Method and apparatus for balancing a rotary tool assembly

The present invention entails a method and apparatus for balancing a high speed rotary tool assembly. A pair of closed balancing rings 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.

FIELD OF THE INVENTION 
The present invention relates to tools and more particularly to balancing 
mechanisms for rotary cutting tool assemblies. 
BACKGROUND OF THE INVENTION 
The balancing of high speed rotary cutting tools, that is tools that 
typically turn 10,000 to 20,000 rpm, is very important. If a high speed 
rotary cutting tool is allowed to operate in an unbalanced state this will 
result in oscillating forces being created by the tool assembly due to the 
centrifugal force effect of the unbalance. This typically leads to 
premature bearing wear or failure and machine vibrations which result in 
uneven cutting action. 
To minimize the unbalanced effects on the tool assembly, the assembly must 
be accurately balanced. This usually entails a balancing operation each 
time there is a significant change (e.g. changing tools or reshaping the 
same tool) in the tool assembly. It is not uncommon to find a tool itself 
out of balance and it is often difficult to precisely place the cutting 
tool where its axis is exactly aligned with the axis of the rotary tool 
holder. Due to the frequent need for rebalancing, there is a need for an 
efficient, integral, adjustable and accurate balancing system for such 
high speed rotary cutting tools. 
There are commercially available balancing units which allow for quick and 
accurate measurements of the amount of unbalance in a tool holder and tool 
assembly. But the problem again is providing a quick and efficient 
mechanism for accomplishing the balancing operation. 
There have been various attempts at designing rotary tool balancing 
systems. One such design comprised three or more radially screws which are 
moved radially, inwardly and outwardly, from the axis of the tool holder. 
This approach is often tedious because typically the various screws making 
up the mechanism have to be moved by different amounts. In addition, this 
approach is quite limited in the amount of unbalance that can be corrected 
without adding more screws and making the method even more complicated and 
tedious. 
A second balancing approach found in the prior art is a system that 
utilizes two or more movable weights. In this design, the weights are 
moved to specific positions on a given diameter in order to balance the 
tool. There are many disadvantages to this approach. Often such a movable 
weight design is expensive to manufacture, especially those designs that 
incorporate rings and grooves, etc. Finally, in certain balancing systems 
incorporating movable weights it can be difficult to incorporate the 
design within the structure of the tool holder and still be within an 
industry standard tool envelope (such as ANSI and ISO) for that type of 
tool. 
Finally, it has been known to utilize cooperating open type rings about the 
axis of a rotating tool holder to achieve a final tool balance. See U.S. 
Pat. No. 4,626,144. This design also has drawbacks. As suggested in this 
disclosure, the opened balancing rings are actually tied together and 
cannot be moved and adjusted independently. This limits the versatility of 
this balancing approach since the individual balancing element cannot be 
moved completely independently of each other. Besides this, the total 
balancing mechanism is complicated and expensive to manufacture. 
SUMMARY AND OBJECTS OF THE INVENTION 
The present inventions entails a rotary cutting tool balancing mechanism 
that comprises a pair of closed balancing rings that are respectively 
journalled around a cylindrical bearing surface forming a part of the 
housing of a rotary tool holder. Preferably, each balancing ring is 
equally and symmetrically unbalanced. In conventional fashion the 
magnitude of unbalance and the angular position of that unbalance is 
determined for the rotary tool holder and a certain cutting tool. Then the 
pair of closed balancing rings are rotated about the tool holder so that 
the net balancing effect of the rings precisely counters the unbalance of 
the tool holder and associated cutting tool. 
It is therefore an object of the present invention to provide a rotary 
cutting tool balancing system that is efficient, integral with the rotary 
tool holder, and capable of accurately balancing the tool holder and any 
cutting tool secured within the tool holder. 
Still a further object of the present invention resides in the provision of 
a balancing mechanism for a rotary cutting tool assembly wherein the 
balancing mechanism provides for positive retention of the balancing 
elements incorporated into the balancing mechanism. 
Another object of the present invention resides in the provision of a 
balancing mechanism for a rotary cutting tool assembly that can be 
manufactured at a relatively low cost. 
It is also an object of the present invention to provide a rotary cutting 
tool assembly with an integral balancing mechanism that is designed to 
precisely and accurately balance a tool holder and associated tool. 
A further object of the present invention resides in the provision of a 
rotary cutting tool balancing system of the character referred to above 
wherein the balancing rings are so designed such that respective rings can 
be easily manufactured such that they possess equal and symmetrical 
unbalance. 
It is also an object of the present invention to provide a balancing system 
for a rotary tool assembly that can be easily and precisely adjusted to 
counter the unbalance of an associated tool holder and a certain cutting 
tool secured within that tool holder. 
Another object of the present invention resides in the provision of a 
balancing system for a rotary cutting tool of the character referred to 
above that is particularly useful for retrofitting existing tooling. 
It is also an object of the present invention to provide a balancing 
mechanism for a rotary tool assembly that has a neat, clean and 
noncluttered appearance when incorporated into the tool assembly. 
It is also a further object of the present invention to provide a balancing 
mechanism for a rotary cutting tool wherein the balancing rings provide 
for quiet operation. 
Still a further object of the present invention resides in the provision of 
a balancing mechanism for a rotary cutting tool assembly wherein the 
design of the balancing mechanism can be incorporated into the tool 
structure such that the tool structure, including the balancing mechanism, 
still meets standard ANSI or ISO tool envelope requirements. 
Other objects and advantages of the present invention will become apparent 
and obvious from a study of the following description and the accompanying 
drawings which are merely illustrative of such invention.

DETAILED DESCRIPTION OF THE INVENTION 
With reference to the drawings, there is shown therein a rotary tool 
assembly incorporating the balancing mechanism of the present invention, 
the rotary tool assembly and balancing mechanism being indicated generally 
by the numeral 10. 
The rotary tool assembly comprises a rotary tool holder 12 having an outer 
housing 12a and central axis 12b. Forming a part of the outer housing 12a 
is a collet chuck 12c that forms an opening within tool holder 12 for 
receiving a conventional collet 14. The collet 14 is in turn adapted to 
receive and hold a cutting tool, such as a drill 16. For purposes of 
clarity, the tool holder 12 is shown in FIG. 1 in schematic form and its 
nose piece or nose assembly that secures about the collet 14 and collet 
chuck 12c is not shown. 
Tool holder 12 is provided with a ring balancing mechanism 28. Forming a 
part of the ring balancing mechanism is a cylindrical smooth bearing 
surface 18 that is formed completely around tool holder housing 12a. 
Defined about one side of cylindrical bearing surface 18 is a shoulder 20 
that in the preferred embodiment shown herein forms a part of the tool 
holder housing 12a. Axially spaced towards the tool end of the tool holder 
12 is a circumferential slit or groove 22 that extends around one side of 
the cylindrical bearing surface 18. A retaining ring 24 is positioned 
within slit 22 thereby defining an annular space between shoulder 20 and 
retaining ring 24. 
Formed in the cylindrical bearing surface 18 is a pair of axially spaced 
set screw grooves 26. 
Rotatively journalled on the cylindrical bearing surface between shoulder 
20 and retaining ring 24 is a pair of balancing rings 28. Each balancing 
ring overlies a respective set screw groove 26 formed in the cylindrical 
bearing surface 18. Each ring 28 includes an inner bearing surface 30, an 
outer circumferential edge 32, and an annular body 34. Formed radially 
through the annular body 34 of each ring is a threaded set screw opening 
36. Threaded into each threaded opening 36 is a set screw that includes a 
threaded portion 38, a ball type head 42 and an intermediate nonthreaded 
portion 40. The purpose of the intermediate nonthreaded portion 40 is to 
impart compressibility to the set screw. In particular, the nonthreaded 
portion 40 would be more susceptible to being compressed than the threaded 
portion 38. As will be more fully appreciated from considering the entire 
disclosure, the entire tool assembly will undergo centrifugal loading 
during operation. The balancing rings may expand or move outwardly under 
centrifugal loading so that the set screws lose contact with the bearing 
surface. To solve this problem, the set screws are torqued to compress the 
non-threaded portion 40 of the set screws. When the rings 28 expand, the 
set screws, due to being previously compressed, will elongate to maintain 
engagement with the set screw grooves 28. 
Each balancing ring 28 is intentionally formed with a certain amount of 
imbalance. Preferably each balancing ring 28 has an equal amount of 
imbalance. The imbalance can be created by forming one or more openings 
34a in the annular body 34 of the ring. In the case of the embodiment 
disclosed each balancing ring 28 is provided with three equally sized and 
equally spaced openings 34a. Each ring 28 is provided with an indexing 
mark on its outer edge which lies preferably on the same radial axis as 
the center of gravity of the balancing ring 28. In the preferred 
embodiment, the center of gravity of the balancing ring 28 lies on the 
radial axis of the set screw opening 36 which serves as the indexing mark. 
The indexing mark is used to position the balancing rings 28 as will be 
described in subsequent portions of the specification. 
As seen in the drawings, the pair of balancing rings 28 are closely 
journalled about the cylindrical bearing surface 18 and are confined 
between the shoulder 20 and the retaining ring 24. To reduce noise 
generated by openings 34a during the operation of the tool holder 12, it 
is preferable that the respective openings 34a be hidden or otherwise 
enclosed. As seen in the drawings, shoulder 20 effectively closes openings 
34a on the rear most ring 28. An optional face plate 35 is interposed 
between the foremost ring 28 and retaining ring 24 so as to effectively 
close the openings 34a of the foremost ring. It is appreciated that this 
could also be achieved by appropriately spacing retaining ring 24 and 
extending the same to cover openings 34a. 
Each ring 28 can be rotated independently of the other ring and can be 
fixed with respect to the tool holder 12 by engaging the set screw ball 
head 42 into an underlying set screw groove 26. By providing the set screw 
groove 26 it is appreciated that the cylindrical bearing surface itself is 
not scarred or otherwise damaged by the engagement therewith of the set 
screws. 
To balance tool holder 12 and an associated cutting tool 16, the balancing 
rings 28 are first placed in a balanced neutral position. In other words, 
the respective rings 28 are rotated and positioned such that the rings 
themselves produce a zero net balance effect. In the case of identically 
unbalanced rings 28, as disclosed herein, the rings are positioned such 
that indexing marks are located 180 degrees apart. This results in the 
unbalance associated with each ring cancelling the unbalance of the other 
ring. 
Next, the particular cutting tool 16 to be used is inserted within the tool 
holder 12. By conventional means, the unbalance of the tool holder 12 and 
the inserted cutting tool 16 is determined. It should be pointed out that 
there are commercially available machines for determining the unbalance of 
such a tool assembly such as a MT-50 Tool Holder Balancer manufactured by 
American Hofmann. In determining the unbalance associated with the tool 
holder 12 and the cutting tool 16, there will be both a magnitude 
unbalance determined and also the angular position of that magnitude 
unbalance will be determined. 
After determining both the magnitude and the angular position of the 
unbalance, the angular position of the balancing rings 28 must be 
calculated which will offset the tool imbalance. This can be done manually 
using a hand-held calculator, or the balancing machine can be programmed 
to provide the correct positions of the rings 28. In any event, the 
angular position of the balancing rings 28 is calculated using 
trigonometric formulas which are well known to those skilled in the art. 
Once the correct position of the rings 28 is determined the set screws are 
loosened and the balancing rings 28 are rotated with respect to the tool 
holder 12 such that the net counterbalancing forces of the rings 28 is 
equal to and exactly opposite the tool imbalance. In other words, the 
rings 28 are positioned such that the magnitude of the net counter 
balancing force is equal to the magnitude of the tool imbalance, and the 
angular position of the net counterbalancing force is disposed 180 degrees 
from the angular position of the tool imbalance. At this time the rings 
can be securely fixed to the tool holder 12 by threading the set screws 
inwardly to where the ball head 42 engage and seats within respective 
underlying grooves 26. As pointed out above, the set screws can be 
tightened to compress nonthreaded portions 40. Thus when the balancing 
rings 28 expand due to centrifugal loading, the set screws will elongate 
to maintain contact with the grooves 26. Thus, the balancing rings 28 are 
firmly fixed to the tool holder 12 such that there is no relative movement 
between the two. 
The following is an example of a balancing operation. A standard 50 
V-flange collet chuck manufactured by Kennametal, Inc. of Latrobe, Pa. was 
equipped with two balancing rings weighing approximately 2 ounces a piece. 
The imbalance of each ring was approximately 14.5 gm.cm, so that a total 
maximum imbalance of 29 gm.cm was possible. The balancing rings were 
initially set 180 degrees apart so that the net balancing effect of the 
rings was zero. Using a Hofmann MT-50 Tool Holder Balancer, the collet 
chuck was found to have an imbalance of 7 gm.cm at 290 degrees with 
respect to a fixed reference on the balancer. The balancing rings were 
then set at 35 degrees and 185 degrees with respect to the same reference. 
The collet chuck was tested again and found to have an imbalance of 0.2 
gm.cm at 110 degrees. The balancing rings were then moved to 34 degrees 
and 186 degrees respectively. Upon retesting, the collet chuck was found 
to have an imbalance of 0.1 gm.cm at 110 degrees. Each adjustment took 
approximately 3 minutes to make. 
The ring balancing mechanism of the present invention could be incorporated 
into any rotary cutting tool holder or could be retrofitted into an 
existing rotary cutting tool holder or assembly. In this regard, many 
cutting tool manufacturers design rotary cutting tools such that the size 
of certain portions of the tool assembly conform to or fall within an 
industry standard tool envelope such as set by ANSI or ISO. Typically, the 
area of concern (i.e. the area to which the standard applies) is that area 
of the tool holder 12, as viewed in FIG. 1, extending upwardly from the 
lowermost balancing ring. This enables conforming tool assemblies to be 
interchangeable. Thus, it is important that the ring balancing mechanism 
of the present invention be of a design that can be incorporated within a 
rotary assembly and still enable the rotary assembly to still conform to a 
particular industry tool envelope standard. It is of particular 
significance that the present ring balancing design can be retrofitted 
into existing rotary tool assemblies in such a manner that the standard 
tool envelope is not exceeded and that the final retrofitted tool assembly 
still conforms to the industry tool envelope standard. 
It should be pointed out that the present ring balancing tool mechanism 
could be incorporated into a series of coaxial assembled rotary tool 
holders. For example, the ring balancing mechanism described herein could 
be incorporated into a boring bar that is secured within a tool holder 
which itself is provided with a like ring balancing mechanism. Thus in 
such a tool assembly design, there would be balancing on different such 
assemblies at different axial locations. 
From the foregoing specification and discussion it is appreciated that the 
ring balancing mechanism of the present invention enable a final balanced 
state to be achieved with a rotary cutting tool assembly quickly and 
efficiently. This is particularly important inasmuch as the above 
described balancing operation is typically formed for each new cutting 
tool inserted into a tool assembly. As indicated above, the tool balancing 
mechanism of the present invention tends to be precise, and easy to use 
and maintained. 
The present invention may, of course, be carried out in other specific ways 
than those herein set forth without parting from the spirit and essential 
characteristics of the invention. The present embodiments are, therefore, 
to be considered in all respects illustrative and not restrictive, and all 
changes coming within the meaning and equivalency range of the appended 
claims are intended to be embraced therein.