High Speed Spindle Technology – Tool Holding

A machine tool spindle is designed to do only one thing: remove material. Our main objective is to remove material until we reach the final part shape. To do this, we must rotate a cutting tool and force the cutter into the work piece, create chips and mill out unnecessary material until we are finished. 

This process has evolved through history step by step.

Treadle Lathe | Courtesy J. J. Holtzapffel c.1870's
 

This was followed by blacksmithing. The common material used was wrought iron. By using hand tools, like a hammer and chisel, and his forge, the blacksmith would heat the iron to forging temperature. When heated to the correct color, the smith would hammer the material until it was formed into the desired shape, be it a horseshoe, sword or plow. The first “additive” manufacturing, I guess.

The Machines Are Coming                                   

As manufacturing evolved, more machines emerged. In the Middle Ages, lathes were developed that could cut metal. One important use was for the boring of canon barrels. The first real boring mill was developed in the mid 1700’s by James Watt to produce steam engines. One of the first milling machines was made by Eli Whitney in 1818 to produce gun parts.

 

Early Milling Machine | Courtesy Eli Whitney Museum
 

Eventually, cutting tool materials, such as tool steel (1.5% carbon), were developed and widely used for drills and milling cutters. However, as we all know, sharp tools wear out and become dull, resulting in poor surface finish and chatter. They must be changed to continue production. The need for a replaceable cutting tool system was clear.

The Wedge Works

Tool holding has three basic requirements:

1. The tool must be held rigidly

2. The tool must be held accurately, particularly regarding run-out.

3. The tool must be easily removable.

The first solution to effective tool holding was by using a taper. The concept of using tapers for secure tool fitting dates to early machining practices. Early tapers like the Morse taper, developed in the 19th century, revolutionized tool-holding by utilizing friction for secure fitting. The Morse taper was patented in 1864, followed by the Jacobs taper in 1902. A tool called a “drift” is used to unclamp the wedge.

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Morse Taper and Drift | Courtesy CADEM.com

As The Tool Turns

The Jacobs drill chuck was developed in 1902 and used on basic drill presses. It provided a simple way to securely and accurately hold a drill bit having a wide range of diameters. It is still widely used today.

 

Drill Chuck | Courtesy Jacobs Company and Dwight Slate Machine Co.

However, as mass production evolved, there was a growing need to utilize a variety of tools during the process in a true machining center. This led to the development of steep taper tooling systems. 

The Steep Taper Is Born

The first steep taper system was developed in 1920 by the NMTBA (National Machine Tool Builders Association). It was a manual system using a taper ratio of 7/24. Available in a wide variety of sizes, it was held within the spindle with a manual draw bolt for retention and used drive keys to transfer torque. When required, the operator would change tools manually. 

 

MTBA 40 Taper | Courtesy Tools N Gizmos.com

However, a growing need for increased productivity called for faster (read automatic) tool changing. Modern machine tool design in the 1970’s now included automatic tool changing magazines that fully automated the tool changing process. The introduction of CAT and BT tapers marked a significant leap. These designs include a wide flange for ATC magazine storage, as well as automated clamping. Instead of a manual draw bolt used to retain the taper in the spindle, a “draw bar” would be used to clamp the tool holder by the retention knob. 

CAT40 Tool Holder With Retention Knob | Courtesy Edge Tech

The Clamping Challenge

The challenge, from the spindle design standpoint, was how to secure the tool holder in the spindle as it rotated and was subjected to axial and radial loads caused by cutting chips. And, the draw bar must only hold the tool holder with grippers surrounding the retention knob. Enter the “power” draw bar.

 

Power Draw Bar | Courtesy Ott Jacob

A basic power draw bar design uses spring washers to provide holding force that was sufficient to hold the tool holder in the spindle shaft as it rotated. To release the tool holder, a hydraulic or pneumatic fluid is applied to a cylinder forcing the draw bar forward, opening the fingers from around the retention knob. Sensors within the spindle feed back the draw bar position to control ATC motions to remove one tool and load the next.

Now I Want To Go Faster

This approach was fine for 40 taper spindles running up to around 15,000 rpm. At higher speeds, particularly in the BT40 size, problems began to develop. In a steep taper system, like BT, the tool holder is pulled snugly into the spindle taper. The holding force created by the draw bar springs results in tool accuracy and rigidity. All is good. 

However, when spindle RPM increases, centrifugal force acts on the spindle shaft taper, causing it to open in a “bell mouth” condition. When this happens, the tool holder is pulled deeper into the taper, compromising the accuracy and location. This instability limited spindle RPM using steep taper systems. A better solution was needed.

 

 

HSK Tool Holder for High Speed | Courtesy Big Daishowa
 

A new tooling standard emerged in the 1990’s. It was invented in Europe and called HSK (hollow shaft cone). The HSK system uses a 1/10 taper ratio and is available in many sizes. The taper is hollow and does not use a retention knob. The clamping for HSK occurs internally to the tool holder. In the HSK system, the grippers clamp internally, not externally on a retention knob. This causes the tool clamping force to increase with RPM, not decrease as seen in CT and BT. And, unlike CT or BT, HSK uses direct flange contact. This ensures the tool does not move under high RPM

                                               

HSK63A Tool Holder | Courtesy Technics

HSK holders are designed for high-speed machining and applications requiring greater precision and accuracy. There are many types specified by the ISO standard for both low speed and high-speed operations. Some have driving keys (A,C) and others, like (E,F) have no keys and are intended for ultra-high RPM applications. 

One major advantage of the HSK system is the increasing tool retention force as the spindle rotates. This is due to centrifugal force acting on the grippers which are mounted internally. This bonus force can be as high as double the static holding force.

  

Tool Retention Force Doubles at Speed | Courtesy Intelligent Concept

Let’s Not Forget Collets

No tool holding conversation would be complete without considering how collets are used to support tool holding production. Clearly, if tool shanks were only one diameter, collets would not have been needed. However, tool shanks come in almost any diameter and must be held rigidly to perform.

 

The first attempt at holding tools used a side-lock set screw to lock the cutting tool into a bore in the tool holder. This proved sufficient, at first, however it presented some issues. With the tool clamped from the side, it tended to push the center of a cutter off-center which affected the rotating accuracy. It was also not accurate enough for precision drilling. Enter the collapsible collet.

Collets | Courtesy Kennametal

A collet is a device produced with a nominal bore and slotted cuts. It is positioned in a solid taper. A nut is screwed against one end, forcing the collet into the taper. As the collet moves it collapses and reduces the claping bore until secure contact is made with the tool shank. Useful for both drills and end milling cutters, the collet provides reasonable rigidity and great accuracy. All tooling systems (CAT, BT, HSK) offer some type of collet system as tooling. The collet would be built into the working side of the tool holder. There are also other tool clampinh techniques used, such as hydraulic and shrink fit.

 

A modern, high-speed spindle will typically have the following features:

Without a safe, accurate and robust tool holding system, the spindle will fail at its main task: quickly removing metal to produce a finished part. Modern HSK tooling systems are rugged and extremely accurate and have a long, useful life.