Basics on shifting the machine motion to the cutting tool
An industry expert answers questions about what toolholders do and the different styles used today.
Figure 1. A toolholder transfers motion from the machine tool to the cutting edge itself.
Toolholders provide the critical connection between a machine spindle and the cutting tool. They are responsible for securely gripping the tool for extremely high speeds, avoiding excessive runout, and maintaining balance.Canadian Metalworking talked with Jack Burley, vice president of sales and engineering for Big Kaiser Precision Tooling Inc. , about toolholder basics, including common systems, a little history, and what the future may hold.
Burley has over 30 years of industry experience. As a member of the American Society of Mechanical Engineers’ (ASME) Technical Committee, he reviews recommended toolholder designs, suggested improvements, and changes requested from manufacturers on an annual basis.
CM: What is the basic job of toolholder?
Jack Burley: A toolholder transfers motion from the machine tool to the cutting edge itself. It pulls the total assembly together (see Figure 1).
Each machine follows a different toolholding standard, so they come in thousands of varieties and adaptations. A manufacturer must know or evaluate what type of standard a machine has for its automatic tool changer and magazine.
CM: What are the most common types of toolholders?
Burley: There are three common steep-taper type toolholders. Steep tapers have a 7-to-24 rate of taper, which means that for every 7 inches in Y there are 24 in. in X, or a 3-1/2-inch taper per foot.
In North America the most common style is the CAT 40 and 50 (see Figure 2a). It is based on the ASME B5.50 standard, which has been around since the late ‘70s. The standard was developed by Caterpillar Inc., which was a significant user of machine tools with automatic tool changers. The company dictated what the toolholders would look like if machine tool builders wanted to sell to them. They wanted standardization.
Until then every OEM had a different way of holding tools. There was no interchangeability. Several toolholder companies and machine tool builders got together with Caterpillar and some other manufacturers, refined the design, and made it a new standard.
Japanese and Asian machine tool builders settled on the BT (see Figure 2b), a Japanese Industrial Standards (JIS) that has been around about as long as the CAT standard. The taper itself, the diameters, the lengths, are the same as the CAT. CAT and BT toolholders will fit into the same spindle if they are hand-loaded. The difference is in the tool changer mechanism—the way the automatic tool changer grips the toolholder to change it from the magazine into the spindle.
Figure 2a. The primary difference between a CAT and a BT toolholder is the V-groove on the shank. Also see 2b.
The European DIN standard followed suit with the CAT design. Toolholder dimensions are the same as the CAT, but measurement is in metric.
CM: What is different about HSK toolholders?
Burley: High-speed machining started getting popular in the ‘90s, especially in aerospace where they began machining monolithic parts like wing struts from billets rather thanusing fabricating processes. A lot of high-speed machining was used to produce these parts quickly.
The toolholding systems available at that time were not as capable of this type of machining as compared to the German hollow taper shank, hohl shaft kegel (HSK) in German, that has a shallower taper at a 1-to-10 ratio (see Figure 3). Since then the HSK has been standardized to ISO specifications (12164-1, -2). Several sizes of HSK toolholders are available in different forms to fit with small to large machines. For example, form A is for general-purpose machining, and form E or F is for high-speed machining. The forms have different features depending on the standard they follow (see Figure 4).
For the most part, the market has settled on the HSK form A that is most common internationally. It is truly one of the only worldwide-side toolholder standards. It has been adopted in Japan, North America, and Europe. We see it and sell it here, but it is not as common as the CAT taper toolholders.
CM: What about the cutting tool connection?
Burley: What I call the toolholder’s second end holds the cutting tools. The tools–drills, end mills, taps, reamers—are typically cylindrical with a straight shank.
The collet chuck is the most common way to hold a drill and comes in sizes from very small to very large. Every collet will hold a range of tool sizes. When the nut clamps a tool down in the chuck, there is usually about a 0.020 in. of collapsibility in the toolholder.
For example, a 5/8-in. collet for a 0.625 diameter tool will still clamp a 0.620 diameter tool. The 0.625 is the maximum diameter, a 0.605 would be the minimum. A collet chuck is very universal and can hold a lot of tools.There is an ER quality standard for collet chucks and there are proprietary systems from collet manufacturers that have slightly different angles designed to provide more concentricity and accuracy (see Figure 5).
The collet chuck is good for general purpose machining conditions. Other tools that should be used for heavy duty rough end milling.
Figure 2b. The angle or rate of taper on CAT and BT toolholders is the same, about 16 degrees.
CM: Is there an alternative to the collet chuck?
Burley: A side lock end mill holder is the other popular way to hold the tool. A set screw engages with a flat section on a straight shank tool to secure it in the holder. It provides a lot of contact, but each holder fits only one size tool (see Figure 6).
Clamping is very secure, but the runout and rigidity are not very good. It’s a fairly old technology for general-purpose end milling but continues to be popular because it is inexpensive compared to other toolholding systems.
CM: How does shrink-fit fit in?
Burley: Shrink-fit technology provides a very solid connection between the tool and the toolholder. It has become fairly popular in the last 20 years.
During the connecting process, the tooling bore is heated to 1,000 degrees F so it expands enough to allow the cutting tool to drop in. When the toolholder cools to ambient temperature (some shrink-fit systems have cooling built into them), it shrinks back to its original size and compresses around the tool (see Figure 7). This pairing results in extremely little runout and a very good gripping force.
Only one size tool fits in each shrink-fit toolholder. It’s a very practical system for end milling where the tool shanks are all one size.
CM: Tell us about hydraulic chucks.
Burley: A hydraulic chamber is located inside a hydraulic chuck. When the piston is compressed, the hydraulic fluid around the toolholder bore compresses to provide a very strong clamping force. One hex screw clamps and unclamps the tool (see Figure 8).
CM: What seems to be the most popular chuck?
Figure 3a. HSK toolholders have a shallower taper than CAT or BT types.
Burley: The milling chuck (see Figure 9). It works on a roll lock bearing system. As the nut is clamped down, it compresses the bore of the toolholder to provide a very high rigidity factor. This type of toolholder typically outperforms others by a factor of two. It is a little more expensive but allows for the use of collets. A clamping wrench is all that’s needed to secure the tool.
The milling chucks have a higher productivity than anything else on the market as far as roughing end mill holders, slotting cutters, or even when used with face mills. It’s not a first choice to be used with drills.
CM: Are there other toolholders we should discuss?
Burley: Besides cylindrical shank toolholders there are systems like shell mill holders that are standard for face mills. Mill lock screws or socket head cap screws, depending on the manufacturer’s specifications, are used for tightening.
Then, with newer technologies like multiaxis machines that turn and mill, we’ve had the introduction of toolholders that can work with both cutting processes (see Figure 10).
CM: How do you choose?
Burley: Particularly with technologies that give higher material removal rates and better surface finishes it comes down to performance quality based on what is spent. Don’t expect a lot of performance from the less expensive systems. More investment provides a higher-performing system and longer tool life because of reduced runout.
I’d like to say for every 0.0001 in. of runout you either increase or decrease tool life by 20 per cent; for example, if I put a 3/8-in. drill in a collet chuck and get 0.0005 in. of runout before use. But if runout is reduced to 0.0004 in., tool life would increase by 20 per cent. On the reverse, if it goes from 0.0005 in. to 0.0006 in., tool life will decrease by 20 per cent.
Cutting tools can be expensive, and if they are thrown away before performing up to their full life expectancy, the costs adds up. Paying a little more for a high-precision toolholder will provide a good ROI and then pay dividends.
CM: What’s next?
Figure 3b. An HSK-F type with no slots or holes makes it better suited for high-speed machining on smaller machines.
Burley: The big question is what the next standard will be based on evolving machine tools. How will the toolholder interface change? Will it be a significant change or an improvement to a current system?
At this point I don’t see anything monumental on the horizon.