Selecting Toolholders

Tool holders play a critical role in connecting elements and maximising performance. With several types available, the most suitable one for your application will depend on the operation. Consider key characteristics as they will differentiate a standard tool holder from one that’s the right choice for precise machining.

Selecting a high-speed tool holder that will consistently produce high-quality, machine shops need to focus on balance, taper contact and accuracy. Alan Miller, Senior Engineering Manager at BIG DAISHOWA, the brand available from Industrial Tooling Corporation (ITC) discusses why these aspects are top priorities for manufacturers and reveals the attainable outcomes of each.

Taper considerations

For precision machining, the taper must be manufactured to extremely tight tolerances for the tool holder to provide a rigid connection for the axial and radial loads. Good repeatability in and out of the machine must be consistent. The requirements will depend on the tool holder used, such as the HSK, CAT/BT, and polygon styles. HSK are hollow shank tapers and available types include the A, E, and F. These offer radial stiffness and high-speed precision. This taper is much shorter (25 to 125mm) and has a mechanism for radial and axial connection.

“Since this is a small hollow taper, material selection is extremely important,” Miller says. “HSK is clamped by fingers reaching inside the hollow core and stretching the taper back into the spindle. The wall thickness is very thin and it’s critical to have a high-quality grade of steel.”

Another consideration for HSK is the finishing of clamping features after heat treatment because the fingers reach inside and grab on a feature inside the hollow taper. So, making sure it’s finished after heat treatment assures it’s in the correct location.

CAT and BT style are steep taper holders with a traditional style cone. Both have a 7:24 taper ratio and require a pull stud to secure the holder. As the machine grabs and pulls, the spindle expands to secure taper contact. With the larger size and the cone shape spindle, the taper will open as speeds increase. As the spindle opens, the force of the clamping unit draws the holder deeper into the spindle, causing the loss of accuracy.

To address this issue, BIG KAISER’s BIG-PLUS spindle system (BBT) available from ITC allows the shank to contact the spindle taper and the spindle face simultaneously, allowing increased tool rigidity due to the larger contact diameter of the tool holder flange face. This works together to resist deflection.

The polygon tapers put the face and taper of a machine spindle and tool holder in contact, ensuring high repeatability. The polygon design also allows for self-centering to improve accuracy. However, these types are more difficult to grind because of the slope, so it takes a high-end machine to achieve the desired accuracy.

As spindle speeds increase, it becomes more important to obtain tool holder balance. Balanced tool holders allow users to run at high speeds and feeds while maximising tool life, surface finish and spindle life. Balancing ensures the tool consistently engages with the material to prevent chatter and poor surface finish.

“For balance, we’re talking about side loads generated by that unbalanced mass running at high speeds,” Miller explains. “So, that can influence the bearings of the machine spindle to the actual cutting performance at the business end of the tool. It’s very important to consider, and there’s a new ISO toolholder balance standard that was released in 2017 to help users.”

The updated ISO:16084 standard accounts for all variables for safe machining. It also considers factors that can lead to imbalance, like the interaction between the spindle and tool, factoring in allowable loads on spindle bearings generated by tool imbalance.

The best way to ensure balance is to measure the tool holder as a full assembly. Ultimately, this results in a better-performing tool. Although each part can be balanced individually, this will not guarantee the highest level of overall balance. The benefits include optimal surface finish, machine sustainability, better part geometry and extended spindle life.

Miller also cautions: “When dealing with balance, there’s also a plateau point, where additional balancing won’t help anymore. The goal could be exact balancing, but we’re going to spend a lot of time on every tools, trying to achieve that same amount of balance. No matter how much more balancing you do, it won’t change the way the tool performs.”

Accuracy

Improved quality will depend on the accuracy of the tool, which is largely affected by tool runout. Very low runout of the holder will give better quality and improved cutting for more consistency and lower costs.

“Runout accuracy is a very important consideration. Low runout will give better part quality and it will improve tool life and allow you to get more consistency, longer run times, and fewer tool breakages,” Miller adds.

BIG DAISHOWA designs and manufactures tool holders to reduce runout, by using precision materials, production processes and heat treatment selected for precision. This includes pull studs built with through-hardened H13 tool steel where all features are precision ground. Each collet is inspected for 100% concentricity, guaranteeing runout within 0.001mm at the nose. Most of the customer base follows guidelines of 0.007mm to 0.0013mm to be a good runout. This may only give 50% to 60% of the potential cutting tool lifespan.

“Our programs are down to 0.0002mm and 4XD to make sure you have accurate tool assemblies,” Miller says. “All of our collet chucks and finishing tool holders guarantee that runout accuracy is about 3 to 5 times better than what is considered good.” Runout can also impact cutting force, which can cause vibration and ultimately inhibit machining accuracy. “Vibration is challenging the function of the cutting parameters, mostly back to the cutting speed and rotational speed,” Miller explains. “The best tip that we have for anyone experiencing vibration is to adjust the speed in one direction or the other, change lanes and get into a harmonic situation.”