How To Achieve and Maintain Tight Tolerances In Part Production

Tight tolerances are not a machinist or workshop’s favorite words. Unless critical to design and functionality, part fabrication workshops and production centers will often advise against designing parts with tight tolerances.

When we receive a part CAD file and notice tight tolerances, we carry out some operations to ensure that we deliver client parts according to specifications. Here are some of our best recommendations for achieving precision and high dimensional accuracy when designing or manufacturing parts with tight tolerances. 

First, What is Tight tolerance?

The term “tight tolerance” is one of the most loosely-used words in the manufacturing industry. It simply refers to a design or production system where the part fabricator and machine shop have been restricted to the lowest possible deviation from the client’s specified part dimensions.

The term “tolerance” itself is used to denote the acceptable deviation (+/-) from the intended part dimension that can be considered acceptable. For instance, if a part is to measure 5mm and has a tolerance of 0.1mm, the finished part is deemed to be acceptable if it measures 4.9mm or 5.1mm in its dimensions. Metrically speaking, a design or part is said to have tight tolerances when its acceptable deviation (tolerance) is 0.005 inches or below. 

Why are tolerances critical?

Tolerances serve as a guideline to the machinist to ensure that parts are made to accuracy. Because of part production’s size and technicalities, an allowance is made for deviations from the specified dimensions. However, these deviations cannot be excessive as certain parts will underperform, lose functionality, or completely fail. This part design failure can lead to tooling overhaul, budget depletion, and business losses arising from custom dissatisfaction.

What are the advantages of tight tolerances?

The key advantages of keeping tight part tolerances include improved fit, enhanced assembly, elimination of post-processing operations, improved functionality, increased success in procuring mating parts, part reliability, and high dimensional accuracy.

What are the disadvantages of tight tolerances?

While there are several benefits of using tight tolerances in your design, the downsides manifest in the production costs and leadtimes for your part. Because of the level of accuracy required, the need for manual intervention, and on some occasions, special tooling, insisting on tight tolerances will ultimately make your part more expensive.

How to achieve tighter tolerances  

·     Perform a design-for-manufacturability (DFM) analysis

Firstpart offers a free design-for-manufacturability analysis for every project through our instant quoting system. A DFM analysis is the single, most effective way to ensure that tight tolerances are intuitively included in your design. Taking corrections and suggestions on your part design will not only reduce cost but ensure that realistic and repeatable tight tolerances are achieved in your part design. As factors like geometry, shrink rates, warp, overall size, wall thickness, and even material choice can influence tolerance control, a lot of emphases must be put into the product and process design to ensure that achievable tolerances and design parameters are agreed.

·     Calibrate your machine

After your part has been optimally designed, the next step is to calibrate your machine before running production. When possible, refer to the manufacturer or service hubs to have your machine serviced and calibrated occasionally. For CNC machines where there is minimal deflection, this may be done once a year. With laser calibration systems, you can return your machine axes positioning and measuring systems to new-like states and ensure improved dimensional accuracy before running your operations. Calibrations also tension belts, tighten bearings, and enhance the drive motors – all of which are crucial to machining accuracy. 

·     Perform warm-up operations

Warm-up operations are normal processes designed to get your machine to the base conditions to operate at least at normal levels. Warm-ups are essential for preparing fluids, lubricating systems, and tooling systems. Machine warm-ups will help to minimize sluggishness in machine operations and prime the system for operation. This will also reduce any dimensional changes that will occur as the machine temperature and performance rise during production. Allow your machine to run for some 10-15 minutes with all moving components active. Running your machine beforehand also helps to mitigate the effect of thermal expansion during operation. After your warm-up is completed, be sure to measure all your tools for precision.

·     Process design and control

Process design and control is essential if any machine shop is to achieve tight tolerances. When working with unforgiving tolerance specifications, factors like thermal stabilization, pressure, and environmental temperature, must be controlled to ensure that your final part comes out in the desired dimensions.

Process parameters can cause the workpiece or machine to change dimensionally. The workpiece and material temperature are also important. Ensure that the workpiece temperature has stabilized around the machine’s temperature to avoid shrinking, warping, and expansion during machining or molding. For injection molding, parameters like number and location of gates, air trap prediction, material flow analysis, flow, cooling, pack, weld line location, cavity/part temperature differences, coolant flow rate, shrinkage, and vent locations can affect the dimensional accuracy of the final part. Finally, ensure that the workpiece is secured correctly or clamped before production commences. This will limit the extent of vibrations during production.

·     On-machine inspection

Using an on-machine inspection system can help you achieve on-the-go quality control. With a digital probe that measures the part as it is being machined, you can see live deviations from the desired dimensions and achieve immediate corrections before finalizing machining on the workpiece.

·     Tooling matters

Tooling plays an inarguable role in achieving tight tolerances. Consider the tool geometry, length, size, number of flutes, sharpness, impact tolerance, cavitation, design, and tool material before you commence your production runs.

Tool sharpness and quality can affect the repeatability of tight tolerances. Tools that have become blunt are less efficient. They drain machine energy, increasing leadtimes, and prevent precision and accuracy in your cuts. For extreme tolerances, you must use the right set of tools, including different ones for milling, roughing, and finishing. Finishing tools must be reserved for making final passes, while the roughing tools should take the brunt of machining other parts of the product.

For injection molding, achieving tight tolerances is more of a challenge because of the heating and cooling of the tools and the cavity number of the mold. Tooling must be designed to make tight tolerances repeatable, as inconsistencies will result in insufficient cooling and varied shrink rates.

FirstPart Rapid Prototyping & Low-Volume Manufacturing in China

Looking to make your parts with tight tolerances? FirstPart is one of China’s leading manufacturing hub for Additive, CNC and conventional manufacturing techniques. In our array of manufacturing techniques, we carefully optimize our systems to achieve tight tolerances and make parts that adhere to client specifications for improved fit, functionality and overall quality.

We boast of excellent in-house capacity, labor force and logistics while delivering exceptional value for money. Our array of services include CNC machining, CNC turning, CNC milling, 3D printing, Rapid Tooling, Die casting, Rapid prototyping, Plastic Injection Molding, Urethane Casting, Aluminium Extrusion, Post-machining/Finishing services and much more.

As we understand the global challenges that is faced by new and existing businesses in these times of the Coronavirus, we offer product tooling, mass production, bridge tooling and low-volume prototyping/manufacturing with very flexible minimum order quantities (1 to 100,000). Our services are online, scalable and innovative, with a team of engineers and design experts available to support you through your entire product development cycle.