It is often the case that precision instruments are expected to operate within certain limits. Even though a component may appear to be produced to exact dimensions, the slightest change in size can impact its performance.
This is why tolerances are important in instrument engineering. They determine how much variation is allowed in a dimension and enable the various parts to function together as intended.
What Is a Tolerance?
A tolerance is the amount of variation that is allowed in a given dimension.
There is no way to manufacture each component to an identical dimension. Instead, engineers establish an acceptable range within which a part will perform properly. For example, a shaft can be designed to have a particular nominal diameter, but allow some variation in size.
Why Do Precision Instruments Need Tolerances?
In regular machinery, a minor change in a dimension might not be significant. In a precision instrument, however, the same difference could impact accuracy, motion, alignment, or calibration. Gears, shafts, sensors, brackets, and other adjustment parts may require a certain degree of precision to work properly.
Using appropriate tolerances can minimize movement, binding, misalignment, or inaccuracies.
Tolerances and Production
Tolerances must be compatible with the method of manufacture. A design that includes very narrow tolerances may demand special tooling, more inspection, or more expensive production methods. Unless this level of precision is essential to the function of the instrument, it can add cost without adding value.
Thus, good engineering practice dictates using the precision that is truly required.
Dimensional Tolerances
Dimensional tolerances apply to linear dimensions such as length, width, height, diameter, thickness, depth, angle, etc. Different parts may have different requirements for tolerances depending on their role.
A bracket or enclosure might not need to be dimensionally as precise as a part that is integral to a measuring system.
Geometric Tolerances
Tolerances are not only a matter of size. A part can be the correct size and still be in the wrong location or shape. Geometric tolerances can specify:
- Straightness
- Flatness
- Roundness
- Parallelism
- Perpendicularity
- Position
Such specifications assist engineers in controlling the relative position and orientation of parts.
Tolerance Stack-Up
When multiple parts are put together, each part’s individual tolerance can contribute to the overall tolerance of the assembled piece. This is called tolerance stack-up. Even if all parts are within their respective tolerances, the combined tolerances can result in an assembled unit that doesn’t meet expectations. Engineers evaluate these interactions during the design phase to make sure the finished instrument works as expected under the range of tolerances.
Measuring Precision Parts
Manufacturing high-precision parts requires measurement tools capable of detecting small variations. Depending on the accuracy required, engineers can measure parts using calipers, micrometers, coordinate measuring machines, optical measurement devices, go/no-go gauges, or other tools. The measurement method itself needs to be accurate enough to detect the desired level of precision.
Tolerances and Quality Control
Tolerances are linked to quality control. Testing or inspection determines whether a part meets the established tolerances. However, quality control isn’t just about making every part as precise as possible. A part that meets the design intent might be better suited for production than a part that has been manufactured to tighter tolerances than necessary. Quality control, then, is about ensuring that the parts meet the design requirements.
Tolerances and Assembly
Tolerances can also help facilitate assembly. The parts should fit together properly, but still allow for any necessary movement or adjustability. Too much clearance can lead to slop or looseness in the assembly. Too little clearance can make assembly difficult or result in binding. The design of a tolerance scheme is a delicate balancing act.
Tolerances and Environmental Factors
Finally, tolerances should take into account environmental conditions. Changes in temperature, vibration, material expansion, wear, and other factors can affect the dimensions of a part over time. An instrument designed for use in a controlled environment might have different tolerances than one meant for industrial use. Tolerancing is thus about taking into consideration the full operational life of the instrument, not just the initial design.
Conclusion
Tolerances bridge the gap between design intent and actual production. They define how much a dimension can vary and still work properly. For precision instruments, the right tolerances ensure accuracy, ease of assembly, performance, and affordability. It is not about achieving perfect parts, but rather, achieving the right precision.