6 Considerations for CNC Turned Parts
Complex machined parts are our bread and butter at Janee Precision, so we’ve built up a robust lathe department equipped with conventional and Swiss style machines to deliver high-precision CNC turned parts fast. Most of our machines are multi-channel which allows us to do work on the main and sub spindles at the same time, and bar feeders allow unattended operation for production runs. Tap to learn more about "What is Turning"
Our CAM software allows us to quickly and accurately program complex profiles for turning and boring as well as milled features that can be machined with live tooling.
We also offer secondary processes for turned parts, such as centerless grinding, cylindrical grinding, and honing.
Simply put, when you come to us for CNC turning, we’re committed to getting you the highest quality parts as quickly as possible—and that process begins before you even submit a quote.
We compiled a handy list of factors to consider when communicating your CNC turned part requirements to any machine shop.
Top Considerations for CNC Turned Parts
1. Thread class
Specifying a preferred thread class, which determines the fit of the thread, is a best practice for any type of CNC machined part. Here’s a not-too-technical breakdown of thread classes:
- Class 1 is the sloppiest fit, consisting of undersized male threads and oversized female threads. This type of fit is common for applications used in harsh or dirty environments.
- Class 2 is considered a standard fit and has a reasonable amount of clearance. If customers don’t specify a preferred thread class, machine shops will typically default to Class 2.
- Class 3 is the tightest fit and is ideal for precision equipment, commonly found in laboratory or cleanroom environments. Class 3 threads are the most difficult to machine, adding cost to the part. There are no real strength benefits in going from a Class 2 to a Class 3 thread, so we don’t recommend doing so unless your application requires this thread type because of accuracy.
2. Corner radii
Standard turning inserts generally have a 0.008” to 0.016” corner radius. Parts are often designed with perfectly sharp corners, but this can’t be machined easily or reliably with standard tooling. Even standard grooving tools have a .002” – .008” tip radius. If you do need a perfectly sharp corner, be sure to call it out in your drawing or most shops will assume a standard corner radius is okay (you can count on us to ask).
If you need the functionality of a sharp corner, and you have some design freedom, try adding a slight undercut on sharp inside corners or a chamfer or radius at the end of ID bores to allow parts to fit. This creates clearance for mating parts while allowing the use of standard tools for a reliable and fast machining process. Reliable and fast equals better and cheaper!
3. Thread relief
It is important to consider thread relief to make sure your parts will function as intended. OD and ID threading is a very common process on CNC lathes. There are many processes to create threads on a turned part, but the most common are single point threading and tapping. With either process, there will be some amount of unusable thread depth. On a standard tap, the first 3-6 threads are tapered to allow free cutting or forming of threads. This means that your drilled hole needs to be at least 6 threads deeper than the amount of usable threads to be easily machinable
For single point threads, the last 1-2 threads will be partial because the machine has to retract while the spindle is running at a high speed. In most cases that won’t cause an issue, but if you need something to thread right up to a shoulder, you will need a thread relief groove that is about 2x as wide as the thread pitch. This can be done for internal or external single point threading.
With any threading process, a burr is created on the end of the part where the thread profile is reduced to a sliver. That burr is usually minimized by adding a chamfer to the part before threading and it is important that the chamfer is larger than the depth of the thread profile. If not, there will still be a burr on the end of the part, which is never a good thing. If you don’t specify a chamfer, it will be added. This isn’t optional because we don’t make bad threads.
Another option for removing the burr is to use live tooling to remove the first revolution of the thread on the end of the part. This is called a Higbee thread and it can be done on ID or OD threads. While this adds a process and can’t be done on all machines, it removes the first partial thread completely. This process ensures parts thread together easily, and eliminates the possibility of cross threading. The most common application of Higbee threads is for firefighting equipment that has to be assembled quickly in any situation. If you want the absolute best quality for your threaded part, this is the way to go. It also ensures consistent thread quality for high volume production runs.
4. Mating part tolerances
If you have two mating parts, it’s important to define the tolerances so that the parts will fit together even if each one is at the extreme of its tolerance range. This callout is especially key for differentiating between press fits (parts that are secured together permanently) and slip fits (parts that slide together and come apart easily).
At Janee Precision, our specialty is complex machined parts, and most of the parts we turn have a diameter of 1” or less. For most small parts, we recommend a starting point of 0.0005” – 0.001” of total clearance for a slip fit and the same amount of interference for a press fit. These can be adjusted depending on material, application, and the size or profile of the mating features.
5. Length to diameter ratio.
CNC turned parts with a length to diameter ratio greater than 3:1 are prone to tolerance and finishing issues due to the possibility of tool or part vibration.
Does this mean you’re restricted to a 3:1 ratio when designing a part? Not necessarily. It is usually possible to machine up to 6:1 by carefully adjusting cut parameters, but it will increase machining time. Adding a center drilled hole in the end of a part allows us to support even longer parts of 10:1 or more with a live center during machining.
Advanced equipment like sub-spindle machines also give us greater flexibility when turning long parts. We can support both ends of a part and machine different sections in a series of steps to preserve our tools and produce high-quality turned parts. We make some parts that are over 200:1 using this process.
6. Milling features
Example of a turned part with milled features.
There’s a common misconception that adding milled features to a CNC turned part requires a second operation. This assumption may have been accurate years ago, but many modern lathes are equipped with live tooling that’s capable of milling basic features.
It’s easy for us to add features like cross holes and flats with little impact on cost and lead time for our customers. We can also add more complex milled features and even have 5-axis capability for turned parts under 1” diameter. Tap to learn more about turning & milling combination processes.
