Come progettare parti per la lavorazione CNC

CNC machining is one of the most efficient and precise manufacturing methods available. However, to get the best results, you need to know how to design parts for CNC machining.

This blog is here to provide guidelines and best practices when designing parts for CNC machining.

What is a CNC Machining Process?

CNC (Computer Numerical Control) machining is a subtractive manufacturing process that you can use to create precise parts. You start with a solid block of material, and cutting tools remove sections to shape the final product.

This method offers high accuracy and works with metals, plastics, and other materials. However, since it relies on cutting tools, you need to consider certain design limitations to ensure manufacturability.

What are the Restrictions of a CNC Design?

CNC machining lets you create complex parts, but there are some limits. These limits come from how the cutting tools work, and they are discussed below.

Tool Geometry

Most CNC tools, like end mills and drills, are cylindrical and have a limited cutting length. When the tool removes material, it leaves behind its shape. This means that inside corners will always have a rounded edge, even if you use a very small tool.

Tool Access

CNC tools cut straight down onto the material. If a part of your design cannot be reached from above, it cannot be machined. One exception is undercuts. These need special tools, and you can learn more about them later.

To make machining easier, try to align holes, walls, and other features with one of the six main directions (top, bottom, front, back, left, right). However, advanced 5-axis CNC machines can handle more complex angles.

If you have deep and narrow features, the tool needs to go deeper, which can cause more vibration and lower accuracy. To avoid this, design parts that can be machined using the largest possible tool with the shortest possible length. This makes the process smoother and more precise.

Tool Stiffness

In CNC machining, your cutting tools are made from strong materials like carbide or tungsten, which are harder than the workpiece. But even strong tools can bend a little when cutting. This bending, called tool deflection, can cause small mistakes in your work.

If you don’t need extreme accuracy, this may not be a big problem. But if you are working on a very precise job, even a small bend can affect the final result and limit your design choices.

Workpiece Stiffness

Even if your cutting tools are very strong, they may not work well on extra-tough materials.

If the workpiece is too stiff, it can shake or bend while you are machining. This can make it harder for you to get accurate results, especially when you need very tight tolerances.

Workpiece Shape

The shape of your workpiece affects how easy or hard it is to machine. If the shape is too complex, you may need to stop and adjust it many times, slowing down the process.

Even if you use an advanced machine, you might still need to turn the workpiece several times, making production less efficient. Keeping designs simple can help you save time and get better results.

Workholding

It’s important to hold your workpiece tightly while machining. If any part of your setup is weak, your machine or tool may vibrate, reducing accuracy.

If the workpiece moves during machining, each finished part may come out slightly different. A strong setup helps you get the same high-quality results every time.

Guidelines for a CNC Design

When designing parts for CNC machining, there are no strict rules because machines and tools keep improving. These guidelines will help you create designs that are easier to machine.

Tolerances

Tolerances define how much a part’s size can vary. Most CNC machining tolerances are ±0.1 mm, but tighter tolerances of ±0.005 mm are possible. If no tolerance is specified, manufacturers will use standard industry tolerances.

Internal Edges

Internal edges should have a corner radius that is at least one-third of the cavity depth. This helps the tool cut smoothly. Increasing the radius slightly (by about 1mm) allows the tool to follow a circular path instead of making a sharp turn, which improves surface quality.

If you need sharp 90-degree corners, consider using a T-bone undercut instead of making the radius smaller. Floor radii should be 0.5 mm, 1 mm, or left without a radius, as these sizes work best for machining.

Cavities and Pockets

Cavities and pockets should be no deeper than four times their width. Cutting tools have a limited length, usually three to four times their diameter. If cavities are too deep, the tool may bend, and chips may not clear properly, causing vibrations.

If you need deeper cavities, design them with varying depths instead of making one deep cut. Special tools can cut cavities deeper than six times the tool diameter. For example, a 1-inch diameter end mill can cut up to 35 cm deep.

Holes

Holes should match standard drill bit sizes whenever possible. Any hole larger than 1 mm can be machined, but using standard sizes ensures better accuracy. The best hole depth is four times its diameter, though deeper holes can be made with special drill bits.

Normally, holes are drilled up to ten times their diameter, but specialized tools can go as deep as forty times the diameter. Drill bits leave blind holes with a conical bottom (135-degree angle), while end mills create flat-bottom holes. CNC machining does not favor through holes over blind holes—either works fine.

Thin Walls

Thin walls can be difficult to machine because they vibrate and reduce accuracy. For metal parts, walls should be at least 0.8 mm thick, and for plastic parts, at least 1.5 mm thick. It is possible to go as thin as 0.5 mm for metal and 1.0 mm for plastic, but this should be checked carefully. Plastic parts may warp or soften due to heat and stress.

Threads

Threads should be at least M1 in size, but M6 or larger is preferred. Threads smaller than M6 are cut with tarps, while CNC threading tools are used for larger threads since they prevent tool breakage. The best thread length is at least 1.5 times the diameter, with a maximum of three times the diameter.

Most of the force on a thread is taken by the first few teeth, so making them longer is unnecessary. If a blind hole is threaded and smaller than M6, leave an unthreaded section at the bottom equal to 1.5 times the diameter. For M6 or larger, the hole can be fully threaded.

Small Features

Small features like tiny holes should have a minimum diameter of 2.5 mm (0.1 inches). It is possible to machine holes as small as 0.05 mm (0.005 inches), but this requires special micro-drills and expert knowledge. The physics of cutting changes at this size, so unless absolutely necessary, very small features should be avoided.

Tall Features

Tall features are difficult to machine accurately because they tend to vibrate. To minimize this issue, keep the height no more than four times the width. You can also improve stability by rotating the part 90° during machining, which changes the height-to-width ratio and makes it easier to machine.

Text and Lettering

Text and lettering should be engraved rather than embossed, as engraving removes less material. The recommended font size is 20 or larger, with a depth of at least 5 mm. Sans-serif fonts like Arial or Verdana are ideal because they are widely supported by CNC machines.

CNC Machine Setups and Part Positioning

When machining a part using a CNC machine, you often need to rotate it multiple times to reach all surfaces. However, each time you rotate the part, the machine must be recalibrated, and a new coordinate system has to be set.

When designing a part, keep these two things in mind:

• Rotating and realigning the part takes extra work and time. If the part needs to be rotated three or four times, that’s usually fine, but anything more than that can be too much.
• If two features need to be very precise in relation to each other, try to machine them in the same setup. Every time you recalibrate, there’s a small chance of error.

CNC Machining Undercuts

An undercut is a shape in a part that a regular cutting tool can’t reach from above. You need special tools to make these cuts. There are two main types: T-slots and dovetails. A T-slot has a horizontal cut inside the part, while a dovetail has angled cuts, usually at 45 or 60 degrees. Undercuts can be on one side or both sides of a part.

A T-slot tool has a horizontal blade attached to a vertical shaft. The width of the cut can be between 3mm and 40mm. It’s best to use common sizes, like whole millimeters or standard inch fractions, so you can find the right tool more easily.

A dovetail tool is measured by its angle. The most common angles are 45 and 60 degrees, but you can also find other angles from 5 to 120 degrees in 10-degree steps. These less common angles are harder to find.

Designing Undercuts for CNC Machining

If you design a part with an undercut inside, make sure to leave enough space for the tool to fit. A good rule is to leave a gap at least four times the depth of the undercut between walls

Standard tools can only cut to a certain depth because their cutting diameter is usually twice the size of the shaft. If you need a special undercut that a standard tool can’t make, a machine shop may have to create a custom tool. This takes more time and costs more money, so try to avoid it if you can.

Material Choices for CNC Machining

Choosing the right material for CNC machining affects cost, ease of machining, and final quality. Metals like aluminum, steel, stainless steel, brass, copper, and titanium are strong and durable, that’s why they are ideal for high-stress parts. They also resist heat and corrosion, which is useful for many industries.

Plastics are lightweight, affordable, and easy to shape into complex designs. Some, like acetal (POM), nylon, polycarbonate (PC), acrylic (PMMA), and PEEK, resist chemicals, so they are suitable for harsh environments. Plastics work well when you need flexibility and lower costs without sacrificing durability.

Surface Finish for CNC Machining

The surface finish of a CNC part affects its look, strength, and performance. An “as machined” finish has small tool marks, but you can request a smoother surface. Bead blasting gives a smooth, matte look by shooting tiny beads at the part. Anodizing adds a protective layer to resist scratches, heat, and corrosion—Type II offers basic protection, while Type III is more durable.

Powder coating creates a tough, colorful layer by baking dry powder onto the part. If you need something unique, custom finishes can add special textures, colors, or coatings. The right finish makes your part stronger, more durable, and better-looking.

What are the Best CNC Machining Practices?

• Use the tool with the largest possible diameter to machine your parts faster and more accurately.
• Add large rounded corners (at least ⅓ of the cavity depth) to all inside vertical edges.
• Keep cavity depths no more than 4 times their width to avoid machining problems.
• Line up your design’s main features with one of the six main directions. If that’s not possible, consider 5-axis CNC machining.
• If your design includes threads, tolerances, or surface finish requirements, submit a technical drawing with clear notes for the machine operator.

How to Design Parts for CNC Machining Summary

CNC machining is a great way to make precise parts, but good design makes the process smoother and more cost-effective. By following these simple guidelines, you can create parts that are easier to machine, stronger, and more accurate.

To ensure you high-quality parts, contact DEK. We are a CNC machining company with years of experience making parts for different industries.

Our shop has a range of CNC machines, from 3-axis to 5 assi, and a skilled team of designers, engineers, and machinists. We can also help you with the best CNC machine design ideas. No matter how complex your project is, we can handle it.