(16 design tips to Minimize the cost of CNC machining parts- Original from Chinese Engineer Mr Zhong)

Summary of contents:

1, Background

2, What factors determine CNC costs?

3, Design for Minimizing the cost #1: Radius at the internal vertical angle

4,Design for Minimizing the cost #2: Reduce the depth of the groove

5,Design for Minimizing the cost #3: Avoid thin walls.

6,Design for Minimizing the cost #4: Reduce screw thread depth

7,Design for Minimizing the cost #5: Design standard size holes

8,Design for Minimizing the cost #6: Avoid precision tolerance requirements

9,Design for Minimizing the cost #7: Reduce the number of clamping

10,Design for Minimizing the cost #8: Avoid small features with a large ratio of height to width

11,Design for Minimizing the cost #9: Remove the text and symbols from the surface of the part

12,Design for Minimizing the cost #10: Consider the processability of the material

13,Design for Minimizing the cost #11: Consider the price of the raw material blank

14,Design for Minimizing the cost #12: Consider the size of the blank material

15,Design for Minimizing the cost #13: Avoid multiple surface treatments

16,Design for Minimizing the cost #14: Mass production

17,Design for Minimizing the cost #15: Design the axially symmetrical parts

18,Design for Minimizing the cost #16: Use other processes methods to replace CNC machining

19,The end

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1, Background

Seeing how CNC processing works: piles of scrap, long processing hours. What comes to mind for you as an engineer?

I’ve been obsessed with the cost-effective design for so long that when I see this, the picture that comes to mind is: this scrap is all bought back in dollars, and the high-speed rotation of the knife is the expenditure of dollars.

Right! CNC machining should be one of the most expensive of the many processes for machining parts.

That’s really a lot of money!

 

2, What factors determine CNC costs?

Processing time. The longer the processing time, the higher the cost of CNC. Machining time is the main reason for CNC costs.

Material costs. The cost of blank material is an important part of the cost of CNC. Material costs can be reduced by optimizing the design to maximize the use of standardized blank and reducing waste.

Installation time. When batches are small, the preparation of geometric models and the specification of the machining process severely affect CNC costs. This part of the cost is a fixed cost that can be amortized through mass production.

Other design factors. When CNC parts are designed with special requirements (e.g., precision tolerance requirements and thin-walled designs), the use of special cutters, more precise quality control, lower machining speeds, and more complex machining steps can have a significant impact on CNC costs.

 

3, Design for Minimizing the cost #1: Radius at the internal vertical angle

All CNC cutters have a cylindrical shape, which creates a rounded corner of the same size as the groove when the groove is machined with the perpendicular face to the face. If the product is designed with small corners at the vertical joints of the groove body, small cutters are used, which means more machining time, because small cutters are not as efficient as large tools – which leads to increased machining time and costs.

In order to minimize the cost:

  • Rounded corners are at least 1/3 of the depth of the groove, the larger the better.
  • All corners are designed to the same size; this allows the same cutters to be used for the entire process.
  • At the root of the groove, design a small rounded corner (0.5 mm or 1 mm), or no radius.
  • The ideal corner size should be slightly larger than the radius of the cutters, which will reduce the load on the cutter during machining, which in turn will reduce machining costs. For example, with a depth of 12 mm and a rounded corner design of 5 mm or more, a cutter with a diameter of 8 mm (radius of 4 mm) can be used to ensure machining efficiency.

 

4,Design for Minimizing the cost #2: Reduce the depth of the groove

The machining depth of CNC cutters is limited to a certain extent, and the best performance is achieved when the depth of the groove body is 2-3 times the diameter of the cutter. For example, a 12 mm diameter milling cutter can process grooves up to a safe depth of 25 mm.

Of course, it is also possible to machine deeper grooves, up to a maximum of 4 times the tool diameter, but this will increase the machining costs, especially when machining with multi-axis CNC machines

In order to minimize the cost:

  • The depth of the grooves do not exceed 4 times than the length

 

5,Design for Minimizing the cost #3: Avoid thin walls.

Unless specifically requested, thin-wall designs should be avoided, as they are not strong enough and are costly to machining.

Thin walls are prone to deformation and fracture, and to avoid this, more complex machining routes need to be added, which can cost more machining time. Thin walls are more prone to vibration and machining thin walls with high precision is a big challenge.

In order to minimize the cost:

  • For metal parts, the thickness of the wall is at least 0.8 mm, the thicker the better.
  • For plastic parts, the thickness of the wall is at least 1.5 mm, the thicker the better.
  • Metal parts can be machined as small as 0.5 mm and plastic parts can be processed as small as 1 mm, which of course is not recommended.
  • When designing holes (including through-holes and screw holes) or grooves, thin walls often appear and you need to be sure to follow the design guidelines above.

 

6,Design for Minimizing the cost #4: Reduce screw thread depth

Unnecessary screw threads depths can increase CNC machining costs because special cutting tools are required.

Remember: excessive screw thread depth (more than 0.5 times the diameter) can not increase connection strength.

In order to minimize the cost:

  • The depth of the screw thread can be up to three times than the screw hole diameter.
  • For blind hole tapping, it is desirable to add at least 1/2 extra length of the hole screw diameter to the root of the hole.

 

7,Design for Minimizing the cost #5: Design standard size holes

Use the standard drills, can machine the holes fast and high-precision. For non-standard holes, the use of end-millings will increases costs.

Also, the depth of the hole should be no more than 4 times than its diameter. Deep holes (up to 10 times the diameter) can be machined, but this increase costs dramatically because of the difficulty of machining.

In order to minimize the cost:

  • Design standard size holes
  • The depth of the hole does not exceed 4 times than its diameter

 

8,Design for Minimizing the cost #6: Avoid precision tolerance requirements

Precision tolerance requirements will increase the costs because this requires complex machining processes, increase machining time, and require more inspections. When defining the dimensional tolerances of the parts must be taken seriously, avoiding every dimension to be marked with tolerances, only when necessary, and only when it’s must mark precision tolerances.

If tolerances are not defined in the part engineering drawings, then the part is machined to standard tolerances, (±0.2mm, or looser), which is sufficient for most non-critical dimensions, in this way can significantly reduce machining costs.

For those internal features, precision tolerances are more difficult to guarantee. For example, when machining intersecting holes or grooves inside, it is easy for small defects such as burrs to appear at the edges due to force deformation. These features require testing and deburring processes, both of which can only be performed manually, which are costly and time-consuming.

In order to minimize the cost:

  • Define precision tolerances only when it’s the must requirements.
  • All dimensions are marked by the same reference.
  • Remember: the decimal point in the tolerance is important, it defines the degree of accuracy and which measuring tool needs to be used. For example, two decimal points can be measured using a vernier calliper and three decimal points can be measured using a micrometre or coordinate measuring device. To reduce costs, it’s better to avoid unnecessary increases in decimal points.
  • Precise tolerance requirements can be avoided through optimized product design, such as shortening dimensional chains and using positioning features, for the details, you can contact us for the Product Guide for Manufacturing and Assembly.

 

9,Design for Minimizing the cost #7: Reduce the number of clamping

Minimum the number of clamping of parts, preferably just one-time clamping. For example, a part with blind holes on both sides would need to be clamped twice, one side being machined and then rotated to re-clamp, then the other side can be machined.

Rotating or repositioning the machining parts will increase machining costs because the clamping action is generally done manually. In addition, for complex part structures, custom clamping fixtures are required, they further increase the costs. Particularly complex structures may require multi-axis CNC lathes, which further increase costs because of the high hourly rates of multi-axis CNC machines.

It is possible to consider a complex CNC machining parts into multiple parts, which are then fastened together by screw thread or welding.

In order to minimize the cost:

  • Design parts that require only one-time clamping.
  • If this is not possible, divide the complex parts into multiple parts and fastened together by subsequent processes.

 

10,Design for Minimizing the cost #8: Avoid small features with a large ratio of height to width

During CNC machining, some small features with a large ratio of height to width are prone to vibration, it makes precise machining more difficult.

To avoid this, these small features should be connected to some thicker walls or supported by some reinforcement.

In order to minimize the cost:

  • Avoid designing features with a ratio of height to width greater than 4.
  • Small features connected to thicker walls or with added reinforcement for support

 

11,Design for Minimizing the cost #9: Remove the text and symbols from the surface of the part

The text and symbols on the surface of the parts can significantly increase the cost of CNC machining due to additional machining processes and more time-consuming machining.

Text and symbols can be added to CNC parts using some surface treatment techniques, such as screen printing or painting, which is a more cost-effective method.

In order to minimize the cost:

  • Remove the words and symbols from the surface of all parts.
  • If the texts and symbols description are must points, choose concave rather than raised, as the latter requires the removal of more material.

 

12,Design for Minimizing the cost #10: Consider the processability of the material

The processability of a material is the degree of difficulty with which the material is processed. The better the processability, the easier it is for parts to be CNC machined and the lower the cost.

The processability of a material depends on the physical properties of the material. In general, the softer and more ductile the material, the easier it is to machining. The brass C360, for example, has the highest processability and can be machined at high speed; aluminium alloys (Al 6061 and 7075) can also be easily machined.

Steel is very low processability and requires more than twice the processing time compared to aluminium alloys. Of course, the processability varies from steel to steel, with stainless steel 304 having a processability index of 45%, while stainless steel 303 has an index of 78%, the latter being easier to process.

The processing properties of plastic materials depend on their stiffness and thermal properties. During CNC machining, plastic materials melt and deform easily at high temperatures.

POM is the easiest material for CNC processing, followed by ABS; PEEK and nylon are common that very processed engineering plastic materials.

In order to minimize the cost:

  • Try to choose materials with good processability if possible.

 

13,Design for Minimizing the cost #11: Consider the price of the raw material blank

Raw material prices are another key factor in CNC machining costs

The figure shows the prices of common metal and plastic raw material blanks, with sizes 150*150*25mm

Aluminium 6061 is clearly a cost-effective material, not only the material is low in price, but also the processability is good, very suitable for prototyping.

The stainless steel 303 and brass C360 are also very processable, with high processing efficiency and time periods, but the high price of blank material makes them suitable only for high volume production. Through the batch effect, high blank prices are balanced by short processing times.

The price of plastic materials such as ABS, nylon and POM is close to aluminium 6061’s price. PEEK is a very expensive material and is only used when necessary.

In order to minimize the cost:

  • Choose materials with low blank prices whenever possible, especially when the batches are small.

 

14,Design for Minimizing the cost #12: Consider the size of the blank material

The size of the blank material affects CNC costs: in order to ensure dimensional accuracy, all sides of the part must be machined to remove the material, which significantly increases material costs, especially in large batches.

In general, the size of the embryo is at least 3mm larger than the part, for example, a part of the external dimensions of 30 * 30 * 30mm, then better to choose the size of 35 * 35 * 35mm embryo; and the external dimensions of 27 * 27 * 27mm, then better to choose the size of 30 * 30 * 30mm blank material, which can save part of the material cost.

In order to minimize the cost:

  • The blank material size is generally at least 3 mm larger than the part size.
  • Standard blank material sizes can be consulted with suppliers to design parts as close to blank material sizes as possible to reduce material waste.

 

15,Design for Minimizing the cost #13: Avoid multiple surface treatments

Surface treatment can improve the appearance and resistance of CNC parts to harsh environments, but they can also increase costs.

If CNC parts require multiple different surface treatments, this can further increase costs as it requires more processes, for example, some surface treatments need to be masked.

In order to minimize the cost:

  • Remove surface treatment
  • Avoid multiple surface treatments

 

16,Design for Minimizing the cost #14: Mass production

The batches production of part affects machining costs because the initial installation cost of CNC machining is spread over each part when batch demand is low. When lots are larger, the cost of installation is less spread over each part. LANLI can process even 1 pcs.

The higher the part quantity, the lower the CNC cost, which is even more evident when the part quantity is very small: even a small increase in quantity leads to a significant cost reduction. LANLI can process even 1 pcs.

In order to minimize the cost:

  • High volume production reduces costs and allows a larger order to be placed rather than multiple scattered orders. High volume production can reduce the costs, so it’s better to order a larger amount of order rather than multiple small orders. But at LANLI, our engineer will find the best way to save the cost all the way.

 

17,Design for Minimizing the cost #15: Design the axially symmetrical parts

Axially symmetrical parts can be machined using a lathe or milling machine, which is small and has a much smaller hourly rate than a 3-axis or 5-axis machining centre.

In order to minimize the cost:

  • Design parts with as much axis symmetry as possible.

 

18,Design for Minimizing the cost #16: Use other processes methods to replace CNC machining

CNC machining is an expensive process method. Therefore, however to minium the cost, the costs are always high. With modern processes changing rapidly, it is important to think about using other processes instead of CNC instead of curing. The previous generation of products used CNC machining, while the new generation of products must use CNC machining. This is not undesirable.

As known, 3D printing costs less than CNC machining in small batches; in larger batches, moulding processes such as melt casting and injection moulding cost are less than CNC machining.

The following table shows the process options for different quantities of parts.

As the old saying goes, there’s plenty more fish in the sea and the next craft will be better.

3D printing is better than CNC machining in many cases.

3D printing for more complex parts with more complex geometry

3D printing can provide samples within 24 hours.

3D printing is usually cheaper when quantities are small.

3D printing can process materials that are difficult for CNC to process, such as superalloys.

If asked ultra-precision parts, CNC Machining will be highly suggested.

In order to minimize the cost:

  • Use other processes instead of CNC.
  • Note: The same is true for any other manufacturing and fastening process, including injection moulding, metal stamping, welding and riveting, etc., in the early stages of product design when choosing the process, silently recite the ancient poem “there’s plenty more fish in the sea and the next craft will be better” times, try the best to find, there will be a better process, only you do not currently know that.
  • Cost-Effective Product Design: Designing to Reduce Costs has three chapters on how to select manufacturing and fastening processes. Welcome to contact us to get more information.

19,The end

The design of minimizing the cost is a very challenging task and there is currently a general lack of knowledge among engineers in domestic China. In the past few years of actual product development experience, the design is also basically not considered cost, one-sided considered that “customer first”, “customer best”, the engineer’s design job is to meet customer needs, as for the cost-effective is handed over to the manufacturing department.

This is a misperception of the cost of the product.

  • 1, product ingredients originally have a very high status in product development. As advocated by inherited product development (IPD), product development is an act of investment, if the product cost is too high to bring profit or even loss to the enterprise, even if the customer is satisfied with the product, such product development is a complete waste of enterprise capital and human resources and does not produce any value. In this way, businesses will eventually be eliminated.
  • 2, more than 85% of product cost is determined by product design and the remaining 15% is determined by product manufacturing. If cost-effective product design is not done at the time of product design, it is bound to cost too much when the product is finally produced. In the transition from “Made in China” to “Made in China 2025”, cost-effective of product designing is necessary.

When it comes to cost-effective product design, companies and engineers must first change their mindset and become more cost-conscious, when one day engineers will “sleep through the night” for each detail of product design, for example.

  • Should the part wall thickness be 4MM or 5MM?
  • Should rounded corners be designed for 0.3MM or 0.5MM?
  • Should the important dimensional tolerance be 0.05mm or 0.1mm?

It can be said that the engineer has an initial cost awareness.

Without such cost-effective awareness, an engineer is not an engineer, just a draughtsman who knows how to use CREO/UG or SOLIDWORKS. Engineers who don’t make changes, don’t study hard, and only position themselves as draughtsmen will never get the career mutation they need to wake up one day in the future to find their jobs replaced by AI.

Other engineers believe that cost is a business matter and that it has little to do with themselves if the business is not profitable. In response, I can only wish that in the event of massive corporate layoffs, they will not reach you.