Peter Jacobs explains why small dimensions are a big deal in manufacturing
Precision manufacturing is based upon manufacturing parts consistently that have dimensions with strict tolerances, typically in the range of microns. Such unprecedented precision is achievable by employing competent CAD/CAM digital equipment alongside high-speed cutting tools.
However, manufacturers must account for practical considerations on such a small scale that extends beyond the equipment used, considering that all materials naturally expand and contract as the environment changes. Therefore, achieving the desired levels of tolerances in the micron range can get tedious. This scenario holds for all significant manufacturing processes, be it CNC machining, pressure die casting, injection moulding, or 3D printing.
Tight tolerances - why do they matter?
Tolerance is a numerical range of measurements associated with the dimensions of a component. It indicates the level of deviation that the manufacturing team can afford from the ideal values.
Some components demand extremely tight part tolerances, where a deviation of merely 0.2mm can render them unusable. Such wastage of parts can be avoided by adhering to appropriate tolerance values for your components during the initial design process.
A robust dimensional tolerance analysis process allows you to determine your component’s optimal tolerances, thereby ensuring that your parts are manufactured efficiently, accurately and offer extensive functionality.
Significance of small dimensions in manufacturing
The impact of small dimensions on your finished components can be examined better by first considering the actual scale of a micron. Its extremely tiny scale makes it evident that seeking tighter dimensions is financially expensive, impractical, and typically undesirable.
The true scale of a micron
To fully wrap your head around the actual size of a micron, you must juxtapose it with other measuring units such as a metre since the powers of 10 can be used to scale down to a millimetre and, eventually, a micron. These dimensions are typically the most relevant for precision manufacturing.
Bringing down three powers of 10 narrows the scale down to a millimetre, which is the same scale used to measure a large bacteria or a salt grain. The millimetre count is even etched into a precisely graduated linear scale. However, naked human eyes cannot see one-tenth of a millimetre, which is 10-4 of a metre. This number, being already quite insignificant, is still larger by a whopping 100 times compared to a micron, which stands at 10-6.
When dimensions shrink down to the size of a micron, manufacturing processes clash against the physical barriers of accuracy. Under such circumstances, thermal expansion and contraction of the workpiece, and the vibration in the cutting tool, substantially influence the finish of the final product.
Comparing a micron with natural phenomena
The microscopic scale of a micron can also be emphasised when contrasted against natural phenomena. For instance, a micron is even smaller than the wavelength of microwave radiation and on the same scale as a beam of violet light. On such a small scale, trivial variations in energy considerably change the physical dimensions.
Therefore, when a manufacturer can make a CNC-turned part that offers a tolerance of +/- 10 microns, they are meticulously splitting a strand of human hair into 20 or more layers. Therefore, small dimensions can be troublesome to deal with for manufacturers.
Small dimensions - a manufacturer's dilemma
To work on minuscule dimensions as low as a micron, painstaking workmanship besides stringent environmental restraints are paramount. Such extreme conditions substantially elevate costs and increase production time. However, exacting tolerances can cause performance hindrances in several applications and are implausible to achieve even on the most efficient CNC machine.
This situation occurs because most products must be able to perform motion freely. Most components must slide past one another, twist in unusual ways, open or shut, and be flexible enough to fit alongside other parts.
The impracticality of small dimensions
If a part is too precise down to the microns, it will bind and seize up. Friction would adversely affect its movement. Besides, it will instantly fall out of its precision when exposed to extreme temperatures and even dirt, fingerprints, or water vapor. Such parts are almost impossible to service or operate, thereby making them impractical. Moreover, trying to achieve unrealistic precision in a particular area of a component leads you to lose it in the other. This scenario happens because parts must be clamped in holding fixtures during machining, which disfigures some dimensions. This caution also applies to cast or injection moulded components that require free space for release from their dies.
The sweet spot for dimensional accuracy
Typically, 10 microns is a reasonable number for precision bearings, shafts, and most rotational parts. CNC milling of metals seamlessly delivers 20~50 microns, which is acceptable for structural components and enclosures. Manufacturers can enjoy some leniency in terms of tolerance values for plastic injection moulded components.
With the evolution of technology and the increasing demand for precision in the manufacturing sector, it has become crucial to deliver tight tolerances. Such a leap is achievable by rendering extreme dimensional accuracy, which is often on the microscopic scale.
However, when things are scaled down to such minuscule levels, the job of delivering unprecedented precision in the manufacturing process becomes counterproductive. Factors such as lack of flexibility and extreme friction make such small dimensions impractical to be employed in the manufacturing sector.
Peter Jacobs is with CNC Masters.