CNC Fixture: Types, Use And Design Guide
When you’re working in machining, you know how important it is to find something to hold workpieces securely in place. That’s is where fixtures come in. Fixtures are what let you accurately locate and clamp parts on CNC machines.
But with different types of fixtures like dedicated, modular, and flexible, how do you know which to use and how to design them? Don’t worry, we’ll walk you through everything you need to know about CNC fixtures in this guide.
What Is CNC Fixture?
Role of CNC Fixtures in the Machining Process
CNC fixtures play an integral part in the machining process. They securely hold the workpiece in place so that the cutting tool can accurately machine the part.
Workholding
The main purpose of any fixture is workholding. The fixture needs to grip the workpiece tightly enough that it does not move during machining, while also not damaging the part. Fixtures use clamps, vises, and locators to hold the workpiece.
Clamps apply pressure to grip the sides or edges of the part. Vises have jaws that clamp onto the workpiece. Locators position and align the workpiece.
Accuracy
Proper workholding is key to achieving tight tolerances and an accurate final part. If the workpiece moves or vibrates during machining, features can end up out of spec. Fixtures should locate the workpiece in the same position for each new part. They should also distribute clamping forces evenly so the workpiece does not become distorted.
Maximizing Spindle Time
Efficient fixtures minimize setup period so that the spindle is actively cutting for the longest possible period. Quick-change fixtures and standardized clamping mechanisms allow new workpieces to be loaded and unloaded swiftly. Poka-yoke design helps prevent mistakes.
Well-designed CNC fixtures improve workpiece quality, reduce waste, and increase productivity. They may seem like a small part of the overall machining process but their role is crucial.
Types of CNC Fixture
A variety of fixture are used on CNC machines depending on the application. The most common are:
Milling Fixtures
Milling fixtures are used to securely clamp and locate the workpiece for milling operations like facing, slotting, gear cutting, etc. The fixture has to withstand high cutting forces and allow easy loading and unloading of the workpiece. They are made from hardened materials like steel alloys. The most common types are the vice fixture and clamping table.
Milling fixtures are used to securely hold and locate the workpiece during milling operations. The most common are:
Vise Fixtures:
Simple fixtures suitable for clamping prismatic parts. They provide quick setup but limited accuracy.
Angle Plates:
Used to hold work at an angle for angular milling. They provide adjustable angle settings but setup takes time.
Dividing Head:
Used for indexing and rotating cylindrical workpieces. They provide accurate rotation and division but are complex to setup.
Turning Fixtures
Turning fixtures are used to hold and rotate the workpiece during turning operations on a lathe. The fixture has to align the workpiece with the lathe axis and allow feeding at different speeds. The most common types are the chuck, collet and faceplate. These fixtures grip the workpiece from the outside diameter.
Turning fixtures are used to securely hold and locate the workpiece during turning operations on a lathe. The common are:
Chuck:
A third jig chuck grasps the workpiece for rotation. 3-jaw jig chucks are versatile but lack accuracy. 4-jaw jig chucks provide precise clamping but setup takes longer.
Faceplate:
A faceplate fixture mounts a plate with workpieces off-center or with an irregular shape. They can handle complex shapes but require careful balancing for smooth rotation.
Mandrel:
A mandrel passes through the center bore or a hollow part to grip from the inside. Mandrels provide good concentric clamping but only suit work with products with holes set in a center bore or hollow shape.
Turning fixtures allow for efficient metal removal, producing accurate dimensions, and a good surface finish. The appropriate fixture depends on the size, shape and features of your workpiece.
Grinding Fixtures
Grinding fixtures are used to hold and position the workpiece during surface grinding, external grinding, internal grinding, etc. They have to withstand high temperatures and allow uniform material removal. Magnetic and mechanical clamps are commonly used.
Drilling Fixtures
Drilling fixtures locate and clamp the workpiece accurately for drilling holes in the required position. They have bushings to guide the drill and pillars to support large workpieces. The most common types are the drill press table, drill jig and drill template.
Boring Fixtures
Boring fixtures are used to bore accurate holes in the workpiece. They have to locate the workpiece in a fixed position so that the boring bar can cut holes to precise dimensions. Boring fixtures with adjustable stops are commonly used for repetitive boring operations.
The type of CNC fixture used depends on the geometry of the workpiece, machining process, required accuracy and production volume. Properly designed fixtures improve productivity, quality and reduce operator fatigue. With the variety of fixtures available, you can achieve efficient metal cutting on CNC machines.
Considerations For CNC Fixture Design
When designing a CNC fixture, several factors need to be considered to ensure maximum effectiveness and quality. The material of the workpiece, tolerance levels required, and the quality and number of important reference surfaces all play a role in the quality of the final design.
Material of the Workpiece
The material that makes up the workpiece determines what kind and types of clamps and locators will work best. Harder materials like steel require more robust clamping, while softer materials need more delicate handling to avoid marking or distortion. The coefficient of thermal expansion of the material must also be accounted for to ensure accuracy is maintained during machining.
Improves the Tolerance Level
The required tolerance level of the finished part impacts the details of the design in terms of locators and clamps. Higher precision parts need extremely rigid fixturing with more locators to minimize any possibility of movement during machining. Clamps should also apply even pressure and avoid deflecting the workpiece.
Reference Important Surfaces
Any critical surfaces that will mate or align the work part with other components must be located and clamped to high accuracy. The fixture should grip the workpiece at these key interfaces to ensure they are located and machined properly. Locators should contact the workpiece on these surfaces at three points to positively define position.
Secure Clamping
The fixture must securely clamp the workpiece in place. If the clamped workpiece shifts during machining, it can lead to errors and scrap parts. The clamps should apply even pressure around the workpiece. Pneumatic or hydraulic clamps are a good option as they provide strong, consistent clamping force. For irregularly shaped workpieces, consider using multiple smaller clamps rather than one large clamp.
Stability and Rigidity
The fixture itself must be rigid and stable to withstand the forces of the machining operations. If the fixture flexes or vibrates, it will negatively impact the precision and finish quality. Construct the fixture from a sturdy material like steel. Brace large fixtures to prevent twisting under stress. Mount the fixture directly to the CNC table for maximum stability.
Alignment and Positioning
The fixture must accurately align and position the workpiece in the correct orientation for machining. Misalignment will result in faulty parts. Include guide pins, registers, and stops in the fixture design to properly locate the workpiece. For repeat jobs, a fixture that consistently aligns the workpiece the same way each set is essential. Consider the order of operations and how the workpiece will need to be repositioned during multiple setups.
Material Compatibility
Choose fixture materials that are compatible with the workpiece material. Dissimilar metals in contact can lead to galvanic corrosion. Coatings may be needed when different materials are unavoidable. If using clamps, choose clamp jaws and pads that won’t damage the surface of the workpiece. Soft jaws or jaw pads are often used for delicate workpieces.
Following these guidelines will help you design a quality CNC fixture and tool that securely holds the workpiece, withstands machining forces, precisely aligns the workpiece, and prevents material incompatibility issues. An effective fixture and tool is key to safe, accurate, and repeatable CNC machining.
FAQ
The fixture offset refers to the position of the fixture relative to the machine coordinate system. When you install a fixture on the CNC machine, you have to measure its exact position and enter those measurements into the CNC control.
The control will then use that information to adjust programmed tool paths to account for the position of the fixture. This allows the CNC machine to machine parts accurately even though the fixture may not be in the exact same position each period.
What are some examples of CNC fixtures?
Some common examples of CNC fixtures include:
- Vises: A vise holds the workpiece in position while allowing access for machining. It is good for simple prismatic parts.
- Clamps: Clamps are used to securely hold sheet materials or odd-shaped parts in position . They come in a variety of styles for a variety of different purposes and applications.
- Jigs: A jig positions and supports the workpiece while also guiding the cutting tool. It is good for repetitive jobs and machining of complex parts.
- Chucks: A chuck mounts directly to the machine spindle to hold and rotate the workpiece. It is commonly used for lathe operations.
- Pallets: A pallet fixture allows quick and easy changeover of parts by pre-loading workpieces onto pallets that can be swapped in and out. This reduces setup time.
What's the difference between a milling fixture and a turning fixture?
The main differences between milling and turning fixtures are:
- Milling fixtures are used on milling machines, while turning fixtures are used on lathes.
- Milling fixtures typically hold the workpiece stationary while the tool moves, whereas turning fixtures rotate the workpiece as the tool feeds into it.
- Milling fixtures usually have a flat base, while turning fixtures mount directly to face the table and slide down to face the lathe spindle.
- Milling fixtures secure the workpiece from multiple sides, while turning fixtures grip the outer diameter of the workpiece.
- Milling tool fixtures must withstand the forces of the cutter rotating at high speed, while turning tool fixtures must handle the resistance of a tool feeding into the rotating table or workpiece.
- The details and specific design details of any fixture depends on the needs of the particular machining operation. But in general, the goal is the same: to hold the workpiece securely and consistently for accurate, efficient metal cutting.
Author
Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.