You don't have to sacrifice part performance or machining manufacturability to make your CNC Machined parts more cost-effective. The Xometry application engineering team has put together 10 tips that will save costs, increase manufacturability, and reduce headaches when scaling from low-volume to production manufacturing.
Xometry’s lead CNC Applications Engineer, Tim Bowman, explains these best practices in the video below.
Watch and make your CNC parts more cost-effective today:
Xometry applications engineer Tim Bowman goes over the best tips to reduce CNC machined part costs.
Let’s take a deeper dive into why these tips will save costs on your project. If you are just getting started with designing for manufacturing, check out Xometry’s free Design Guide for CNC Machining. The common theme in all these tips is that decreasing time and risk will decrease both upfront costs and repeating costs over bulk orders.
Tip #1: Round Internal Corners using ≤ 3:1 Ratio
Details: All internal corners in machined pocket features should be rounded, not squared. Be generous with the radii of internal corners in pockets. An inside radius of 3:1 (Length to Depth) or less will allow a single tool to create the features smoothly and quickly.
Reason: CNC machining uses spinning tools to cut metal and plastic parts into shape. Most commonly, an endmill is used to make features like internal pockets. Since an endmill creates a rounded vertical cut, the internal corners cannot be sharp or squared. Additionally, the radius on an internal corner should be above a standard endmill size to facilitate faster cutting of the feature. This allows the cut action to engage an even amount of material regardless of it being a side or corner keeping the cutting movement, or feed, consistently fast.
Tip #2: Avoid Deep Pockets
Details: As a best practice, we suggest parts with lengths up to 4X their pocket depth. Anything greater than that becomes exponentially more expensive to manufacture. Note that per tip #1, the deeper the feature is the larger the tool radii, as well as radiused corners, should be for optimal machining. For example, round drills can make holes at 1:10 radius to depth ratio where endmills should hold closer to a 1:4 ratio.
Reason: Cutting tools are made with very hard materials so they remain rigid during operation. This also means that materials, such as carbide, are brittle. Anything that causes flex or vibration in the tool may cause it to snap or chip. Parts with deep internal cavities are often time-consuming and expensive to manufacture because the tool must be progressively stepped down in smaller increments to form the features.
Tip #3: Limit Use of Tight Tolerances
Details: Design as much as possible to the General Manufacturing Tolerances of the process. Only assign tighter tolerances and GD&T to mission-critical features and surfaces. Also, try to define a single datum, like the intersection of two sides, or the center of a hole, and then dimension everything else from that common reference.
Reason: CNC machining is inherently a very accurate process. Typical distance tolerances are +/- 0.005.” To achieve tighter tolerances, an operator must carefully control the operation and even make changes to the machining code on the fly. This adds time for in-process inspection (IPI) and increases machining time.
Xometry can produce tolerances as close as 0.001” repeatedly, but specific inspection tools may be required for measurement. You can request an inspection report when generating an online manufacturing quote. General Dimensioning and Tolerances (GD&T) call outs like surface profile will inherently require a CMM report, adding cost and time to the project.
Tip #4: Expand Thin Walls
Details: Thin walls should have a minimum width of 1/32” with double that, 1/16,” recommended for standard features. If very thin walls are needed, it’s usually more economical to utilize other manufacturing methods, such as sheet metal fabrication services.
Reason: Chatter on a part looks like rough, scale-like features on a flat edge. This is typically caused by the part moving or flexing during cutting. Managing chatter requires specialized tooling and slower feed rates when machining, increasing costs. Designing thin features to be thicker makes that surface more resilient to flexing, which will reduce or eliminate chatter.
Tip #5: Optimize Tapped Holes
Details: Avoid tap depth call-outs that are more than 3X the diameter of the hole. Ensure there is at least 1/32 clearance beyond the tap depth in the hole. This does not include clearance for drill tips.
Use standard thread sizes for your parts.
Reason: The hole-tapping depth and tap size both play a role in costs. Excessive tapping, such as full-thru tapping, may require special tools or risky manufacturing. These can increase costs as well as production scrap likelihood. Mechanically, increasing tap length beyond 3X the diameter of the hole does not increase the strength of the hold. Most of the work is done by the first few turns. Standard threads mean standard taps, which are highly commoditized. Custom threads can add hundreds of dollars for special purchases and check gauges.
Tip #6: Leverage Standard Drill Sizes
Details: Keep hole sizes to standard fractional, number, or letter drill sizes.
Reason: Drill bits are used to cut circular holes equal to their diameter. Unlike an endmill, drill bits are not designed to move laterally. Circular holes designed outside of standard drill sizes would need to be manufactured with an endmill or reamer, adding to part costs.
Tip #7: Use Less Expensive Materials
Details: When possible, use less expensive materials in your design. Aluminum 6061-T6 is the least expensive CNC metal. Acetal (Delrin) is typically the least expensive machined plastic.
Reason: Softer materials, like Aluminum 6061-T6, are easier to cut than materials like steel. This reduces cycle time during machining and allows these materials to be cut with standard drills and endmills. Harder materials require more expensive tools and can add to the project cost.
Tip #8: Avoid Multiple Finishes
Details: Reduce surface finish requirements when possible. This includes part marking, surface smoothing, and post-processing finishing services.
Reason: Finishes like reduced surface roughness (below 125 Ra), knurling, and other texturing is applied during the machining stage and can increase cycle times. Post-processing finishes like anodizing, chemical conversion coating, or paint are applied after the part is machined. Each finish requires setup and potentially masking. With multiple finishes, like chemical conversion and anodize on the same part, there are additional handling steps increasing time and cost per part.
Tip #9: Split Up Complex Parts
Details: When possible, split up large and complex parts into an assembly. The use of screws and pins can achieve flush assembly after production. Weldments are also great ways of fabricating complex metal shapes by combining simpler designs.
Reason: Large parts, like those with deep pockets, require significant material removal from solid billet, adding machine time. Internal features may also require specialized tooling to produce. Producing shallow features can be significantly quicker.
Tip #10: Order Larger Quantities
Details: Ordering multiple quantities of the same part helps generate production efficiencies and lower the cost per piece.
Reason: Machining requires CAM programming and setup per operation. These are non-recurring costs, which means that the first part produced will be significantly more expensive than any additional parts. As quantities increase, setup costs amortize and most parts costs are associated with direct material costs and machine time.
Need a daily reminder of these tips? Download our PDF poster version to keep the Top Ways to Save on CNC Machining handy for your whole team! Also be sure to check out our full blog post with annotated images. To test these tips on your own designs, get an instant quote today.
This article was originally published on 11/13/17 and was updated on 11/17/20.