CNC turning is a form of machining that uses a computer-controlled lathe to make cuts in a rotating workpiece. It is an essential process for creating cylindrical components like shafts.
Turning is one of the most common machine shop processes. However, CNC turned parts must be designed using very different criteria to milled parts. Lathes can create near-perfect curves impossible to achieve on other equipment, but they are limited in the types of cuts they can make. Furthermore, different features require different tooling and turning operations, such as tapered turning, knurling, parting, and facing.
This article explains how to design for CNC turned parts when requesting CNC turning services from a trusted machine shop like 3ERP. It shows what can and can’t be achieved via turning and explains exactly how you can communicate your requirements to our engineers.
How Does CNC Turning Work?
CNC turning is a type of CNC machining that uses a unique mechanical process. Unlike CNC milling, which uses a rotating cutting tool to remove material from a stationary workpiece, turning or lathe machining involves rotating the entire workpiece at high speed while a non-rotary tool removes material from it.
Because the workpiece rotates on a fixed axis, any point on the cutting tool’s path creates a perfect circle around that center. It works a bit like a potter’s wheel: push a finger into the outside of the spinning clay, hold it in place, and you create a ridge spanning the entire circumference of the workpiece.
Different turning operations create different cuts. Some of the most common include:
- Facing: The tool moves across the end of the spinning workpiece, reducing its length and making the end perfectly flat.
- Grooving: The tool cuts into the side of the spinning workpiece to make a narrow channel around its circumference.
- Parting: A wide tool cuts right through the side of the spinning workpiece to cut it in two, usually to remove the finished piece from the bar stock.
- Straight Turning: The tool moves parallel to the side of the spinning workpiece, reducing its thickness or diameter.
- Threading: The tool cuts the spinning workpiece while moving sideways at a consistent speed, making a continuous spiral groove such as a mating thread.
Our detailed blog post on CNC turning contains more information on the operations, tooling, advantages, and applications of the process.
General Design Considerations
All manufacturing processes benefit from proper Design for Manufacturing (DfM), but CNC turning has particularly narrow design limitations. It is an incredibly powerful process capable of producing high-quality CNC turned components like shafts and bushings, but it is not the most versatile.
When considering how to design for CNC turned parts, the main thing to remember is that most turned parts are primarily cylindrical, have circular features like bores or grooves, or are symmetrical around a central axis. That being said, some parts may use CNC turning to create cylindrical or tapered areas, then use CNC milling to create the asymmetrical details. However, more setups and more equipment increases the cost of the part.
Another factor to consider is that, when it comes to turning vs milling, milling wins the size contest hands down. For example, our Haas ST-20Y lathe has a 21-inch (533 mm) maximum swing (largest possible workpiece diameter), while our biggest machining centers can produce larger parts.
When ordering turned parts from a machine shop and prototyping specialist like 3ERP, remember to specify tolerances, material requirements, and inspection requirements.
For more information, consult our CNC machining guide.
7 Design Tips for CNC Turning
This section provides seven must-remember design rules for CNC turning, provided the part meets the basic requirements of the process.
1. Design Around Standard Bar Stock Sizes
Cylindrical parts should be designed around standard bar stock sizes. This minimizes material waste and machining time, saving you money on your part. Talk to us directly to find out what stock we can offer in your chosen material.
Importantly, this doesn’t mean you need to compromise on your design, as standard bar stock sizes are available in regular increments.
2. Locate Features on One End
Adding features to several sides of a turned part increases the need for secondary operations, increasing lead time and cost.
If possible, locate features like screw threads on one end of the part. Even if that means a bit of a redesign, it’s probably worth it.
3. Keep Parts Axisymmetric Whenever Possible
CNC turning prizes rotational symmetry. If your part doesn’t contain asymmetrical features, it can be turned quickly and efficiently, resulting in lower machining costs.
Introducing asymmetry may be necessary for certain features, but remember that re-fixturing adds to machining complexity and cost.
4. Minimize Length-to-Diameter (L/D) Ratios
The CNC turning length-to-diameter ratio is an important factor to consider, because it affects how much the workpiece is affected by flexural forces. Challenges in CNC machining of long slender shafts include machine CNC vibration or chatter, which reduces the accuracy of cuts.
When making long CNC turning parts, an ideal L/D ratio is about 3:1.
5. Avoid Unnecessarily Tight Tolerances
As with other machining processes, tight CNC turning tolerances require slower machining, increasing the overall cost of the job. Sometimes tight tolerances are important—such as with mating parts—but sometimes they can be avoided.
When submitting your technical drawing with your CAD files, specifying liberal tolerances on non-critical areas will reduce lead times and ultimately lower the quote we can offer you.
6. Design Practical Thread Depths
Threading is one of the most useful CNC turning operations, allowing the machinist to create spiral grooves in a part. But overly deep cuts during thread machining increase the risk of tool deflection or part damage. Besides, very deep threads often provide minimal fastener strength improvement over moderate ones.
Note that you generally don’t need to model the threads on your CAD design; instead, provide a thread callout in your technical drawing, noting the standard and size of the required thread.
7. Include Relief Grooves
Relief grooves or undercuts are recessed channels in a turned part, usually found where a shaft meets a shoulder (the “step” between different diameters) during shaft machining or at the end of a thread. Machining relief grooves allows mating components to fit flush against a shoulder.
Undercut machining is important because without a relief groove, an imperfect radius is left against the shoulder, preventing proper mating.
CNC Turning with 3ERP
Turning machining is the best way to create cylindrical parts with circular features. Because CNC turning parts can be made from standard bar stock with precise operations, parts can easily meet dimensional and performance requirements—especially when you use a machine shop with years of experience such as 3ERP.
However, good CNC manufacturing design means thinking carefully about the process during the CAD stage, not after. This allows our machinists to make your parts exactly how you need them.
