BY CHARLES BATES
Single-unit constructed toolholder
Collet chuck toolholders
Internal workings of a hydraulic toolholder
High-quality/ high-precision toolholders cost more, but highquality toolholders pay off in the long run because they increase tool life. For example: two shops that use the same $30 endmill but different toolholders for a milling operation in mild steel could have as much as a 40 percent difference in annual costs because of their toolholder choice. Here's how:
The first company opts for a low-precision toolholder that costs $100 and delivers only 80 percent of the endmill's rated lifespan. The second company spends $150 for a high-precision toolholder. It costs 50 percent more than the first company's toolholder, but it increases the expected life of the endmill by 30 percent.
Over the course of a year, the first company will end up needing 120 endmills — at a cost of $3,600 — but the second company will need only 70 endmills — at a cost of $2,100. After subtracting the $50 added cost for the precision toolholder, the second company saves $1,450 in tooling costs for the year, about 40 percent less than the first company.
This does not mean that the most expensive toolholder is right for every operation. The key is to determine the type of toolholder (conventional, collet, heatshrink, hydraulic and so on) that is the most appropriate for the job, then opt for a highquality/high-precision model of that type to improve tool life.
High-precision toolholders typically have low levels of total indicated runout (TIR) and provide good clamping forces, rigidity, balance and repeatability. All of these contribute to extending cutting tool life and reducing scrap that is directly related to tool life.
Most scrap is produced immediately before and after toolchanges — before changes due to tool wear and after changes because tooling is not immediately "dialed in." So, by extending tool life with a high-precision holder, a shop can reduce the frequency of toolchanges and reduce scrapped parts.
A toolholder's ability to dampen vibration also increases tool life, especially for shops that use expensive perishable tools such as PCD and CBN. While hydraulic holders are considered the common way to control vibration, other types of holders offer the same benefit. These holders have chambers filled with a polymer material that works the same as a hydraulic fluid for vibration dampening.
Shops also should understand that they can save money without sacrificing precision or tool life by opting for conventional, centering-type toolholders. These conventional toolholders have been given improved accuracy, concentricity and balance that make them suitable for more processes than in the past.
Toolholder manufacturers recommend combining conventional and centering toolholders, such as collet or one-piece, with high-end toolholders such as shrinkfit or hydraulic. However, often a shop will purchase a high-end holder system for one application, then feel compelled to justify the cost of that system by using it for as many applications as possible, even if its use is unnecessary. For example: shrinkfit holders provide a substantial benefit for only a small percentage of the tools in a machine's tool magazine, and using them for all tools in the magazine would be overkill.
Shops also often overestimate the amount of torque capacity they actually need in a toolholder, falling prey to the more-is-better/tighter-is-better mentality. This can cause increased costs for toolholding systems that are not necessary to do the job.
As an alternative to the costly hydraulic or heatshrink-type toolholders, a one-piece or single-unit constructed, toolholder could be the correct choice. One-piece toolholders have accuracy, balance and torque capacities that lie between hydraulic and heatshrink-type holders, and are less costly. These toolholders include one-piece systems that have special, non-round I.D. holes that rely on the spring tension in the steel of the holder itself to secure cutting tools. They work in a way that is similar to heatshrink toolholders, but without the need to heat them: shops place the holder in a compressing unit that squeezes the non-round I.D. hole to a round shape, load the tool, and release the pressure. The holder constricts around the tool shank to hold the tool in place.
Contributors to this article include: