Balancing Of Tool2

Balancing of Tool Holder

ABSTRACT Today, high speed machining has become a fact of life for many machine shops. In industries tool and machine technology has advanced to allow increasingly higher spindle speeds, one clear benefit of this trend is greater efficiency and greater productivity of organization. So at high speeds, Machine operator various problems of tools like breaking, vibrating have face unbalancing etc, therefore for overcoming all these problems the balancing in essential plays a very important role in producing quality parts.

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Balancing of Tool Holder

INTRODUCTION Manufacturing of high speed spindles often consider 10,000 rpm a starting point, speeds of 30,000 rpm are common and some spindle approach 1,00,000 rpm also. At these elevated speeds time in cut can be reduced drastically. When milling softer metals or composite materials, as spindle speeds continue to increase the potential, the adverse effect on machine and w/p increases due to vibration increase exponentially because centrifugal force increases with square of speed. One of the major contributor to vibration and the easiest to control is unbalance. Definition of unbalance and aerospace it has become the norm lather than expectation one clear benefit of this trend is greater efficiency and productivity through increasingly higher spindle speeds. As spindle speeds continue to increase the potential for adverse effects on Machine and w/p due to vibration increases exponentially because centrifugal force increases with square of speed. One of the major contributors to vibration and the easiest to control is unbalance. Unbalance defined :Unbalance is defined as that condition that exists in a rotor when vibratory force of motion is impaired to its bearings as a result of C.F.

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Balancing of Tool Holder

UNBALANCING TOOL

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Balancing of Tool Holder

WHAT TYPE OF TOOLHOLDERS ? There are several practical issues surrounding the question of when to use balanced or balanceable toolholders. One of the issues is obviously cost. Balanced toolholders are more expensive than non-balanced, and balanceable toolholders are the most expensive. However, the technical issues should override the short-term cost concerns. If balancing (of any kind) is not considered solely on the basis of the cost of the toolholder, and balancing is in fact required, the ultimate cost will be much greater. This will lead to out-of-specification parts, extensive downtime of the machine, and possibly significant cost to replace bearings or a very expensive precision spindle. Although

10,000

rpm

is

generally considered the threshold for the balancing of toolholders, other variables also determine whether balancing is required. These include the size and shape of the tool and toolholder. For example, a simple half-inch endmill in a CAT 40 toolholder at 12,000 rpm would generally be okay with a balanced toolholder, because the endmill itself would not contribute much variation in unbalance. On the other hand, a long boring bar in a CAT 50 holder might require balancing at 5,000 rpm. To summarize, there are three decision-making levels: to balance or not to balance; balanced vs. balanceable holders; and single-plane or two-plane balancing. C.O.E. & T. Akola.

Balancing of Tool Holder For most applications, 10,000 rpm is still the threshold at which to consider balancing. For small, relatively simple tools such as endmills and drills, balanced toolholders will suffice up to about 15,000 rpm. Above 15,000 rpm, the force generated by even the small amounts of unbalance created by endmills and drills will generate enough force to negatively affect the machining process, so balanceable toolholders will be required. Furthermore, at speeds below 20,000 rpm, only relatively long tools will require twoplane balancing. Above 20,000 rpm, two-plane balancing should be considered for all applications. Remember, these guidelines are suggestions only. For your specific requirements, you should discuss the balancing aspects with a toolholder or balancing specialist. Balance Tolerance Like other machining processes, toolholders are balanced to a tolerance. In North America, the convention is to balance to what is known as ISO Quality Grade G2.5. Briefly, this means the toolholder balance is consistent with the spindle as recommended by the International Organization for Standardization. The actual value is a function of the weight of the assembly and its maximum rotational speed. This value can be easily calculated, or determined by the balancing machine.

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Balancing of Tool Holder

BALANCING THEORY Unbalance in a rotating body is defined as the condition that exists when its principal mass, or “axis of inertia, ” does not coincide with its rotational axis. There are three fundamental types of unbalance. Static (or Single Plane) unbalance.

STATIC UNBALANCE F1 = F2 ∠ F1 = ∠ F2 The mass axis does not coincide with the rotational axis, but is parallel to it (figure 1). The force crated by this unbalance is equal in magnitude at both ends of the rotating body, as is the force vector’s angular position at both of the bearing journals. Couple Unbalance

COUPLE UNBALANCE C.O.E. & T. Akola.

Balancing of Tool Holder F1 = F2 ∠ F1 = ∠ F2 + 180º The mass axis does not coincide with the rotational axis, but intersects it at the rotating body’s center of gravity (fig.2)/ The force vectors are equal, but 180º apart. Dynamic (or two-plane) Unbalance.

DYNAMIC UNBALANCE F1 ≠ F2 ∠ F1 ≠ ∠ F2 The mass axis doesn’t coincide with the rotational axis, is not parallel to it, and doesn’t intersect it (fig.3). F

m

r

S

e

M

w

VARIABLE BALANCING

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Balancing of Tool Holder Figure 4 illustrates the variables involved in balancing a rotating body. When figuring unbalance, assign the following variable to the listed Characteristics: M = Rotor mass S = Center of mass e = Displacement of mass center r = Distance from center of rotor to center of gravity of mass (m) w = Angular velocity u = Rotor unbalance To determine the unbalance (U) in a rotating body, use the eqn; U = M x e, or U = M x r Couple Unbalance Unbalance is expressed as the product of mass x distance, that is “grammillimeter,” “ounce – inch “ or “kilogram – meter.” To find the amount of centrifugal force produced by a given unbalance, use the formula: F = U x W2 Where W is given as the angular velocity in radians/sec. To fine W, use the formula; 2π × rpm 60

Combining formulas, we get;

 2π × rpm  F = m×r ×   60 

As the formula shows, centrifugal force caused by tool holder unbalance increases at the square of the speed. This is important to remember as cutting speeds rise. Even if a tool holder has low unbalance initially, unbalance becomes significant when speeds exceed 10,000 rpm. C.O.E. & T. Akola.

Balancing of Tool Holder

WHEN IS BALANCING NECESSARY ? For low –speed machining operations involving spindle speed of less than 10,000 rpm balancing isn’t important factor, unless tool holders are extremely asymmetrical. When machining at high speeds, however relatively small unbalances can produce dangerously high forces on the spindle bearings. For example, consider a well- balanced tool holder with an unbalance of 1 gmm. It produces a radial force of only 0.56 lbs, at 15,000 rpm. But most tool holders aren’t balanced to such stringent values. Research has shown that the average initial unbalance for most tool holders is 250 g-mm. At 15,000 rpm, 250 g-mm produces a continuous radial force for 140 lbs.

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Balancing of Tool Holder

EFFECTS Unbalance can have a detrimental effect on both the work piece and the machine tool. Tool holder unbalance primarily causes chatter. This degrades work piece finish because vibration makes the cutting tool deviate from its intended path. Close tolerances can’t be maintained, which often leads to the scrapping of parts. Unbalance can have even more devastating effects on the machine tools. The high centrifugal forces generated can cause the spindle to experience tremendous internal stress, leading to premature bearing failure. Repairs are time consuming and costly. Finally, unbalance reduces tool life. A balanced tool holder, can increase tool life by as much as 50 percent.

This aluminum work piece illustrates tool holder unbalance affecting surface finish.

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Balancing of Tool Holder

SOURCES OF UNBALANCE To discover the most common sources of unbalance, researchers at American Hofmann Corp. tested a variety of tool holders. The researchers found that because of the strength tool holders need to withstand the forces encountered during metal cutting, they can be classified as rigid rotor. A tool holder that’s strong enough to support a cutting tool in operation is stiff enough to prevent flexing. Right rotors don’t suffer from unbalance caused by flex-induced run out. Tests also revealed that couple unbalance usually isn’t serious problem. Most couple unbalance results from two parts being attached with their axes misaligned. For example, a flywheel which is not perpendicular to its shaft will have high couple unbalance. Tool holder, however have tapers that align precisely with spindles. All surfaces rub true to the taper, generally eliminating couple unbalance. Most tool holders tested exhibit high static (single-plane) unbalance. This unbalance could be traced to several different sources. On CAT V – flange tool holders, the most common source is the different depths of the drive slots, which is part of the inherent design. Some end mill holders have setscrews that cause unbalance. In other cases unbalance is traced to an ungrounded V- flange base. These types of unbalance can be eliminated by changing the tool holder’s design. The primary recurring source of unbalance in high-speed tool holders, how ever comes from the collet and or collet nut. Tests have shown that a tool holder that is perfectly balanced with the tool in place and collet nut properly tightened can be thrown C.O.E. & T. Akola.

Balancing of Tool Holder off balance simply by loosening and retightening the collet nut. As much as 30 g-mm of unbalance can be introduced this way. Because balance tolerances usually range from 2 to 5 g-mm, the tool holder must be rebalanced. Two-plane unbalance, though more rare, can be equally as troublesome. Several tool holders American Hofmann tested displayed a significant amount. Most high-speed tool holders with two plane - Unbalance have a high length to diameter ratio. They hold the collet far enough away from the V- Flange. (Usually two or more diameters) to crate unbalance sources in two distinct planes. This is especially true on CAT V- flange tool holders, unless the inherent unbalance has been rectified in the tool design. Even if the tool holder is balanced, the cutting tool itself can cause unbalance. Tests indicated that unbalance in end mills as small as 0.375” dia. can put a 5 –1 b. tool holder out of tolerance. Tool unbalance can result from flat son the shanks, varying flute lengths on the same tool, and nonsymmetrical shapes, like that of boring bar.

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Balancing of Tool Holder

CORRECTION The most common method of balancing rotating bodies is to remove material from the unbalance location or “heavy spot,” by drilling, milling or grinding. This is not practical with a tool holder, as it would have to be remachined after every tool change, quickly ruining it. A successful method employed by several toolmakers is to drill and tap four or six equally spaced holes in the tool holder’s V-flange area. A setscrew is inserted into each hole, which usually has either 6 – 32 or 8 – 32 threads. The tool holder is balanced by turning the screws in or out to create a force vector 180 o from the unbalance force vector. Extra screws can be added to correct extreme unbalance. The tool holder can easily be rebalanced when the cutting tool is changed. The setscrews are held in position with a non- hardening-locking compound. Nylon- insert lock screws also can be used. This strategy can be applied by users to balance their existing tool holders. Bench top tool holder balancers such as Hofmann’s MT – 50 enable tool room and setup personnel to easily check and correct tool holders for balance. Other tool holder manufactures offer innovative methods for correcting unbalance. One has designed a system, which integrates two adjustable rings into the body for the tool holder. Each ring has equal unbalance. During setup the rings are placed with their unbalance masses 180o apart. After the tool is installed, the rings are adjusted until they produce a force vector equal to and opposite the unbalance vector. The result is a balanced tool holder.

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Balancing of Tool Holder Another manufacturer employs a method whereby small weights are placed into one or more of several equally spaced pockets near the V – flange. These pockets are sealed after the proper weight has been added to balance the tool holder/ One swiss firm builds a micrometer adjustable boring ban that incorporates an internal counterweight. As the cutting tool is moved readily, the counter weight moves in the opposite direction keeping the tool holder in balance.

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Balancing of Tool Holder

BALANCE TOLERANCE A final consideration is balance quality, or what level of unbalance is acceptable. The American National Standards Institute’s standard for Balance Quality of Rigid Rotating Bodies(s. 19 – 1975) defines the permissible residual unbalance for rotating body relative to its maximum service speed. The standard assigns different balance – quality “grades” to related groups of rotating bodies based on experience gained with various types, sizes and service speeds. The balance – quality grade (G) equals the specific unbalance (e) the rotational speed (W) or :

G=exW

The units for balance quality G are in mm/sec. e = As shown earlier

U M

Where U is g-mm and M is in grams substituting, we get: G =

U M

x W

or

G =

U M

Solving for U, we obtain: U =

9 .5 x M x G rp m

The ANSI tables show balance quality for machine tool drives is G 2.5, and for grinding machine drives, G 1.0. A tool holder’s balance quality should fall between these limits. It would serve no practical purpose for a tool holder to be balance better than the spindle. C.O.E. & T. Akola.

Balancing of Tool Holder Using the ANSI limits for machine tool and grinding machine drives, we can calculate the balance tolerance for a tool holder of a given weight operating at U

(u p p e r)

=

9 .5 x 3 0 0 0 g x 2 .5 m m /s e c 2 5 ,0 0 0

U (upper) = 2.85 g-mm U

(lo w e r)

=

9 .5 x 3 0 0 0 g x 1 .0 m m /s e c 2 5 ,0 0 0

U (lower) = 1.14 g-mm The balance tolerance for this tool holder is between 1.14 and 2.85 g-mm

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Balancing of Tool Holder

CONCLUSION As spindle speeds creep higher, the need for balanced tool holders becomes more acute. When buying tool holders, users should ensure that their vendors offer balanced products. Companies with small tool holder inventories can have them balanced by independent balancing services. Firms with large numbers of tool holders will find it more and more practical to install balancing machinery in their tool rooms. Whichever course they follow, one thing is certain those old, unbalanced tool holders simply won’t do the job any longer.

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Balancing of Tool Holder

REFERENCE 1) American Journal of Cutting Tool Engineering. 2) Theory of Machines by P.L. Ballaney. 3) Production Technology by R.K.Jain. 4) Websites : •

www.google.com Search Topics

1) Effects of tool unbalance. 2) Selection of tool holder for machines.

•

www.mmscuttingtoolszone.com

•

www.mmsHighspeedMachiningzone.com

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Balancing of Tool Holder

Static (or Single Plane) unbalance.

STATIC UNBALANCE F1 = F2 ∠ F1 = ∠ F2

Couple Unbalance

COUPLE UNBALANCE F1 = F2 ∠ F1 = ∠ F2 + 180º

C.O.E. & T. Akola.

Balancing of Tool Holder

Dynamic (or two-plane) Unbalance.

DYNAMIC UNBALANCE F1 ≠ F2 ∠ F1 ≠ ∠ F2

The mass axis doesn’t coincide with the rotational axis, is not parallel to it, and doesn’t intersect it (fig.3). F

m

r

S

e

M

w

VARIABLE BALANCING

C.O.E. & T. Akola.

Balancing of Tool Holder

This aluminum work piece illustrates tool holder unbalance affecting surface finish.

C.O.E. & T. Akola.