Posted June 15, 2021

Design Rules for CNC Machining: The Basics and Beyond

A part design according to design rules for CNC machining

The adage “rules are meant to be broken” may be true for some creative endeavors but don’t apply to the design of aerospace, medical, or other precision parts. The design rules for CNC milling and turning prevent the addition of unnecessary features or creating a component that is not manufacturable, saving time and money. The following includes information on design rules for CNC machining and how using them will help create better components.

Design Rules for CNC Machining and How They Apply

While CNC milling and turning can create many different contours and shapes in the parts they fabricate, the equipment has some limitations. For instance:

  • The cutting tool of a mill is unable to create right angles at the bottom of a hole.
  • Milling tools also have restrictions on how deep they can cut, and corner radiuses.
  • Mills have limitations on the sizes of parts they can work on.
  • Lathes are only able to produce round or cylindrical shaped parts.
  • The cutting tools of a lathe are restricted to only a specific range of motion.
  • The size and shape of a part may restrict how it is fastened to the mill or lathe, which could limit the approach angles of the tools.

If designers aren’t aware of these limitations, or if they don’t follow basic design rules and good practices, the manufacturing of the component may experience the following:

  • Delays: If the machine shop cannot produce the part as designed, they will have to resolve the problems with the designer before proceeding. Additionally, the resolution of the problems may force a redesign of the part. All of this adds time to manufacturing, which may disrupt the project’s development schedule.
  • Expense: Losing time due to design problems will add unexpected costs to the project budget. These expenses will continue to escalate if it requires multiple redesigns to correct the errors. At the same time, unnecessarily tight tolerances in the design will cause additional manufacturing expenses to perfect those tolerances.
  • Failure: In some instances, the part may not be manufacturable as designed, causing the machine shop to “no-quote” the job, resulting in its cancelation.

The key is to understand the cause and effect of the manufacturing process for a design. For example, including hollow spots in a part design may seem like an excellent way to reduce weight and save money. In actuality, though, you will be increasing the part’s fabrication costs, which may cost more in the long run. Similarly, understanding the capabilities of the machining equipment prevents designing something a manufacturer can’t build.

Unfortunately, no industry-wide standards exist for CNC machining design rules. The reason for this is because machining equipment and technology constantly improve to add new capabilities. Something impossible today may change tomorrow, nullifying a standardized design rule covering it. Also, machine shops often don’t want to assume additional liability by posting specific rules and standards. Shops will share their capabilities but don’t get involved in managing international rules and standards.

However, many CNC machining best practices serve well as design rules, as explained below.

The Six Basic Design Rules Engineers Should Know for CNC Machining

Many aspects of component design need to conform to the capabilities of the CNC mills and lathes used for manufacturing. Six primary design rules will help to create a fully manufacturable part.


Due to the cylindrical shape of the cutting tools, designers should draft milled corners with a radius. A common recommendation for this radius is ⅓ the cavity depth, or larger if the part will allow. Larger cutting tools reduce the time for machining and produce a higher-quality surface finish inside the part cavity.


Avoid designing tall thin walls in parts. The milling process can stress the material and cutting tools enough to cause bending and deflection due to vibrations. This vibration can create ripple effects on the walls’ surface or cause material defects such as chipping or fractures. It can also make it difficult to manufacture the part to the required tolerances, resulting in out-of-tolerance walls. Machine shops often recommend a minimum wall thickness of 0.8mm for metal and 1.5mm for plastic.


When possible, use standard drill bit sizes to avoid milling the hole, which takes more time. The maximum depth of the hole should be four times its nominal diameter, with deeper holes possible at increased manufacturing costs. When threading the holes, limit the maximum length of the threads to three times the nominal diameter of the hole. And remember not to design right angles at the bottom of the holes.


Minimize the machining of text on your part. Adding text also adds time and expense, and the smaller the text is, the higher the cost. Simple engraving is the most efficient option. Raised or embossed text requires removing a lot of material, and complex engraving takes time in the machine shop that is better suited for production.


Use a standard tolerance on your component’s 2D drawing, such as ± 0.003” for metal and ± 0.005” for plastics, and only use unique tolerances when required for the function of the part. Unnecessary, additional tolerances only add complexity, time, and expense to manufacturing.

Unnecessary machining

Don’t over-design the part. Adding superfluous features or cutouts adds manufacturing time and expense to the project.

How Your CNC Machine Shop can Help with the Basic Design Rules and Beyond

As stated above, many design rules and guidelines will help increase the manufacturability of a new part. Partnering with a machine shop will apply these best practices to ensure that the manufacturing of a component goes as smoothly as possible. For example, it isn’t unusual for designers to update a CAD model without changing the accompanying documentation. The resulting inconsistency of design data can create confusion and slow down the part’s manufacturing. Working collaboratively with a machine shop to develop robust design rules and best practices saves time and money, ultimately designing a better part.

At Plethora, our account managers have strong engineering backgrounds and use those skills to consult with all of our customers on their projects. Whenever possible, we encourage clients to share their entire project designs with us. This information gives us as much data as possible to help develop the best solution for each part. We are an ISO 9001 certified shop, and our primary goal is to manufacture your parts to the highest level of quality. Our online DFM and quoting systems receive your data to begin working on your next project immediately. To get started, upload your design files to Quote My Part or call us at 415-726-2256.

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The Plethora Team

The Plethora team is your go-to CNC manufacturer for hardware done right the first time. We have the tools and experience needed to create high quality custom parts quickly and with precision, whether you need a prototype or production run.

Topics: materials, finishing, CNC machining, Prototyping, quality design