The Value of Workholding

Workholding refers to standardized or custom fixtures tasked with the important job of bracing the part during the CNC machining operation. A work holding piece may be called a fixture, bracket, jig, or clamp which also often defines the method used to hold the part. Precision demands that all parts of the CNC machining process prevent vibration and material slip. The value of work holding makes that possible for any shape or size of raw material. High precision and high speed can only be achieved when reliable rigidity is present. That stiffness is achieved through a combination of the machine, tool, raw part, and the work holding all working together to prevent unintended movement.

Value of Workholding

Tolerances/Precision – Only when a part is secure can the tooling perform fast and precise movements. Tolerances can only be achieved when there is zero movements from the high pressure of cutting and drilling operations.

Tool Life – Tool life is greatly reduced when the tool path does not provide clean movement of the tool due to vibration or misalignment. This provides poor quality work also. One of the costliest mistakes in CNC machining is tool breakage. The unstable clamping of a part will interrupt the tool path leading to wear and damage.      

Safety – Tools are expensive investments to start, but damage can extend to other parts of the machine. Any breakage also has the possibility of personal injury as well due to flying parts and scrap even from within an enclosed machine workspace.

Cost – In all CNC operations, keeping costs down benefits both the customer and the shop. Higher levels of scrap will result from poor work holding choices. Even if work holding options are limited, reworking the part in several operations that are properly clamped is less expensive than broken parts and broken equipment.

Efficiency – Efficiency is gained through the avoidance of machine downtime, breakage, tool wear, scrap, and injury. The best cost control a machine shop has is in the proper design and execution of a CNC program that takes into account tool path, work holding, and raw material for each project.

Securing the Fixture

Approaches to clamping the work holding in place are just as important as the design of the fixture. Where the work holding provides a variety of shapes and methods of attachment to the part, the way it is secured to the machine is a constant of the CNC machine design.

Vise – a vise is a typical assembly that can clamp a large range of sizes for mostly vertical (Z axis) cutting approaches. Caution is recommended for softer materials, such as brass, or very thin raw materials, as the vise can distort the part easily when tightened securely. A modified vise can be used with parts that do not have parallel sides by machining a piece of aluminum (or steel) into the reverse form of your part shape on one side and parallel sides on the opposing side to fit the vise. This is called a Soft Jaw.

Tooling Plate – A tooling plate provides a solid mounting solution for operations where multiple-axis cutting is required. A standard plate consists of smooth holes and threaded holes to align and fasten the raw material. The use of dowels formed into the part requires a secondary tooling operation to remove them or, similar to the vise, a custom secondary plate can be formed to receive the shape of the raw machined part with alignment dowels and fastening to the tooling plate. Also known as a fixture plate, this is an excellent choice for repeatability in machining operations.

Securing – Many fixtures are held in place with simple levers, clamps, and grippers to secure the part. The work holding piece may be secured to the machine in a similar fashion or with the use of vacuum or electro-magnetic systems or assisted pneumatics for automated solutions.

Designing Fixtures

One of the greatest values of work holding is in performing tooling operations to several parts at once for a multiple-piece order. Designing custom plates to hold the raw material for several parts saves setup time, tool change, and run time. It takes time to change tools – even 2 to 10 seconds per part is a lot of time when running thousands of units. Using any of the fixture types mentioned above, adding a custom work holding piece can take the movements for one piece and apply them to 2 or even 8 pieces or more in one program, depending on the size of the final part and the machine bed.

Place Emphasis on Workholding Design

Securing the part for any tooling operation is a critical step in the high-precision world of CNC manufacturing. As part of the whole operation working together, holding the workpiece can be the trickier factor to overcome. Tools are designed to be held securely in chucks. Worktables and machines are levelled, and input from the CAD/CAM system is precise. Paying particular attention to the best approach for machining should also consider the value of work holding. In programmable and multi-axis machining this still allows for the design and engineering skills of a machinist to shine. Designing smart work holding devices improves both accuracy and efficiency in any operation.