ManufacturingCNC machiningWhen it comes to parts, it's important to find out the best way to load and hold the workpiece first. This step occurs before the toolpath is drawn and plays an important role in the process.
Workholding and loading is a critical aspect of the CNC machining process. Your workpiece is only as good as how well it stands and stabilizes on the machine's working surface. As a result, machinists and engineers have come up with a variety of solutions to keep the workpiece stable.
In this article, we will introduce some of the most well-known workholding methods, evaluating the advantages and disadvantages of each.
T-slot
The T-groove is essential for workholding and provides a T-cut where the T-crossbar is located on the underside of the table or within the table itself. This allows the nut to fit into the groove to provide a movable position for inserting the bolt or clamp.
There are many different types of fixtures, each with its own advantages and disadvantages.
Gluing
The addition of bakelite blocks to the table is a popular method of workpiece clamping, especially for prototype machining. The workpiece can be easily glued to the bakelite table, which is what most Chinese prototyping companies do.
Gluing is still a very simple and popular solution for fixing workpieces. It can be used for flat and irregularly shaped materials. It can provide higher strength than carpet tape while eliminating the need for workpiece retention tabs. Removing parts from the bed requires the use of a paint scraper or manual peeling. Glue is especially useful for prototyping and plastics.
Proper gluing requires even gluing to keep the workpiece level. It is a fast, inexpensive, and effective workholding option, but how it works varies depending on the material used. For example, hot glue can sometimes remove large chunks from materials such as foam and wood. The best way to prevent damage is to apply glue selectively and apply it to areas that are not aesthetically or functionally important, or use a thin layer for easy removal.
Similarly, glue hardens faster on metal. When machining metal, one way to avoid the glue hardening too quickly is to place the material on a non-metallic scrap board that is fixed to the bed. The material should be on top so that the glue has a chance to stick to both surfaces and not harden on its own.
Advantage: Multiple parts can be glued to the table at once, which reduces the operator's workload. This is an effective way to create plastic or aluminum prototypes.
Disadvantages: The workpiece may peel off. Parts may also be damaged during disassembly.
Moss-pumping
Bolts are a good way to fix the workpiece directly with a T-shaped table for machining. Frankly, it's even a better idea to add aluminum blocks, as it's easy to create threaded holes where they need to. Threaded holes go hand in hand with fixtures for better leveling and stability.
Items used to secure materials include T-nuts, studs, and flange nuts. Bolts and nuts are especially useful if the work surface has threaded inserts instead of T-slots. In these cases, bolts can be screwed into the insert to secure fixtures of various designs, holding them in place.
Advantages: Bolted joints are a very stable method of fixing the workpiece, and then you can cut the workpiece very hard. Complex geometries will not be a problem to hold.
Disadvantages: The use of bolts requires additional material as a frame. When machining high-volume parts, material waste can be very high and loading times can be long. Hinge marks can be problematic when cutting parts from the frame.
Vise
Machinists often use vises as a standard method of fixing workpieces. When the tool is processing a material, the vise usually holds the workpiece from both ends, between the two jaws. They are especially useful for those parts that have straight edges, but they can also be used for parts with rounded edges.
Depending on the type of vise, machining rounded edges may require the help of soft jaws, which can be milled to support the workpiece. Modern vises are designed with a quick-switch base that allows the machinist to load the workpiece from the CNC machine. Once the part is complete, the next batch of parts can be loaded quickly. Vise is also the first choice in our CNC workshop.
Pros: The vise can cut at high speeds and the workpiece can be easily positioned when making repeating parts. They are an effective way to manufacture high-volume CNC parts. You can also place multiple vises on your CNC machine to make different parts at once.
Disadvantages: The part must have regular geometry and parallel surfaces. Otherwise, custom grippers are required.
Clips
There are several types of fixtures available:CNC machiningWorkpiece clamping in the process. They include combination clamps, pliers, and C-clamps.
Combination clamps or "block clamps" are composed of a series of components with standardized design, different functions, and different specifications and sizes. These machine fixtures can be assembled like building blocks according to the machining requirements. Due to their nature, combination fixtures are very time-saving in design and manufacturing, making them suitable for small batch production cycles. The combined fixture also has the advantages of high positioning accuracy, large clamping flexibility, reusability, energy saving and material saving, and low cost. It is ideal for low-volume production that requires slightly more complex shapes.
Another advantage of fixtures is the ability to improve specific components and replace them according to the specific requirements of the workpiece. For example, the use of combination precision vises allows for more versatility, higher standardization, easier use and more reliable clamping. This type of precision vise has the advantage of quick and easy installation, clamping. It can shorten the production preparation time and improve the efficiency of small batch production.
Simpler clips, such as C-clamps, can be easily found at hardware stores. They** are cheap and easy to use, but their simplicity also has drawbacks. Clamping the material on only one side can sometimes cause it to lift on the other side, so it's important to make sure the material is flat before machining it. Similarly, cutting out pieces and parts from the material can be tricky, requiring multiple jigs and possibly other methods of securing the material. For heavier projects, it's best to use different types of fixtures to streamline your workflow.
Other ways to improve these fixtures include using a smooth fixture base or using special magnetic materials. The latter is used for electromagnetic permanent fixtures, which can effectively improve the machining efficiency of CNC machine tools and machining centers. Positioning parts and clamping elements of traditional fixtures take up a lot of space, but because electromagnetic permanent fixtures utilize the magnetic force inherent in the material, they do not have these space-taking parts. This saves a lot of hassle for a variety of machining operations and minimizes clogged parts that can get in the way of the machine. However, care must be taken to ensure that the magnet's suction is strong enough to resist the cutting force.
Hydraulic and pneumatic grippers use an additional power source to properly position, support and compress the workpiece by means of hydraulic or pneumatic components. As a result, they can accurately and quickly determine the position of the machine and the cutting tool in relation to each other and adjust the workpiece. They can also offer other benefits, such as compact design, multi-station clamping, high-speed heavy cutting, and automatic control, all of which speed up material processing. This provides them with additional practicality, making them ideal for CNC machines, machining centers, and flexible production lines.
Vacuum table
A vacuum table compresses the workpiece by creating a pressing force – this compression force is caused by the difference between the vacuum under the part and the atmospheric pressure pushing down from above. Vacuum tables are often the best solution for flat machining on CNC milling machines.
These are very accurate, flexible, and efficient. They are suitable for a wide range of materials and are particularly useful for materials that cannot be clamped, such as high-tech composites or advanced synthetics. They can also minimize roadblocks in ways that fixtures and other solutions cannot, while also making setup and changeovers more efficient.
However, vacuum tables also have their drawbacks. Vacuum tables are best suited for flat materials, as parts with different geometries require additional preparation. Some small parts may also not have enough surface area for the vacuum cleaner to accommodate them, although more advanced types of vacuum tables with more uniform or concentrated pull-down force can handle these items.
Advantage: The loading time of the vacuum table is very fast and it is suitable for materials that cannot be clamped.
Disadvantages: Most vacuum tables are only suitable for simple, flat parts.
Magnetic table
The magnetic table uses a magnetic base to hold the workpiece made of suitable metal. They are a popular solution in the mold making industry because most mold cavities are made of steel. However, they are also used for steel workpieces for general CNC machining operations.
Most magnetic tables are electromagnetic, so the magnetic force can be stopped after machining. They allow for quick set-up and excellent cutting tool operation because no physical fixtures or other equipment are required. They are suitable for light or heavy machining, but cannot be used for non-magnetic workpieces (e.g. aluminum workpieces).
Advantage: The magnetic table allows for a full cut and can be set up quickly. They are also very safe and secure.
Disadvantages: Magnetic tables can only be used with magnetic metals, limiting their use in aluminum-based manufacturing.
Custom fixtures
When milling large volume parts, standard workholding methods may not be sufficient. If that's the case, it might be worth making a custom fixture to CNC machine the part. In mass production, shortening the time can make a big difference.
Fixtures can guide the toolpath and greatly improve product quality, but they are only needed when absolute stability is required. The workpiece is usually slid into or dropped into the fixture and held in place with one or two quick fixtures.
Custom-produced workholding offers distinct advantages and can be manufactured in different ways. Fixtures that require high stiffness and tight geometric tolerances are best manufactured using CNC machining, while fixtures designed to conform to bent parts may be easier to produce by FDM or SLS 3D printing.
Advantage: Custom fixtures can be customized for oversized or irregular workpieces, reducing set-up time and ensuring stability.
Cons: Extended turnaround times and increased spending. These fixtures are not suitable for other common parts.