The CNC Process
by Edgar Andrade, Tooling Manager
Observe an 18K wedding ring, an 18K heart pendant with a diamond cut design, and 18K hoop earrings with a satin finish. What is the common denominator for this jewelry? If the first thought that comes to mind is that they all have 18K gold, that is correct…but there is a more subtle similarity. Each of these pieces could have been made through a CNC process. What does “CNC” mean? The acronym stands for “computerized numerical control.” Before CNC technology was available, it was common to observe jewelry machinists performing manual machining processes on machine tools, for example the machining of rings in a lathe engine or milling the outline hub in a Bridgeport milling machine. These machines were controlled by the manual rotation of the hand wheels, with the machinist using his or her eye-hand coordination, mathematical and machining skills to advance the “tools” in the machine’s x, y, and z axis. Some machines were not equipped with digital readouts, and the reading of the manual movements was done on the hand wheel scale. With the introduction of the CAD/CAM programs and CNC machines, the machining process changed, and the machine movements became programmed, digitized as “numerical control” commands. The CNC process allows for modifications on the fly, so it is very easy to quickly update a program, or change tool parameters, to achieve the desired design aesthetic, without waiting for the next day. The aforementioned is the case of a lost wax casting process, where shrinkage factors can be “estimated.”
What is involved in the CNC process? Here is a rundown.
CNC MACHINES TYPES
There are many types of CNC machines used in the jewelry industry, such as those designed for light removal of metal, as in the case of diamond cutting operations. Other CNC machines need to have a robust structure for withstanding heavy machining. Some of the special features of the latter are automatic loading and unloading of parts, probes to measure amorphous parts, and turning capabilities in milling machines, amongst others.
This type of CNC machine has a main spindle that rotates around 6000 RPM. The tools are mounted on a tool changer, and it is possible to a have one or more live tools with an independent motor and higher spindle RPM. The feeding of a CNC Lathe can be either manual or automatic. For example, a CNC Lathe can be used for the turning and profiling of a ring. In this case a tube is feed into the machine, the program starts the spindle, the machine starts the pump for the coolant, then Tool # 1 faces the tip of the tube, while Tool # 2 cuts the inside diameter, Tool # 3 cuts the outside diameter and, finally, Tool # 4 cuts the width of the ring. The full cycle is completed in 1 minute.
CNC MILLING MACHINE
This machine is the automated version of the Bridgeport type milling machine. There are many accessories that can be added to it; the most common is a vise and/or a rotary table. Typically, CNC Milling Machines have a main spindle (around 12000 RPM and up) and flat parts are machined directly in a vise, whereas a rotary table is used for round parts, like rings or bracelets. There are an infinite amount of designs and shapes that are CNC milled, such as: diamond cutting with fly wheels, milling recessed designs, or piercing through, or milling the prongs, for stone settings. An example of use would be the milling of a ring that has being previously turned in a CNC Lathe, which is mounted on a rotary table chuck. A 0.020” end mill pierces a design.
5-AXIS MILLING MACHINE
This machine has built in 5-axis capabilities, without the need for a rotary table. The advantage of having 5-Axis milling machines is the ability to complete parts that could require secondary operations. For example, for a ring with a design on the side, the ring could be machined on the outside diameter, then set up in another, and a fixture would hold the ring to machine the sides. Usually the spindle runs at very high speeds of over 30000RPM.
This type of machine is the most expensive of all the CNC machines. The Swiss version has multiple spindles and many special features. The type of feeding used can be automatic, through a bar feeder, or manual, by mounting the part in the chuck. This type of machine is common in the medical and aerospace industries. In the jewelry industry, stone setting and finishing features are available for a Swiss machine.
Designs are drawn in a 3D file, typically in Rhino. The programmer then imports the drawing into a CAD/CAM program, such as Master CAM. Many moving parts, such as design, material, and size are critical for quality elements being studied in detail, before the actual program starts. For complex pieces, the file is imported in SolidWorks, or a similar program for surface repairs. Taking the time to inspect the 3D file before programming reduces the time and effort exerted behind unnecessary and costly revisions. The CNC program is composed of G and M codes. There are instructions for tool motions, including the X, Y, and Z axis, as well as ( a –b in 5 axis) cutting rates, spindle speed, feed rate, machine tool change, coolant, etc. After the program is completed, a “simulation” feature allows for virtual testing of the program. Programs are then loaded into the machines via a USB port, or the machines network. Then, the set up operator follows the setup instructions, including the tools list and their position in the tool magazine.
TOOL SETTING AND MACHINING
The tooling for CNC is usually made of solid carbide, drill bits, end mill or turning inserts. Some materials, like platinum, require tools with a special coating, such as TiB2 or diamond coating. Based on the program, the operator chooses between different shapes, ball nose, taper, straight, and 24 flutes. For each design, different sizes are required; usually, micro end mills are used, but for large quantities of material to remove, a 1/8 end mill could be used. Paying attention to the tool list is critical for avoiding mistakes. A 0.010” end flat end mill can have different LOC (length of cut) if the wrong tool is used. This will cause the final result to not be acceptable, even if the program was done correctly. Every tool must be carefully inspected before the set up and the amount of hours is recorded. This system allows tools to change before the break in a maching cycle, causing repairs or scraps. After tools are checked and set, the operator sets up the part in a rotary table, or holding fixture.
Another important note is about holding clamping parts. The holding fixture’s design is critical; it directly impacts the quality of the surface, and vibrations due to improper holding clamping could yield in rough surface, or premature, tool wear, including tool breakage.
When deciding on the right CNC process and machine, it is important to analyze the costs: programming, tools, and machine cycle times and maintenance are expensive, when compared to other jewelry manufacturing processes. CNC benefits include high precision machined parts, well-defined edges, and the repeatability of close tolerances. There is no high cost of tooling, as with stamping, where tools and dies are built for each design, and where it can be time consuming to make.