Updated for April 2025
Want better, faster, and less expensive CNC turned parts from your precision machining partner? At Focused on Machining, we provide CNC turning services at whatever level your project demands. We consistently turn parts to precise tolerances using sophisticated equipment and our extensive breadth of tooling.
Our talented lathe operators can machine almost any part that comes our way. Still, there are cost and time-saving considerations that we want our customers to know about.
Today, we’ll review 6 Design for Manufacturing (DFM) best practices to help you get the highest quality CNC turned parts quickly and cost-effectively.
6 Time and Cost-Efficient Considerations for CNC Turned Parts
1. Add allowances when designing for stock material specifications
Engineers will often design CNC turned parts as closely as possible to stock material specifications under the assumption that they’ll save on material costs and machine time. The problem with this approach is that the sizing for bar stock can vary up to 10%.
If, for instance, your part needs to measure close to ½”, then ½” bar stock may not be a viable option. Instead, we’ll need to size up to ⅝” or even ¾” bar stock to account for any dimensional variance.
Another important consideration is material straightness. Even with correctly sized stock material, bars can have slight bends or warping, and we may not have enough material to clean up the entire surface.
If costs are a major concern for you and you want us to order the minimal amount of bar stock, be sure to add adequate allowances when designing for stock material specifications.
2. Note “stock finish okay” on your print if applicable
Stock material can become damaged during transit, arriving with gouges, scratches, and other imperfections. At our Colorado machine shop, we always machine the external surfaces of the parts we make to improve the appearance of the surface finish—unless instructed otherwise.
If a stock finish is acceptable for your CNC turned part, let us know by indicating “stock finish okay” on your print. We’re happy to leave the outside of the part untouched if you only need internal features, and requesting a stock finish can save you up to 75% on the cost of your project.
3. Indicate if a center mark is permissible or not
If your CNC turned part is longer than it is round and is longer than 4 inches, we may need to engage the tailstock inside the lathe to support the part on both ends instead of just one. This technique creates a more rigid structure, allowing us to turn more features, hold tighter tolerances, and produce higher volumes faster. However, it does leave a small center mark behind where the tailstock enters the part.
The center mark may or may not be a problem depending on the part, but this determination is ultimately up to the customer. If it’s not permissible, we’ll need to complete an additional operation to remove it, which will affect cost and lead time.
Be sure to let us know up front whether a center mark is permissible or not so we know how to proceed.
4. Factor in corner radii
It’s possible for us to machine sharp corners in CNC turned parts, but doing so requires special (i.e., expensive) tooling. Even then, the tooling is prone to chipping and breaking.
A cost-effective alternative is to include a radius wherever two faces intersect to eliminate 90-degree turns. That way, we can use our standard tooling. A simple rule of thumb is to design radii measuring a minimum of .008”, but keep in mind that the larger the radius, the faster we can machine the feature. And we often remind customers: a larger radius in a perpendicular corner actually leads to a stronger part.
5. Avoid over-tolerancing the surface finish
If you were to take a milled part and a turned part directly off of each machine, the turned part would have an exceptionally smooth surface finish, while the milled part would show tooling lines from the end mill. But if you then measured the surface finish of both parts, the turned part would, counterintuitively, have a higher Ra value.
The takeaway is to be mindful of over-tolerancing CNC turned parts because the surface finish often looks much better than the measurement conveys. Extremely tight tolerance specifications may require custom grinding operations, and they can significantly increase cost and lead time in our production machining services. Not only that, high volumes require significantly more labor, as team members must check a large number of parts to ensure they match specifications.
6. Maintain a length-to-diameter ratio of 4X in parts
The further the workpiece is from the workholding, the less rigid and stable it becomes. If, for instance, your part has a length-to-diameter ratio of 10:1, chatter resulting from the tool vibration can compromise tight tolerance machining and surface finish specifications. A part with a 2X length-to-diameter ratio, on the other hand, is exceptionally rigid, allowing for efficient and precise machining.
If your part exceeds the recommended ratio, we can still machine it—our CNC machine shop has the problem-solving ability to deliver on our promises—but we’ll have to use a workaround, like engaging the tailstock to add a center-drilled hole.
Bonus Tip: Aim for a length-to-diameter ratio of 4X in holes
It’s possible to create holes with a greater length-to-diameter ratio than 4X, but the process becomes extremely slow. Essentially, we have to peck out the hole, withdrawing the tool repeatedly to remove the chips. Failing to withdraw the drill creates high heat and pressure that will ultimately break the drill, resulting in costly delays.
When you need a precision machining partner for your CNC turned parts, our precision machine shop is here to deliver. Contact us today, and let’s discuss optimizing your design for CNC turning services.