Part II: Spindles and Their Relationship to High-Speed Toolholders

There are a number of issues to consider when selecting the proper toolholder for hard milling and high-speed applications. A number of these issues also can be applied to toolholder selection for even conventional machining in order to maximize tool life and improve surface finish.

There are three major issues to think about when considering the proper toolholder for a high-precision application: (1) rigidity (2) accuracy and (3) balance. Much has been said in the industry today about the issue of balance, but it is important to consider the combination of all three of these issues when providing the best solution for the end-user.

 

Rigidity

Rigidity is developed when there is good taper contact between the toolholder and the machine tool spindle. Taper contact is commonly expressed in a percentage. High quality toolholders will provide 80 percent or better taper contact with a machine spindle. The majority of the contact should be in the thicker part of the taper where the spindle of the machine is at its most narrow point.

Toolholders that are able to maintain an 80 percent or higher taper contact must go through a much more detailed machining process than the common toolholder. After heat treatment, a quality toolholder will first ground the chamfer at both ends of the tool. This chamfer will serve as the datum point in the grinding process. In other words, toolholder manufacturers will rough grind approximately sixty toolholders. Then they will check the amount of stock remaining on the holder and take a light finish grind. If the chamfer on both ends of the toolholders is not ground, the tool will not have a solid datum point to relocate between the centers of the OD grinder. Therefore, the finish grind may not completely clean up the taper and the toolholder will lose taper contact.

Also, in order to maintain roundness, the finished grind should be a traverse grind rather than a plunge grind. Another key element to successful taper contact is the back end of the taper should be ground so the retention knob has a square surface.

A surprising number of toolholder manufacturers do not follow the above-mentioned suggestions for maintaining taper contact, and consequently they lose a tremendous amount of rigidity. In other words, the tool can be balanced well, but if the taper contact is lacking, there still will be vibrations caused in the spindle.

Measuring Taper Contact
In order to maintain taper contact, it is important to know how to measure taper contact when one is machining a toolholder. It is best to consider using all three of the following elements when measuring the toolholder's taper contact: taper gauge, AT rating and taper roundness. In order to guarantee a high level of taper contact, it is important that the toolholder manufacturer inspect 100 percent of the tapers with a taper gauge after the final grind process. To do this, the manufacturer must place a thin layer of dye at 120 degrees along the length of the taper. He must then place the taper of the toolholder in the female taper gauge, and rotate the tool ten degrees in both directions. When the toolholder is removed, the place where the holder is actually making contact with the taper gauge will be visible.

A quality toolholder will have 90 percent or better contact with the taper gauge. This sort of contact will translate into 80 percent or better contact with the spindle of a new machining center.

Another measurement developed to monitor toolholder tapers is the AT rating. AT means angle tolerance, which decreases in ten steps of length ranges. There is an ISO and an ANSI AT rating chart. The ISO AT rating chart takes into account the length of the taper, consequently, making it more difficult to maintain. It is important that toolholder manufacturers maintain an ISO AT3 or better taper angle when machining their toolholders for high-speed machining (HSM) in order to maintain taper contact.

The third way to check the taper of a toolholder is by testing the roundness. A quality toolholder manufacturer will inspect the roundness of its tapers in order to guarantee taper contact. This process assures the manufacturer that the holder is not left with an elliptical taper.

Checking the roundness is done on a digital roundness tester. This step is crucial in assuring a quality product. It takes about fifteen minutes to check a toolholder. Although it is time consuming, the extra time usually translates into a more expensive toolholder.

There are standards that have been developed to measure toolholder taper contact specifications. The first contact the toolholder has with the machine tool is between the toolholder taper and the machine spindle. This is the foundation for the cutting edge. The first look at this critical connection is to understand a typical taper fit. The standard states that the allowable tolerance is split between the shank and the socket. Two accuracy standards cover the measurement of this taper.

First there is the ANSI B 5.10 standard, which gives a constant rate of .001 inches per foot independent of taper length. The lack of specifications as to the taper length puts those who use this standard at a disadvantage, as can be seen when comparing this to the second accuracy standard ISO-1947.

The ISO 1947 taper rate error method is considerably more flexible. Twelve accuracy grades are listed; cone length governs the error rates, with ten being the standard. It is all organized with logical steps of tolerance progression. Thus, the ISO 1947 method will provide the most effective marriage of tool taper shank angle tolerances with machine tool spindle socket angle tolerances, so that an effective performance will result in high-speed rotating tooling situations.

Accuracy

Accuracy is the second major issue of maintaining effective HSM conditions. In order to maintain accuracy with a toolholder, you must minimize the run-out, grip the cutting tool well and make sure that the shank on your cutting tool is accurate. When thinking of the entire setup, it is important that the end user is using a high quality cutting tool with a properly manufactured shank so that it will sit in the toolholder correctly.

The cutting tool must be held in a toolholder that can minimize run-out. The run-out is measured at the tip of the cutting tool. If a tool is running out a great deal, the cutting tool will be only cutting with one side. This will lead to a diminished tool life and a poor surface finish. Run-out can ultimately affect the balance of the toolholder's setup as well.

A high quality toolholder merged with a high quality cutting tool should be able to guarantee at least .0002" run-out at four times the diameter of the tool. With these specifications, the customer will see a huge increase in tool life and a much better finish on his workpiece. The tool life corresponds directly with the amount of run-out. If the toolholder has three times more run-out than a high quality toolholder, the customer will end up with one-third the cutting tool life.

Toolholder Styles
In today's toolholder market, there are a variety of styles of toolholders that can maintain good run-out accuracy for HSM.

The first is shrink-fit toolholders. These toolholders are talked about a great deal in HSM. In order to grip the cutting tool, the customer actually heats up the metal of the toolholder to expand the ID. Then, he or she is able to drop the cutting tool into the ID of the holder. Shrink-fit tooling has proved to be effective for run-out and balance repeatability. However, it has proved to be cumbersome and costly with the heating mechanism needed, and each holder is dedicated to one shank diameter. Also, the effectiveness of the run-out minimization is dependent on an extremely close tolerance on the cutting tool shank. The biggest advantage of shrink-fit toolholders seems to be extended length reach applications.

The next option for minimizing run-out is hydraulic chucks. They have proved to be effective and maintain their balance, but they are not good for heavy side-loads as a result of the distortion of the hydraulic bladder. They also have proved to require some extra maintenance because of hydraulic fluid leakage, as compared to alternative toolholders.

The final option is milling chucks and high precision collet chucks. Milling chucks have shown to be effective for run-out and gripping torque. High precision collet chucks have proved to be effective for run-out and balance, very simple with moderate cost, and extremely versatile with interchangeable collets for a variety of shank diameters.

However, when selecting a collet chuck for high-speed machining, it is important to choose one with high precision as a standard. The following are a few standard tips for selecting the correct collet system for HSM.

• An eight-degree collet is recommended for use in high-speed applications. A standard ER collet uses a sixteen-degree taper. Eight-degree tapered collets give twice the gripping force due to the taper being twice as shallow. The shallower the taper, the less deflection of the gripping force away from the tool shank. Collet bore engagement by the tool shank and gripping power run hand in hand. The less the engagement, the weaker the gripping power. It is recommended that a minimum engagement of 2/3 of the full collet bore length be used with 3/4 collet bore engagement preferred. This is important due to the loss of gripping power because of centrifugal force and spindle growth.

   

• Where possible, use the minimum collet envelope relative to the bore size. In other words, use the largest bore size in the smallest collet envelope to achieve high gripping force with reduced toolholder mass. This means simply to use a 1/2" shank in a 1/2" capacity collet system instead of a 1/2" shank in a one-inch capacity collet system. This helps in keeping centrifugal force low and will allow the highest speed to be achieved relative to the balance specification limitations. This is logical since the chuck diameter will be smaller and the chuck will weigh less.

 

Balance

Balance is the third and final component for selecting a toolholder for high-speed/high-precision machining. In HSM, the need for a balanced toolholder is crucial. This balance is necessary in order to maintain the life of the machine tool spindle, and to provide a good finish on the workpiece. Today, so much of the high-speed market is the mold industry, and customers are using high-speed spindles to machine hardened steel. This helps them minimize EDM and polishing time. However, they must maintain excellent finishes in order to successfully machine hardened material. Therefore, a balanced toolholder assembly is critical.

It is important to identify the issues of balance and how to properly measure and maintain the balance of a toolholder. What is a balanced toolholder, and at what rpm should you consider balanced tooling? A good general definition of a balanced toolholder is a state of equilibrium in which rotational forces are countered by equal, opposite forces. According to ISO-1940, as mentioned earlier, 8,000 rpm is the approximate speed at which unbalance begins to have adverse effects on the application.

What are the causes of unbalance? Several factors contribute to toolholder unbalance. One factor is excess mass on one side of the rotating toolholder. Another factor is flaws in the base material. Flaws such as voids, seams, porosity, etc., may appear in the raw material at any of the fabrication stages. Since it would be extremely rare for such a flaw to be balanced by an equal flaw 180 degrees from the original, the resulting difference in density causes unbalance, as well as structural weakness.

Poor tolerances during fabrication can also contribute to unbalance. Unmachined portions of forging, out of roundness and improper placement of through holes on the rotational axis can create a shift in the toolholder axis. Any machining performed on the toolholder that diminishes the absolute concentricity about the rotational axis contributes to unbalance. An additional factor is asymmetrical toolholder design.

A good example of an extremely unbalanced toolholder is the set-screw endmill holders that, by design, have one or two set screws located on one side of the toolholder to clamp on the Weldon flat-a flat surface ground into many cutting tool shanks—of the endmill. Due to this design, endmill holders are inherently unbalanced because there is no offsetting weight 180 degrees from the set-screws. Consequently, set-screw endmill holders are not at all recommended for HSM. However, if a high gripping torque is required on the cutting tool, milling chucks are recommended to maintain the cutting tool on the centerline of the toolholder.

All toolholders used in high-speed spindles should be of symmetrical design. Drive slots should be of equal depth and width, or have a correction mark on the smaller drive slot (in the case of CAT tooling). Another potential source of unbalance due to design is DIN notches in the flange of the toolholder.

The use of asymmetrical components, cutting tools, drivers, spacers or even retention knobs all can be a contributing factor to toolholder unbalance. Any component in the assembly of the toolholder cutting tool has the potential to upset the balance. Therefore, when the toolholder is sent to be balanced, all components should be assembled. This includes retention knobs, collets, gauge pins and any other components. If the retention knob does not have a ground pilot—standards for CAT shank retention knobs don't call for a ground pilot under the shoulder—it should be cleaned and lock-tighted to the toolholder.

In summary, when looking at a toolholder you must focus on the toolholder design starting from the retention knob and working your way to the nose of the toolholder, reviewing each component that is to be used at high speeds. As a general rule, all toolholders used in high-speed applications should have an AT3 or better taper specification. The taper should also be at least 60 millionths or better in round, and have 90 percent or better taper contact with a taper gauge. All toolholders should be symmetrically designed with no off-setting weight, and an effort should be made to keep the toolholder body dimensions as small as possible. As you move outward from the centerline of the toolholder, the allowable error tolerance decreases.

Also, it is preferred that the toolholder is as short as possible but, of course, longer toolholders are often needed in mold work. Therefore, if you need extended-length tooling, it is best to use a one-piece construction. In fact, a tool with a one-piece construction is better than putting an extension in a collet chuck at any rpm. A one-piece constructed toolholder will not only be more balanced, but it also will provide much better rigidity and accuracy.

The retention knob used in high-speed applications should be piloted, such as with BT tooling. This emanates from the ISO 7388/2 international standard. The objective is to have the pull back forces as near as possible to the centerline in order to help reduce even the smallest effect of a taper error that results from the AT3 taper specification. Unfortunately, CAT tooling does not call out for a specification of having a ground pilot connection between toolholder and retention knob.

It also is critical to have a round, ground and piloted lock nut assembly. The turned and milled HEX nut design has given way to a round and ground shape with shallow elongated spanner wrench slots. The ground diameter is slightly smaller that the former HEX diameter. The normal unpiloted acme thread has been modified by the addition of a close fitting pilot diameter located immediately at the rear of the threads. These features combine to produce a more repeatable positional lock nut with reduced mass. The smaller diameter and round shape gives reduced centrifugal force as speeds are increased. The pilot feature provides for higher quality balancing of an assembled chuck and cutter assembly. Some toolholders claim to be balanced, but when you look at the inside of the nut, you see a "C" snap ring. This means that each time you tighten the nut, the snap ring will be located in a different location, leading to extreme unbalance.

In order to standardize the level of balance required by toolholder manufacturers, each machine builder calls out different requirements based on the machine's maximum rpm. Many machine builders have spoken about requiring toolholders to be balanced at G 2.5. In order to understand what this expression means, we will look at the formula that determines the G rating as far as balance is concerned.

To get an idea of the balance tolerance, let's calculate an expression for the allowable unbalance, U, and then determine the formula for G 2.5. Then we will calculate the balancing universe for our 8,000 to 20,000 rpm speed range that is appropriate for the #40 taper example collet chuck. The general expression for allowable unbalance is U = G X 9549 X W/ rpm. U is allowable unbalance in gram millimeter, W is tool assembly weight in (kilograms), 9549 is a constant, rpm is the rotational speed of the assembly, and we know what the G number is—2.5.

Now let's calculate balance tolerances for any rpm for G 2.5. The tool weight for our example tool assembly is 2.75 kilograms.

So: U2.5 = 2.5 X 9549 X 2.75 divided by rpm or U2.5 = 65.659 divided by rpm.

So: solving for G 2.5 at 8,000 rpm gives us an allowable unbalance of 8.2 gram millimeter and for G 2.5 at 20,000 rpm gives us an allowable unbalance of 3.3 gram millimeter.

You will notice that as the rpm gets higher, the permissible unbalance becomes less. The different machine manufacturers usually assign the acceptable G rating for HSM.

 

Balancing a Toolholder

When balancing a toolholder assembly, the unbalance must be identified and located so it may be corrected. This identification process is done on a toolholder-balancing machine. At this point, a choice has to be made between one-plane and two-plane dynamic balancing. A phenomenon known as dynamic unbalance becomes more pronounced as gauge lengths increase. When one is balancing in two planes, the center of gravity of the toolholder is included in the equation. So all toolholder assemblies have two planes—one that extends from the center of gravity toward the nose of the toolholder and one that extends from the center of gravity back toward the retention knob.

Dynamic unbalance arises when there is an unbalance in the bottom plane and in the top plane. The result is that the toolholder's assembly centerline can be affected. When a toolholder assembly's unbalance is corrected via two-plane balancing, the toolholder assembly will rotate around the centerline. When the correction is made via one plane, the rotation of the toolholder assembly may be affected. Instead of rotating about the centerline, the toolholder assembly may now rotate between three points, 180 degrees from the correction mark. This can create a wobbling effect, which is as detrimental to tool performance as not balancing the assembly at all.

Once the unbalance has been measured, it is important to decide how a shop is going to deal with balancing. Many adjustable or balanceable holders attempt to correct all unbalances in a single plane. The longer the gauge length of the toolholder assembly, the more negative effects can be seen from dynamic unbalance. All balanceable toolholders require a balancing machine to truly determine the location of unbalance, and where and how to adjust the unbalance. If you do purchase a balancing machine, you are not required to purchase balanceable toolholders. Many shops hard balance their toolholders (remove weight by drilling material out of the toolholder).

The second option is to purchase your toolholders pre-balanced (or hard balanced) from the manufacturer. This solution works well if you have taken the initial steps of choosing the correct toolholder in the first place. In other words, the choice should have been made to purchase toolholders that are rigid (excellent taper contact), accurate (very little run-out), and symmetrical (no off-center weights). Also, when having your toolholders come balanced from the factory, the recommendation is to assemble the holder with the retention knob, the coolant accessories, a collet, the nut and the cutting tool to assure best results. The balance level will be affected every time you remove a tool from the toolholder. If a significant change is made to the toolholder assembly, it should be sent back to the toolholder manufacturer to be re-balanced. For longevity of spindle bearings, quality of finish and overall performance in high-speed applications, it is recommended that toolholder assemblies be dedicated to a specific shank diameter.

In conclusion, quality toolholders for HSM applications must have rigidity, accuracy and balance. If they do, the customer will see extended cutting tool life, extended spindle life and a better surface finish. These results are true at any rpm, but the toolholder must definitely be under these criteria for HSM.

Also, as mentioned before, there are many machine tool spindle options for running at high rpm's. Ultimately, one of the dual face contact options can provide the best interface of the toolholder to the machine tool. As the market becomes increasingly competitive, hopefully the advances in the combination between the toolholder and machine will continue to provide more options to maximize a manufacturer's time and profitability.