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 has been around for something like 150 years, but recent developments with the technology have allowed broaching to be integrated into the CNC machining environment. It is now possible for parts to be broached in a rotary process or a linear process and machined in a single setup, as opposed to 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 allowed a single setup to be used to accomplish precise cuts and to lower the possibility of tolerance stack-up when parts had to be moved from one machine to another. This signaled the arrival of CNC rotary broaches. Since that point, technology has continued 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 needed 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 are typically used on a lathe, but they can also be used on a Swiss-style machine that makes use of a special holder, and they can additionally be used on a mill. Other types of CNC broaches are index or punch broaches which are largely used on Swiss lathes that use a sub-spindle for free machining.
Single-point broaching is generally performed using lathes or mills which generate a keyway, or which can produce square radiused corners. Indexable CNC broaches are commonly used 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.
CNC broaches can be used in both low-production and high-production environments, and they can also be coupled 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 broaching jobs performed with CNC machines generally include 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 very fast speeds can be used to create final forms with great accuracy and consistent repeatability, while also producing a low volume of heat.
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, with broaching tools now being used 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 Swiss-style CNC machines are used for broaching. This makes it possible to use smaller and smaller broaches and to achieve extremely precise levels of tolerance.
Challenges to be Overcome
Despite the promise of CNC broaching, there are always challenges associated with technology. Many of these are presently being overcome or at least reduced 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 for polygon forms to be machined into a blind hole right at the end portion of a piece being worked 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, an undercut should be added, 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 are equipped with 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.
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.
