How to Avoid Common Design for Manufacturing (DFM) Errors in CNC Machining?

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CNC (Computer Numerical Control) machining provides tremendous accuracy and flexibility when making complex individual parts, however, no amount of advanced design innovation will aid our production if we do not think about Design for Manufacturing (DFM) principles.

In CNC machining DFM mostly refers to maximizing the design for efficient, economical, and high-quality manufacturing. If DFM is not taken into account, it could result in extensive rework, delays, and compromises in part performance.

This article will discuss the common DFM mistakes in CNC machining and How to Avoid Common Design for Manufacturing (DFM) Errors in CNC Machining so that your designs flow smoothly from concept to reality.

How to Avoid Common Design for Manufacturing (DFM) Errors in CNC Machining

What is Design for Manufacturing (DFM) in CNC Machining?

Design for Manufacturing (DFM) is an approach to engineering, that seeks to enhance a product’s design to make manufacturability easier, decrease production costs, and improve product quality.

It can also be inferred that DFM in CNC machining encompasses assessing the capabilities and limitations of CNC equipment and tooling during the design stage.

If design incorporates manufacturability at an early enough stage, designers will be able to mitigate the chance of expensive rework, unnecessary material waste, and receive products to market sooner.

Importance of DFM in CNC Machining

Design for Manufacturing (DFM) is not just a cost-saving exercise but a total approach to developing a successful manufacturing process. You can achieve the following when you consider manufacturing limitations and capabilities in the product design stage:

  • Decrease manufacturing costs: Reduce material waste, machine time, and tool wear.
  • Enhance the quality of parts: Produce tighter tolerances, better surface finishes, and repeatable results.
  • Reduce processing time: Combine machining operations and minimize the number of setups and specialized tools.
  • Increase reliability: Design parts that are always within specifications and always work as designed.
  • Reduce rework and scrap: Can prevent costly problems on the shop floor with forethought.

Common DFM Errors and How to Avoid Them

Here are some of the most common DFM traps for CNC machining and useful how to tips to avoid them:

1. Designing with Unmachinable Features

There are several geometries that simply cannot be made or very difficult to made with a standard CNC machining process.

Error:

  • Sharp Internal Corners (0- radius): CNC milling tools are round making it impossible to manufacture perfectly sharp internal corners.
  • Deep Narrow Slots/Pockets: These require super long thin tools which are prone to vibration, deflection and breakage leading to poor surface finish and poor accuracy.
  • Complex Internal Channels/Cavities: Features like curved holes, helical channels or complicated internal lattices may be tricky or even impossible for standard cylindrical tools to reach and machine.
  • Undercuts requiring special tools: Undercuts are generally not a problem with undercut / T-cutters or lollipop cutters, but undercuts that are more complex, or those located in inaccessible areas detrimentally affect cost and lead-time.

How to Avoid:

  • Add radius to internal corners: Always use a radius for any internal corners. A general rule of thumb is to ensure the radius is at least slightly bigger than the smallest end mill you expect to use (ex, minimum 0.8 mm or 0.03 inches). The larger the radius, the better as they allow the use of larger, more rigid tools that require less machining time and cost.
  • Adjust Depth-to-Width ratios for pockets/slots: When designing pockets, limit the pocket depths at a maximum no greater than 3-6 times the smallest internal corner radius or tool diameter, depending on the material. Pockets that are too deep, may need to be machined using multiple passes, special tooling, or slower machining speeds.
  • Simplify internal geometry: Whenever possible, always simplify internal channels and cavities to allow for the ability to access the features directly with tools and provide straight-line paths for tools to travel through. In extreme cases, you could even consider an alternative manufacturing process such as EDM for very complicated internal geometry.
  • Design for tool access: Always consider the ability for standard cutting tools to access all features you’ve designed. Avoid any geometry that might obstruct the tools, or create overhangs or any other features that could limit access.
  • Use “Dog Bones” or Tear Drops on sharp Internal Corners: If you have a function-critical requirement for a near 90-degree internal corner, you might at least consider providing a “dog bone” or tear-drop relief that would allow a round tool to cut in (even though there is a radius) while allowing the main feature to maintain a square edge.
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2. Over-specifying Tolerances and Surface Finishes

Excessive tolerancing, or surface finishes for non-critical features of parts increase time, cost and the chance of a parts being rejected.

Error:

  • Unnecessarily Tight Tolerances: Tolerancing, such as ±0.001″ instead of ±0.005″.
  • Excessive Surface Finishes: Specifying a mirror finish, or a surface that is not visible, or a mating surface.

How to Avoid:

  • Specify Only Tolerances that are Functional in Nature: Specify tolerances when it is important to the function of a part, such as a mating surface and give the widest allowable tolerances dimensionally that don’t effect function.
  • Identify Critical Specs as Priorities: Show with arrows/dots in the drawings what particulars, or what features, which are critical for the overall function of the part.
  • Know Capabilities and Standards: Know what the standard default tolerances of CNC machines are (±0.13mm (±0.005 inches)). Tighter tolerances can be machined, but may cost more, take more time, require special tooling and may affect machine speed.
  • Aesthetics and Function: If the tolerances required for design feature are requiring a smooth surface for aesthetics, try to achieve that with a post machining process, not requiring tight tolerances during machining. Most operations only usually apply a standard, or a “as machined” finish and a standard finish of 3.2μm Ra, which is usually acceptable for functional purposes and usually less costly.

3. Designing Thin Walls and Features

Thin walls vibrate, deflect, and warp while machining, leading to errors and potential broken parts.

Error:

  • Insufficient Wall Thickness: Thin walls have little rigidity.
  • Long, Thin Features: Long thin features are more susceptible to chatter and deflection.

How to Avoid:

  • Maintain wall thickness: The minimum wall thickness recommended for metals is 0.8mm (.031 inches) and plastics 1.5mm (.059 inches).  While it can be less, as the wall thickness decreases the chances of encountering a problem increase along with the costs.
  • Consider adding stiffening features: If you must consider thin walls, you can add stiffening features like ribs and gussets to increase rigidity.
  • Optimize your width to height ratios: If the wall is unsupported or freestanding, the most common width to height ratio is 3:1.

4. Neglecting Tooling and Setup Considerations

Avoid designs that create complications with accessibility of standard tooling / machining set ups as cost and lead times are likely to increase.

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Error:

  • Use of Non-standard tools: Designing features that require a custom or specialty tool.
  • Unnecessarily multiple setups: Designs that require the part to be turned around or set up several times on a machine.
  • Difficult workholding: Parts that cannot be clamped without deforming or vibrating.

How to Avoid:

  • Design for standard tooling: Familiarize yourself with the typical range of standard end mills, drills, and other cutting tools ITU would find at a machine shop. Design to feature that can be cut with standard tools.
  • Design setups to be minimized: A good rule of thumb is to have the geometry cluster on the same side of the part. The more setups that are needed, the more chance for human error, and costs increase while lead time increases.
  • Think about workholding initially: Design parts with flat surfaces or features that can be gripped easily in a vice or fixture. If custom fixturing is needed you should think about important design features to make custom fixturing easy to design and use.
  • Avoid deep holes small diameter holes: An acceptable hole depth is approx. 6 times the diameter for standard machining as the possibility of a broken/failure tool or getting excessive wear on a tool increases. For greater hole depths we could use a method called gun drilling or work with your machinist to develop a workable solution.

5. Ignoring Material Machinability

The materials that you choose in machining can have significant effects on the cost and difficulty of machining them. impacts the ease and cost of machining.

Error:

  • Selecting Difficult to Machine Materials When You Don’t Need To: Use of hardened steels or exotic alloys when this job could have used a material or steel that is much easier to machine.
  • Not Thinking of Material Characteristics: Miscalculating tooling wear based on impact resistance; heat created; and chip evacuation characteristics of the materials that you selected.

How to Avoid:

  • Select Machinable Materials First: If your material characteristics allow, choose the machined material that is easiest to machine. Aluminum alloys – 303 stainless steel rather than 304 stainless steel.
  • Ask for Help from Your Manufacturer: Talk to your machinist or supplier about the material. Ask them for the best material recommendation for your application from a manufacturability perspective.
  • Understand Tool Wear and Heat: Understand that selecting a harder material and tool will result in more wear on your tooling; and therefore different cutting parameters, which then affect the cost and lead time.

6. Overly Complex or Aesthetic-Driven Designs

Adding additional complex attributes or decorative attributes for decoration for no other reason will add unnecessary labor for machining.

Error:

  • Complex Pattern/Engraving: This is anything that has no function, and very possibly will drastically increase machining time, if not time on the entire project.
  • The Complex Object: Don’t forget all the features, and unique components or geometry you designed to be ‘functional’.

How to Avoid:

  • Think about the function, and keep it simple: Design a part that will be the simplest part that meets all functional and structural requirements. Then ask yourself if it is functional or structural, then ask yourself if you want it at all.
  • Also, use post machining if aesthetics are a large part of the design: If aesthetics will be a large portion of the design consider polishing, anodizing, painting, etc., after machining, if the object can be machined without decoration or as digitally designed!
  • Standardize features and components: Specify all screw sizes, write down all screw thread pitches, standard sizes for all components, etc Components will always have some element of uniqueness, but the more you can standardize, the less complexity you will add to your design, and the more efficiency or time you can add to your project!
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7. Poorly Defined or Incomplete Drawings

Ambiguous or missing information on technical drawings creates confusion, delays, and wrong parts.

Error:

  • Missing Dimensions and/or Tolerances: You forgot to specify a critical dimension or tolerance callouts.
  • Unclear Geometric Dimensioning & Tolerancing (GD&T): You applied GD&T incorrectly, or the GD&T is confusing.
  • Missing of Features of Criticality: You did not specify which features are critical to the function of the part.

How to Avoid:

  • Provide full information: Make sure you include every dimension, tolerance, material specification, and requirement of surface finish on your drawings. The more information the better.
  • Use GD&T: If you are going to use GD&T, be sure it is clear, concise, and accurately reflects the functional requirements of the part.
  • Peer review drawings: Allow another engineer or experienced designer review your drawings before you send them out for quote or make.
  • Talk to your manufacturer: It is always best to keep the line of communication open with your CNC machining partner. Provide them with your CAD models, and be receptive to their feedback and suggested changes to improve your design.

Conclusion

When it comes to CNC machining, dodging them is crucial to the successful, cost-effective, high quality parts we all want.

If engineers choose to continue to prioritize manufacturability from the get-go, they will effectively limit production issues, improve lead times and be able to deliver excellent products.

Understanding CNC machine shortcomings and advantages, reducing complexity and geometries, allowing tolerances, and keeping open communication with partners are the foundation of DFM.

Remembering this will allow for a more efficient, enjoyable, and successful CNC machining project.

References