CNC Milling vs. 3D Printing

For someone new to the jewelry industry, the sheer assortment of tools and equipment in the typical workshop can be overwhelming. That's especially true when it comes to technology, which changes so rapidly. If you've invested in your own computer-aided design (CAD) software program and you're ready to produce your own models, you have two computer-aided manufacturing (CAM) methods to consider: CNC mills and 3D printers.

Which is the right choice to bring your designs to life?

To help you decide on which is best suited for your operation, we've assembled a short "primer" on the basics of each method; it covers where they shine, where you might need to make accommodations, and what to expect in quality, time savings, and cost. First, though, you must look at the type of work you produce—for instance, do you specialize in small, hollow earrings, or are your best sellers large personalized pieces with lots of lettering? Your answer will help to determine the technology that's right for you and your business.

CNC Milling Machines

Milling machines use rotary cutting tools to carve a jewelry model from a block of wax, allowing jewelers to touch up or refine their models before casting. The number of axes in the machine can vary, with three- and four-axis machines the most common in the industry. Five-axis machines are available and can give the user more abilities and flexibility, but they're more expensive, tend to be slower to mill, and require more work to keep the machine in alignment.

What You Might Want to Mill

• Pieces with simple patterns or designs that have no undercuts. Any designs for which the cutting tool can come straight down onto the surface (if it is flat) or perpendicular to a band surface (on a rotary cut) are ideal.
• Items with lettering or that have designs with fine detail. Because mills carve wax, which burns out with little expansion, they can create sharp, crisp letters that won't be compromised during the casting process.
• Pieces on which you want to do hand-carving work, such as engraving, before casting.

Things to Be Aware Of

• Because the standard cutting tool has a taper to it, any straight 90-degree edges will have a slight tapered effect. This can be compensated for during the CAD design process by including additional material in those areas.
• Although mills are very good for carving fine, sharp details, there is a limit to the fineness that is possible. Even the smallest cutting tools have a small flat on the tip that measures 0.003 inch. While pretty small and not something easily detectable to the naked eye, it is something to keep in mind.
• Mills with fewer than five axes cannot easily be used to create undercuts on pieces, although such designs can be milled in multiple pieces and later assembled.
• Avoid carving very long, thin models on a mill. As the mill physically cuts away material, the wax will vibrate. Although all models require some form of support during the cutting process for stabilization, long, thin models require additional supports that will not only need to be removed, but can also break from the vibration.
• As with undercuts, most mills have difficulty creating hollow pieces. Milling designs in separate segments for later assembly may be possible with some designs.
• Although mills can run multiple models at a time depending on geometry and size, most mid-complexity and up pieces will need to be run by themselves.
• Although the technology has been around for a while, it can be difficult to find tech support, as many users and companies have moved to 3D printing.

The Materials You Use

There are many suitable types of carving wax that can be used with milling machines, although typically those with higher hardness tend to perform better.

What You Might Pay

Machine costs vary widely depending on the number of axes and the size and complexity of the mill. Some basic desktop units capable of milling a range of materials can be had for as low as $2,200. Larger units with five axes capable of milling metal can run more than $100,000.

3D Printers

Most of the 3D printers currently on the market work by growing models layer by layer. Many of these printers use a photopolymer resin material that hardens when exposed to light. Other machines use a jetting process of wax-based material, which can help avoid the casting issues many users encounter with the photopolymer resins.

What You Might Want to Print

• Pieces with complex geometries or freeform shapes. Because the printers grow the models layer by layer, they are ideal for pieces with undercuts and hollow insides.
• Jobs requiring production in volume. Printers allow users to print multiple items on the same build plate, saving users time.

Things to Be Aware Of

• On most printers using photopolymer resin, all parts of a model must be supported during printing, so locations for supports must be considered: Those areas will need to be cleaned off after building. Some CAD programs can automatically assist with adding supports where needed, but manual manipulation may still be required.
• Because printers build models layer by layer, finished models can show layer lines, and the surfaces may not be as crisp and precise as a carved wax model.
• Before photopolymer resin models can be cast they first must be cleaned with a solvent and then post-cured since the printer only partially cures the model as it's built.
• With large, solid pieces, the photopolymer resin material may leave an ash residue during burnout. Larger items result in a larger amount of ash, which could create porosity issues with the cast piece.
• Photopolymer resin models have different investment requirements. While many can be cast with regular gypsum-based investment, the water-to-powder ratios often must be adjusted. Dental investments may need to be used with certain models, but may involve additional work to remove the material after casting.
• Photopolymer resins expand during the burnout process, which can result in pieces of investment breaking off or cracking. To prevent this, avoid sharp angles or edges, anything with thin holes built into the design, or lettering more than 0.6 mm high.

The Materials You Use

Materials vary by printer; each machine's manufacturer has its own "secret-sauce recipe." Many printers use photopolymer resin, but others use wax-based solutions that can be easier to cast. Consult with a printer's manufacturer on what it offers and how you should use it to meet your needs.

What You Might Pay

The 3D printer market has exploded over the last few years. There are now DIY kits that allow users to construct their own printers for only a few hundred dollars. Larger machines more geared toward production use begin for as little as $3,000 and can run upwards of $150,000.

MJSA Journal would like to thank Steven Adler of A3DM Technologies Corp. in Portland, Oregon; Darla Alvarez of GIA in Carlsbad, California; Russ Hyder of the Jewelry CAD Institute in Las Vegas; Lee Krombholz of Krombholz Jewelers/Just Like You Designs in Cincinnati; Bob Lynn of Lynn's Jewelry Studio in Ventura, California; and Mark Maxwell of Maxwell CAD in Oceanside, California, for their help with this article.