Design Tips for Low-Volume CNC Machining Production Runs

Originally published on fastradius.com on October 19, 2022

In the manufacturing business, more money put into production generally means less profit or higher prices for customers. But careful planning and informed decision making about your design will help save time and money during the CNC machining process.

CNC machining costs can be divided into two main buckets: non-recurring costs and piece costs. Non-recurring costs refer to all of the upfront costs required to get any production run (large or small) off the ground; these costs are amortized over the production quantity. Piece costs are expenses associated with each individual part, and scale with quantity. The bottom line cost to manufacture a single part, taking into account all cost buckets, is often referred to as the fully burdened part cost.

To put this in mathematical terms: 

Fully Burdened Part Cost = Non Recurring Costs / Quantity + Piece Costs

Non recurring costs include:

• Part design
• Planning (manufacturing process, inspection, etc.)
• CAM programming
• CMM programming
• Workholding, fixtures, gauges, and tooling identification and sourcing

Piece costs include:

• Machine time
• Labor time (loading and unloading parts)
• Consumable tooling
• Raw material
• Cleaning, deburring, and finishing
• Manual inspection

Following the equation above, in lower volume runs, non-recurring costs are amortized over fewer parts, which means the expense is higher per part. For example, if the non-recurring costs for a job are $5,000, and the production quantity is 100 parts, then each part will have a burden of $50 for non-recurring expenses. On the other hand, if that same job is ordered for a quantity of 200 parts, this would result in a non-recurring burden of just $25 per part, a potentially significant cost delta. The good news is that you can minimize CNC machining costs for shorter production runs by optimizing your part designs.

Design tips for short production runs

To maximize your time and budget, try these design tips for short production runs:

Use standardized, simple designs

Keep your design as simple and standardized as possible. Overly complicated part designs can require multiple manual rotations and repositions, more expensive CNC systems, or specialized tools. With this in mind, it may be worthwhile to break up a complex piece into simpler components that can be assembled later.

You’ll also want to:

• Design standard-sized holes: The deeper the hole, the more complex the metal chip evacuation process, and the more expensive your part. Try to make each hole’s depth no more than five to six times the drill’s diameter to ensure your machinist can quickly evacuate the metal chips. Also, make sure to use standard drill sizes when designing your holes or you may end up needing to purchase a custom tool, which can drive up costs and overall production time further.
• Use standard threads: As with drill sizes, using standard thread depths and diameters can save time and money. In practice, this means simply using an existing thread class instead of creating a custom thread via CNC machining or 3D printing. Try to use the H2 thread tolerance, since H3 is tighter and more expensive to create. Also, it’s best to keep your thread depth no deeper than required by your part’s functional and structural specifications.
• Design with standard stock sizes in mind: You can also cut costs and machine times by designing your parts to align with standard stock sizes. Instead of ordering a larger piece of material only to have to machine it down, your machinist will be able to buy a standard stock piece and then make few (if any) changes before starting on your part. Beyond choosing standard stock sizes, it’s also a good idea to allow a large enough tolerance so that the outside of the part does not need to be machined.
• Use loose tolerances: Tight tolerances can require specialized manufacturing processes or secondary operations, and can increase set-up, machining, and inspection time and cost. With that in mind, your part’s tolerances should only be as tight as required by functional and structural constraints.

Avoid thin walls, tall walls, and narrow pockets

CNC machining can be a delicate and time-intensive operation, especially if you have thin walls, tall walls, or narrow pockets.

Since they can increase the risk of cutter deflection, deformation, compromised surface finishes, and part failure, and make it difficult to meet specified tolerances, thin walls are generally less stable and more expensive to machine. Beyond those considerations, your machinist may need to take numerous passes over a part to create a thin wall without accidentally fracturing or snapping it with excessive vibrations, which will, of course, further drive up machining times and costs.

There are options for addressing the increased machining times, higher production costs, and fracture risks associated with thin walls. For example, if you have a metal part, try to design walls that are at least 0.8 mm thick. For plastic parts, try to aim for walls at least 1.5 mm thick. Similarly, you’ll want to avoid including tall walls, deep cavities, and narrow pockets in your part, since these features require longer cutting tools and the removal of more waste material. Like the problems associated with thin walls, these issues can lead to increased cutting tool deflection, chatter, inaccuracies, reduced tool life, and sub-par surface finishes, all of which drive up the final part cost.

Avoid unnecessary text or finishing processes

Adding text to your part will enable you to clearly number components, include a description, or apply a company logo. While machined text looks aesthetically pleasing, and can be functionally useful, it’s an expensive and time-consuming process since you’ll need to trace each character with a small ball end mill or engraving tool. Unfortunately, including raised text on a part is even slower and costlier since even more material will need to be milled away from the part to achieve the effect.

As an alternative to machining, if your part requires text, lettering, or a logo, you may be able to save time and money with a post-production surface finishing method. For example, silk screening, laser marking, rubber ink stamping, and painting will all enable you to add text to your parts faster than direct engraving. Of course, given that adding any CNC finishing process will increase your parts’ turnaround time and overall costs, it’s best to avoid text and other non-critical finishing processes whenever possible.

Choose the proper material

Material selection is important in both long and short production runs since using the wrong metal or plastic for your parts can drastically increase project costs. Some materials are more difficult to machine than others, which means longer turnaround times and higher machining costs. Even if two materials are equally machinable, the chances are that one will be more expensive than the other.

For example, while titanium is ideal for aerospace applications, it’s expensive and difficult to machine. On the other hand, softer, less costly metals, such as aluminum, are easier to machine and strong enough for everything from airplane parts to architectural components. Similarly, if you want to create a plastic part but don’t require the strength or high heat resistance of polyetheretherketone (PEEK), you can save money by using acrylonitrile butadiene styrene (ABS) or another high-performance engineering thermoplastic.

To put it simply, if you don’t really need the properties of an expensive and/or rare material, you should use a less expensive and/or more common material. As an added benefit, using a more readily available material also likely means a reduction in your parts’ overall lead time. It’s worth applying this mentality during the prototyping phase, too; for example, if you only need to create a proof-of-concept model, you should use the least expensive plastic that will still demonstrate the feasibility of your idea.

Additive manufacturing as an alternative for short production runs

While designing with CNC machining lead times in mind can help reduce turnaround and cut costs, depending on your material requirements and the size of your run, additive manufacturing might represent a better option

In contrast to CNC machining, which is a subtractive process that involves cutting away material to shape a final product, the additive manufacturing process builds parts layer by layer. Additive manufacturing is typically far less labor-intensive than CNC machining, sometimes only requiring limited mid-production repositioning and limited post-processing to enhance a part’s functionality, or to achieve a desired finish. Like CNC machining, additive manufacturing can be used to make metal parts, but plastic additive manufacturing is far more common. In fact, many of the plastics used in additive manufacturing are mechanically equivalent to those used in injection molding. 

Generally, additive manufacturing is a better choice for extremely low-volume production runs or for projects in which finished parts or prototypes must be delivered fast, and where tolerance requirements are not tight. By contrast, CNC machining is more appropriate for low-volume production runs with part quantities in the higher double digits or low hundreds, or in projects that have high tolerance requirements.

Working with an experienced manufacturing partner

Each of our design tips for short production runs can help you ensure that your production process is as smooth, cost-efficient, and fast as possible. However, there’s a lot to keep track of when optimizing parts for low-volume production, so working with a trusted manufacturing partner is also a valuable opportunity to take some of the weight off your shoulders.

From design through fulfillment, when you work with SyBridge, our team of experienced engineers will provide you with the support you need to make your project possible. Our cloud-based tools are designed to help you pinpoint design flaws, determine optimal order quantities, and find the right material for your part’s specific application. Simply create an account and upload your part files. We can generate instant quotes for both additive manufacturing and CNC machining projects, which means you can get started making your new parts and products today. Contact us today.