Achieving Precision in Machining
Tool holders play a critical role in connecting elements maximizing machining performance, however, several different styles are available, and the most suitable one 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, high-speed machining.
Selecting a high-speed tool holder that will consistently produce high quality and safe operation, machine shops should focus on balance, taper contact, and accuracy. Alan Miller, Senior Engineering Manager, BIG DAISHOWA—a premium industrial tooling company—discusses why these three aspects are top priorities for medical manufacturers and reveals the attainable outcomes of each.
Taper Considerations
For high machining precision, the taper must be manufactured to extremely tight tolerances for the tool holder to be accurately positioned within the spindle to provide a rigid connection for the axial and radial loads. Good repeatability in and out of the machine maintains consistency. The necessary requirements will depend on the tool holder type used—HSK, CAT/BT, and Polygon styles.
HSK are hollow shank tapers—available types include A, E, and F—and offer radial stiffness and high-speed precision. This type of taper is much shorter (25mm, 32mm, 40mm, 50mm, 63mm, 80mm, 100mm, and 125mm) and includes an internal mechanism for radial and axial connection.
“Since this is a hollow taper and is small, material selection is extremely important,” Miller says. “HSK is clamped by fingers reaching inside that hollow core and pulling and stretching the taper back into the spindle. The wall thickness is very small, and it’s really critical to have a very high-quality grade of steel, which is why we use die steel for the very small tapers.”
Another important consideration for HSK types is the finishing of clamping features after heat treatment. Because the fingers reach inside and grab on a feature inside that hollow taper, making sure it’s finished after heat treatment assures it’s in the correct location, so you get more accurate repeatability in the spindle.
CAT and BT style common steep taper tool holders have a more traditional style cone. Both have a 7:24 taper ratio and require a retention knob or pull stud to secure the holder within the machine spindle. As the machine grabs and pulls, the spindle expands to secure taper contact. With the larger size and the cone shape of the spindle, the taper will open as speeds increase significantly. As the spindle opens, the force of the clamping unit draws the holder deeper into the spindle, which causes the tool holder to lose some accuracy.
To address this issue, BIG KAISER’s BIG-PLUS spindle system 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. Larger face contact, combined with the taper contact, works together to resist deflection.
Polygon taper types 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 accuracy.
Tool Holder Balance
As spindle speeds increase, it becomes more important to obtain the necessary tool holder balance. Balanced tool holders allow users to run at the highest spindle speeds and feed rates while maximizing tool life, surface finish, and spindle life. Balancing ensures the cutting edge of the end mill consistently engages with the material to prevent chatter and poor surface finish quality.
“For balance, we’re talking about side loads generated by that unbalanced mass running at high speeds,” Miller explains. “So, that can have influences on the bearings of the machine tool spindle to the actual cutting performance on the business end of the cutting tool. It’s very important to consider, and there’s a new ISO toolholder balance standard that was released in 2017 to further help users meet requirements.”
The updated ISO 16084 standard accounts for all variables for safe and productive machining. And Miller says it also takes into consideration additional, more complex factors that can lead to imbalance. It focuses on the interaction between the spindle and tool by factoring in the allowable load on the spindle bearings generated by the tool’s imbalance.
The best way to ensure balance is to measure the tool holder as a full assembly. Ultimately, this will result in a better performing tool. And although each part can be balanced individually, this will not guarantee the highest level of overall balance.
Benefits of balancing include Optimal surface finish; Machine sustainability, better part geometry; and Extended spindle life. Miller also cautions that “when dealing with balance, there’s also a point that we can get to a plateau, where additional balancing won’t help anymore. The goal could be exact balanced, but we’re going to spend a lot of time on every tool they’re trying to achieve that same amount of balance, and it becomes more or less stable. 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. A very low runout of the tool holder will give users better quality and improved cutting for more consistency and lower costs. “Runout accuracy is a very important consideration,” Miller adds. “Low runout will give you better part quality and it will improve cutting tool life and allow you to get more consistency, longer run times, and less tool breakage.”
BIG DAISHOWA designs and machines tool holders to reduce runout, by using:
● Precision materials – Material, production, heat treatment selected for precision, including pull studs built with through-hardened H13 premium tool steel, all features are precision ground
● High concentricity – Each collet is inspected for 100 percent concentricity, guaranteeing runout within 0.00004" at the nose
● Slim design options – Various interfaces reduce the need for extensions to help avoid interference
● Specialized nuts – Available for a more efficient coolant supply, protects small holder components that can cause runout, other issues
● Increased chuck body internal taper contact length – Collet overhang is reduced, improving rigidity, clamping, runout accuracy
● Shallower collet taper, thick-body design – Improves concentricity, reduces chatter, deflection for better surface finishes, longer cutting tool life
Most of the general customer base, as Miller has found, follows the guidelines of 0.0003" to 0.0005" to be a good runout. However, this may only give about 50-60 percent of the potential cutting tool lifespan. “Our programs are down to 0.0001" and 4x diameter 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 3x to 5x 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, 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.”
Cooling Systems
There are future developments that BIG DAISHOWA has developed, such as tool holders for CO2 cooling systems to eliminate liquid coolant for metal cutting. They reduce the amount of contamination that medical parts have within the machining process but still cool the cutting tools, so users get a good tool life. These capabilities are built into Mega micro chucks, featuring a slim nut and taper design to prevent interference in applications with micro drills and end mills. The notch-free nut prevents vibration and noise and offers superior balance and concentricity. “The jet coolant nut was designed around those types of developments where we can offer unique components specifically for carbon dioxide cooling style machining that’s becoming more important to the medical community,” Miller concludes.
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