Introduction to Tensile and Compressive Stress in CNC Machining(steel vs cast iron Una)

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Computer Numerical Control (CNC) machining is a manufacturing process that uses pre-programmed computer software to control machine tools like mills, lathes, routers and grinders. CNC machining is used to produce precision parts out of various materials like metals, plastics, wood and composites by removing material from a raw stock to achieve the desired shape and size.
During any machining operation, the workpiece is subjected to stresses. These stresses can be tensile or compressive in nature. Understanding the difference between tensile and compressive stresses is important for optimizing the machining parameters in CNC machining.
What is Tensile Stress?
Tensile stress refers to the stress that tends to stretch or elongate the material it acts on. Tensile stress causes two sections of the material to be pulled apart. This causes the material to become thinner at the region where tensile stress is applied.
Tensile stress is measured in units of force per unit area like pounds per square inch (psi) or megapascals (MPa). The ability of a material to withstand tensile stress before breaking is known as tensile strength.
Sources of Tensile Stress in CNC Machining
Several steps in the CNC machining process can induce tensile stresses in the workpiece material:
- Clamping Force: The workpiece must be securely clamped on the machine bed or fixture to prevent movement during machining. This clamping force exerts tensile stress on the material.
- Tool Engagement: As the cutting tool engages the workpiece and removes material, it exerts tensile forces perpendicular to the direction of feed. This happens as grains are sheared while cutting.
- Rapid Tool Movement: When the tool quickly reverses direction during slotting, pocketing or profiling operations, alternating tensile-compressive stresses are produced in the workpiece.
- Residual Stresses: Plastically deforming operations like rolling and forging induce residual tensile stresses in the part. These stresses remain even after clamping and cutting forces are removed.
- Thermal Effects: Non-uniform heating and cooling during machining produces thermal stresses. Plastically deformed surface layers cool faster than the hot core, creating tensile residual stresses.
What is Compressive Stress?
Compressive stress pushes condensed sections of the material together. This stress acts to shorten the material by crushing and buckling. Compressive stress is also measured in psi or MPa. The capacity to withstand compressive load before buckling is called compressive strength.
Sources of Compressive Stress in CNC Machining
Compressive stresses are also produced in the workpiece by various actions in CNC machining:
- Clamping Force: The force applied by clamps, vices and fixtures to hold the workpiece creates compressive stress underneath them.
- Tool Engagement: The leading edge of the cutting tool exerts compressive force on the workpiece material ahead of it. This happens due to the wedging action.
- Built-up Edge: Frictional heating and welding of work material on the tool face forms built-up edge. Periodic breakage of this edge sends compressive shock waves into the workpiece.
- Residual Stresses: Plastically deforming actions like rolling, forging, grinding and machining induce residual compressive stresses in the surface layers of metal parts.
- Machine Tool Deflection: Bending of thin workpieces under cutting forces causes the material on the machine-side to be under compression.
Importance of Controlling Stresses in CNC Machining
Properly managing and reducing tensile and compressive stresses is crucial for high quality CNC machined parts. Some key reasons why controlling stresses is vital are:
- Minimizing Distortion: Balancing tensile and compressive stresses prevents warping and distortion after unclamping the finished workpiece.
- Reducing Dimensional Errors: Uncontrolled stresses lead to inaccurate dimensions, poor surface finish and reduced tolerance.
- Preventing Tool Damage: Excessive stress can fracture the cutting tool or cause it to dislodge from the holder. This damages the tool and workpiece.
- Improving Tool Life: Optimized stresses extend cutting tool life by reducing abrasive and adhesive wear.
- Eliminating Chatter: Alternating cyclic stresses from vibration leads to chatter which affects surface finish and precision.
- Increasing speeds/feeds: Judicious stress reduction allows more aggressive machining parameters.
- Avoiding Residual Stresses: Controlled stresses prevent undesirable residual stresses in the machined components.
Strategies for Controlling Stresses in CNC Machining
Here are some proven strategies that CNC programmers and machinists employ to minimize tensile and compressive stresses during machining:
- Optimizing Fixturing: Use larger clamping surfaces, more clamping points and positive clamping methods to reduce fixturing stresses.
- Choosing Suitable Materials: Select workpiece materials with appropriate strength to withstand cutting, clamping and thermal stresses.
- Analyzing Stresses: Use FEA software to identify sections with highest stresses. Modify part design, supports and machining sequence to reduce stresses.
- Employing Stock Allowance: Provide extra stock allowance on the casting or forging to account for workpiece deformation under stress.
- Using Stiff Machines/Tooling: The machine tool and cutting tool holder should have high static and dynamic stiffness to minimize deflection under load.
- Applying Coolant: Use high pressure coolant directed at cutting zone to minimize thermal stresses.
- Controlling Chiploads: Optimize feed, speed and depth of cut to maintain suitable chiploads that prevent excessive cutting forces and vibration.
- Balancing Operations: Balance turning and face milling operations to equalize tensile and compressive stresses.
- Choosing Suitable Tool Geometry: Select inserts with optimized rake angles, edge preps and chipbreakers to lower cutting forces and vibration.
By properly analyzing and controlling stresses in the CNC machining process, manufacturers can fabricate high precision components faster, with superior finish and longer tool life. This allows optimizing the production efficiency. CNC Milling