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Balance basics for job shops: understanding the current balance standards helps shops get the most out of their equipment, and some tool holders and noses prove better at high-speeds than others.

When machining, especially at a high rpm, operators need to understand the negative effects of unbalance. The machine's performance should be evaluated during a production runoff at the machine manufacturer's facility. The operation should also be monitored after installation and any unbalance, bad tooling, chatter or other problems should be eliminated or minimized.

Surface Finish

Ideally, surface finish is determined by feed rates, depth of cut and rpm. Vibrations coming from a good cut should be multiples of the spindle rpm. Forces that show up at the spindle frequency include unbalance forces and the force of each tooth slicing away a "chip" of material. In order to produce a good cut, the tool tip has to be relatively rigid. The feed rate in ipm (and depth of cut) needs to be constant. If the radial depth of cut is light, say 0.0004" per tooth, and varies due to runout by 0.0002" (one tooth has 0.0003" chip load, the next 0.0005" chip load), then the tool holder will cut part walls that have a poor surface finish. Balance forces can also induce additional runout in the assembly. In some instances, Command Tooling Systems (Ramsey, MN) has estimated that the added runout from well balanced tooling was 25 - 50 [micro] in. If the chip load is small, this effect may show up and be noticeable.

Force Overload

Surface finish and a force overload on the bearings of the spindle are two problems to confront in machining. Force, represented by the formula F=m*r*[[omega].sup.2], is easy to define as far as balance is concerned. [omega] is defined in radians per second, with a circle defined as 360[degrees]. M*R is the definition of unbalance, mass times a radius. The units for any mass can be defined as gram, kilogram, pound and ounce. Usually inches or millimeters are preferred for the radius. For larger unbalances pound-inch or ounce-inch are acceptable, but for the finer unbalances that are required in tool holder balancing or spindle balancing, gram-millimeters are often the preferred unit of measure utilized. Unbalances themselves have units of gram-millimeters. The levels are a vibration velocity and are specified as G numbers with units of millimeters per second.

Two standards, ISO 1940 and ANSI S2.19, are usually cited when tool holder or spindle balance is specified. The committees that created these standards recognized that unbalance could be detrimental to rotating assemblies and wanted to provide their knowledge to the manufacturing community. They divided balance control into three methods: vibration control, force control and part production.


Vibration Control

Vibration control, which refers to the G number, is the most widely used method. Many different numbers, such as acceleration, velocity or a peak-to-peak displacement, can describe vibration. All three of which can describe the same part spinning at the same rotational speed.

The standards committee defines vibration units to be mm/sec. The G numbers are vibration threshold limits. G1.0 corresponds to a free spinning vibration of 1.0 mm/sec, which is usually specified for grinding spindles because the closer tolerance work is easier with a stable work piece and spindle. 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.

The G numbers are generally assigned to an overall assembly. A machine spindle assembly may have an overall design balance level of G6.3 in operation. The spindle itself can be balanced to G2.5 levels. The weight of the spindle and the cylindrical configuration of the spindle allow it to be easily balanced to G2.5, G1.0 or G0.4 levels. In addition, the main rotating mass of the spindle can be balanced between or over centers, allowing for good resolution and repeatability.

All major subcomponents of the assembly have the same vibration level requirements. Tool holder balancing is more difficult because the locating surface is a tapered cone. The light weight of the tool holder coupled with the tapered cone/balancing machine, make balancing difficult at levels of G1.0, G2.5 or G6.3.


Force Control

Another way to prevent spindle damage is looking at machining forces and balance forces. As mentioned before, forces from balance are given by F=m*r*[[omega].sup.2], which is why balance becomes more critical as speeds go up. This method requires a good understanding of the operation's machining loads. At a higher rpm, the vibration control method is not practical because the formula yields very low (sometimes impractically low) allowable residual unbalance.

Part Production

While the vibration method may overstate the importance of balance, force control methods overlook the fact that, while the machine's tool may not be damaged, the parts may be unacceptable. 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 surface finish if the tool has more than one cutting edge. If there is sufficient unbalance rotating at an rpm to create 100 lb. of force, that force will be directed in the same orientation relative to the spindle face.

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 initiated to determine whether tight tolerances can be produced. Many manufacturers sell balancers that can resolve to the tolerances required. An understanding, though, of the mechanisms involved in gripping and locating a tool holder show that spending even five minutes to get below 1 g-mm is wasteful and puts job shops at a disadvantage.


High-Speed Tool Holders

Some tool holder shanks and tool holder noses do perform better than others from a balance perspective.

ANSI style or CT style shanks have the largest uncorrected balance. These shanks have offset drive keys that can contribute 90 g-mm to a 40 taper and 400 g-mm to a 50 taper. The unpiloted retention knob can also add 5 - 25 g-mm in any orientation. Other tool holder standards perform much better. The piloting of the retention knob allows for improved 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 tool holders do not have offset drive keys and are totally symmetrical, offering a better design.

HSK tool holders have an inherent unbalance that is difficult to correct in two planes. Because of the higher spindle speeds usually used with this standard, there is information to statically balance this type of tool holder. High-speed version, Forms E and F, are designed to have no asymmetries, making them perfectly balanced off the drawing board.

Shell mill holders can be balanced after assembly, but there is usually an error introduced by size tolerances allowed on the pilot. Also, some cutters are not symmetrical. Keep in mind that symmetry doesn't refer to an even number, for example three and five tip assemblies can easily be symmetrical. Just because a cutter has four teeth, it is not guaranteed to be in balance.

Often used in high-speed, high-power machining, milling chucks feature excellent accuracy and gripping strength, but tend to have non-repeatable balance. While this is not always the case, usually a higher number of moving elements relates to balance inconsistency.

Hydraulic chucks can be repeatable and accurate with the gripping strength depending on the age and maintenance of a holder. The dynamic seal releases minute leakage, which, given enough actuations, substantially limits performance. The hydraulic holder also has a thin flexible membrane that limits its effectiveness for high-speed milling. In slower drilling applications, this type of holder is considered a good choice.

Widely used for high-speed machining, collet chucks provide adequate gripping force. In general, this holder can be dangerous. DR collet chucks have a fine pitch with a 60[degrees] v form. The combination provides a system that is resistant to loosening at elevated rpm; centrifugal force does expand the nose piece at high rpm. 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.

Lastly, the connection that has the best high-speed capability is the shrink fit. Shrink fit holders have no moving elements. Without moving elements, the one piece design locates the tool perfectly and is symmetrical. Centrifugal force does not relax the grip during operation, which becomes a concern when spindle speeds pass 50,000 rpm.

Balancing a tool holder means measuring it after production operations are complete and making necessary corrections. Holders should be hard balanced (material removed) to bring them into good balance performance. Balanceable features should then be used to correct variables from the presetting process if they are of significant to the operation. Command Tooling Systems, LLC

[Editor's Note: Special thanks to Bill Keefe, former head of engineering, Command Tooling Systems (Ramsey, MN) for assistance with this article.] or Circle 206 for more information
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Copyright 2006 Gale, Cengage Learning. All rights reserved.

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Title Annotation:TOOLHOLDING
Publication:Modern Applications News
Date:Aug 1, 2006
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