CNC machining is the backbone to modern manufacturing. That being said, even the most sophisticated CNC machine cannot spare you from a design that is poorly prepared. Whether you are an experienced designer or a budding designer, avoiding horrible design shortsightedness will save you and your employer time, money, and headaches.
Here we will briefly walk through 10 mistakes that happen regularly and the associated countermeasures to assist you with improving your CNC machining process.
1. Not considering material properties
Material should never be an afterthought when you are designing a part. Each material has a unique way of behaving when machined.
For example, aluminum is soft and easy to machine, while titanium might be tenacious and stubborn.
When you ignore variations in material behavior, your design may look incredible on paper but become a nightmare when it reaches your CNC table.
Properties of materials such as hardness, ductility, thermal conductivity, and brittleness affect tool wear, cutting speeds, and surface finish.
For example, stainless steel requires slower cutting speeds with tougher tools than aluminum.
Subsequently, if your design neglects important material properties, you will experience additional tool failure, longer production times, and higher costs.
Common Blunders with material choice
One common mistake is to select materials only by their strength or appearance with little or no consideration for machinability.
Delays in production can often be attributed to material choices that neglect availability, a serious amateur (or worse) error. A smart designer chooses a material that is a mix of function, cost, and machinability.
Always check machinability charts and speak with the CNC operator before embracing material choices.
2. Over-Complicating the Design
Designing for CNC isn’t about impressing people with your CAD skills but for you to get the most effective/functional part you can create.
Most individuals (engineers) tend to over-complicate designs with excessive curves, excessive internal geometries and then they throw in non-standard features.
Most importantly, every unnecessary detail you add to your design can effect the costs in regards to longer machining time or having to buy additional tooling.
Although CNC machines provide precision, every detail you add will add programming time and the potential for programming/usage errors.
When you incorporate radii, avoid cavities that are too deep, and don’t incorporate tight corners, you can improve costs.
Always ask yourself: “Can this be done with a 3 axis mill? Do I need 5 axis?”. If you can break a design down into smaller components, or make a small adjustment to the geometry to allow for easier machining, you should always do it.
Use standard hole sizes, rape consistent wall thicknesses, and try not to have features that are super thin. If you are unsure, having the machinist look at your designs early on will probably save you considerable money value in the long run.
3. Tolerances that are too Tight
One of the places where design and reality meet, are tolerances. Here is the hard reality: The vast majority of parts do not need to be made to ultra tight tolerances.
However, the majority of designers are slapping ±0.001″ everywhere “just in case”. Just this naive act will totally ruin your budget.
If you are working to someone else’s tightly specified tolerances, tighter tolerances translate to more precise machining and inherently more expensive manufacturing (quality checks, faster CNC programs, programmed speeds, etc).
Unless your part is required to be high precision as in aerospace or medical manufacturing be conscious to not simply use tighter tolerances whenever heavy tolerancing is not specifically required. For many consumer or industrial product parts, (+/-.005″ or even +/- .010″) is quite acceptable.
Best Practices for Tolerances
To begin, make sure you know what the purpose of all of the part features will be. Using unreasonable tolerances only where necessary is good practice.
For non-critical spaces use general tolerance notes, and definitely call out critical dimensions. The machinists are your friends most will advise changes to your tolerances that will help the machinist create your part effectively without sacrificing function. Besides, always provide a tolerance-drawing even as a CAD file it eliminates assumptions.
4. Tool Selection Problems
Tooling is where the rubber hits the road in CNC machining, and using the wrong tool is like driving your sports car through a swamp you aren’t going to get far! Most design errors originate from the incorrect assumption that all tooling is the same. Spoiler alert… it is NOT.
Each tool is designed to suit specific materials and geometries. A flat end mill may be great for facing surfaces, but terrible for finishing internal radii.
For example, if your design has a deep cavity, consider whether there is a long reach tool available to use, and how stable it will be in the cutting process.
Not every shop will have the specialty tool available, and using the wrong tool will result in chatter, poor finish, and broken cutters.
Using generic or “one size fits all” tools will result in inefficient machining without the possibility of achieving good results. You are better served to design your features around tooling that most shops commonly stock.
Generic drill sizes or threading tolerances are considered “stock” tooling and save money and time for machinists. Possibly the most important set of consideration in tooling is in tool deflection.
Long tools will deflect slightly when under pressure, potentially affecting accuracy in your part. Designing with generic tooling options in mind produces less hurdles and expense.
5. Overlooking Toolpath Optimization
Toolpath programming is not that different from programming a GPS for your CNC machine.
A good toolpath program will guide the machine to operate correctly in a smooth and repeated path, needless to say, a bad toolpath program would result in high cycle times, tool wear, and bad parts.
Yet, design needs to anticipate the manner in which toolpaths will be created, and the toolpath execution.
Toolpaths Affect Effeciency and Surface Finish
Toolpaths are what make the difference between machining time and surface quality. For instance, a zigzag path will produce more surface marks on the finished surface, but maybe creates better cycle time on the rough cutting.
Paths like circular or spiral may take more time, but may improve your finish. Without a toolpath strategy, your part may end up rough, excessive cycle time or may be in accurate.
Optimal Toolpath Strategies:
- Start with a plan to minimize tool marking or gouging action by focusing on the tool’s entry and exit paths.
- Use climb milling when possible, this helps with your finish and can reduce tool wear.
- While in the roughing pass, eliminate rapid moves for minimal repositioning.
- It’s also important to collaborate with the CNC programmer to develop toolpath options based on speed vs. accuracy.
- Be sure to simulate the toolpaths before machining to catch toolpaths that can result in issues.
6. Consider the Machine Capabilities
Remember that designing a part without knowing the limits of the CNC machine is the same as writing code for a program you are not sure will run on a device, it may run but it will probably crash.
Every CNC machine has limits on the machines travel, spindle speed, axis movement, tool capacity, etc. Not considering the limits of the machine only creates an unrealistic design that is not able to be made efficiently – or worse.
Not every CNC machine can make every part. A design intended for a 5-axis machine may be physically impossible on a 3-axis mill without exorbitant costs.
For instance, undercuts or deep internal pockets or complex angles can only be created on specialty machines, and even then, under special setups.
There are also parts which simply exceed the machine/ fixtured part volume limits. Understanding your machine limits where it is physically constrained including travel across the Z axis, access of rotary axes for even a small part, or access to other tool changer types is critical to developing a realistic and machinable design.
Design within operational boundaries
Here’s where collaboration becomes crucial. Always consider having an open conversation with the machinist, or the shop manager, to get a feel for the particular machines available in the shop.
In a primarily 3-axis machine shop, designing for a 5-axis machine may be impossible, and any tooling or setups they may have will be much more cost-heavy than simply designing for the machines in their facility from the onset.
You can imagine the optics of the machinist having to create complex setups and flipping the part multiple times that would have come under 1 setup.
Considering they are forced to work under your specifications, be sure to ask the machinist to help show how features of certain parts could be exploited when making the part.
In the same sense, while some CNCs have slower feedrates, or can be more angry and less rigid, too, and definitely won’t yield well when expecting deeper cuts and slowing down in the cut.
If you do this and design with the kind of machine you have made every effort to qualify with the terrain and forage made from your shop, totally, then you will qualitatively improve your turnaround time, lower the cost, and accuracy on the part when it comes time to assemble and maybe perform testing.
7. Forgetting-Fixturing and Workholding
You’ve got the design right, you selected the right material, and you’ve even optimized your toolpaths, but this means nothing if your part can’t be held secure when you machine it, it just becomes noise.
While fixturing and workholding are essential to making parts (the whole point is you are expected to be wrong as much as right), the concepts are often left out of the design process; without proper consideration, your part is likely to move, slip, possibly, fly off the table itself.
Importance of fixture secure and repeatable
Success and accuracy in machining is largely of how securely the part is held.
If something is displaced or damaged from being clamped poorly, or allowed to move, more than likely you’re going to experience tolerance-movement, chatter, and poor finish issues, if not scrapping the part altogether.
Repeatable fixturing is also important for manufacturing processes with several parts to be run. If we cannot load and hold parts repeatably, it will be hard to control the quality.
Designing with Fixturing Considerations
The first step should be to ask yourself; How will this part be held during every operation? Is it clamped from the sides or bottom? Is there enough area to clamp with a vise or vacuum fixture? Also, don’t place critical features on the same face that will be used for workholding, and how many setups the part will require.
If possible, design in features for fixturing flat edges, through-hole, and good boss locations.
Work with your machinist to have your design be workholding-friendly. This single discussion can significantly reduce machining time and setup errors.
8. Ignoring Cutting Speeds and Feeds
Let’s be real – CNC machining is no different than any other process in the fact that it’s not a plug-and-play technology at any point. Each material/tool combination has its best practices regarding the different cutting parameters, including set speeds, feeds, and depth of cut, etc.
If any of your designs require some form of setting besides “average,” there is a chance you will push the machine too hard or not push the machine hard enough, with the potential of ruining the part you are making by manipulating into unintended loads.
Ignoring speeds and feeds will lead to tool wear, heat buildup, and poor surface finishes at best.
The determination of speeds and feeds is dependent on the, material hardness, tool geometry, cutter diameter and sometimes the rigidity of the machine.
Tool speeds and feeds are important to keep efficacy and efficiency during production, too slow and we are going to have rubbing rather than cutting; too fast and the tool can get too hot.
If the tool gets too hot, we can have built-up edge, or warping. In more catastrophic cases, we could have tool failure. Different operations–roughing, finishing, drilling–will have different settings.
The design needs to factor in whether or not these operations will be performed at optimum parameters.
Avoiding Tool Wear and Part Defects
While making a deep slot, tight corner, or thin wall can feel like a rewarding challenge, those features will often require ultra-fine tuned speeds and feeds that not every shop will produce consistently.
Tools will wear faster creating dimensional accuracy and surface defects. You can minimize this by producing engineered features that can be cut at standard parameters.
If the feature needs specific speeds or feeds, put it on your drawings. This warning could possible save hours of grumbling, troubleshooting, and rework.
9. Poor CAD/CAM Integration
You might have heard, “garbage in, garbage out.” This is the plight of poorly transitioned CAD models into the CAM software world. One of the pitfalls of a great design is when it gets truncated in translation from design to programming.
When CAD and CAM are poorly integrated errors take hold: misaligned features, improper toolpaths, or missing operations. The design needs to be CAM friendly from the outset.
CAD and CAM should be a seamless continuation of work. Still, the lack of knowledge in CAD designers is that what looks good on a monitor only possibly does so in the same state in the machining world.
A radiused corner in CAD may require a specific tool size in CAM. Alternatively, a draft angle may inhibit tool selection. When it comes to CNC, assumptions made by CAD and CAM teams are dangerous.
Common Software Pitfalls
Often times the software or file formats don’t match or are old, which can transfer into the CAD as dimensional loss, distortion geometry and/or just skipped layers.
Then there’s the inconsistent naming conventions and lack of layer management that left a CAM programmer scratching their head.
Solutions to these issues are to keep the CAD models clean, organized, and basic; use a native format whenever possible, and always have a documentation supplement. Schedule a design review with your CAM team as an investment that returns big rewards.
10. Not Prototyping or Iterating
You would never release a new app without testing it – why would you do that with a CNC part? Not prototyping is one of the most expensive things to forget when designing machined products.
A miss on your part for something small could end up you redesigning a part while wasting materials and time. Prototyping is not only a step – it is a strategy.
Prototyping allows you to fix design errors, verify tolerances, and validate machining and setup strategies before mass production. Prototyping is also where theory meets practice.
A design may look great when you ran simulations but a bad vibration, heat build-up, or material inconsistency may have you quickly redesigning. The prototype makes you aware of these issues early enough that they are still fixable.
How do you build a prototyping and iterative design process?
Starting with low-cost material such as plastics, or softer metals for your first run. Use this feedback to improve your CAD model, then generate a second version, if necessary.
As much as this iterative process may seem to slow you down, it will save you money in the future. It also builds rapport (trust) with machinists, clients, and stakeholders. Make prototyping a part of your design DNA, instead of an afterthought.
Conclusion
CNC machining is as much art as it is science. Designing for this process requires a careful blend of creativity, precision and pragmatism.
The missteps we have reviewed, such as overlooking the consequences of material properties, over-designing, failing to consider tolerances, and forgoing prototypes, are all common mistakes especially among designers with a proven history in product design. But they are all avoidable mistakes.
Avoiding these false steps requires a deliberate change of mindset from design-focused to manufacturing-aware.
This involves thinking about how the material will behave, what machining operation the design needs to consider, what constraints exist in work-holding, and how your design will affect tooling wear early and often.
Accessibility of real life wisdom from machinists is a unique value that can help proactively address design challenges. The sooner we can address your assumptions with prototypes, the sooner we can find a simple path forward.
There is great potential in CNC machining when designs are done well, efficient, and clearly communicated. So, feel free to pull all this thinking into your next project—you will be amazed at how much more efficient your workflow will be from CAD to cut.
Always remember: the best CNC design is machinable, not just beautiful.
FAQs
1. What is the most common CNC machining mistake that beginners make?
One of the most frequent beginner mistakes is applying tight tolerances to every dimension of their designs. Although it seems logical to guarantee precision, tight tolerancing costs you more and creates complexity without add any functional value. At the very best, apply critical tolerances, and leave the other tolerances general to ease machining.
2. How do I select the best material for my CNC project?
Start by figuring out the application of your part and understand if it is under stress, has heat, or is in contact with corrosive liquids after all. Then, you can try to balance requirements with machinability and cost. As an example, aluminum is a good material for lightweight applications and is easy to machine; stainless steel is good for durability but harder to cut. Don’t forget to consider availability of material and lead time.
3. Do I have to use the same tool for different materials?
Yes, technically—however it won’t be desirable. Many tools wear differently depending on the material being cut. We don’t want to use a tool that was made for cutting aluminum on hardened steel, because the tool will wear out quickly and the overall finish will be poorer quality. Always match your tooling to the material and process for best overall results.
4. When should I go back and look at my CNC design again?
Go back as many times as is necessary to ensure functionality and manufacturability. You may revisit your design after you’ve performed prototype testing, communicated with machinists for feedback, or made any changes to material or manufacturing processes. The more we iterate to improve your part, the smoother production will go.
5. What are the best practices for CNC prototyping?
Always prototype using a low-cost material to confirm fit, function and machinability. After getting your prototype you should always review with your machinist for any improvements. Keep your documentation up to date after each version. As a nice option to have, test your design using the exact machine and tooling that will be used in production. The performance may surprise you!