Reducing CNC machining costs (14 tips)

what affects the costs of CNC machined parts?

CNC machining can be a complex process and because of that costs can rise quickly. This can be a real deal breaker for any engineer trying to get their parts machined. Which is why, we made a list with 14 cost reduction tips to help you keep your CNC machining costs to a minimum.

Welcome back to another episode in our series: what is CNC machining? Today we are going to go discuss 14 tips that will help you keep your CNC machining costs to a minimum. In the description below you’ll find the link to our essential cnc cost reduction checklist which includes even more information. Before we start discussing the tips let’s take a quick look at what affects the costs of CNC machined parts.

The first: machining time and model complexity. The more complex the geometry of a part is, the longer it takes to machine and the more expensive it will be.

Second: start-up costs. These are related to CAD file preparation and process planning. They are significant for smaller volumes but are fixed. With these, there is an opportunity to reduce the unit price by taking advantage of economies of scale.

Third: material cost & finishes. The cost of the bulk material and how easily that material can be machined greatly affect the overall cost.

As a rule of thumb: to minimize the cost of CNC machined parts, stick to designs with simple geometries and standardized features.

Now that we know what affects the cost of a CNC machined part it’s time to move on to our cost reduction tips.

Tip 1: Add a radius in internal vertical edges

CNC milling tools have a cylindrical shape and will create a radius when cutting the edge of a pocket. To minimize cost add a radius of at least 1/3 of the depth of the cavity (the larger the better). Preferably, use the same radius for all internal edges. On the floor of the cavity, specify a small radius (.5 or 1 mm) or no radius at all.

Tip 2 - Limit the Depth of Cavities

Machining deep cavities affects the cost of CNC parts drastically. A lot of material needs to be removed which is very time consuming and special tools might be required.

When it comes to the depth of cavities it’s important to keep in mind that CNC tools have a limited cutting length. Typically they‘ll work best when cutting cavities with a depth of up to 2 to 3 times their diameter.

It is possible to cut even deeper cavities but this will increase the machining cost, as special tooling or multi-axis CNC systems are required. To minimize costs, always limit the depth of all cavities to 4 times their length.

Tip 3 - Increase the Thickness of Thin Walls

Thin walls are prone to vibrations, deformation and fractures. Machining them accurately is therefore time and cost consuming. So, unless weight is a major factor, thick solid sections are more stable and are preferred. To minimize cost for metal parts, design walls thicker than 0.8 mm (the thicker the better). For plastic parts, keep the minimum wall thickness above 1.5 mm.

Tip 4 - Limit the Length of Threads

Threads that are longer than necessary can increase the cost of CNC parts, as special tooling may be required. To minimize costs, design threads with a maximum length of up to 3 times the hole diameter. For threads in blind holes, it is recommended to add at least 1/2 the diameter of unthreaded length at the bottom of the hole.

Tip 5 - Design Holes with Standard Size

Holes can be CNC machined fast and accurately using standard drill bits. For non-standard sizes, the hole must be machined using an end mill tool, potentially increasing the cost. Therefore, standard drill bits are prefered.

To minimize costs, design holes with a diameter that is an increment of 0.1 mm for diameters up to 10 mm and 0.5 mm above that.

Also, limit the depth of all holes to 4 times their diameter. Deeper holes (up to 10 times the diameter) can be manufactured, but they will likely increase the cost, as they are problematic to machine.

Tip 6 - Specify Tolerances only when Necessary

Defining tight tolerances raises the cost of CNC machining, as it increases the machining time and requires manual inspection. If a specific tolerance is not specified the parts will be machined using the standard tolerance (at least ± 0.125 mm). To minimize cost define a single datum (for example the cross section of two edges) as reference for all dimensions with tolerances.

Tip 7 - Keep the number of Machine Setups to a Minimum

Design parts that can be machined in as few machine setups as possible - preferably in only one setup. Repositioning a part adds to the cost, as it usually needs to be done manually. For complicated geometries this is even more important to consider as they may require a multi-axis CNC system, further increasing the price. Consider splitting the part into geometries that can be CNC machined in a single setup and can be bolted or welded together later.

Tip 8 - Avoid Small Features with High Aspect Ratio

Small features with high width-to-height aspect ratio are prone to vibrations, and therefore difficult to machine accurately. To improve their stiffness, add bracing support or connect the small features to a wall. Also, design features with a width-to-height aspect ratio less than 4.

Tip 9 - Remove all Text & Lettering

Adding text on the surface of a CNC machined part can increase the costs, as additional and time-consuming machining steps are required. If text is necessary, choose engraved over embossed lettering. Surface finishing methods, such as silk screening or painting, are a more cost-effective way of adding text on the surface of a CNC machined part.

Tip 10 - Consider the Machinability of the Material

If you have the option to choose between materials, choose the one with better machinability. Machinability refers to how easily a material can be cut. The higher the machinability the faster a material can be CNC machined, which lowers the cost.

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

Brass C360 is the metal with the highest machinability, allowing for high-speed machining. Aluminum alloys can also be machined very easily.

Steels have 10 times lower machinability than aluminum and will take at least 2 times more to machine. Note that different steel grades have different machinability. You can find more information on the machinability indexes of the different steel grades in our downloadable cnc cost reduction checklist.

The machinability of plastics mainly depends on their stiffness and thermal properties. During CNC machining, plastics are prone to melting and warping.

POM (Delrin) is the easiest plastic to machine, while ABS comes in close second. PEEK and Nylon are other common engineering plastics that are a bit more difficult to machine.

Tip 11 - Consider the Cost of the Bulk Material

The cost of the bulk material is another factor that can affect the price of the CNC machined parts. To minimize costs, choose a material with low bulk cost (especially for low-volume orders).

This table summarizes the price of metal alloys and plastic materials commonly used in CNC for a sheet with dimensions of 6’’ x 6’’ x 1’’ (or approximately 150 x 150 x 25 mm).

Aluminum 6061 is clearly the most cost-effective way of creating metal prototypes as it combines low cost and very good machinability.

Notice that the metals that offer greater machinability, like Stainless Steel 303 and Brass C360, have a higher cost and, for this reason, are better suited for larger volume production, as the larger material cost will be overshadowed by the much shorter machining times, taking advantage of economies of scale.

When it comes to plastics, ABS, Nylon, and POM (Acetal,Delrin) are priced approximately the same as Aluminum 6061. They are more difficult to machine though, so expect the price to be higher. PEEK is a very expensive material and should be used only when necessary.

Tip 12 - Avoid (Multiple) Surface Finishes

To minimize costs, choose the as-machined surface finish. Only request multiple surface finishes, when absolutely necessary. Surface finishes improve the appearance and resistance to harsh environments of CNC machined parts, but also increase their cost.

Tip 13 - Think Blank Size

To minimize costs, design parts with dimensions 3 mm smaller than a standard blank size. 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 effect on the material cost (especially for high volume orders). As a rule of thumb, the blank must be a minimum of 3 mm (.125’’) larger than the end part.

Tip 14 - Taking advantage of Economies of Scale

Take advantage of economies of scale by ordering higher quantities or placing a repeat order.

In CNC machining, the quantity greatly affects the unit price. This is because the startup costs are relatively high and, when quantities are low, they represent a big percentage of the cost. BUt for large quantities, the startup costs are almost eliminated as these costs are divided over many parts.

In this graph, we summarized the average unit price of 12 different parts machined in Stainless Steel 304. The drop in unit price is almost exponential: even a small increase in quantity can decrease the unit price by a lot. Notice that ordering very high volumes (> 100 parts) reduces the unit price by 5 to 10 times. The effect of economies of scale are noticeable also for low volumes.