Tensile vs Compressive Stress in CNC Machining(manufacturing drawing Cheryl)

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Computer numerical control (CNC) machining utilizes automated machine tools like mills, lathes, routers, and grinders to shape metal, plastic, wood, foam, and other materials. During the machining process, the material experiences different types of stresses that impact the final product quality. Understanding tensile and compressive stresses is key for CNC operators to avoid defects like cracks or distortions.
Tensile Stress
Tensile stress occurs when a material is pulled in opposite directions, creating elongation. This type of force tends to stretch the material thinner. Tensile stress is measured in units of force per unit area, such as pounds per square inch (psi) or megapascals (MPa).
During CNC machining, tensile stresses commonly occur when holding the workpiece, gripping with cutting tools, or separating the finished part from the raw material. For example, when securing a workpiece to the machine table using clamps or vices, the clamping force puts localized tensile stress on the material. As the cutting tool presses into the material, it induces tensile stress around the cutting zone. Similarly, parting off a finished part from the remaining stock applies tensile force perpendicular to the parting line.
Excessive tensile stresses can cause undesirable results in CNC machined components:
- Plastic deformation - Stretching the material beyond its elastic limit causes permanent elongation and reduced thickness in those areas. This can lead to workpieces being out of tolerance.
- Necking - Localized reduction in cross-sectional thickness of the material when pulled. Necking exacerbates non-uniform plastic deformation.
- Cracking - When the tensile stress exceeds the ultimate tensile strength of the material, cracks form and propagate. Brittle materials like cast iron are prone to cracking under tension.
- Workpiece slipping - Insufficient clamping force leads to loss of friction, allowing the workpiece to slip and move during cutting operations.
- Tool chatter - Chatter primarily occurs from vibration, but excessive tensile stress in the workpiece can also enable dynamic twisting that contributes to chatter.
Compressive Stress
Compressive stress pushes inward on a material rather than pulling outward. Compressing makes the material shorter in one direction and causes bulging in other directions. Like tensile stress, compressive stress uses units of psi, MPa, etc.
In CNC machining, compressive stresses mainly occur from the pressure of cutting tools pressing into the workpiece to remove material. The localized heat and friction in the shear zone adds to the compressive force. The material beneath the cut tries to flow away from the tool, creating internal compression.
Potential defects from excessive compressive stress include:
- Plastic deformation - Exceeds the yield strength, causing permanent compression and distortion after the forces are removed.
- Buckling - Long, thin features on a workpiece may buckle and bend under compressive loads.
- Cracking - While rare, excessive compressive force can theoretically surpass the material's ultimate strength and cause subsurface cracking.
- Tool wear - Pushing harder into the material accelerates tool wear. The heat and friction from compressive forces degrade the tool cutting edges.
- Chatter - Similar to tension, localized vibrations from compressive release can enable chatter while machining.
Minimizing Tensile and Compressive Stresses
Here are some methods CNC machinists use to control tensile and compressive stresses during machining operations:
- Optimize toolpaths - Program toolpaths to balance forces and minimize rapid direction changes that induce stress spikes. Avoid unnecessary tool lifting.
- Adjust feed/speed rates - Higher feeds and speeds exert greater forces. Reduce rates to lower forces. But don't sacrifice cycle time efficiency.
- Use proper workholding - Clamps, vises, fixtures, and jigs should hold workpieces securely without exerting excessive clamping forces.
- Employ indexable tooling - Indexable inserts with multiple cutting edges handle compressive forces better than solid end mills and reduce tool changes.
- Choose suitable materials - Pick workpiece materials that have higher strength to withstand greater stresses without plastic deformation.
- Utilize peck drilling cycles - Peck drilling involves a series of rapid drills and retracts to clear chips. This intermittent cutting action lowers temperatures and drilling forces.
- Minimize overhang - Hold workpieces as close to tool contact as possible to reduce vibration and leverage forces. Overhang admits instability.
- Apply coolant - Coolant washes away heat and chips to decrease cutting forces. This lubricative effect relieves compression and friction.
In summary, understanding tensile vs compressive stress empowers CNC operators to make adjustments to avoid workpiece or tool damage.Analyzing the stress mechanics during programming, setup, and machining helps guarantee better quality outcomes. With proper stress control, manufacturers can push material properties to the limit while retaining part accuracy and precision. CNC Milling CNC Machining