From CAD to the workshop: simple parts that complicate machining

In product design, there is a very common pitfall: assuming that a piece is cheap to manufacture just because it looks simple. No complex contours, no organic shapes, no mechanisms… and yet, when it comes to the budget, the price skyrockets. 

Why does this happen? Because in industrial manufacturing, complexity is not always visible. It is often hidden in geometric details that require changes to processes, tools, or machining strategies. 

In this article, we review the geometries that most often increase the cost of a part without the designer being aware of it.  

Walls that are too thin in CNC machining

A thin wall may seem like a minor detail in CAD, but in manufacturing it is a real red flag. Everything looks fine on screen, but when it comes to the machine, it’s a different story. 

• In CNC machining, thin walls require reducing feed rates and cutting depths to prevent vibrations. 
• Increases the risk of deformation during machining and also when the part is unclamped. 
• In many cases, specific tools or special strategies are required to avoid rejects. 

In addition, a wall that is too thin increases process variability, which translates into more checks and more rejected parts. The result is more machine time, more risk, and therefore higher costs. 

Deep holes and small diameters in machined parts

Deep, narrow holes are one of the biggest enemies of cost, even though they are not visually striking. 

• They require long, thin tools that are much more fragile. 
• They reduce process stability and increase the likelihood of tool breakage. 
• They are forced to mechanize more slowly and with greater supervision. 

Each breakage involves stopping the machine, replacing the tool, and, in the worst case, repeating the part. As a rule of thumb, when the depth exceeds 4–5 times the diameter, the cost begins to increase non-linearly. 

Small interior radii and manufacturability issues

Very small or nearly square inner radii are easy to draw in CAD, but complicated and expensive to manufacture. 

• They force you to use tools with a reduced diameter. 
• They significantly increase machining time. 
• Increases wear and tear and the risk of tool breakage. 

In many cases, increasing the inner radius by a few millimeters does not affect the function of the part, but it can significantly reduce manufacturing time and cost. Whenever possible, larger radii are cheaper. 

Tight tolerances and extra costs in the manufacture of parts and prototypes

One of the most common causes of cost overruns in seemingly simple parts is setting overly strict tolerances without functional justification. 

• Tight tolerances require additional controls. 
• In many cases, they require secondary processes such as manual rectification or adjustments. 
• The percentage of parts rejected due to minor deviations is increasing. 

The key question that should always be asked is: is that tolerance functional or merely aesthetic? Adjusting tolerances only where they add value can drastically reduce costs. 

Inaccessible surfaces and poor orientation

A part can be geometrically simple and yet very complex to manufacture if tool access has not been considered. 

• Poorly oriented surfaces require more fastening changes. 
• Each repositioning requires more preparation time. 
• In some cases, it requires the use of more expensive machines with more axes. 

Designing with how the part is actually held and machined in mind allows the process to be simplified and costs to be significantly reduced. 

False symmetries in the design of machined parts

Parts that appear symmetrical but are not often cause unexpected problems in manufacturing. 

• They prevent the reuse of tools or machining strategies. 
• They force you to reposition the part even if the change is minimal. 
• They break the repetitiveness and efficiency of the process. 

Sometimes a small displacement, an off-center drill hole, or an asymmetrical detail turns a cheap part into a complex and more expensive one. 

Demanding surface finishes

A visually clean or aesthetic surface finish is not always trivial or inexpensive to achieve. 

• Requires slower, more controlled final passes. 
• In many cases, this involves manual polishing operations. 
• Increases total manufacturing and handling time. 

Defining high-end finishes only in visible or functional areas allows for a balance between aesthetics, cost, and feasibility. It is advisable to specify finishes only where they add real value.