Know when shrink fit is a good fit

Shrink-fit toolholders are suited for high-production environments and have been widely adopted in automotive component manufacturing. Techniks

High runout and vibration can frustrate any machinist, which is why it is important to choose a toolholder that makes sense for the application. Historically, Weldon flat holders were used to ensure security and reduce tool pullout during the machining process. However, this option required operators to run the machines at a lower RPM due to varying runout of the tool and balance of tool assembly. Instead, some manufacturers turned to collet chucks, which offered better runout but did not have the increased gripping torque needed in many applications.

In the early 1990s, hydraulic chucks were introduced to deal with some of these issues. And while they were great for straight up and down work, like drilling and reaming, offering great accuracy, they weren’t always great for other operations. They also required a lot of maintenance.

In the late 1990s, shrink-fit holders had emerged as an alternative to the other toolholding options available, particularly hydraulic chucks. Shrink fit offers the ability to drill and ream with high accuracy without the maintenance necessary with hydraulic chucks.

“Shrink fit, from an engineering standpoint, is theoretically the perfect toolholder, with no moving parts or accessories,” said Alan Miller, senior manager of engineering, BIG DAISHOWA Inc., Hoffman Estates, Ill. “It just involves the holder, the cutting tool, and some heat, which is great. But sometimes, in practice, it can get a little more challenging.”

While shrink-fit toolholders came onto the market as an alternative to other toolholding options, their development really accelerated in the die and mold industry.

“Shrink fit offers a very slim profile, which enables deep-reach-type applications like the in the die and mold industry,” said Brendt Holden, president, Haimer USA, Villa Park, Ill. “They even allow for shrink extensions to expand capabilities into those deep-reach cavities. It really solves the problem of many hard-to-reach, complex part production.”

Particularly, shrink fit makes sense in high-production environments and has been widely adopted in automotive component manufacturing.

“Shrink fit doesn’t offer a lot of flexibility when it comes to different size cutting tools,” said Mike Eneix, vice-president of sales, Techniks, Inc., Indianapolis, Ind. “A dedicated size is needed for every tool that will be used, which is different than a collet chuck or hydraulic toolholder, where you can have one holder for many tool sizes. For some small job shops where they don't know what job is going to be coming in the door, shrink fit may not provide the flexibility they need.”

For the most part, shrink-fit holders are very slim, which is one of their advantages. They are designed with deep-reach, slim profiles for high speeds and feeds and a low depth of cut.

“However, for shops looking at doing heavy rouging applications, shrink fit wasn’t always a good option because it was very rigid, but also very thin-walled, so that led to vibration,” said Holden.

Shrink fit offers a very slim profile and some allow for shrink extensions to expand capabilities into deep-reach cavities. It really solves the problem of many hard-to-reach complex part production, said Haimer USA President Brendt Holden. Haimer

Generally speaking, standard shrink-fit toolholders are best-suited for light to medium milling applications. Super aggressive, heavy roughing is not recommended.

“On the other side of that, standard shrink-fit toolholders only really offer middle of the road runout, which is not recommended for high-surface-finish work,” said Miller.

Many of today’s toolholder manufacturers have addressed these issues by developing shrink-fit toolholders with heavy-wall designs and a thicker profile to handle different types of operations.

“Geometries have evolved to now allow for different models of shrink-fit toolholders to perform both heavy roughing and finishing applications,” said Holden. “What is more, with slim profile holders, there are extended reach options where operators can shrink multiple extensions together to add 15 to 30 in. to get those deep-reach applications and still have the accuracy maintained.”

The best place to apply shrink fit is in tight clearance areas, places where larger collet chucks won't physically fit. They are typically suited for 5-axis machines and parts where aligning the toolholder in the part is essential to get as much clearance as possible.

“Within the overall classification, you need to make sure that you're picking the right style of base,” said Miller. “Whether it's something for heavy roughing or more of the standard operations, just be cautious when you get into the very thin profiles or are working in heavier operations.”

Shrink fit toolholders have become increasingly popular in recent years for a multitude of reasons. Here are some benefits of choosing this type of toolholder:

1) Consistency. The setup of these toolholders is very consistent from one operator to another. This is particularly important in production environments where reliability and repeatability are necessary.

“Everyone in a shop who sets a shrink-fit tools will do it the same way,” said Holden. “You don’t need an expert, and experience won’t make much difference. This is a huge advantage, especially with the skilled labour shortage. Whether it’s the first day on the job or 20 years in, an operator will set up the tool the exact same way and get the exact same results.”

2) Accuracy. A well-constructed shrink-fit toolholder will offer a 0.00012 in. (3 µ) maximum runout at three times the cutting tool diameter. This accuracy is very repeatable from operator to operator.

Induction shrink machines are very powerful, explained Techniks Vice-Presidnet of Sales Mike Eneix. Heating the toolholder, even for a few seconds longer than needed, can reduce the life of the toolholder, especially if this is done repeatedly through the life of the tool. Techniks

3) Balance Repeatability. Because this type of toolholder has no moving parts, it has great balance repeatability, especially at higher speeds.

“If a shop purchases a properly balanced shrink-fit toolholder with the right accessories and works with a well-balanced end mill, then they will have good balance while operating at high speeds with little to no fine tuning,” said Holden.

4) High Gripping Torque. Shrink-fit toolholders grip the cutting tool 360 degrees around the shank. This prevents the cutting tool from moving during operations and controls the rigid connection.

“It’s important to make sure that the cutting tool shank is situated through the entire clamping area of the toolholder,” said Miller. “With short tools, the toolholder will try to compress around a tool area that doesn’t exist, creating small steps or deviations in the bore. Unfortunately for the U.S. market, a lot of the imperial nominal tools have very short cutting tool shank length compared to the DIN and ISO counterparts, which makes this a bigger consideration for this market.”

5) Reduced Tool Change Time. With a good induction shrink-fit machine, tool changes are very quick, in about five to 10 seconds. There is no need for additional steps or accessories, like nuts or discs, that need to be tended to. This allows the tool and toolholder to be returned to the machine quickly to restart operations.

Shrink fit is a universal system and most toolholder manufacturers make them, but because they are universal, shops are not locked into a proprietary system. Any branded toolholder can be shrink fit in any branded machine, making them extremely flexible. However, it’s important to remember there are both good and bad toolholders and shrink fit machines on the market.

“A quality toolholder with a quality shrink-fit machine can ensure that the holder lasts for the life of the machine tool,” said Holden. “If you have a good machine in combination with a good holder, you really don't have to think about anything else. There are so many things to worry about in the shop, you don't want your toolholder assembly to be one of them. Investing in a quality system can allow shops to put this aspect right out of their minds.”

Toolholder Material Matters. The toolholder should not only be made out of the right material. During the purchasing process, shops should pay attention to the type of material but also material consistency.

“There can be variances with the material type,” said Eneix. “Most manufacturers use a certified H13 steel. The benefit of this material is that it can be heated and cooled thousands of times without losing the integrity of the tolerance of the holder.”

Angle Tolerance Matters. With any toolholder, the only part of that holder that makes contact with the machine spindle is the back end of the taper, so it is important to ensure a quality angle.

BIG DAISHOWA Senior Manager of Engineering Alan Miller explained that cleanliness is one of the top things to keep in mind when working with shrink-fit holders. The BIG DAISHOWA Alpha Wiper Cleaner cleans smaller cylindrical bores by simply inserting and removing cleaner before cutting tool insertion. BIG DAISHOWA

“This can be measured with an AT scale or angle tolerance,” said Eneix. “The angle of the taper should be held to an AT3 taper tolerance or better. That’s going to maximize contact with the toolholder and eliminate any issues with interference or poor connection.”

Cleanliness Matters. A clean toolholder will ensure a good connection with both the cutting tool and the spindle. Operators should make sure that the inside bore of the toolholder is clean. Debris present in the bore during the induction process can score the inside and create issues with quality and eventually degrade the toolholder, shortening its life cycle.

“Cleanliness is one of the first things to watch out for,” said Miller. “During heating and cooling, even the most minor of contamination can get baked in there, leading to runout issues. And this can build up overtime, causing consistency problems.”

Induction Machine Quality Matters. Choosing a toolholder is one thing, but shops also need to consider the quality of their induction machine, which is essential to the shrink-fit process.

“These machines are designed to expand the bore of the toolholder enough to accept the cutting tool,” said Eneix. “They should expand by only 0.01 to 0.03 in. However, getting a good quality machine that is able to perform this task quickly and effectively can be costly. Some small or medium shops may not want to invest in a $15,000 shrink-fit machine, and that’s understandable.”

The benefit of this system is that once a shop purchases the shrink-fit machine, it can be used for any of these types of toolholders because it is a universal machine.

“A good quality machine should have a shrinking process that allows tools to be changed in five seconds and then cooled in 30 seconds,” said Holden. “Other systems might take up to 15 minutes to cool the tool, which is a significant amount of wasted time.”

Choosing a shrink-fit machine with modern coil technology is essential. This will provide safe, efficient energy to the toolholder without overheating. In combination with controls, shops should look for a machine with smart features like automatic sensing, which will sense the distance from the outside of the hold to the inside of the holder.

“It will be able to determine how much power, time, and frequency is needed to open up the bore,” said Holden. “These smart features really take any variability out of the equation and out of the operator’s hands.”

Temperature Matters. Don’t overheat the toolholder. Overheating, even for a short amount of time, can damage the holder. Smart features can eliminate this issue, but for those that don’t have automatic sensing, it’s important to pay attention to how long the toolholder is being heated.

Induction machines vary when it comes to quality. Choosing one that can both heat and cool quickly is important. Techniks’ Quencher ShrinkFIT machine’s cooling cycle uses a coolant bath that cools in 30 seconds. Techniks

“Induction shrink machines are very powerful,” said Eneix. “If you heat it even for four or five seconds too long, it can affect the life of the toolholder, especially if this is repeated throughout the life of the tool.”

Overheating the toolholder can change the material properties and grain structure.

Oftentimes, when operators are having trouble removing the tool from the holder, there is a tendency to keep heating it over and over until the tool is free.

“Operators may not realize how damaging this is to the holder,” said Miller. “In some cases, it can completely destroy the holder. Some of the latest machines have technology like a thermal limiter, which ensures that the toolholder is never overheated.”

Not all systems are equipped with this technology, which is why it is important to properly train operators who will be working with shrink-fit toolholders. While there is a great deal of consistency with the setup, training will help ensure that overheating is not an issue.

“It does take a lot to damage the toolholder,” said Eneix. “The tool has to get over 1,500 degrees to lose integrity, but it does happen. This is typically seen during extraction of the cutting tool. With many different operators performing these tasks, a little bit of training can go a long way to maximize the life cycle of the holder.”

Associate Editor Lindsay Luminoso can be reached at [email protected].

BIG DAISHOWA, www.bigdaishowa.com

Haimer USA, www.haimer-usa.com

Techniks USA, techniksusa.com