Balance is often discussed, and is misunderstood by many. While everyone who is still reading this article has probably seen the formulas and read many papers on the subject, the purpose here is to:
- Describe some of its effects,
- Loosely explain balance,
- Define why balance is important--sometimes,
- Discuss how different holder configurations perform at higher rpm.
Surface finish can be influenced by background vibration. Ideally, surface finish is determined by feed rates, depth of cut, and spindle speed. Vibrations coming from a good cut should be multiples of the spindle rpm. Forces that show up at the spindle frequency or a multiple thereof include unbalance forces and the force of each tooth slicing away a "chip" of material. Unbalance forces can act like additional runout in the assembly. Our company estimates that the added runout or "dynamic runout" addition from well balanced tooling is 25 - 50 µin. (0.64 - 1.3 µm). On toolholders balanced to G12 at 15,000 rpm, the additional runout could be as high as 0.00016" (4.1 µm). In actual cutting tests at a major manufacturer, there was no discernable difference between G2.5 and G12 on a given set of cutting tests looking at surface finish. This was one test and can be misleading if broad generalizations are made. If the chip load is small, this effect may show up and be noticeable.
Surface finish/tool life and a force overload on the bearings of the spindle are two problems to confront in machining. Force is easy to define as far as balance is concerned: F= m X r X v2, where omega is defined in radians per second, with a circle defined as 360º or 2p radians. Also, m X r (mass times a radius) is the definition of unbalance. Mass units can be defined as gram, kilogram, pound, or ounce. Usually inch or millimeter are preferred for the radius. For larger unbalances pound-inch or ounce-inch are acceptable, but for the finer unbalances required in toolholder balancing or spindle balancing, gram-millimeters (g-mm) are often the preferred unit of measure. Many people confuse g-mm with the G number from the ISO or ANSI standard. Another cause of confusion is thinking that the G number from the balance standard relates to some kind of gravity forces that military pilots endure during high-speed maneuvers. Unbalances themselves have units of gram-millimeters. Unbalance levels are a vibration velocity and are specified as G numbers with units of millimeters per second (mm/sec).
Two standards are usually cited when toolholder or spindle balance is specified. They are ISO 1940 and ANSI S2.19. Each standard was put together by a committee. Committee members recognized that unbalance could be detrimental to rotating assemblies, and wanted to provide their knowledge to the manufacturing community. They broke balance control into three categories:
- Vibration control,
- Force control,
- Part-production ability.
Of these three methods, vibration control has been used most often. This is the G number manufacturers often specify. Many different numbers, such as acceleration, velocity, or a peak-to-peak displacement, can describe vibration. All three can describe the same part spinning at the same rotational speed.
The standards committee defined their vibration units to be mm/sec. The G numbers are vibration threshold limits; the number G1.0 corresponds to a free spinning vibration of 1.0 mm/sec. Because closer tolerance work is easier with a stable workpiece and spindle, the number G1.0 is usually specified for grinding spindles.
The G2.5 and G6.3 vibration levels are used for machine-tool spindles and machine-tool parts. More vibration is allowed because tolerances being held are generally larger, and the overall operation will not benefit from stricter balance tolerance. Using our sound analogy, if the music is loud enough so that you cannot hear the background noise, trying to eliminate the background noise is a waste of time and money.
Each grinding machine typically has a constant force (except in the case of interruptions) from the thousands of workpiece interactions. This averaging effect makes the operational forces very uniform. A machining operation usually has higher loads, each with its own frequency and amplitude. Tooth impact creates a vibration that is larger than that experienced during grinding, and limits the effectiveness of very restrictive balancing. Variation in runout can also influence this vibration, and can often overcome the gains balancing has delivered.
The G numbers referred to in the standards are generally assigned to an overall assembly. A machine spindle assembly may have an overall design balance level of G6.3 in operation, while the spindle and the toolholders will be balanced to G2.5 levels. Moreover, the weight and cylindrical configuration of the spindle allows it to be easily balanced to G2.5, G1.0, or G0.4 levels. The spindle's main rotating mass can be easily balanced between or over centers, allowing very good resolution and repeatability.
All major subcomponents of the assembly have the same vibration-level requirements. Because the locating surface of the toolholder is a tapered cone, toolholder balancing is more difficult. The toolholder's light weight, coupled with the tapered cone/balancing machine, make balancing the toolholders difficult at G1.0, G2.5, or G6.3 levels of balance at very high spindle speeds.
This vibration-level method is the most restrictive way of looking at balance. Often a more-restrictive balance level is specified because of a lack of understanding, or a desire to compensate for balancing that is not done or is done incorrectly.
Force control, another way of dealing with balance, requires looking at machining forces and balance forces, and managing the overall level to prevent spindle damage. As mentioned before, forces from balance are given by F=m X r X v2, which explains why balance becomes more critical as speeds go up. This method requires that you have a good understanding of the operation's machining loads. At low spindle speeds, the relation between speed and balance-induced forces explains why balance is less important. At high speeds, the vibrational method is not practical, because the formula yields very low (sometimes impractically low) allowable residual unbalance.
While the vibration method may overstate the importance of balance, force control methods overlook the fact that, while you may not damage the machine tool, the parts may not be acceptable. A particular unbalance may not generate a high enough force to cause damage to a spindle, but the unbalance force vector can function like runout and spoil part surface finish if the tool has more than one cutting edge. This is because the unbalance force rotates with the cutting tool. If there is sufficient unbalance rotating at a speed to create an unbalance force of 100 lbf (445 N), that force will always be directed in the same orientation relative to the spindle face. Even a slight unbalance can create 100 lbf if it is spun fast enough, and this force will pull the spindle centerline around in a circle with a radius equal to the spring constant of the spindle times the force created by the unbalance. This spindle-centerline runout is caused by toolholder unbalance.
The vibration method is conservative, and guarantees that balance issues are addressed; however, in some instances it is not economically or physically possible to adhere to this method. In these cases, the force created should be evaluated and, if low enough, a trial-and-error process can be used to determine whether tight tolerances can be produced. This should be initiated with the help of the machine tool manufacturer and your toolholder supplier. We are not suggesting running unbalanced toolholders, but when toolholders are spinning above 30,000 rpm, common sense requires us to recognize that G2.5 is not attainable. While many manufacturers sell balancers that can resolve to the tolerances required, an understanding of mechanisms involved in gripping and locating a toolholder makes it evident that to spend even five minutes of time to get below 1 g-mm is wasteful, and will put you at an economic disadvantage.
In many low-speed applications, a balancing operation or feature of any kind is wasted. Some toolholder shanks and toolholder noses are, however, better than others from a balance perspective. While a shrink-fit toolholder may be balanced and grip a cutting tool very well, it does not have good size flexibility. This limitation may be important, but for the purpose of this discussion, toolholders are considered strictly from a balance perspective.
From a balance standpoint, ANSI-style or CT-style shanks have the largest uncorrected balance. They have offset drive keys for timing of cutting tool edges. In addition, there is also an unpiloted retention knob. The offset drive keys can contribute "90 g-mm to a 40-taper toolholder and "400 g-mm to a 50-taper. The unpiloted retention knob can contribute 5 - 25 g-mm in any orientation. Other toolholder standards perform much better, and piloting the retention knob allows for much better balance repeatability. The DIN69871 standard and the MAS 403 (BT) both are similar to the CT, but with a metric piloted thread for the knob. The BT flange toolholders do not have offset drive keys and are totally symmetrical, which is the best design for balance.
From the viewpoint of balance, when compared to ANSI-style toolholders, Form A HSK toolholders are initially in worse shape. They have an inherent unbalance that is difficult to correct in two planes. Because of the higher spindle speeds typically used with this standard, it also includes information to statically balance the HSK toolholder. The high-speed versions, Forms E and F, have no asymmetries in their design, so they are perfectly balanced off the drawing board.
Accuracy and gripping strength should be of the utmost importance when considering which toolholder cutting-tool interface to employ. In many operations, there are many choices that will work. When this is the case, balance can be a deciding factor. Although they are symmetrical, frictional interfaces like a Morse or Jacobs taper should not be operated at high speed, and will therefore not be considered. Weldon-type end mill holders can be balanced, but they usually perform poorly for runout, and have a structurally weak configuration 90º from the setscrews. These two problems will limit an end mill holder's performance even though they can be balanced quite well, and with a consistent cutting tool they repeat balance beautifully.
Shell-mill holders can be balanced after assembly, but there is usually an error introduced by size tolerances allowed on the pilot, and also some cutters are not symmetrical. Understand that symmetrical is not the same as even-number. Three and five-tip assemblies can easily be symmetrical; just because a cutter has four teeth, it is not guaranteed to be in balance. Most indexable cutters will have a maximum rpm noted, and even though the cutter may be balanced for a higher rpm, it should not be run faster than the maximum determined by the cutter manufacturer.
Milling chucks are often used in high-speed, high-power machining. While they have excellent accuracy and gripping strength, they tend to have nonrepeatable balance. While this is not always the case, usually a higher number of moving elements relates to balance inconsistency.
Hydraulic chucks can be very repeatable and accurate. Gripping strength depends on the age and maintenance of a holder. With a dynamic seal, which the piston is, there is always minute leakage. Given enough actuations, this leakage substantially limits performance. The hydraulic holder also has a thin flexible membrane that limits its effectiveness for high-speed milling. In a drilling application, (usually slower), the hydraulic holder is always considered a good application.
The toolholder most widely used for high-speed machining is the collet chuck. Collet chucks provide adequate gripping force for high-speed machining. While high-power machining--especially at low rpm--may slip in this interface, usually high-speed machining produces lower forces. Generalizing collet chucks can be dangerous: DR collet chucks have a fine pitch with a 60º V form. This combination provides a system that resists loosening at elevated rpm. Centrifugal force does expand the nose piece at high speeds. As this expansion occurs, the V form of the threads allows the nose piece to unload, and then loosen. As a rule of thumb, the smaller a nose diameter, the faster it will be able to run without limitations. Collet interfaces and collets are symmetrical, have very good balance, and work well together. Nose pieces that extract the collet need to have a balanced or symmetrical extraction device.
The connection that has the best high-speed capability is the shrink fit. Shrink-fit holders have no moving elements. The one-piece design locates the tool perfectly, and is symmetrical. The only thing that will relax the toolholder's grip during operation is centrifugal force, and it only begins to be a concern as spindle speeds pass 50,000 rpm.
One final caveat: balance is only as good as the company supplying it. While a BT40 shrink-fit toolholder may be perfectly symmetrical, and therefore design-balanced for high-rpm work, only after manufacturing variability has been correctly removed can it actually run at 15,000 rpm. Some manufacturers treat balance as smoke and mirrors. If you can't measure it, they can assure you it was done properly.
Balancing a toolholder means measuring it after production operations are complete, and making necessary mass corrections. Balanceable holders should be hard-balanced (material removed) to bring the holder into good balance performance. Then balanceable features should be used to correct variables from the presetting process, if they are of significance to the operation. In many cases, a shrink-fit holder or collet chuck only needs to be hard-balanced once at the factory. Unless spindle speeds are above about 30,000 rpm, a balanced toolholder can adequately address all balance necessities.
Notice I did not refer to balance requirements. What is being required by some of today's machine tool manufacturers is overly restrictive and not deliverable. While we recommend following all manufacturer's requirements, we also predict these will be relaxing within the next five years as understanding of the current standard improves, or a new standard is developed.
The only way to make decisions about balance is to improve your understanding. The ANSI S2.19 standard is available from many sources. It goes into considerable detail regarding single-plane and two-plane balance, and how to calculate acceptable unbalances when using the vibration method or--as everyone refers to it--the G-number specification. The only way to change is to understand the status quo and question it when necessary. Hopefully a new standard will emerge.
This article was first published in the March 2005 edition of Manufacturing Engineering magazine.