Minimize warpage when CNC-machining aluminum parts

When machining materials such as aluminum, internal stresses can create warped parts and in some cases result in cracking after machining. That’s why aluminum manufactures such as Alcoa™/Arconic™ have developed processes to temper internal stresses, and stress-relief methods to minimize them.

Although these internal stresses are minimized by the mill, they can still exist. So industry has developed methods to further improve part accuracy by utilizing some of the following guidelines. (These guidelines are used for dimensionally critical parts, such as components used in aircraft, missiles, space vehicles, telescopes and satellites.)

Some methods to minimize the effect of internal material stresses

1. Pre-qualifying methods

1. Pre-qualify the raw material with ultra-sonic testing
   When designing and selecting aluminum, typically Alum Alloys 7000 series (7018, 7049, 7050, 7075) are ultra-sonically tested for void and defects from the mill.If your requirement uses 6000 series Alum (6061), it is not mill US tested and can be performed for an upcharge if it is required. This is one of the reasons 7000-series materials are typically double the cost of 6000 series materials. (For more info see Ultrasonic Testing of Wrought Metals ASTM-E2375, AMS-STD-2154)
2. Pre-qualify the raw material by measuring internal stresses
   Measure the internal stresses with the X-Ray diffraction method. This benchmarks the provided    raw material and provides comparison for pre- and post-stress relief.

2. Machining Methods

1. Light cuts
   Light cuts with relatively small diameter cutters at high RPMs and feed rates minimize impact  stresses.
2. Low-stress machining strategies
   Strategies such as spiral paths, rather than incremental step downs, minimize the cutting tool  impact of each step down and step over. The tool is in a constant load rather than creating multiple impacts during each tool path. Another way of stating this is “continuous tool  engagement vs an interrupted cut”.
3. Non-compressive part holding
   Use a method of holding that minimizes compressive or tensile forces on the part. Use tabs,  zero-point, vacuum, magnetic, top clamp or other method.
4. Multi-flip or release hold
   This  method is used for all material and is not limited to aluminum. This method has oversize  rough, semi-finish and finish passes. It requires unclamping the part and flipping for each  operation.
5. Centrally located
   The part should be made in the middle of the raw material. This naturally balances the internal  stresses. This may require slightly larger stock thickness, but will help minimize warpage  significantly.

3. Process methods intertwined with machining methods

1. Surface stress-relieving with bead blast /shot peen
   In the oversize part condition, another method is to normalize surface stress with a glass bead blast of shot peen. This is typically done uniformly for best results.
2. Stress Relieving/Heat Treated
   This method can be done with three techniques: Thermal, Mechanical and Thermal Mechanical.  Thermal methods are typically cryogenic with an uphill quench. Mechanical methods are tensile  or compressive in nature.

Less internal material stress. Life-Saving Precision.
This only a glimpse of the thinking and techniques used by Protomatic to minimize material stress and optimize the manufacturing of your critical aluminum parts. It’s another way we show our commitment to delivering quality, efficiency, and Life-Saving Precision in every part and device we produce.

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About the Author
Doug Wetzel is Vice President and General Manager of Protomatic, a CNC precision machine shop serving the medical and aerospace industries.