Importance of Toolholders

Toolholders are often one of the last pieces in the manufacturing process that are evaluated, but toolholders play a critical role in connecting all other elements to maximize machining performance. The medical industry invests a lot of money on high-quality machine tools with high-speed, expensive spindles and expensive cutting tools to cut very difficult exotic alloys such as cobalt chrome, Inconel, stainless steel, and titanium. A high-quality toolholder however, is often overlooked. The reality is that you can't buy a low-quality toolholder for a medical application and expect to get high performance and a good finish, or hold a tight tolerance and maintain spindle integrity. Ultimately, the life of the cutting tool and machine tool will be compromised. The result of this would be machining inferior parts that may be out of spec or require secondary operations to improve surface finish. When using a lesser-quality toolholder, the results that were expected from making a large investment in high quality machines and cutting tools have been diminished. The bottom line is that a high-performance machine coupled with all high-performance accessories allows the machine to perform to its maximum potential and produce the highest quality medical parts.

As the toolholder is the critical connection between the machine spindle and the cutting tool, selecting the right toolholder for the application is also critical. In general, by using a quality toolholder that has excellent taper contact, accuracy and balance, you will get a better fit, more rigidity and fewer vibrations – a main cause of premature spindle wear or failure. For example, in a Lyndex-Nikken customers' medical application a spindle lasted more than 10 years with a Lyndex-Nikken toolholder, however, a newer machine they purchased and used with a lesserquality toolholder, that only lasted three years before needing the re-ground. Using a quality-balanced toolholder is essential to protect the investment of high-quality machines, spindles and cutting tools.

With the higher speeds and feeds demanded today, a higher-performance toolholder becomes more critical. In order to maximize machining performance, four main things are needed from a toolholder:

1. Rigidity
2. Accuracy
3. Power
4. Balance

Advantages of using an advanced toolholder that holds rigidity, accuracy, power and balance is extended cutting tool life, extended spindle life, better surface finish and accuracy of parts.

Rigidity

In a quality toolholder, rigidity is achieved through a high percentage of taper contact. As a guideline, ISO 1947 has a selectable taper tolerance that varies by the "AT" number. This means that with a toolholder, there is allowable socket tolerance and allowable shank tolerance to achieve the perfect taper.

Ideally, a good quality toolholder will have a better taper contact ratio than the ISO guidelines. Typical taper choices include CAT40, CAT50, various HSK forms, BT30, BT40 and BT50. Benefits of high quality tapers are:

• Greater rigidity;
• Higher concentricity – the connection between the spindle and the toolholder;
• Less damage to machine spindle – eliminates spindle fretting; and
• Increased Tool Life – When the runout accuracy of 21µm is reduced to 3µm, the tool life can be improved by approximately five times.

If a machine is equipped with a 7/24 taper dual-contact spindle, another consideration is utilizing a special dualcontact toolholder. A 3Lock toolholder is a dual-contact toolholder that allows for both taper and face contact – increasing the rigidity and thereby accuracy. Lyndex-Nikken's 3Lock toolholder takes dual contact toolholders to the next level with their construction. Lyndex-Nikken's 3Lock toolholder consists of a main body with an internal taper configuration housed inside a taper cone. This taper cone is pre-loaded on the body with the disc springs. The combination of the taper cone and the disc springs create a dampening effect that, in conjunction with flange contact, reduces cutting vibration, thus extending cutting tool life.

An example of the 3Lock toolholder is shown in the diagram below and explained as follows:

Before Clamping: When the 3Lock tool is inserted into a 3Lock spindle (before clamping), the gap between the spindle flange and the tool flange is #40:0.2mm, #50:0.5mm

After Clamping: When the tool is clamped, the taper cone pre-loaded by the disc springs deforms radially and slides axial to allow the face contact between the spindle face and the tool flange.

Accuracy

Accuracy is greatly affected by tool runout. The total incremental runout (TIR) at the cutting edge of the tool is more important than measured at the nose of the toolholder. Some stated accuracies are at the nose of the toolholder. But, what is really important is the TIR at the cutting edge of the tool.

For runout accuracy, less then 3µm at 4XD produces better stability and improved surface finish.

Tool runout can cause vibration that can inhibit machining accuracy. Tool runout adversely effects constant cutting force and decreases tool life. For example, with a four-flute endmill, each flute should be removing the same amount of material. But, if you have tool runout, it can cause an uneven cut and the endmill will not properly machine. In effect, each flute will not remove the same amount of material. You will have varied chip load from flute to flute. This can affect accuracies when trying to hold tight tolerances and will decrease cutter life. Due to the risk of infection with medical parts, good finish is critical in medical applications and unless you have tremendous accuracy you will not have good finish.

Power

Higher gripping torque allows for greater metal-removal rates with more aggressive cutting depth, feeds and speeds. This higher gripping force allows for more overall system rigidity by producing a more secure connection between the toolholder and the cutting tool. Holding force can be an important consideration when combating chatter. Clamping power is measured by gripping torque tests. Gripping torque is essential to hold the tool firmly in position. High gripping torque eliminates the possibility that a tool will slip within the toolholder while machining.

Gripping Torque Data: The graph on the following page shows the comparison for the expansion for the nose piece between Nikken's C 1 ¼ G milling chuck nose ring (68mm OD) and the competitors copied chuck nose (80mm OD) ring at high-speed rotation. According to the graph, the difference of the expansion at 20,000rpm will be 6µm, this means the axial tightening length is reduced by approximately 0.2mm, which is equivalent of 400Nm gripping torque reduction.

Balance

If balanceable tools are required, an ideal tooling assembly (toolholder, cutting tool and retention stud) is balanced in a state of equilibrium in which rotational forces are countered by equal and opposite forces. This is ideal for maximizing cutting performance. The toolholder mass is greatest of the components in the tooling assembly and therefore is the greatest factor when considering the balance of toolholder, cutting tool and retention stud assembly. The specific advantages of using balanced toolholders are the ability to run at maximum spindle speeds and higher feedrates while maximizing tool life, surface finish and machine spindle life. Balanced toolholders are also ideal for high-speed machining. Balance is most important at higher speeds. Balance also becomes more important when tools are longer, as chatter can be a problem.

Balancing is best achieved through a series of tapped holes surrounding the holders' centerline. If the holes are parallel (not perpendicular) to the holders' centerline, balancing is optimized with the set screws not being depth sensitive. This parallel configuration prevents set screws from being unscrewed during high speed operation.

The effects of imbalance in a toolholder assembly are:

• Spindle bearing stress due to unbalance will result in premature spindle bearing failure;
• Vibration due to unbalance can cause poor quality surface finish, diminished tool life and out of tolerance parts; and
• Spindle expansion can cause loss of spindle taper contact and therefore less rigidity.

The causes of imbalance in toolholders can be flaws in the base material, poor tolerances during manufacturing or an asymmetrical toolholder design. Any voids, seams and porosity in the base material will all result in imbalance and structural weakness. As well, any machining performed on the toolholder during manufacture that diminished the absolute concentricity regarding the rotational axis contributes to unbalance. These include un-machined portions of the toolholder, out of roundness and improper placement of through holes. Finally, any offsetting weight that is not countered by equal, opposite forces causes an asymmetrical toolholder design and that causes imbalance.

High Performance

To maximize machining performance, a quality toolholder should offer excellent rigidity, good accuracy, high clamping power and dynamic balancing to suit the machines maximum spindle speed.

With a high-quality toolholder such as Lyndex-Nikken's VC toolholder, medical parts manufacturers often see productivity increases. What a medical manufacturer needs is a toolholder that is highly-balanced for high speed, is a solid tool that absorbs any vibration of the cutting tool and has minimal runout. Used in medical applications for machining hip and knee components, Lyndex-Nikken's high-performance VC toolholder exceeds all of those requirements. The VC operates at up to 40,000rpm, offers the ultimate in high-speed, highaccuracy and ultra-smooth surface finishes. Nikken's engineers have designed a completely new collet system unlike any other where the collet sits deep.

VC Features:

1. Simple external design without notches for ultra highspeed rotation.
2. Eight-degree internal taper proven with the Slim Chuck for accuracy and gripping torque.
3. Pilot shank on the collet for further stability and accuracy.
4. The thick wall design of the VC toolholder body improves cutting rigidity.

The design characteristics of the VC result in high accuracy (runout is within 0.00012" at 4xD) and superior finishes. Its slim and smooth profile eliminates noise related to high-speed rotation, while it reduces the air current created by the hex nuts and slots on the periphery of most toolholders. Dampening of microharmonic vibrations is achieved with the use of a groove under the nut. A thicker wall at the base of the VC, and a short gage length, allow for greater rigidity and static stiffness. As an option, axial adjustment is easily achieved with an adjustment screw. High-pressure, coolant-thru is also available. The VC design features a TiN-bearing nut that makes greater torque possible by reducing the friction associated with nut tightening.

The eight-degree collet design of the VC further contributes to rigidity and gripping torque. The extended straight portion of the collet pilots and maintains the concentric placement of the tool. Its flat shoulder design results in heightened perpendicular force transfer. The VC is balanced with the collet for operation up to 40,000rpm.

Medical Manufacturing

The good news right now for U.S. medical manufacturing is that medical parts require complex machining with exotic alloys and have strict regulatory standards that are not well suited to outsourcing and production overseas. In all manufacturing, striving to improve processes, increase quality and reduce costs is a constant – and medical manufacturing is no different. Internal controls are tighter which has magnified the issue of cost justification. We're already seeing this type of thinking with some medical manufacturers. Some manufacturers now have a dedicated manufacturing engineer with the sole responsibility to cut costs on current manufacturing processes. More and more they understand that cheaper, less-quality toolholders do not cut costs. In reality, with lesser-quality toolholders the machining process takes longer and the spindles and cutting tools wear out faster and overall costs increase.

What medical manufacturers need to think about is overall smarter manufacturing processes. For example, both the tool life and spindle life are affected by the toolholder. You can have lost productivity and degradation of spindle due to poor quality toolholders. However, manufacturers don't always look at them until there are problems. It is beneficial to do true long-term cost analysis, not cost analysis on each specific element. Lyndex-Nikken has a broad spectrum of toolholders and has a unique way of looking at applications. Depending on the material, size of cutting tool and other factors will determine which specific toolholder is ideal for the application. It is also important to look at the part that is being machined and to consider any secondary processes that are needed such as polishing, coatings or surface finish.