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. 

If material 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 Denver 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, 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 the customer’s to make. 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. 

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. High volumes of turned parts with extremely tight tolerance specifications may require custom grinding operations, increasing cost and lead time. 

6. Maintain a length to diameter ratio of 4X

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 tolerance 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, but we’ll have to use a workaround, like engaging the tailstock to add a center drilled hole. 

When you need a precision machining partner for your CNC turned parts, Focused on Machining is here to deliver. Contact us today, and let’s discuss optimizing your design for CNC turning services.