Decision Drivers for Selecting Solid Carbide or Indexable Tools

Most CNC milling machine operators will offer a variety of opinions on their preferred type of cutting tool for a specific application that is based strictly on past experience because it is natural to rely on strategies that have resulted in success. However, a better-educated decision can result in even better results.

For example, a common challenge across shops is the proper application and programming of solid carbide end mills for roughing applications in milling operations where an indexable cutting tool was previously used. In a nut shell, the way solid carbide is being used in a high-speed approach (high depth of cut and light width of cut) for roughing applications.

This article focuses on the factors that drive machinists to select solid carbide or indexable cutting tools. This requires a look at the criteria driving the cutting tool selection process for a given application. Examples of these factors include the machine tool’s spindle power and speed capabilities, workpiece geometry and material, CAD/CAM for CNC programming and fixturing.

Machine Tool

Most milling machine tools purchased in the United States today are lighter-duty machines with CAT40 or HSK 63 spindles, which generate low horsepower and torque when they are operated at slow spindle speeds. Price is one of the main purchasing criteria for buying machines with these light-duty spindles. However, in moldmaking the trend toward finishing cores and cavities in a hardened state over producing electrodes and using EDM to complete a workpiece, has driven mold shops to invest in light-duty machining centers with high-speed spindles.

Higher spindle speeds are necessary for the hard-milling process to be effective. Today, it is not uncommon to see spindles capable of running between 20–30K revolutions per minute (RPM). Although many of these spindles have a fair amount of power (20–30 horsepower), many lack the horsepower and torque at slower spindle speeds and are better suited for a high-speed approach at higher (or maximum) RPM.

Although it is not always possible to do so, the best-case scenario is to choose the cutting-tool diameter that peaks machine spindle power while operating within given surface-speed parameters (SFM) for a particular material. This leads to using solid carbide end mills in smaller diameters (¾ inch and smaller). Based on the trend toward lighter-duty machines with peak performance at higher RPM, many roughing operations that previously involved an indexable-type milling tool are now being performed with a solid carbide end mill.

Advanced toolpaths benefit both indexable and solid carbide cutting tools, but it is the application of solid carbide end mills in a high-speed approach that benefits the most from these toolpath advancements.

Programming

Advancements in CAM systems also are driving the selection of solid carbide cutting tools over indexable cutting tools. The ability to effectively maintain control of cutting tool engagement (or the width of cut) has provided the opportunity to optimize and maximize roughing applications, especially those involving light-duty, high-speed milling machines. These new algorithms can control the engagement of the cutting tool throughout an entire operation, no matter how complex the workpiece geometry.

Without these algorithms, catastrophic cutting tool failures would result that could damage the part or the machine spindle over time. This is especially true on a high-speed milling machine, since high-speed spindle design does not hold up well against excessive abuse.

Prior to these toolpaths, operators would often set speeds and feeds for an entire operation based on a worst-case scenario in which the cutting tool engagement changed drastically at some point in the program. This was because of the tool path algorithm’s inability to control the tool based on some variation in the part geometry that the algorithm could not effectively handle. This worst-case programming sacrificed productivity for the sake of eliminating a cutting tool failure.

Advanced toolpaths benefit both indexable and solid carbide cutting tools, but it is the application of solid carbide end mills in a high-speed approach that benefits the most from these toolpath advancements. Without them, it would be virtually impossible to program the vast amount of code that occurs when using the high-speed approach, and it is now very common to see a solid carbide tool roughing a part in one axial pass (up to 6xD cutting-flute length) rather than using an indexable tool with multiple axial passes.    

Workpiece Geometry and Material

It goes without saying that workpiece geometry plays a major role in determining cutting tool type and size. However, workpiece geometry alone should not be the deciding factor for selecting a cutting tool diameter. For example, a given workpiece geometry may provide access for a 2.00-inch diameter cutting tool or larger. But, can a lighter-duty spindle be effective in running this cutter? Maybe not. 

A very common mistake in milling is to choose a cutting tool that is too large for a given machine to effectively operate. Machinists are forced to sacrifice the effective and efficient use of carbide by limiting the depth or width of cut when choosing a cutter diameter that is too large. It is critical to match the cutting-tool diameter to the machine tool capability while staying within the limits of what the cutting tool can handle in terms of speed in a given material. Using solid carbide end mills in a high-speed approach offers more freedom when matching the cutting tool to the machine and material, especially on light-duty machines.

Another important feature is the stress or strain placed on the material during roughing. Here, machinists are reconsidering traditional roughing techniques that employ indexable tooling, since the high-speed approach with solid carbide tooling has proven to put less stress/warp on the workpiece. 

Using solid carbide end mills in a high-speed approach offers more freedom when matching cutting tool to machine and material, especially on light-duty machines.

Fixture and Part Clamping

Rigidity is the most important variable in machining. When rigidity is lacking, a high-speed approach that reduces force may be the only option for productive milling. For example, rigidity is lacking when the workpiece geometry makes it difficult to clamp the part, when holding onto a slim piece of material in a vice or when milling operations distort the workpiece with thin walls or ribs.

Of course, when faced with these circumstances, reducing the cutting tool diameter is a go-to alternative, which results in using solid carbide over indexable tooling. By design, solid carbide tooling (in variable pitch or helix or in the sharpness of cutting edge) almost always imparts less force or stress than that of an indexable tool taking the same cut. Again, using solid carbide is a better strategy when facing less rigidity.

Cutter Advancements

Are you still using two-, three- and four-flute indexable or solid carbide milling tools? Most solid carbide and indexable cutting tools (up to a one-inch diameter) do not have more than four flutes. Indexable tools are constrained from the addition of more flutes because of their design, since more room is needed to create the insert pocket and chip gullet. And, solid carbide tools with two, three and four flutes are still the most common due to their versatility.

However, solid carbide end mill designs are evolving to optimize high-speed milling strategies by adding more flutes. Because of the small or light width of cut used in the high-speed approach, the end mill’s chip gullets can be smaller, which provides more room to add extra flutes on the end mill.

It’s not uncommon to see solid carbide end mills with seven- and nine-flute designs, or one flute for every 0.040 inch (or 1 millimeter) of diameter. For example, ½-inch diameter end mills with 12 flutes or a 1.0-inch diameter end mill with 25 flutes are becoming more common. These end mill designs take the high-speed approach to new extremes in terms of higher feed rates, which gives shops a productive reason to replace an indexable tool with a solid carbide end mill.

Taking the machine, the material and the programming into consideration, the use of solid carbide cutting tools in a high-speed approach may offer more freedom to adapt to common machining variables and to match or surpass what has previously been done with indexable tooling. This is especially appropriate in a light-duty environment where working at lower speeds constrains spindle power.