SafeLock: The ‘Seatbelt' for Cutting Tools

Nobody gets into his or her car expecting to get into an accident. They do, however, usually put on their seatbelts, just in case. The same can be said for placing a cutting tool into a tool holder. No one expects the cutting tool to come out of its tool holder, but in aggressive applications it can happen, potentially scrapping out very expensive parts and material. Wouldn't it be nice to have a "seatbelt" for cutting tools? Now there is - the Haimer SafeLock system.

The aerospace industry is constantly searching for new technology that will allow for the ability to produce parts faster with greater accuracies and reliability. Adding to the challenge is the increasingly larger varieties of materials that are machined for aircraft manufacturing, along with the high price of the raw material to be machined.

Engineers typically have a great number of factors to study in order to find the best solution for their machining process. The first element of the machining process looked at is typically the machine tool selection for the particular machining process, followed by an investigation into the fixturing of the workpiece.

Once the base setup is established, the aerospace engineers can look closely at the cutting-tool technologies available. There are a number of specific carbide grades and coatings available that, when used with specific geometries, provide the best machining process possible for efficient cutting. Finally, the method of holding the cutting tool is examined. This tool-holding method is crucial in the overall machining process.

Cutting performance plays an important role in a plant's production. Special materials (aluminium, titanium and composites) and part geometries always present difficulties in aircraft machining processes. A specialty in the aerospace industry is the cutting of large structure components, which, in most cases, show a high number of cavities. These work pieces are cut from the solid, with often 90% of the original material being machined. All of these processes have to be carried out with top quality and precision. Not only the machine tool, but also the cutting tool and its tool holder are responsible for providing a properly machined workpiece. Further challenges include titanium machining because the cutters are moving quite slow, aggressively chipping away at the material.

Competition forces the aerospace industry to keep high productivity in mind. High-Performance Cutting (HPC), with the driving power used to full capacity to cut into the material with a high feedrate, is very popular in this industry, especially since there are special cutting tools on the market that support this kind of machining. They resist the conditions of high torques, feeds and tensile forces without breaking.

The danger of pulling the cutter out of the chuck with this force has increased. This is at least the case for tool holders that offer precise clamping with high runout accuracy, such as shrink-fit chucks, milling chucks, hydraulic chucks or press-fit chucks. They use friction to clamp the tool; therefore, their clamping force is limited and sometimes insufficient with HPC. As an alternative, many users take conventional set-screw endmill holders, where the clamping screw grips the tool via a screw locking down on a flat. With that, the user can generally transmit any torque he or she wants until the cutter finally breaks. However, with the set-screw endmill holders, the users have disadvantages: imprecise runout, which causes poor clamping as the shank in the chuck needs a little clearance, and a short cutting tool life.

Until now, there were no further alternatives - a challenge for the clamping specialists of the company Haimer in Igenhausen, Germany. They have a patented combined solution called SafeLock, which combines a shrink-fit chuck or another high-precision chuck with locking elements. The principle is the following: helical grooves are ground on the cutter's shank to form closed drivers. The helical form of the groove protects the tool against overturning and pulling out. The drivers are integrated in the chuck, and can be balls or rods. With this, the user can unify the advantages of high accuracy of clamping and the advantages of positive locking. With the helical form of the grooves, the tool's length can be adjusted. The clamping process is easy, as the chuck is heated as usual and the tool is inserted with a turning move. The balls or rods locate themselves into the grooves. A spring supports the accurate fitting of the tool. After a few seconds, the holder gets cold so that there is friction, and a closed connection is formed.

The modification to the cutting tool shank is easy and symmetrical. In fact, cutting tool suppliers have found that they are able to add the SafeLock grooves to standard stocked endmills. The grooves do not greatly affect the balance, keeping the cutting tool stronger.

SafeLock's clamping procedure is simple. The chuck is heated as usual and the tool is inserted with a rotating movement.

With extreme titanium machining, there are additional problems with high precision chucks: the cutting tools were pulled out of the chuck during tough applications after machining for a period of time. The cutting tool essentially would creep out of the chuck, potentially scrapping out very expensive titanium parts.

Alternatively, set-screw endmill holders or whistle notch holders are not able to reach the precision, and result in insufficient cutting tool life and a limitation in feedrates.

For this reason, the people in charge of production at a large American aircraft manufacturer wanted to test the new Haimer Power Shrink Chucks with the SafeLock System in order to compare them to the currently used weldon holders. On a vertical milling machine, a Power Shrink SafeLock chuck, which had a 32mm bore and a gage length of 120mm, was inserted.

For roughing and finishing, the same coated, solid-carbide tool type with an effective cut length of just more than 80mm was used. The test workpiece was a critical aircraft component, made of titanium. In the test, the runout accuracy was measured at the tip of the tool in the machine spindle. It reached 5µ (0.0002") with the Haimer SafeLock chuck. Under the same preconditions, a set-screw endmill holder reached only values between 0.05mm and 0.08mm (0.0020" and 0.0030").

The results of this test have exceeded the engineers' expectations. The tool life more than doubled because of the SafeLock chuck. They also found that they were able to produce 120% more parts with the same cutting tool because of the stable cutting and better runout conditions. The cutting tool did not pull out of the chuck, and expensive material was spared from the scrap bin. Finally, when combining the SafeLock chuck with more aggressive multi-flute cutters, the customer was able to drastically increase feedrates in order to machine parts faster. The surface also showed considerable differences. When roughing, as well as finishing, there were no vibrations with the SafeLock-System.

The cutting-tool manufacturer, Helical Solutions, Gorham, ME, was the first licensed manufacturer in North America and it is now providing the SafeLock option standard on its cutting tools.

"Facing increasing pressure to machine their parts faster, with no pullout, our customer base was desperate for a solution to aggressively raise their metal removal rate," says David McCulloch, President of Helical Solutions LLC. "The SafeLock modification on the shank of our cutting tools allowed our customers to aggressively machine with full confidence that their cutting tool would not come out of the chuck."

The SafeLock System has provided a seatbelt for the cutting tool - assuring a lack of cutting tool pull-out (no scrapped parts) and higher accuracies, allowing for more aggressive machining and a higher metal removal rate. Ultimately, it has given companies the ability to machine their parts faster to keep up with production.