Computer Numerical Control (CNC) consists of the automated control of drills, lathes, 3D printers, and mills, to achieve a consistent result with any kind of machining operation.
Broaching existed for something like 150 years, but recent developments with technology allow broaching to integrate into the CNC machining environment.
It is now possible to machine broach parts in a rotary process or a linear process in a single setup, instead of having to use a secondary operation. The reason this is important is that it’s possible to broach features on a CNC machine at a mere fraction of the cost that other more traditional methods might cost.
Development of CNC Broaching technology
As CNC technology developed, it became possible to integrate broaching with CNC equipment, so that additional cutting tools could be used on the same machine. This triggered broaching companies to begin designing tools, tool bodies, and various holders that could be included in the CNC environment. It allows using a single setup to accomplish precise cuts and to lower the possibility of tolerance stack-up when moving parts from one machine to another. This signaled the arrival of CNC rotary broaches.
Since that point, technology continues to evolve with broaching systems now featuring indexable insert broaching tools, along with the introduction of tool holders. These kinds of tool holders make it possible to accept several specialized inserts for specific customer applications.
Benefits of CNC Broaching
CNC broaches come in a variety of different forms. They can be rotary or wobble broaches which you can typically use on a lathe, but they can also run on a Swiss-style machine that makes use of a special holder, and they can additionally run on a mill. Other types of CNC broaches are index or punch broaches on Swiss lathes that use a sub-spindle for free machining.
Single-point broaching generally happens using lathes or mills which generate a keyway, or which can produce square radiused corners. Indexable CNC broaches are common on Swiss machines and mills, and sometimes also on lathes, to produce any kind of size or shape, with features ranging from a standard keyway all the way up to a much more complex profile having multiple radii or splines.
You can use CNC broaches in both low-production and high-production environments, and you can also couple them to a driven head which would permit an increase of broaching speed by as much as 300%. It’s possible to perform broaching on any machine which has a turning center, but performing broaching jobs with CNC machines generally includes internal as well as external profiles and shapes.
These might be splines, squares, hexagons, double hexagons, keyways, triangles, serrations, numbers, letters, and all kinds of custom forms that might be produced from different materials. One of the big benefits of broaching on CNC machines is that you can use their very fast speeds to create final forms with great accuracy and consistent repeatability, while also producing a low volume of heat.
CNC Machine Broaching
Broaching on CNC machines also virtually eliminates the need for any kind of secondary operation. Broaching applications that make use of CNC equipment are continually growing, using broaching tools now to create serrations on turn parts. When using conventional machining, it is necessary to use a separate process to produce serrations on turn parts.
However, with a broaching tool using CNC equipment, it’s now possible to use a conventional lathe with a c-axis, all in one operation. One of the biggest areas for growth concerning CNC machining is in the medical and dental sector, where you can use Swiss-style CNC machines for broaching. This makes it possible to use smaller and smaller broaches and to achieve extremely precise levels of tolerance.
Challenges to Overcome
Despite the promise of CNC broaching, there are always technology-associated challenges. You can overcome and even reduce any of these by tweaking or adjusting the technology involved. One challenge as an example is that it is currently not feasible to broach an internal square, hexagon, or keyway in a blind hole using a traditional linear broach.
Conventional broaching calls for a broach to be pulled through the entire length of a given part, and that necessitates having a through-hole. Rotary, on the other hand, allows to machine polygon forms into a blind hole right at the end portion of a piece you are working on. This process is achievable very quickly on any vertical machining center, lathe, or mill.
When broaching a blind hole, it should be attempted to drill slightly deeper than the form depth which is undergoing the broaching. This will provide room for the accumulation of chips which inevitably occurs at the bottom of a hole. Ideally, you should add an undercut, so that chips can break away freely of their own accord.
Another challenge is that of alignment and centering. Rotary is ideal for creating small forms such as squares and hexagons and softer metals such as brass, aluminum, and steel. However, when it’s necessary to align several holes using a rotary tool, it can be an operation that presents extraordinary demands. Most rotary holders have a spindle that turns freely, and this does not permit the alignment of holes. That makes it important to center a rotary in the middle of the workpiece to the greatest extent possible.
What happens when you cannot center the piece?
If this centering does not take place, the results may be an uneven form configuration or possibly oversized forms. One last challenge associated with CNC broaching relates to the tensile strength of the material, which can be a huge factor in determining how any workpiece reacts during the broaching process. The greater the tensile strength, the more difficult it will be to achieve successful broaching.
This challenge is now being overcome by applying a TiN coating which helps broaches to perform well even when high load conditions are present, or when it’s necessary to broach materials that are difficult to machine. Even these materials can now be broached when properly coated, so this is an example of how industry challenges are being met and resolved.