CNC Machining: Tooling Fundamentals

  • Today's CNC machining centers for wood, plastics, and composites come standard with steep taper ISO/SK/BT style tool holders, or most commonly HSK style tool holders. The automobile (i.e. the CNC machine), the rims (the tool holders), and the tires are my favorite analogy when presenting tooling principles (the cutting tool). With inadequate rims and inexpensive or broken tires, even the most well-designed vehicles would not go very far. Let's have a look at the significance of correct cutting tool selection, as well as the external elements that influence tooling selection and other elements that have a direct impact on the machining cycle.

    Also, check the common CNC machining tools types.


    PCD (Process Controlled Deposition) Tooling
    Begin by defining parameters. Understanding the machine settings, output, spindle, clamping, and tooling options is critical depending on the cutting operation that needs to be completed. The quality, density, abrasiveness, and surface finish of the material to be machined, as well as the material hold-down and dust collecting system, will all play a role in the process. Cutting tool selection will be based on an understanding of the features listed above that have already been determined, as well as the expected feed speeds (capacity) and cost efficiency. All of these factors play a role in determining which tooling is most appropriate and cost-effective for the job. Today's CNC machining centers for wood, plastics, and composites come standard with steep taper ISO/SK/BT style tool holders, or most commonly HSK style tool holders (Picture 1). With a runout tolerance of 0.003mm (.0001”), the basic HSK tool holders have extremely high levels of precision. It is preferable to use a tool holder that comes with a ball bearing collet nut rather of a single-piece static nut. The inner ring of the nut is uncoupled from the torque/threaded part of the nut (Picture 2), allowing the nut's circular motion to be totally transformed into clamping force with no frictional loss. Advantages include increased clamping power, reduced wear, and the ability to run the tool holder both clockwise and counter-clockwise. ER32, ER40, or RDO35 (SYOZ25) collets are used in the most common variants of tool holders used in wood, plastics, and composites.


    Holders for specialized tools
    While normal collet chucks are excellent clamping alternatives for most applications, more specialized applications can benefit from heat shrink and hydro tool holder solutions. Heat shrink tool holders are useful in high-speed machining processes because the tool shank is attached directly into the heat shrink chuck, eliminating the need for a collet system. A heat shrink chuck can only accommodate one size tool shank, and mounting and removing the tool from the chuck usually necessitates an extra (and often significant) expense investment in heating equipment. It's an excellent solution for carbide insert tooling, for example, if the tool and chuck are built at the factory and won't need to be removed for maintenance. Refer to Figure 3. Another option is the hydro chuck, which has the added benefit of avoiding the use of a collet system, which reduces compounded tolerances between the machine and the tool. Hydro chucks are available in all standard metric diameters and are balanced to 25,000 rpm. (Dimensions range from ten to twenty-five millimeters).


    Options for cutting tools
    When it comes to cutting tool possibilities, it's crucial to remember that the initial tool cost isn't the spot to be concerned. The cost of an accurate cutting tool is determined by the cost per linear foot machined. Selecting the incorrect tool can significantly limit and limit the machine's capabilities, as well as negate some of its selling points. Tool holder and cutting tool choices are critical for accuracy, cost-effective production, excellent finish quality, waste reduction, and machine and spindle integrity. Poorly made tool holders and out-of-balance cutting tools will wind up costing significantly more than they appear. Whether you choose solid carbide spiral tools, insert tools, custom profile tooling, or PCD (polycrystalline diamond) tooling, a thorough examination of the advantages and disadvantages of each kind is essential. If a corporation uses 34” 2-flute solid carbide compression bits to mill table tops, for example, it can easily squander thousands of dollars when an equivalent 2-flute carbide insert compression bit can readily outperform for a fraction of the cost. A phenolic fabricator may go through a lot of solid carbide bits in a day when a polycrystalline diamond (PCD) bit (designed specifically for phenolic) will last much longer and cost less. When compared to a carbide-tipped round-over bit for use on a CNC machine, an insert tool will retain constant diameter and dimensional precision throughout at a lesser cost.


    Use the appropriate tools
    Whatever tool is chosen, the most crucial aspect will be how it is used appropriately. The best cutting tool will only work well if it is utilized within the parameters for which it was created. A combination of machine quality/integrity, material hold-down, dust extraction, the clamping system (tool holder/collets), and tool and material composition operating under correct machining conditions is the foundation of successful cutting tool performance. The most crucial factor to consider when choosing a cutting tool is the amount of chip load it will produce during the cutting cycle. If the chip load is not within the appropriate range for the material being machined, the tool may either overheat and have a limited tool life, or the tool will be pushed past its limits and fail (breakage). For example, it is a widely held belief that a router bit with more flutes will produce a superior finish. This is not the case at all. The cutting edge going through the material at the correct speed, i.e. chip load, is what leads to the optimum cutting results. This one aspect is most likely the most important in determining tool life.

    Identifying the chip load
    So, what does it mean when someone says "chip load"? Simply described, it refers to the size/thickness of the chip that is eliminated per flute/cutting edge with each tool revolution. If the feed rate is not altered, the chip size will be reduced by 33% when switching from a 2-flute bit to a 3-flute bit. Because the chips cannot be removed from the cut quickly enough, they are re-cut into even smaller particles, a smaller chip will increase the heat generated during the cut. Chip load charts available online or provided by tooling makers should only be used as a starting point/reference range, and it is the user's responsibility to discover the ultimate "sweet spot" that gives the best combination of tool life, finish, and cost effectiveness.

    The direction of the cut
    Another factor to consider is whether to use a climb cut or a regular cut. The direction of the feed is identical to the direction of the cutting edge for climb cutting. (As a side note, this kind of cutting should never be attempted with a hand feed operation since material kick-back can be quite dangerous.) Climb cutting yields a higher-quality end result. In traditional cutting, on the other hand, the material feeds against the direction of the cutting edge, putting less cutting force on the tool and extending its life. In conclusion, cutting tools and clamping systems are critical to a CNC machining center's ability to deliver on its promises of optimization, capacity, waste reduction, and cost savings, as none of these benefits can be fully realized without the contributions of high-quality tooling and accessories.