CNC Prototyping: Everything You Need To Know

Though 3D printing is now considered the main rapid prototyping technique, CNC rapid prototyping is ideal for injection molding prototypes, as well as functional part testing where getting as close to the final product as possible is key.

CNC prototyping is known for its precision and versatility, with a wide material range and range of different CNC machines to create with and choose from.

Key and coil inserts can easily be added as extra features, and for very complex parts 5 axis CNC machining can handle all but the most intricate geometries. 

What is CNC Prototyping?

CNC rapid prototyping is simply using CNC machining to create prototype parts for uses such as for functional testing, using as a visual example for within your company, or to use for marketing or fundraising.

• For more information on CNC, read our guide to CNC machining

With CNC’s versatility, CNC prototypes range from low cost plastic or acrylic mock-ups to full stainless steel metal prototypes.

The CNC Prototyping Process

Key factors to determine for CNC prototyping:

• Prototype size — to pick the correct material block (stock) size and determine the best suited machine for the CNC project.
• Material used — cheaper plastics like ABS will obviously be more affordable than stainless steel, though there are more affordable metals like aluminum for CNC prototyping.
• Finishing options — do you want the part to be left as-machined, or do you want the part to be bead blasted, anonized, or plated?
• Complexity — more complex parts with difficult geometries or with intricate extras like threaded holes or engraved brand names add to the time it will take a CNC machine to finish the part. It may also require a more advanced machine with more axes, and therefore the cost will be higher.
• Tolerance (accuracy) — the tighter the part tolerance required, the more expensive the prototyping job will be, as better precision takes far longer.
• Type of CNC prototyping machine — CNC turning machines are generally considered cheaper to use for prototyping than milling machines — but have limited applications that may not suit your needs.
• One, or many parts — the more prototypes or parts you order, the less the cost per part due to economies of scale. A CNC prototyping company factors the set up cost into the total project cost, and making multiple parts is faster as you don’t have to keep preparing again for a different part. The actual machining takes the same length of time to create the same part, but the setup time is lower.

Types of CNC machine for prototyping

CNC Milling Prototyping

Depending on the complexity of the prototype, you may need up to a 5 axis CNC machine. These extra axes of movement allow for much more precise cutting and more intricate part creation — but cost more. CNC milling involves cutting the final prototype out of a larger block of material sheets, using a computer controlled tool head cut material from this larger block.

CNC mills are overall the most commonly used CNC prototyping type, offering great accuracy — tolerances of around 0.001” can be expected — and able to be used on a wide range of geometries and materials. Milling prototyping is the best all-round versatile CNC prototyping solution overall.

• We also have a buyer’s guide for CNC mills

CNC Turning Prototyping

Turning machines — or lathes — created rounded parts by holding the block in place while turning it quickly, while a tool cuts parts off of the original stock based on the CNC programming code.

Lathes are ideal for prototyping parts with round mid-sections, but have limited all-round CNC prototyping applications due to only being effective for rounded parts.

• We also have a buyer’s guide for CNC turning machines

However, you may instead opt to do your own in-house CNC rapid prototyping:

CNC Laser Cutter Prototyping

More powerful laser cutters for small businesses can cut metals like aluminum, as well as plastics and acrylics, fabrics, and make wood CNC prototypes. This versatility makes them attractive to smaller studios looking to create a variety of prototypes without breaking the bank.

However, the laser’s heat can leave “scorch” marks on the prototype, and complexity is often limited as most CNC laser cutters only operate on 2 axes.

• We also have a buyer’s guide for laser cutters and engravers

CNC Router Prototyping

CNC routers act in a very similar way to CNC mills and machines, they’re just cheaper versions built with a gantry design to maximize work area for the size of the machine — but with limited complexity as a result.

They’re more suited to hobbyists and small businesses who aren’t focused on complex metal cutting, with routers preferred for wood, plastic and soft metals like aluminum.

They often use 3 axes, offering good complexity for the price — but they lack accuracy compared to CNC mills and there is still a significant learning curve to create CNC files based on the tool and machine.

• We also have a buyer’s guide to CNC routers and cheap CNC machines

Some in-house CNC prototyping is also done with plasma cutters, and waterjet cutters.

Reasons to use CNC for rapid prototyping vs 3D printing

Best option for functional prototypes 

The best comparison you can make between CNC prototyping and 3D printing is that while 3D printing is usually the best option for aesthetic prototypes for shape and aesthetic design testing, CNC prototypes are best for functional testing as they can be made to more effectively mimic the mechanical properties of the final part.

CNC parts offer better strength and mechanical stability than 3D printed prototypes, so for high-strength applications where the part needs to undergo significant strain and testing, such as in aerospace and medical part testing, CNC prototyping is an effective option.

Whereas 3D printers print prototypes with a percentage infill which could be anywhere from 10% to 50%+, CNC prototypes are made from solid material. This is important, especially in parts that will eventually be injection molded, as the prototype will be very similar to the injection molded part. Therefore, CNC prototypes are ideal for low numbers of prototypes that don’t require a mold.

Range of finishing options

CNC machining prototypes offers very high quality finishing options, with a range of different options available. These include:

• Leaving the part as-machined — the default finish for a CNC machined prototype, and fine for less aesthetic requirements.
• Bead blasting
• Sanding with grits to polish 
• Plating — chrome plating, or other finishes including zinc, nickel, tin, or gold or silver.

These options create smooth, crisp and very high quality surface finishes, ideal for pitching your prototype within the company, or for using it as a marketing material to a client or fundraising campaign.

Whereas with 3D printing, FDM printed parts especially leave clear layer lines, and can warp, leaving inaccuracies.

Versatile range of materials

Within CNC prototyping you have a diverse range of materials you can choose to get your designs produced in. Metal, plastic, foam and wood prototypes can be produced with ease on most industrial CNC machines, and even on cheaper CNC routers you can cut wood and acrylic parts.

Some of the materials you can create CNC prototypes in include:

• Metals: aluminum, stainless steel, copper, brass, titanium, sterling silver, bronze, zinc, magnesium.
• Plastics: acrylic, ABS, PP, Nylon, PC, PTFE, HDPE, LDPE, Teflon, PVC.
• Woods: almost any softwood or hardwood.

Though 3D printing is used more for rapid prototyping, niche CNC rapid prototyping uses work when you need a particular material produced into a prototype, especially with metals.

Whereas FDM 3D printers work with a good range of thermoplastics, metals can be slower and expensive to create parts for testing with, such as via DMLS or DED, and using a metal CNC machine may be preferable.

• If you’re considering 3D printing instead, read this guide to 3D printing rapid prototyping
• See also: our buyer’s guide for metal CNC machines

For wood, CNC prototyping is the best option — especially since 3D printers cannot produce pure wood prototypes. There are partially wood filaments available, usually with sawdust mixed with PLA to create wood-mimicking plastic filaments, but does cannot compare against the wood prototyping CNC offers, and even lower cost CNC routers can comfortable cut most soft and hard woods.

Limitations: CNC prototyping vs 3D printing rapid prototyping

Expensive

Whereas PLA filaments start at around $20 per kilo and can be printed on affordable 3D printers starting at around $250, CNC machining metal costs exponentially more.

• We recommend this article for buying a 3D printer under $300

For small businesses, 3D printers that can prototype effectively for around $500 may be preferred, especially as they take up less space in a studio that a CNC machine, and a prototype-capable CNC machine will cost at least $2,000.

Less effective for certain geometries

Additive manufacturing, in building a part up from scratch, can print prototypes such as interlocking chains and other intricate geometries that cannot be matched by any other technology.

While CNC prototyping has fantastic repeatability and can create precise geometries and excellent surface finishes on parts, subtractive manufacturing cannot create the same geometries.

However, CNC prototyping is usually more expensive than 3D printing, with metals costing significantly more than cheap PLA or ABS filament. And even 4 and 5 axis machines can struggle to match the geometries 3D printers offer, especially for geometries with holes and other features that 3D printing has advantages in as it is an additive manufacturing process.

Steeper learning curve to do in-house

A major benefit of 3D printing is its accessibility in both price and ease of use — cursory knowledge of standard 3D slicers like Cura are all you need, and 3D model design skills. CNC programming is more complex, and you need to account for additional factors like the machine’s end mill in your design.