Prototyping Technologies: A Practical Guide for Engineers and Product Developers

Section 2. CNC Machining

2.1 What CNC Machining Offers That Additive Processes Cannot

CNC machining is a subtractive process: it starts with a solid block of material and removes everything that isn’t the part. The fundamental consequence of this is that the material is homogeneous and isotropic throughout. A CNC machined aluminium part has the same material properties as a billet of aluminium 6082; a CNC machined acetal part behaves exactly as the material datasheet describes. There is no layer interface to weaken, no build orientation to consider, no residual stress from a curing process.

For functional prototype testing, this is critical. Where a prototype will be loaded, fatigue tested, subjected to thermal cycling, or used to validate a structural analysis, the test data needs to represent the behaviour of a part with consistent material properties. CNC machining provides this. Most 3D printing processes do not.

Dimensional accuracy is the other major consideration. The quoted general tolerance at Prototype Projects is ±0.1mm, which is a conservative starting point for estimating. In practice, the machine tools hold considerably tighter on critical dimensions, and specific tolerances can be agreed on individual features when the application requires it. This level of control is particularly relevant for parts with bearing bores, mating surfaces, threaded features, and close-tolerance interfaces that will be assembled with other components.

2.2 CNC Milling

CNC milling uses rotating cutting tools to produce profiles, pockets, slots, and complex 3D surfaces from a solid block. Three-axis milling is capable of producing a wide range of features from a fixed workpiece orientation. Five-axis milling adds two additional rotational axes, allowing the tool to approach the workpiece from a much wider range of angles. This is not merely a matter of convenience: it means that complex parts with features on multiple faces can often be completed in a single setup, which reduces both cycle time and the accumulation of positional errors that can occur when a part is repositioned between operations.

Prototype Projects operates two Haas VF-2SSYT five-axis vertical machining centres alongside a range of three-axis centres and mini mills. The five-axis machines are equipped with Schunk air vices and Haas robot packages for automated part loading and unloading, enabling unattended batch production and supporting the low-volume manufacturing service alongside the prototype offering.

Offline programming using Mastercam processes customer STEP, IGES, and X_T files directly, generating toolpaths for complex geometries without the need for redrawing or remodelling.

2.3 CNC Turning

Turning is used for cylindrical parts: shafts, bushes, connectors, fittings, and any component with a primary rotational form. The workpiece rotates against a fixed cutting tool, which traverses along it to produce the required diameter, profile, and length. Turned parts may also have drilled, bored, or tapped features added during the turning operation.

Many prototype parts require both milling and turning operations. Having both processes available in-house avoids the delays and handoff risks that arise when work has to be split between suppliers, meaning a single quote, single point of contact, and single delivery covers the complete part.

2.4 The 3-Day CNC Service

For projects where speed is the defining constraint, Prototype Projects offers a 3-day CNC machining service for parts that meet defined criteria. The service is built around in-stock material billets in fixed sizes, a defined set of standard operations, and a workflow specifically organised to deliver dispatch within three working days of receiving final CAD data.

Available materials for the 3-day service are aluminium 6082, acetal (black and natural), and clear acrylic, in billet sizes from 50 x 50 x 40mm up to 200 x 100 x 90mm for milling, and up to 60mm diameter for turning. Standard threads from M2 to M12 are supported. All parts are vapour honed as standard and inspected to basic visual and CAD comparison before dispatch.

The importance of this service extends beyond simple convenience. In a compressed development timeline, the number of design iterations possible before a programme milestone is determined partly by the speed at which each iteration can be turned around. A three-day turnaround from CAD to physical part means that a design change identified on a Monday can be in the engineer’s hands and on the test rig by Thursday.

2.5 Low-Volume Production

The same characteristics that make CNC machining well suited to prototype parts, accuracy, material integrity, and repeatability also make it practical for low-volume production of parts that are not suitable for, or do not yet justify, injection mould tooling. The five-axis machines with robotic loading are configured to run batches of identical parts unattended, which brings per-part cost down as quantity increases.

This service is relevant for products with annual volumes in the range of tens to low hundreds, for bridge production while mould tooling is being made, and for parts in regulated sectors where the traceability and material integrity of machined components are preferred over moulded equivalents.

2.7 Inspection

All CNC machined parts are deburred, cleaned, degreased, and inspected before dispatch. Standard inspection uses callipers, micrometres, and thread gauges to check key dimensions. For applications requiring a higher level of dimensional verification, Prototype Projects operates an in-house co-ordinate measuring machine (CMM), which uses a contact probe to measure surface positions in three dimensions and compare them against the CAD model.

CMM inspection is particularly relevant for parts with critical bores, mating interfaces, hole patterns, and geometric tolerances, including flatness, parallelism, perpendicularity, and concentricity. Formal inspection reports can be provided where required, which is important for customers in regulated sectors. Having CMM capability in-house, rather than relying on a third-party metrology supplier, means dimensional issues are identified before a part leaves the site rather than after it has been assembled into a test rig or submitted for regulatory review.

Section 4. Injection Moulding and Its Relationship to the Prototyping Process

4.1 Why Injection Moulding Belongs in a Prototyping White Paper

Injection moulding is not a prototyping technology. It is a production technology, and in most product development programmes, it represents the destination rather than the journey. Understanding it matters in a prototyping context for a straightforward reason: every decision made during the prototype stage has implications for the injection moulded part that will eventually follow. The earlier those implications are understood, the less likely they are to become expensive problems.

A prototype that has been designed without reference to mouldability constraints may validate the design intent perfectly well, but then require significant rework before a mould tool can be cut. Wall thickness transitions that cause sink marks, features that prevent ejection, internal geometries that can’t be filled, draft angles that haven’t been considered: these are all issues that a good prototyping programme should surface and resolve before tooling investment is committed, not after.

Working with a bureau that understands injection moulding during the prototype phase, not just at the production stage, means that design for manufacture considerations can be introduced at the point where they’re cheapest to address.

4.2 Prototype Injection Moulding: The Options Between Casting and Full Production Tooling

The economics of injection moulding are well understood. A production-quality steel mould tool represents a significant capital investment, with lead times typically measured in weeks. For high-volume production, it is the obvious choice: per-part cost is low, cycle times are fast, and the full range of thermoplastic materials is available. But for quantities of a few hundred parts, or for a design that hasn’t yet been fully validated, the investment is harder to justify.

Between vacuum casting, which is practical up to around twenty-five parts per mould, and full production steel tooling, there is a middle ground that is often underused: prototype or bridge injection moulding using aluminium tooling.

Aluminium mould tools are less expensive and faster to produce than steel equivalents. They don’t have the longevity of steel – an aluminium tool is typically rated for thousands of shots rather than hundreds of thousands – but for bridge production, market testing, regulatory submission quantities, or early customer supply while the production tool is being made, they are a practical and cost-effective option. Parts produced from an aluminium mould are genuine injection mouldings in genuine thermoplastic materials, which means the test data and the approval documentation they generate are directly relevant to the production part.

4.3 When to Move from Prototyping to Tooling

One of the most common questions in a product development programme is when to commit to mould tooling. The answer depends on volume, confidence in the design, and the cost of getting it wrong.

As a rough framework: vacuum casting makes economic sense up to quantities of around twenty-five parts per mould, sometimes more for simple geometries and less for complex ones. CNC machining can bridge to higher quantities for the right parts and materials, particularly on five-axis equipment with automated loading. Beyond that, the per-part cost of non-moulded production typically exceeds the amortised cost of tooling, and the move to injection moulding becomes justified.

The design confidence question is equally important. Committing to a steel production tool for a design that hasn’t been fully validated through functional prototyping is a risk. Changes to a steel tool after it has been cut are expensive and time-consuming, and some changes are simply not possible without scrapping the tool and starting again. The prototyping programme exists, in part, to reduce that risk to an acceptable level before tooling is ordered.

A useful discipline is to treat the final vacuum casting run as a pre-production sign-off. If the vacuum cast parts pass functional, dimensional, and cosmetic review, and if any feedback from user testing or regulatory review has been incorporated into the design, that is a reasonable point at which to commit to tooling. The silicone mould used for vacuum casting and the SLA master that preceded it represent a design history that supports the tooling decision.

4.4 Design Considerations That Bridge Prototype and Production

Several design features that are straightforward to produce in prototyping processes need to be reconsidered when the same part moves into injection moulding. Being aware of these during the prototype stage, rather than encountering them as surprises at the tooling stage, saves time and cost.

Draft angles are not required for CNC machined or vacuum cast parts, but they are essential in injection moulding to allow the part to eject cleanly from the tool. Vertical walls in a prototype design will need draft added before tooling. The appropriate angle depends on surface finish and material, but 1 to 2 degrees per side is a common starting point.

Wall thickness should be as uniform as practical in an injection moulded part, to promote even filling and cooling and to avoid sink marks on external surfaces. CNC machining and vacuum casting are more tolerant of varying wall sections, which means a prototype may pass functional testing with a wall design that would cause cosmetic or structural problems in a moulding.

4.5 Injection Moulding at Prototype Projects

Working with a single supplier across both the prototyping and moulding phases means the transition from prototype to production part carries a continuous design history. The friction of transferring design intent, inspection criteria, and programme context to a new supplier at a critical point in the development timeline is avoided, and with it, the risk of something important being lost in the handover.

It also means that the team advising on the prototype design is aware of the moulding implications from the outset. That awareness is most valuable early, when design decisions are still easy to change, rather than at the point where tooling is being quoted, and the cost of revision is already high.

Section 6: Emerging Developments

6.1 AI-Assisted CAM Programming

Programming CNC toolpaths has traditionally required a skilled programmer to work through the geometry, select appropriate tooling and cutting strategies, and optimise feeds, speeds, and approach paths manually. AI-assisted CAM tools, including platforms such as CloudNC, are beginning to automate significant portions of this workflow by learning from libraries of successful cutting strategies and applying them to new geometries. The practical effect is a reduction in programming time for complex parts, which has implications for lead time and quoting speed. These tools are most effective on well-understood part types and materials; complex, novel geometries still benefit from experienced programmer oversight.

6.2 Robotics and Automation in Machining

The integration of robotic loading and unloading into CNC machining centres extends the operational hours of expensive capital equipment beyond the working day. When a five-axis machining centre can load its own blanks, machine parts, and unload finished components unattended overnight, the effective capacity of the machine increases substantially without a proportional increase in labour cost. This matters for the economics of low-volume production, where per-part cost needs to be kept competitive with alternatives such as injection moulding for the service to be viable.

6.3 Material Development in Additive Manufacturing

The rate of new material development for industrial 3D printing processes continues to accelerate. In SLS, high-performance polymers and composite-filled grades are extending the mechanical performance of printed parts toward territory previously occupied only by CNC machined engineering plastics. In DLP, fire-retardant and ultra-high-temperature resins are opening applications in aerospace, automotive, and electronics that were previously inaccessible to resin-based printing. Biocompatible materials validated to ISO 10993 are extending micro 3D printing into medical device applications where direct patient contact is involved.

6.4 Convergence of Inspection and Manufacturing

The separation between manufacturing and inspection is becoming less distinct as measurement capability moves closer to the point of production. In-process measurement, where a CMM or optical measurement system verifies dimensions during the machining cycle rather than after it, is increasingly practical on well-equipped machining centres. For prototype work, where each part may be unique and first-off inspection is critical, this reduces the time between machining and confirmed dimensional acceptance, supporting faster iteration.

6.5 Sustainability in Prototyping

Environmental considerations are increasingly part of the procurement discussion for prototyping services. The relevant factors include material waste, which is inherently higher for subtractive processes than additive processes; energy consumption per part, which varies considerably between processes; and the availability of recycled or recyclable materials. In SLS, unsintered powder is typically recycled back into subsequent builds at a proportion determined by the supplier, reducing material waste. In CNC machining, swarf (metal cutting chips) is collected and recycled through established metal recycling channels. These are not marginal considerations: for companies with published sustainability commitments, the environmental profile of their supply chain is subject to scrutiny, and a prototyping supplier that can account for its material and energy usage is better placed to support those commitments.