By Robert Marshall
Once a sign shop has invested in a new computer numerical control (CNC) router, whether for the first time or to replace an earlier machine with a next-generation model, the next step is to ensure in-house operators select the right tooling, based on the specific materials to be cut. Using the optimal tooling can help achieve greater productivity and higher quality. There may of course be refinements and corrections to make as time goes on, but it is important to undertake the following basic considerations when initially choosing router bits for sign cutting applications.
Quick and clean cuts
At the most fundamental level, the choice of CNC router tooling determines (a) how quickly a material can be cut and (b) how clean the cut will be.
The cleaner the cut is, the fewer post-routing rework operations will be required. With this in mind, micrograin solid-carbide router bits are the industry norm nowadays. Some of these bits come with a specialized coating, adapting them for use with specific materials. These coatings can both improve the cut and extend the tool’s life.
The geometry matters. The flute design of the bit determines how the cut material will be ejected from the routing table. The most popular geometries are ‘spiral up-cut,’ ‘spiral down-cut,’ straight flutes and compression bits. Selecting the correct diameter of bit helps determine the spindle speed and the cutting depth that may be achieved.
There are other part design considerations, too. Sign designs featuring tight, internal corners will require a smaller router bit diameter than those that are more basic,
like a simple square or rectangle with only external corners.
In general, it is best to select as large a bit diameter as the design will allow, subject to the presence of a spindle with a large enough power rating for the bit. Larger-diameter bits both provide greater rigidity for high-quality cut edges and allow higher cutting speeds to be used.
If polished edges are required, a dedicated bit designed for use with acrylic can eliminate a further step in the process. Sometimes, a more ‘finished’ look can be accomplished with a second pass after the first cut, to remove most of the material. The second pass can be made at a fractionally smaller size (perhaps using a brand-new bit), sloughing off any remaining ‘whiskers’ and smoothing out the edges. There are also durable diamond-tipped tools that can achieve the most polished cuts, but these can be very expensive, so they may not be feasible for many sign shops.
If the wrong router bit design or size is selected for the specific material to be cut, then the operator may well wind up with chipped, sharp edges or burrs that have to be filed down. The wrong choice of bit can even result in a cut that renders the piece out of tolerance and, thus, headed to the scrap bin.
With tooling, sign shops usually get what they pay for; the less expensive bits may not last as long or cut as well as the more expensive options. As such, tooling with a higher initial cost will pay for itself through greater longevity and improved cut quality. To some extent, the difference is a judgment call.
The selection process is certainly not always black and white. With the wide variety of materials used by sign shops today and demanded by their customers, including many types of plastics, acrylics, wood, aluminum, composites and resins, there may always be some degree of trial and error when it comes to selecting the optimal router bit for a CNC router. There is an ongoing learning process.
Ready access to knowledgeable support from the outside can prove not only helpful, but essential. Sign shops should insist on benefiting from their CNC router vendors’ expertise. Manufacturers and distributors alike should make information about router bit selection available on their websites and on posters that can be mounted to the walls of sign shops.
Generally, when something goes wrong with CNC routing, it is not an issue within the router itself, but with the selected router bit or spindle speed. As such, these problems can be easily fixed, in most cases.