Guide To Make Tight Tolerance CNC Machining Parts

cnc machining tolerances

Achieving tight tolerance parts with CNC machining is no small feat. For small parts, one encounters thermal and mechanical damage to the material, difficulties chamfering and deburring, and an ineffectual drill the deeper one bores. This article shows you how to overcome most of these challenges and get a tight tolerance with CNC machining.

What are Tight Machining Tolerances?

Tolerances are the degree to which a machined part adheres to design specifications or measurements. The closer the adherence, the tighter the tolerance. Typically, tight tolerance ranges from +/-0.002 to +/- 0.0001 inches and are only made possible with CNC machining.

Tight tolerance machining improves fit and function and reduces underperformance and part failure, but it is time-consuming and costly. The nature of the material and part complexity cause significant deviations, especially when many parts are required. As the tools wear out, they lose precision and require constant readjusting and refitting.

Tight tolerance parts are only necessary for a limited number of applications, notably in the aerospace and medical industries.

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Factors affect to Achieve Tight Tolerance in CNC Machining

To achieve tight tolerances, it is essential to understand some of the factors that affect tolerance. These include:

  • Hardness of the material

How hard a material is affects the ability to achieve tight tolerance. Common materials for tight tolerance machining include stainless steel, titanium, aluminum, plastic, and HDPEs. Being harder, materials like stainless steel and titanium are harder to mill, cause more vibrations, generate more heat, and wear out cutting tools faster – all of which promote deviation from design measurements.

Softer materials like plastic may be easier to mill but are susceptible to thermal alterations, which affect tight tolerance. HDPEs, on the other hand, flex easily during machining, which also undermines tolerance.

  • Complexity of the part

It is hard to achieve tight tolerance with comlex designs. Complex parts may require multiple machining processes involving measurements, programming, and setups. All these increase the likelihood of error. For instance, milling along three axes is simpler and more accurate than milling along five. Milling along more than five axes requires more than one setup, which may sacrifice accuracy.

Increased complexity also makes it harder for the tool to trace the right path, making it necessary to be vigilant with measurements during machining.

  • Size of the part

Part size also affects tolerance. For instance, thermal damage to the material is more destructive for smaller parts than larger ones as they heat up faster. Smaller parts also offer smaller surfaces to work on, which may be inappropriate for standard-sized tools, especially if the design is complex.

Conversely, larger parts come with greater complexity and the associated challenges, as well as the potential for unnecessary vibrations, which also throw out measurements. Because of the larger surfaces, cutting and milling tools wear out faster, causing measurement deviations. All these make tight tolerance harder to achieve.

  • Accuracy of measurements

The more accurate measurements are, the tighter the tolerance that can be achieved. The +/- signs in front of the dimensions are essential for conveying the deviation allowable and must be considered. Similarly, complex designs may require collaborative efforts to machine, which makes communication concerning measurements paramount if you wish to achieve tight tolerance.

Typically, measurements come in millimeters and inches. Sometimes calibrating in millimeters rather than inches may offer tighter tolerance.

  • Precision of tools

Tool precision directly correlates with adherence to design specifications. However, due to wear and vibrations, tools will likely lose accuracy during machining. Also, though tools are calibrated well, they may not be as precise if machining an inordinately smaller surface area.

  • Method of machining

Different machining methods offer different tolerances. The most popular CNC machining method is Geometric Dimensioning and Tolerancing, which provides the best tight tolerance along different axes and facets of the part.

GD&T puts you in total control of various elements of the part design specifications, leaving little room for deviation.

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Tips to achieve tight tolerances in CNC machining

Here are the tips to achieve tight tolerances in CNC machining:

Run machines on the first floor

CNC machines generate significant vibrations, especially when working on large parts. To reduce the effect of such vibrations on tight tolerance, install the machine on a firm surface. The firmest surface is usually on the first floor of a building. Also, ensure the machining plant is firmly secured through periodic checks.

Control workshop temperature

Tight tolerance means sometimes the deviation is no more than half a hair’s breadth. Thermal fluctuations in the material and the tools increase deviation from design specifications. To achieve such dimensions, you must completely control all the parameters, including temperature.

Control workshop temperature by placing the machine where the AC or direct sunlight will not affect it. Warm the machine before using it. Also, ensure no significant difference in temperature in the machine and material when you begin working.

Use quality CNC machines

Quality CNC machines give you greater control over design parameters, making achieving tight tolerance easier. You find it easier to program in specifications, which are more stable while in use. Upgrade to take advantage of the technological advances in CNC machine efficiency.

Choose high-quality tools and cutting fluids

CNC machining imposes a lot of stress on the cutting tools. Inferior quality tools rapidly wear out on tough surfaces like titanium and stainless steel and damage easily under thermal stress. As their tips grind down and lose their edge, they lose their calibration, making it harder for you to achieve high tolerance. Besides, they may damage the material and the CNC machine itself.  

Do not skimp on the quality of the cutting fluid, as it plays a significant role in cooling the high temperatures generated during machining, which affect the integrity of the material and tool, causing deviations from design specifications.

Using high-precision fixtures

Calibrate your fixture so that you can get higher precision. Using the metric system, for instance gives you more variations on the machine than you get with inches. Such fine resolutions help you achieve a tighter tolerance. Also, in calibrating your fixtures, account for the inevitable deviations as the machine runs and heats up.

Use good material supplier

Tight tolerance machining requires that you begin with superior materials that will hold under machining and will not damage the machine. Though the compositional structure of most materials is guaranteed by certification, minute variations in the manufacturing process mean they behave differently under the machine.

Establish a solid relationship with the supplier whose product meets your specifications.

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Implement strict quality control measures

Making tight tolerance machining parts is a costly endeavor. Besides, the need for a tight-tolerance part may be so absolute that an entire engineering project may depend on it. Quality control is imperative right from the start of the process.

Strict quality control measures will help you prevent and catch mistakes before they can become catastrophic. Quality control should include even an on-machine inspection system that alerts of deviations during the machining process.

Other considerations

  • Experienced engineers – CNC tight tolerance machining is a precise art that begins from the design stage. Work with experienced engineers who can design with an understanding of the potential and limitations of the CNC machines.
  • Well-trained operators – well-trained operators know how to program and operate the CNC machine by selecting appropriate tools to achieve tight tolerance. Besides, they can make adjustments to keep true to the design and carry out ordinary maintenance.
  • A strong quality control department –  The quality control department ensures that parts that do not meet specifications are not delivered to the client and protects your reputation as a practical, tight-tolerance machinist.
  • Communication – CNC tight tolerance machining is a collaborative process between the client, design engineers, and plant operators, who are often not in physical touch. They all must be adept at relaying, receiving and interpreting the information contained in the designs. Effective communication between these entities is essential for success. It is upon you to facilitate communication between all of them.
  • Program software – There is software that lets you enter various aspects of the design into the CNC machine for execution. This software enables you to choose a machining method and lay out an execution path for the different tools. Note, however, that the software has different capabilities, and decide what suits your design purposes best.
  • Workshop operating software – A centralized workshop operating software helps you manage various aspects of your enterprise, leaving you free to focus on tight tolerance machining with tremendous success.
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The advances in CNC machining technology offer unparalleled benefits in tight tolerance parts machining. With CNC machines, one can produce high-precision parts of uniform quality in bulk. However, human intervention in the form of skill and quality management is indispensable in making the process possible.