There are several factors that affect CNC machining cost, including material, design, finish, quantity, and lead time. Typically, delivery time has the greatest impact. This affects cost even more than material cost, setup time or type of machining.
The choice of material and the design of the part directly affect the machining cycle. Part geometry and tolerances also affect the number and type of machines required, and the skill level of the machinists required to use those machines, which in turn affects cost.
The following article will introduce you to 10 things that can help you optimize your design and save the CNC machining cost.
1. Optimize material choices
When choosing materials, we must consider both the cost of raw materials and the machinability of raw materials. If the material is difficult to process, even if the cost of the raw material is low, the final cost may be higher than that of a slightly more expensive raw material that is easier to process. In general, softer materials are easier to cut, so they require less machining time and can be cut with less expensive tools. Hazardous materials that require extra safety precautions can also increase cnc machining cost.
2. Choose volume and turnaround time trade-offs
The number of parts manufactured by a CNC milling machine directly affects the unit cost: Even if the total cost is higher, within 10,000 pieces, the higher the quantity, the lower the unit cost. Also, the lead time you expect the part to have affects the cost: parts delivered in a few weeks are more affordable than those delivered in two to three days.
3. Carefully evaluate finishes
Surface finishing and other treatments, such as heat treating, special coatings, and anodizing, add to project cost and should therefore be carefully evaluated. Multiple finishing processes or different surface finish types on a part can also increase the number of machining steps, thereby increasing costs.
4. Avoid complex part geometries
Part dimensions, including size and complexity, have a large impact on cost. Larger parts consume more material. Complex, highly detailed parts require multiple processes and possibly multiple machines, increasing programming, fixturing and setup costs. Some complex parts, such as those that require manipulation on more than one face, may be less expensive to produce if designed as separate components that are machined and joined together.
5. Avoid thin walls
Parts with extremely thin walls—usually defined as less than 0.794 mm (1/32 inch)—are not good candidates for CNC machining. Walls that are too thin can cause distortion, making it difficult to maintain tolerances. They also cause chatter, which slows down the machine. Both result in additional costs in machine and operator time. Other fabrication methods, such as sheet metal fabrication, may be more cost-effective for building walls thinner than this minimum.
6. Minimize internal cavity
Deep cavities (commonly called deep cavities) are one manifestation of complex parts. This design has a large impact on both processing time and material quantity cost. Creating a cavity can require many hours of machining. It is both a waste of material and difficult to remove chips.
The long, thin cutting tools needed to create these cavities can also break easily. A best practice is to make the part length at most four times the part depth.
7. Keep rounded inner corners
Sharp 90-degree angles require stopping the machine, rotating the part, and continuing the process. As you can imagine, these extra steps take up machining time and often end up costing you extra money. Changing the internal corner design of a part can affect the reduction of CNC machining costs.
If the design is adjusted to include rounded corners, the machine can run continuously to create the part and most likely reduce profits. Going a step further, make sure that the radius of the inside corners is at least 1/3 of the cavity depth. Larger radii at corners mean that larger tools can cut them efficiently – rather than smaller tools having to go over smaller radius corners multiple times.
8. Utilize standard drill sizes
A CNC machine can use a drill to create holes quickly and efficiently if you plan accordingly and make sure the holes in your part are a standard size. However, non-standard sizes often require the use of end mills, which can have a negative impact on cost. The depth of the holes also has an effect – although it is possible to create holes ten times the diameter, they are harder to machine and will cost more.
To minimize CNC machining costs, design in increments of 0.1 mm if the hole diameter is up to 10 mm. For anything over 10mm, use 0.5mm increments. If you are designing in inches, refer to fractional inch drill sizes and use fractional inches. Whichever size hole you use, make sure the hole is no longer than four times the diameter.
9. Limit the use of tight tolerances
When a part design includes tight tolerances, there is an increase in cost due to increased machining time and increased manual inspection. Unfortunately, achieving tight tolerances on the inside surfaces of parts is especially difficult because machining holes and other cavities creates burrs on the edges. These glitches require an additional step beyond manual inspection – deburring. Deburring is a manual and time-consuming process that also adds to the cnc machining cost..
If the design does not define specific tolerances, use a standard tolerance of ±0.125mm or better. This standard tolerance is fine for many features – so you should reconsider all tight tolerances and specify tight tolerances only when necessary. If you have the advanced design knowledge required to use geometric dimensioning and tolerancing in your designs, you can further reduce the cost of CNC machining because of the often looser tolerances that come with it.
A final note on tolerances – the numbers representing dimensions are critical as they specify the level of precision required for the part. In CNC machining, that level of precision translates to which tool will be used for the machining. The more decimal places you include, the more complex and costly the tool. Make sure to eliminate any unnecessary decimal points from your design.
