up. It is relatively easy to remove metal from a tool if the touch-up involves making a feature bigger: these are referred to as “metal-off” changes. But if a part of the resulting moulding needs to be made smaller, then the more difficult process of a “metal-on” change requires new metal to be welded into the tool and once again machined or spark-eroded away with a new copper pattern to create the revised geometry.
When we are happy with the mouldings, and are sure that they are performing well with each other and together with all the other metal and electronic components, the tooling will be textured to create the required surface finishes.
For many of the tricorder parts a medium satin finish has been specified, but for the screen ‘glass’, the LEDs, watch crowns and buttons, where a highly polished surface is required, finishing the tool continues by hand. Progressively finer diamond pastes are used to buff the inner surfaces of the mould to a near-perfect mirror-smooth finish. Finally, to ensure the surface is as smooth as possible, the inner surfaces of these tools are chrome-plated. For the exterior panels of the tricorder where the original replica was made from vacuum-formed Kydex, after polishing, the tools will be sent to the same specialist texturing company that created the surface finish for our Communicator, where they will have the distinctive Kydex pig-skin pattern laser-etched into the surface.
Over the course of this project, replicating the tricorder hero prop has required the specialist skills and knowhow of a number of different engineers, powerful software, and some seriously heavy equipment. Yet still, at each stage the importance of expert hand finishing is evident. Even in the toolmaker’s workshop, surrounded by enormous computer-controlled milling machines and EDM spark eroders, hand polishing, fitting, and assembly account for almost 30% of the time it takes to make the tool. It seems that when making a replica, however, it is made, old-fashioned artisan skills are always needed to get the job done.
Coming next time
Data discs – what they do and how they work
Fans who register with us will be the first to read our news and the progress of this exciting Tricorder development and, later, where and when to purchase it. You can catch up with the story so far here on this blog, but if you haven’t already done so, why not register your interest in the Tricorder – you’ll then receive a personalised registration certificate and early access to these updates (before we publish them on our website).