A machinist spends days setting up and running a complex five-axis job. The part returns from the finishing department beautifully painted, anodized, or plated. It comes as a surprise when quality control finds the mating surfaces are too thick and the critical bore is too small - the once-perfect dimensions are now out of tolerance, effectively rendering the part scrap.
Computer numerical control (CNC) surface finish tolerances are often measured in microns, meaning even a thin layer of material can ruin painstaking machining work. Engineers, computer-aided manufacturing (CAM) programmers and machinists must account for every single step - including finishing - from the very beginning.
Achieving tight tolerances with 5-axis machining
Positioning accuracy is critical in five-axis machining. Even a minute deviation in axis alignment can ruin part geometries, resulting in a poor fit or assembly failure. This becomes a problem when designers set tight tolerances without considering necessary allowances.
Residual stresses from machining forces and heat build-up can create microscale cracks and increase surface roughness, making parts unsuitable for precision applications. Spindle speed can also increase the nonconformance rate.
On a five-axis machine, a high-speed CNC spindle can reach milling speeds of 25,000 to 90,000 rotations per minute. While this reduces cycle time by six to 10 times compared to standard spindle speeds, a high RPM can induce micro vibrations that create irregular tool marks, thereby degrading surface finish.
Five-axis CNC machines can operate continuously with minimal downtime. However, minor miscalculations stemming from a lack of coating allowances make it challenging to meet tight tolerances, increasing the likelihood of perfectly machined parts becoming little more than scrap.
Unmodeled surface coatings can introduce a layer of material that was not accounted for in the original design and machining plan. It can range from a few microns to several millimeters, ruining tolerances. Professionals must not take up the entire tolerance. There should be enough wiggle room for functional coatings.
The tolerance stack-up
Engineers that program or machine parts for assembly tend to make CNC surface-finish tolerances tighter than necessary to avoid tolerance stacking problems. While over-tolerancing seems like a practical solution to tolerance stack-up, it unnecessarily complicates CNC programmers’ jobs, increases set-up time and limits tooling options.
Tolerance stack-up represents the cumulative effects of part tolerance in an assembly. If a standard anodized layer adds 0.001 inches of material to a high-precision aerospace part with a total tolerance of +/- 0.0005 inches, the part becomes scrap instantly.
Because five-axis machining typically is used for complex, multisided or curved surfaces, an unmodeled coating will not be uniform. The sophisticated toolpaths rely on a part’s geometry being exactly as modeled. A tool that encounters unexpected thickness will not cut in the intended location, resulting in dimensional inaccuracies.
Additive vs. penetrative finishes
Although finishing mistakes can be costly, the process is essential: unfinished metal is vulnerable to chemicals, moisture, ultraviolet radiation, and corrosion. There are dozens of types of metal finishing. Powder coating is among the most popular for fabrication projects, as it provides uniform coverage, even on complex shapes.
Additive finishes add a layer to a part via spraying, electrostatic discharge or dipping, increasing the outside dimensions. Common application processes include painting, plating and powder coating. Penetrative coatings - applied via anodizing or chromate conversion coating - fully integrate into the substrate, requiring smaller surface finish tolerance adjustments.
In precision machining processes, the toolpaths are generated based on a CAD or CAM model of the final part. If the surface coating’s thickness is not accounted for the added material can easily push the part's dimensions outside of the acceptable range.
For instance, after coating a precisely machined hole can become smaller, while an external dimension becomes larger. To ensure accuracy, simulation software must differentiate between coatings that sit on the surface and those that penetrate the substrate.
The blame game that ensues when parts fall out of tolerance pits engineers, programmers and finishing professionals against each other when the real culprit is a mathematical flaw. For five-axis machines, achieving high accuracy is challenging due to the complexity of coordinating the movement of two rotary axes and three linear axes.
