What affects the cost of CNC parts?

2021/11/10


The price of CNC machined parts depends on the following factors:

 

1. Processing time: The longer it takes to process parts, the more expensive the price. Machining time is usually the main cost driver for CNC.

 

2. Start-up cost: These are related to CAD file preparation and process planning, and are of great significance for small batch production. This cost is fixed, and there is an opportunity to lower the unit price through the use of "economies of scale".

 

3. Material cost: The cost of bulk materials and the difficulty of material processing greatly affect the overall cost of CNC. Optimizing your design while considering certain material factors can greatly reduce the price.

 

4. Other manufacturing costs: When you design parts with special requirements (for example, when you define strict tolerances or design thin walls), you may need special tools, stricter quality control and more processing steps ( At a lower processing speed). Of course, this will have an impact on the total manufacturing time (and price).

Now that the source of the CNC cost is clear, let's see how to optimize the design to minimize it...

Curious about the CNC pricing of parts? Upload your files to get a free instant quote

 

Tip #1-add a radius to the inner vertical edge

Add an inner radius at the corners at least 1/3 of the cavity depth

All CNC milling tools have a cylindrical shape and produce a radius when cutting the edge of the cavity.

Using a smaller diameter tool can reduce the corner radius. This means that multiple passes need to be made at lower speeds—smaller tools cannot remove material in one pass as quickly as large tools—increasing processing time and cost.

Minimize costs:

Add a radius that is at least 1/3 of the depth of the cavity (the larger the better).

It is best to use the same radius on all inner edges.

At the bottom of the cavity, specify a small radius (0.5 or 1 mm) or no radius at all.

Pro tip #1: Ideally, the corner radius should be slightly larger than the radius of the tool that will be used to machine the cavity. This reduces the load on the tool and will further reduce your manufacturing costs. For example, if your design has a 12 mm deep cavity, add a 5 mm (or larger) radius at the corners. This will allow ø8 mm cutters (i.e. 4 mm radius) to cut them at a faster speed.


Tip #2-Limit the depth of the cavity

Pro tip #2: If you need an inner edge with sharp corners (for example, when a rectangular part needs to be fitted into the cavity), do not reduce the radius of the inner edge, but use a shape with an undercut, as shown below Show:

 

When rectangular parts need to be inserted into the cavity, add undercuts at the corners of the cavity

Limit the depth of the cavity to 4 times its length

Machining deep cavities will greatly affect the cost of CNC parts, because a large amount of material needs to be removed, which is very time-consuming.

It is important to remember that the cutting length of CNC tools is limited: in general, they work best when the cutting depth reaches a cavity 2-3 times its diameter. For example, a ø12 milling cutter can safely cut cavities up to 25 mm deep.

It is possible to cut deeper cavities (up to 4 times the tool diameter or more), but this will increase costs because special tools or a multi-axis CNC system are required.

In addition, when cutting the cavity, the tool must be chamfered to the correct depth of cut. A smooth entrance requires sufficient space.

Minimize costs:

Limit the depth of all cavities to 4 times their length (that is, the maximum size on the XY plane).

Tip #3-increase the thickness of the thin wall


Increase the thickness of the thin wall to reduce processing time

Unless weight is the main factor, thick solid sections are more stable (lower processing costs) and should be the first choice.

In order to avoid deformation or breakage when machining thin walls, multiple passes are required at low cutting depths. Thin features are also prone to vibration, so machining them accurately is challenging and can significantly increase machining time.

Minimize costs:

For metal parts, the design wall thickness is greater than 0.8 mm (the thicker the better).

For plastic parts, the minimum wall thickness is kept above 1.5 mm.

Remember: the minimum achievable wall thickness is 0.5 mm for metal and 1.0 mm for plastic. However, the machinability of these features must be accessed on a case-by-case basis.

Pro tip: When placing holes (and threads) very close to the edge of the part, thin walls are usually encountered. Ensure that the above guidelines are also followed when positioning the holes in the design.


Tip #4-limit the length of threads


Limit the maximum thread length to 3 times the hole diameter

Specifying a thread longer than the required length increases the cost of CNC parts because special tools may be required.

Keep in mind that a thread longer than 0.5 times the hole diameter will not actually increase the strength of the connection.

Minimize costs:

Design a thread with a maximum length of 3 times the hole diameter.

For threads in blind holes, it is best to add at least 1/2 diameter unthreaded length to the bottom of the hole.


Tip #5-design a standard size hole


Use standard drilling sizes when designing holes

The standard drill bit can be used for CNC machining of holes quickly and with high precision. For non-standard sizes, an end mill must be used to machine the holes, which may increase costs.

In addition, limit the depth of all holes to 4 times their diameter. Deeper holes (up to 10 times the diameter) can be made, but they may increase costs because they are difficult to machine.

Minimize costs:

Design holes with a diameter of 0.1 mm increments, with a maximum diameter of 10 mm and greater than 0.5 mm.

When designing in inches, please use traditional inch fractions or refer to this fractional inch drill size table.

Design a hole with a length up to 4 times its diameter.


Tip #6-Specify tolerances only when necessary


Specify tolerances only when necessary

Defining tight tolerances will increase the cost of the CNC because it increases the processing time and requires manual inspection. Tolerances should be defined carefully and only when necessary.

If no specific tolerances are defined on the technical drawings, the parts will be machined with standard tolerances (± 0.125 mm or better), which is sufficient for most non-critical features.

It is especially difficult to achieve tight tolerances on internal features. For example, when machining intersecting holes or cavities, small defects (called burrs) may occur on the edges due to material deformation. Parts with such characteristics require inspection and deburring, which are manual (and time-inefficient) processes, which increase costs.

Minimize costs:

Specify tighter tolerances only when necessary.

Define a single datum (for example, the cross section of two sides) as the reference for all toleranced dimensions.

Remember: the decimal point in the size is important. They specify the accuracy and the instrument that will be used for the measurement. For example, two decimal places means that a mechanic can use a caliper to make measurements, while three decimal places means that a micrometer or equivalent CMM tool must be used. In order to minimize costs, please avoid adding any extra decimal points to the dimensions beyond what is necessary.

Pro tip: Using geometric dimensions and tolerances (GD&T) in your technical drawings (such as flatness, straightness, roundness, and true position) can reduce the cost of CNC machining because they usually define more relaxed tolerances, but require advanced Design knowledge can be applied effectively.


Tip #7-Keep the number of machine settings to a minimum


Example of a part that requires two machine settings: Rotate the part to machine side features

It is recommended to design parts that can be processed in as few machine settings as possible, preferably in only one setting. For example, a part with blind holes on both sides will be machined in two settings because it needs to be rotated to enter both sides.

Rotating or repositioning parts increases manufacturing costs because it usually needs to be done manually. In addition, for complex geometries, custom fixtures may be required, which further increases the cost. Particularly complex geometries may require a multi-axis CNC system, thereby further increasing the price.

Consider splitting the parts into geometric figures that can be CNC machined in one setup, which can then be bolted or welded together. This also applies to parts with very deep pockets.

Minimize costs:

Design parts that can only be machined in one setting.

If this is not possible, divide the geometry into multiple parts for later assembly.


Tip 8-avoid small features with high aspect ratio


Consider adding support for small features with an aspect ratio greater than 4

Small features with high aspect ratios are prone to vibrations, so they are particularly difficult to machine accurately.

In order to increase their stiffness, they should be attached to a thicker wall or reinforced with support ribs (preferably four: one on each side).

Minimize costs:

A design feature with an aspect ratio of less than 4.

Add supports or connect small features to the wall to increase its stiffness.


Tip #9-delete all text and lettering


Adding text on the surface of CNC parts will increase the price

Adding text to the surface of a CNC machined part will significantly increase the cost because of the need for additional and time-consuming processing steps.

Surface finishing methods, such as silk screen or spray painting, are more cost-effective ways to add text to the surface of CNC machined parts.

Minimize costs:

Delete all text and letters on CNC machined parts.

If you need text, prefer to engrave instead of embossed text, because the latter requires more material to be removed.

In addition, it is best to use a Sans-Serif font (such as Arial or Verdana) that is at least 20 points in size.


Tip #10-Consider the machinability of the material


Machinability refers to the ease with which a material can be cut. The higher the machinability, the faster the CNC can process materials, thereby reducing costs.

The processability of each material depends on its physical properties. Generally, the softer (and more ductile) a metal alloy, the easier it is to process.

Brass C360 is the metal with the highest machinability and can be processed at high speed. Aluminum alloys (such as Al 6061 and Al 7075) can also be easily processed.

The workability of steel is 10 times lower than that of aluminum, and it takes at least 2 times longer to process. Please note that different steel grades have different machinability. For example, stainless steel 304 (the most common stainless steel alloy) has a machining index of 45%, while stainless steel 303 (an alloy with a very similar chemical composition) has a machining index of 78%, making it easier to process.

The processability of plastics mainly depends on their stiffness and thermal properties. During CNC processing, plastics are easy to melt and warp.

POM (Delrin) is the easiest plastic to be processed, and ABS follows closely behind. PEEK and nylon are other common engineering plastics, and their processing is slightly more difficult.

Minimize costs:

If you can choose between materials, please choose materials with better machinability (especially for large-volume orders).


Tip #11-consider the cost of bulk materials

The cost of bulk materials is another factor that can greatly affect the price of CNC machined parts.

The following table summarizes the prices of metal alloys and plastic materials commonly used in CNC for plates with dimensions of 6” x 6” x 1” (or approximately 150 x 150 x 25 mm).

Material batch cost

Aluminum 6061 25 USD

Aluminum 7075 80 USD

Stainless steel 304 90 USD

Stainless steel 303 150 USD

Brass C360 USD 148

ABS USD 17

Nylon 6 $30

POM (Dellin) 27 USD

Peeping $300

 

Aluminum 6061 is clearly the most cost-effective way to make metal prototypes because it combines low cost with very good workability.

Please note that metals with higher machinability, such as stainless steel 303 and brass C360, are more costly and therefore more suitable for mass production, as shorter processing times will mask greater material costs and take advantage of economies of scale.

For plastics, the prices of ABS, nylon and acetal (Delrin) are about the same as aluminum 6061. But they are more difficult to process, so the price is expected to be higher. PEEK is a very expensive material and should only be used when necessary.

Minimize costs:

Choose materials with low batch costs (especially small batch orders).


Tip #12-Avoid (multiple) surface treatments

Surface treatment improves the appearance and resistance of CNC machined parts to harsh environments, but also increases their cost.

Requiring multiple different surface treatments on the same part will further increase the price because of the additional steps required (for example, masking the surface).

Minimize costs:

Select the surface finish after processing.

Multiple surface treatments are required only when absolutely necessary.

Tip #13-consider blank size


The blank size must be approximately 3 mm (.125'') larger than the part

The size of the blank (stock material) may affect the overall cost: to ensure good accuracy, some material must be removed from all edges of the part. This can have a significant impact on material costs (especially for large-volume orders).

As a rule of thumb, the blank must be at least 3 mm (.125'') larger than the end. Let's take an example to see how the size of the blank affects the pricing of CNC:

If you design a part with a 30 x 30 x 30 mm envelope, the next larger blank will be used, in which case it is usually cut from a 35 mm thick sheet. If the part has a shell of 27 x 27 x 27 mm, a 30 mm thick sheet can be used, which saves a lot of material.

Minimize costs:

Design parts that are 3 mm smaller than the standard blank size.

 

Take advantage of economies of scale (extra tip)

In CNC machining, the quantity has a great influence on the unit price. This is because start-up costs are relatively high, and when the number is small, they account for a large proportion of the cost. However, for large batches, they are almost eliminated because these costs are spread over many parts.

In the figure below, we have summarized the average unit prices of 12 different parts machined in 304 stainless steel.

 

The decrease in unit price is almost exponential: even a small increase in quantity will make the unit price drop a lot. Please note that ordering very large quantities (> 100 parts) will reduce the unit price by 5 to 10 times. The impact of economies of scale is also obvious for low production.

Actually:

Increase the amount from 1 to 5 by reducing the unit price by more than 50%!

Minimize costs:

Take advantage of economies of scale by ordering larger quantities or repeating orders.

Understand how your design changes affect costs

 

In short: keep the standard...

The complexity of CNC is costly: geometries that require special tools or fixtures, multiple machine settings, or special materials will have a higher cost.

In order to minimize costs, please consider the following questions before submitting a quotation order:

 Is my part optimized using design guidelines for machinability?
 Are all the features in my model required? Can I delete or simplify any of them and still retain the full functionality of my part?
Can my design be split into multiple parts for CNC machining and then assembly?
 Is there a way to modify my design to eliminate the need for multiple machine settings or special tools?
Is there a cheaper or easier-to-process material that can meet my design requirements?
If above explain good for your work pls leave a message: 

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