BACKGROUND OF THE INVENTION It is of increasing importance to balance rotary tool assemblies which are operated at high rotation speeds up to 15,000 rpm and higher. If not properly balanced at the speed of operation; the cutting action of the tool can be unsuitable for precision work and the tool can be subject to excessive wear and early failure and the spindle bearing life of the associated machine tool can be adversely affected.

U. S. Patent 5,074,723 describes a method and apparatus for balancing a high speed rotary tool assembly which includes a pair of unbalanced rings which are journalled about a cylindrical bearing surface formed around a rotary tool holder, with each ring being rotatively mounted about the bearing surface independently of each other. The tool holder is adapted to receive and hold a cutting tool. After determining the unbalance of the tool holder and cutting tool, the rings are rotatively adjusted about the bearing surface to create an unbalance within the tool rings and the position of the tool rings is adjusted such that the created unbalance of the tool rings is disposed opposite the determined unbalance of the rotary tool holder and cutting tool. The afore described prior art method and apparatus are effective but require a specially designed tool holder which incorporates a pair of unbalanced rings. For reasons of economy and shortened "down-time", it is important to have a tool balancing assembly and method by which almost any machine tool assembly can be quickly balanced using, as balancing rings, rings of a type which are readily available and of ordinary and conventional shape. This is particularly important in high speed operations where tool changes are frequent and precision cutting is essential, e. g. in the fabrication of lay-up molds for intricate aerodynamic and other shapes of special configuration.

SUMMARY OF THE INVENTION The present invention is directed to a combination for use with a machine tool comprising: (i) a rotary machine tool holder having a cylindrical cross-section which is coupleable to a rotating spindle of a machine tool, said rotary machine tool holder having a cylindrically cross-sectioned cutting tool mounted therein to form a rotary tool assembly; and (ii) an annular ring surrounding a portion of the cylindrically cross-sectioned machine tool assembly which is rigidly, directly, affixed thereto, the annular ring having sufficient mass to permit removal of a portion thereof to enable balancing of the combination of annular ring and rotary tool assembly without requiring removal or addition of material to the rotary machine tool holder or cutting tool.

BRIEF DESCRIPTION OF THE DRAWINGS Figure I and Figure 2 show, in cross-section, a rotary tool assembly in accordance with the present invention; Figures 3-5 show rings suitable as balancing rings for use in the present invention; and Figure 6,6 (A)- (C) show a further embodiment of the present invention for balancing a cutting tool.

DETAILED DESCRIPTION OF THE INVENTION With reference to the drawings, Figure 1 shows at 10, a rotary machine tool holder of cylindrical cross-section which is removeably coupled to a rotating spindle schematically indicated at 20 of a conventional machine tool (not shown), such as a milling machine. A cutting tool is indicated at 30 having a flute length as indicated at 32 and a shank 34 of cylindrical cross-section. Gage length is indicated at 33. The cutting tool 30 is mounted in rotary machine tool holder 10, for example by a shrink fit indicated at 40, obtained by heating and expanding tool holder 40 to receive shank 34.

The unbalance of the rotary tool assembly 50 comprising rotary machine tool holder 10 and cutting tool 30, is determined conventionally, e. g. by using a commercially available machine such as a Hofmann 2 Plane Tool Holder Balance Machine EZ-20.2 (American Hofmann Corporation). This machine provides readings of magnitude unbalance and the angular position of the magnitude unbalance for the rotary tool assembly. In the event that the unbalance of the rotary tool assembly is not within the unbalance limits desired for the rotary tool assembly, a ring of nominally balanced dimensions, i. e. (the ring is not designed to be unbalanced) for example of a type shown at 60 or 70 in Figures 3-5 is attached to rotary tool assembly 50, i. e. to rotary machine tool holder 10 as shown at 60 in Figure 1, or to cutting tool 30 as shown at 70 in Figure 2. Ring 60 is secured to machine tool holder 10 by adjusting screw 120 to tighten ring 60 which is in the form of a squeeze clamp as shown in Figure 3. Ring 70 is secured to cutting tool 30 by adjusting set screw 130 which bears against the cylindrically shaped shank 34 of cutting tool 30. Ring 70 is shown separately in Figure 4.

Figures 5,5A show a ring suitable for use in the present invention which is attached to the rotary tool assembly 50 by heat shrinking. The rotary tool assembly with attached ring, 60 (or 70) is measured for unbalance, for example using the above-described balance machine and the amount and angular location of the unbalance is observed. The balancer machine indicates, for an angular location, the unbalance due to an excess of mass (or deficiency of mass) at that angular location. Excess material is removed from the ring 60 (or 70) e. g. by grinding, drilling, milling, as indicated at 80 in Figure 1 and 82 in Figure 2, until the balancer machine readings show that the balance of the combination of ring 60 (or 70) and rotary tool assembly 50 is within the desired tolerance. Mass can also be removed by tapping holes in ring 60 (or 70) as indicated at 90 in Figure 1 and 92 in Figure 2. Where there is a deficiency of mass, a set screw can be threaded into a tapped hole in ring 60, as shown at 100 in Figure 1 and 102 in Figure 2 with the protruding portions 104,106 providing addition of mass.

The following examples will further illustrate the present invention: EXAMPLE I An HSK 63A shrinker tool holder, 5.900 inch gage length manufactured by Tooling Innovations/Brinney Corporation is used as the rotary machine tool holder. A 2 inch diameter x 1-1/4 inch shank x 4 inch flute length, HSS, two flute ball end mill cutter is used in the machine tool assembly. The total assembly weight is 8 pounds and the gage length is 10-1/2 inches. The tool assembly is required to run at 14,000 rpm as determined by the TPF, (Tooth Pass Frequency) method. This is set as the rpm figure for calculation of the desired 6.3 g-spec tolerance. The tool is a stock cutter which has a cylindrical shank except for a weldon flat and whistle notch on the shank of the cutter. The cutter is shrink fitted into the machine tool holder and an unbalance reading for the assembly is determined with a Hofmann 2 plane Tool Holder balance measuring machine designated EZ-20.2. Readings of 136.9 gram-mm at 284 degrees in Static and 71.5 gram-mm at 284 degrees in Couple are displayed by the balance measuring machine for this tool assembly.

This unbalance is too high for this assembly at the desired rpm. The desired 6.3 g-spec tolerance for this particular machine tool is 15.5 gram-mm in Static and 5 times (77.5) that for Couple.

The cutter tool is unshrunk from the tool holder and a ring is added to the shank of the cutter. The cutter is reshrunk back into the tool holder but now with a metal ring of the type shown at 60 in Figure 3 is fixed to the cutter tool at a cylindrical portion of its shank between the body of the cutter and the end face of the shrinker tool holder. The total mass weight has increased to 8.25 pounds so the tolerance of this assembly has changed to 16.0 gram-mm in Static and 5 times that for Couple The ring chosen has one 5/16-18 SHCS, (Socket Head Cap Screw) placed parallel to the end face of the ring for use of squeezing the ring onto the shank of the cutter for rigid placement. This type of ring is very rigid because of the contact area engagement of it's squeezing design. The SHCS of the ring is tightened to obtain the squeeze effect. The resulting assembly is placed in the Hofmann balancer and the unbalance is measured. The readings displayed are now 76 gram-mm in Static and 25 gram-mm in Couple. To further adjust the balance, the 5-1/6-18 SHCS is loosened on the ring and the ring is rotated on the shank to effectively minimize the unbalance of the tool holder assembly. The Hofmann balance machine indicates the direction and amount of rotation that the balance ring will need. In this case, rotation of the ring could not bring this assembly into it's tolerance of 16 gram-mm in Static. A reading of 20 gram-mm in Static was as good as this ring could establish by mere rotation. In order to bring the balance of this assembly into tolerance, material is then taken off the ring until the balance is within tolerance as indicated by the Hofmann balance machine. This is achieved by grinding material off the ring at the location indicated by the angle indicated by the reading of the Hofmann balance machine.

Material is removed from the ring until a new balance reading of 8 gram-mm in Static and 13 gram-mm in Couple is achieved which is within the desired tolerance.

If a single ring does not conveniently achieve the desired Couple reading another ring may be affixed to the assembly in the same manner, but in a different vertical plane. Two rings in separated planes will then conveniently enable the dynamic balance of the tool holder assembly.

EXAMPLE II An HSK 63A cylindrical shrinker tool holder, 5.9 inch gage length manufactured by Tooling Innovations/Brinney Corporation is used as the rotary machining tool holder. A 1 inch diameter x 1 inch shank x 3 inch flute length x 8 inch overall length, 2 flute ball, carbide center cutter is used in the machine tool.

The total assembly weight is 7.5 pounds and the gage length is 12.050 inches.

The tool assembly is required to run at 16,000 rpm as determined by the TPF, (Tooth Pass Frequency) method. This is set as the rpm figure for calculation of the desired 6.3 g-spec tolerance. This example involves a special cutter with a cylindrical shank which has no weldon flats or whistle notches on the shank of the cutter. The cutter is shrink fitted into the machine tool holder and an unbalance reading for the assembly is determined with a Hofmann 2 plane Tool Holder balance machine EZ-20.2. Readings of 20 gram-mm in Static at 127° and 15 gram-mm in Couple at 308'are displayed as the unbalance of this tool assembly.

This unbalance is too high for this assembly at the desired rpm. The desired 6.3 g-spec tolerance for this particular assembly is 12.7 gram-mm in Static and 5 (63.5) times that for Couple.

A ring of the type shown at 75 in Figure 5 is shrink fitted to the lower bottom end of the tool holder. The ring is placed on the tool holder instead of the cutter to ensure clearance for the intended machining operation. The total mass weight has increased to 7.75 pounds so the tolerance of this assembly has changed to 13.1 gram-mm in Static and 5 times that in Couple. This ring is attached in a shrink fit. The ring is smaller in mass than that of Example I because there is less unbalance in this tool holder assembly. The tool holder assembly is placed into the Hofmann balancer again and new readings are taken. The readings are now 22 gram-mm in Static and 13 gram-mm in Couple. In order to bring this assembly into the desired tolerance, material is then removed from the ring by grinding material off the ring at the location indicated by the Hofmann balance machine.

Material is removed from the ring until a new balance reading of 2 gram-mm in Static and. 70 gram-mm in Couple is achieved which is in the desired tolerance.

In a further embodiment of the present invention where the unbalance of a rotary tool assembly comprising a rotary machine tool holder and a cylindrically cross-sectioned cutting tool shrink fitted thereon is determined to be only slightly out of balance, the rotary cutting tool, 100 in Figure 6,6 (A) is released from the tool holder and inserted into a compressible sleeve-shaped member 110 wherein it is closely, slidably engaged. The cylindrically shaped sleeve-shaped member 110 is slidably engaged in cylindrically shaped chamber 150 in tool holder 155. Tool holder 155 can separately receive a plurality of sleeve-shaped members 110,110', 110"as shown in Figures 6 (B), 6 (C) which have the same outside diameter 142 but different inside diameters 152,152', 152"to accommodate different sized tools. Sleeve-shaped member 110 is adjusted by set screw 130 to cause the compressible sleeve-shaped member 110 to be slightly compressed by set screw 130 slightly closing slot 135 by bearing on flat 145 of sleeve-shaped member 110 to fixedly engage cutting tool 100 and remain fixedly engaged at rotational velocities up to a pre-determined value, e. g. 16,000 rpm. The sleeve-shaped member 110 with cutting tool 100 affixed thereto is mounted in a balance machine of the type described abowe and readings of magnitude unbalance and angular position of magnitude unbalance are obtained at different angular locations of cutting tool 100. Mass is removed from the cylindrical shank 103 as indicated at 105 to reduce the unbalance of cutting tool 100. Cutting tool 100, when within the balance tolerance, is coupled to its rotary machine tool holder and the unbalance measured as before. If necessary, the foregoing procedure is repeated.

The following example will further illustrate this embodiment of the present invention.

EXAMPLE III A cutting tool is picked for use in a HSK 63A shrinker tool holder. The tool holder has a 5.9 inch gage length manufactured by Tooling Innovations/Brinney Corporation. The cutting tool is 1 inch diameter x 1 shank x 3 inches flute length x 8 inches overall length, 2 flute ball, carbide end mill. The total assembly weight is 7.5 pounds and the gage length is 12.050 inches. This tool is required to run at 16,000 rpm as determined by the TPF, (Tooth Pass Frequency) method. The rpm figure is set at 16,000 rpm for calculation of the desired 2.5 g-spec tolerance. The 2.5 tolerance is used because of the type of cutting this cutter will be required to perform. It is required that the tool last longer and generate a better surface finish, which this g-spec will provide. This cutter which has no weldon flats or whistle notches on its shank. The cutter is solid carbide, NC Ground. The cutter is shrink fitted into the tool holder and an unbalance reading of this assembly is determined with a Hofmann 2 plane Tool Holder balance machine called the EZ-20.2. The readings of 10.8 gram-mm in Static and 5.1 gram-mm in Couple are displayed as the unbalance of this tool assembly.

This is too high of an unbalance for this assembly at the desired rpm. The desired 2.5 g-spec tolerance for this assembly is 5.05 gram-mm in Static and 5 times that for the Couple reading.

The tool is just slightly out of tolerance so the practical method of balancing this assembly will be to balance the cutting tool by itself before it is put into the shrinker tool holder so the assembly will fall into tolerance. The shrinker tool holders are pre-balanced in two planes to 1.0 gram-mm or less readings in both planes. The cutting tool needs to be pre-balanced in the same manner.

A special tool holding member in the form of a compressible sleeve is mounted onto the Hofmann balance machine for the purpose of pre-balancing the cutting tool. The cutting tool is then checked in the (Hofmann 2 plane Tool Holder) balance machine in the same way a conventional tool holder is checked.

The unbalanced readings of the cutter itself are 10 gram-mm in Static and 5 gram- mm in Couple. To correctly balance the cutter it must have material ground off at certain places of the cutter to correct the unbalance. The Hofmann balance machine will indicate the vector angle and planes in which the material will need to be removed from the cutter. After 10 minutes of using the foregoing procedure to balance the cutter, the readings from the (Hofmann) balance machine for this cutter were 1.5 6 gram-mm in Static and. 87 gram-mm in Couple. The tool is then shrink fitted into the above-noted tool holder and the total unbalance assembly reading is now 3.7 gram-mm in Static and 1.2 gram-mm in Couple and the assembly is in tolerance. The cutting tool can be balanced in single plane (Static) or two planes (Couple) if desired. Material can be ground off the cutting tool in different planes to remove any unbalance in Couple. The Hofmann balance machine identified herein above and used in the example is capable of giving appropriate readings needed for two plane balancing.