Tips on Designing Parts for Machining​

In this article we’ll go over some of the top tips on how you can improve your designs and decrease cost while optimizing your parts for manufacturing using CNC machining processes.

A set of CNC machined parts on a workbench in front of a CNC machining center

Here are some design guidelines for machining:

  • We know this may sound unintuitive coming from a CNC machining service but, if possible, parts should be designed such that they do not need machining. If this is not possible, then minimise the amount of machining required on the parts. In general, a lower-cost product is achieved through the use of net shape processes such as precision casting, closed die forging, or (plastic) molding; or near net shape processes such as impression die forging. Reasons why machining may be required include close tolerances; good surface finish; and special geometric features such as threads, precision holes, cylindrical sections with high degree of roundness, and similar shapes that cannot be achieved except by machining.
  • Tolerances should be specified to satisfy functional requirements, but process capabilities should also be considered. Excessively close tolerances add cost but may not add value to the part. As tolerances become tighter (smaller), product costs generally increase because of additional processing, fixturing, inspection, sorting, rework, and scrap.
  • Surface finish should be specified to meet functional and/or aesthetic requirements, but better finishes generally increase processing costs by requiring additional operations such as grinding or lapping.
  • Machined features such as sharp corners, edges, and points should be avoided; they are often difficult to accomplish by machining. Sharp internal corners require pointed cutting tools that tend to break during machining. Sharp external corners and edges tend to create burrs and are dangerous to handle.
  • Deep holes that must be bored should be avoided. Deep hole boring requires a long boring bar. Boring bars must be stiff, and this often requires use of high modulus materials such as cemented carbide, which is expensive.
  • Machined parts should be designed so they can be produced from standard available stock. Choose exterior dimensions equal to or close to the standard stock size to minimise machining; for example, rotational parts with outside diameters that are equal to standard bar stock diameters.
  • Parts should be designed to be rigid enough to withstand the cutting forces and workholding clamping. Machining of long narrow parts, large flat parts, parts with thin walls, and similar shapes should be avoided if possible.
  • Undercuts should be avoided because they often require additional setups and operations and/or special tooling; they can also lead to stress concentrations in service.
  • Materials with good machinability (e.g. EN AW-6082-T6 aluminium) should be selected by the designer. As a rough guide, the machinability rating of a material correlates with the allowable cutting speed and production rate that can be used. Thus, parts made of materials with low machinability cost more to produce. Parts that are hardened by heat treatment must usually be finish ground or machined with higher cost tools after hardening to achieve final size and tolerance.
  • Machined parts should be designed with features that can be produced in a minimum number of setups – one setup if possible. This usually means geometric features that can be accessed from one side of the part.
  • Machined parts should be designed with features that can be achieved with standard cutting tools. This means avoiding unusual hole sizes, threads, and features with unusual shapes requiring special form tools. In addition, it is helpful to design parts such that the number of individual cutting tools needed in machining is minimized; this often allows the part to be completed in one setup on a machine such as a machining center.

At CNC Proto we specialise in the machining of non-ferrous metals, mostly 6000 and 7000 series aluminium alloys, and engineering plastics. You can see a list of our most used alloys in our Services page.

For more articles on CNC machining, materials and design of machined parts, be sure to check our Resource Library.


Fernando Duarte Ramos

Written by

With an MSc. in Mechanical Engineering, Fernando has worked 8 years at CERN designing and building the next generation of particle detectors alongside top European engineers and physicists. Being passionate about CNC machining, he started CNC Proto, an online CNC machining service in Europe dedicated to providing fast turnaround, high quality parts at a competitive cost. Learn more About Us

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