In short run manufacturing, it’s hard to name a better technology than CNC machining. It offers a well-rounded mix of advantages including high throughput potential, accuracy and repeatability, a broad selection of materials, and ease of use. While almost any machine tool can be numerically controlled, computer numerical control machining typically refers to multi-axis milling and turning.
To find out more about how CNC machining is used for custom machining, low volume production and prototyping, engineering.com spoke with Wayken Rapid Manufacturing, a Shenzhen-based custom prototype manufacturing service about the materials, technology, applications and operation of CNC machine tools.
When it comes to materials, if it comes in sheet, plate or bar stock, chances are you can machine it. Among the hundreds of metal alloys and plastic polymers that can be machined, aluminum and engineering plastics are most common for prototype machining. Plastic parts designed to be molded in mass production are often machined in the prototype phase in order to avoid the high cost and lead time of mold making.
Access to a wide range of materials is especially important when prototyping. Because different materials have different cost and different mechanical and chemical properties, it may be preferable to cut a prototype in a cheaper material than what is planned for the final product, or a different material may help optimize the strength, stiffness or weight of the part in relation to its design. In some cases, an alternate material for a prototype may allow a specific finishing process or be made more durable than a production part to facilitate testing.
The opposite is also possible, with low cost commodity materials replacing engineering resins and high-performance metal alloys when the prototype is used for simple functional uses like fit check or mockup construction.
Although developed for metalworking, plastics can be successfully machined with the correct knowledge and equipment. Both thermoplastics and thermosets are machinable and are very cost effective compared to short run injection molds for prototype parts.
Compared to metals, most thermoplastics like PE, PP or PS will melt or burn if machined with the feeds and speeds common to metalworking. Higher cutter speeds and lower feed rates are common, and cutting tool parameters like rake angle are critical. Control of heat in the cut is essential, but unlike metals coolant is not typically sprayed into the cut for cooling. Compressed air may be used to clear chips.
Thermoplastics, especially unfilled commodity grades, elastically deform as cutting force is applied, making it difficult to achieve high accuracy and maintain close tolerances, especially for fine features and detail. Automotive lighting and lenses are particularly difficult.
With more than 20 years of experience with CNC plastic machining, Wayken specializes in optical prototypes such as automotive lenses, light guides and reflectors. When machining clear plastics such as polycarbonate and acrylic, achieving a high surface finish during machining can reduce or eliminate processing operations such as grinding and polishing. Micro-fine machining using single point diamond machining (SPDM) can provide accuracy less than 200 nm and improve surface roughness less than 10 nm.
While carbide cutting tools are commonly used for harder materials such as steels, it can be difficult to find the right tool geometry for cutting aluminum in carbide tools. For this reason, high speed steel (HSS) cutting tools are often used.
CNC aluminum machining is one of the most typical material choices. Compared to plastics, aluminum is cut at high feeds and speeds, and can be cut dry or with coolant. It’s important to note the grade of aluminum when setting up to cut it. For example, 6000 grades are very common, and contain magnesium and silicon. These alloys provide superior workability compared to 7000 grades, for example, which contain zinc as a primary alloying ingredient, and have higher strength and toughness.
It’s also important to note the temper designation of an aluminum stock material. These designations indicate the thermal treatment or strain hardening, for example, that the material has undergone and can affect performance during machining and in end use.
Five axis CNC machining is more expensive complex than three axis machines, but they are gaining prevalence in the manufacturing industry due to several technological advantages. For example, cutting a part with features on both sides can be much faster with a 5-axis machine, since the part can be fixtured in such a way that the spindle can reach both sides in the same operation, whereas with a 3 axis machine, the part would require two or more setups. 5 axis machines can also produce complex geometries and fine surface finish for precision machining because the angle of the tool can be conformed to the shape of the part.
Besides mills, lathes and turning centers, EDM machines and other tools can be CNC controlled. For example, CNC mill+turn centers are common, as well as wire and sinker EDM. For a manufacturing service provider, flexible machine tool configuration and machining practices can increase efficiency and reduce machining costs. Flexibility is one of the principal benefits of a 5-axis machining center, and when combined with the high purchase price of the machines, a shop is highly incentivized to keep it running 24/7 if possible.
Precision Machining refers to machining operations which deliver tolerances within ±0.05mm, which is widely applicable in automotive, medical device and aerospace parts manufacturing.
The typical application of Micro-Fine Machining is Single Point Diamond Machining (SPDM or SPDT). The main advantage of diamond machining is for custom machined parts with strict machining requirements: form accuracy less than 200 nm as well as improve surface roughness less than 10 nm. In manufacturing optical prototypes such as clear plastic or reflective metal parts, surface finish in molds is an important consideration. Diamond machining is one way to produce a high-precision, high-finish surface during machining, especially for PMMA, PC and aluminum alloys. Vendors that specialize in machining optical components from plastics are highly specialized, but offer a service that can dramatically reduce costs compared to short run or prototype molds.
Of course, CNC machining is widely used in all manufacturing industries for production of metal and plastic end-use parts and tooling. However, in mass production, other processes such as molding, casting or stamping techniques are often faster and cheaper than machining, after the initial costs of molds and tooling is amortized across a large number of parts.
CNC machining is a preferred process for producing prototypes in metals and plastics because of its quick turn time compared to a process like 3D printing, casting, molding or fabrication techniques, which require molds, dies, and other extra steps.
This ‘push-button’ agility of turning a digital CAD file into a part is often touted by 3D printing proponents as a key benefit of 3D printing. However, in many cases, CNC is preferable to 3D printing as well.
It can take several hours to complete each build volume of 3D printed parts, while CNC machining takes minutes.
3D printing builds parts in layers, which can result in anisotropic strength in the part, compared to a machined part made from a single piece of material.
A narrower range of materials available for 3D printing may limit the functionality of a printed prototype, while a machined prototype can be made of the same material as the final part. CNC machined prototypes can be used for end-use design materials to meet functional verification and engineering verification of prototypes.
3D printed features such as bores, tapped holes, mating surfaces and surface finish require post processing, typically via machining.
While 3D printing does provide advantages as a manufacturing technology, today’s CNC machine tools provide many of the same advantages without certain drawbacks.
Fast turnaround CNC machines can be used continuously, 24 hours a day. This makes CNC machining economical for short runs of production parts that require a wide range of operations.
To find out more about CNC machining for prototypes and short-run production, please contact with Wayken or request a quote through their website.
Post time: Nov-30-2019