Tool holding device

The tool holder, which serves principally for receiving a thread cutting tool, has a length compensation under tension and compression. This purpose is served by two compression springs which comprise massive elements of foamed, open and closed cell polyurethane elastomer. This material has good spring and damping properties. The compression springs operate without fatigue. The tool holder also has a coupling which permits radial movements of the thread cutting tool transverse to the axis of rotation in order to compensate for alignment errors.

FIELD OF THE INVENTION 
The invention relates to a tool holding device for machine tools, 
particularly for thread cutting machines. 
BACKGROUND OF THE INVENTION 
When thread cutting the tool is applied to the bore in the workpiece with a 
high feed force in order to achieve as immediate a commencement of cutting 
as possible. A resilient length compensation under compression prevents 
impermissible shock loads. Later in the thread cutting process a feed is 
used which is somewhat smaller than the pitch of the thread. The 
difference is compensated for by a resilient length compensation under 
tension. The latter is also effective if the tool runs on after the rotary 
drive and the feed have been switched off. A coupling which permits the 
tool holding fixture to perform radial movements transverse to the axis of 
rotation compensates for alignment errors between the tool and the bore. 
It has been found in practice that the spring system, which compensates for 
differences in length of the holder and returns the axially movable 
portions of the holder into a neutral position, is subject to such high 
loads that it rapidly fatigues. This applies particularly to the length 
compensation under compression. In this case a fatigue of the spring 
device leads to the feed force, which is necessary for the beginning of 
cutting, building up ever later. An increasing proportion of the 
pre-programmed feed is thus used up before the tool penetrates into the 
bore. The consequence is that the depth of the thread does not achieve the 
necessary and pre-programmed dimension. In order to counteract this 
disadvantage a frequent readjustment of the programming is necessary to 
compensate for the spring fatigue. 
THE INVENTION 
It is the object of the invention to improve the tool holder in such a 
manner that a frequent readjustment of the programming necessitated by 
fatigue of the spring system is avoided. 
The invention provides a tool holder for machine tools, particularly for 
thread cutting machines. The tool holder has a holding portion on the 
machine side and a holding portion on the tool side which are rotatably 
coupled together but can effect relative movement in the direction of the 
axis of rotation. A spring system attempts to hold the two holding 
portions in an axially neutral position. The spring system includes first 
spring means, which attempt to counteract an axial moving apart of the 
holding portion on the tool side from the holding portion on the machine 
side, and second spring means, which act as a compression spring and 
attempt to counteract a deflection by axial compression of the two holding 
portions. The invention provides that at least the second spring means, 
and preferably also the first spring means, are constructed as compression 
springs and comprise at least one, and preferably a plurality, of 
substantially massive elements of elastic plastics material. The first 
spring means, which attempt to return the two holding portions into the 
neutral position following a deflection as a consequence of moving apart 
is referred to below as "length compensation under tension" and the second 
spring means, which attempt to counteract a deflection as a consequence of 
the two holding portions being pressed together, is termed "length 
compensation under compression". 
Surprisingly, it has been found that the fatigue of the spring device is 
caused less by comparatively long-amplitude alternating loads but instead 
predominantly by the short-amplitude but high frequency vibrations which 
occur during the cutting process. Conventional spring devices, which 
operate with coil springs of spring steel, are not capable of dealing with 
these vibrations in the long term. The invention makes use of the 
recognition that the damping of the vibrations is the crucial factor for 
fatigue-free operation of the spring device. Surprisingly, the preferably 
massive element of elastic plastics material combines very good spring 
properties with very good damping properties. Practice has shown that 
readjustment of the program is only necessary, if at all, after a very 
long service life. As a rule, rest periods caused by production factors 
occur beforehand which enable the element of plastics material, which may 
be cheaply manufactured as a mass produced article, to be replaced. 
In principle, all plastics materials which combine the desired spring and 
damping properties can be used. Currently preferred is foamed, open and 
closed cell (mixed cell) polyurethane elastomer. 
The compression spring enabling the length compensation under tension is 
preferably softer than the other compression spring since the former has 
to accommodate lower forces. A differing spring hardness can be set by the 
material choice and/or shape. If every compression spring comprises a 
packet of similar elements of plastics material, very widely varying 
combinations of spring hardnesses can be pre-selected on the basis of a 
single elastic starting element. 
In a preferred embodiment the compression springs, but at least the 
compression spring enabling the length compensation under tension, are so 
constructed that they have a constant spring force substantially 
independent of the spring displacement. The mode of operation of the tool 
holder may thus be considerably improved. 
In order to achieve the desired spring characteristic the compression 
springs can comprise elements of different Shore hardness. It is also 
possible that each compression spring comprises elements of differing 
shape. In this connection there are numerous possibilities for 
modification. Thus the outer shape of the elements can be different or 
merely their diameter. Furthermore, a compression spring can comprise 
elements with interposed spacer discs of differing thickness and/or 
differing diameter. 
The tool holder in accordance with the invention is suitable, in 
particular, for thread cutting but can also be used with the same 
advantage for counterboring and milling since comparable working 
conditions are present in these processes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in FIG. 1, the tool holder includes a holding portion 1 on the 
machine side and a holding portion 2 on the tool side. Arranged between 
the two holding portions 1 and 2 is a spring system 3 which is responsible 
for length compensation under tension and compression. Furthermore, the 
holding portions 1 and 2 are rotationally coupled so as to be 
longitudinally movable by means of a carrier device 4. Arranged between 
the holding portion 2 on the tool side and a tool receiving sleeve 5 is a 
coupling 6 which permits radial movements of the tool receiving sleeve 5 
transverse to the axis of rotation. 
The spring system 3 includes a compression spring 7 for the length 
compensation under compression and a compression spring 8 for the length 
compensation under tension. Both compression springs comprise a packet of 
elements 9 of foamed, mixed cell polyurethane elastomer. This material has 
good spring properties and simultaneously good damping properties. The 
spring system 3 therefore has no tendency to fatigue. 
The compression spring 7 comprises two elements 9 whilst the compression 
spring 8 includes four elements 9. The latter is correspondingly softer. 
The elements 9 are constructed as lens-shaped rings. In order to avoid 
relative displacements, a respective spacer disc 10 is arranged between 
the individual lens-shaped rings 9. The number of elastic elements used 
for both springs can be selected at will in accordance with the 
application and construction. 
The two compression springs 7 and 8 are surrounded by a housing 11 which is 
secured by adhesive in the holding portion 1 on the machine side and whose 
base 12 and screwed-on lid 13 form axial abutments for the two compression 
springs. The elastic rings 9 are pushed onto a rod 14 which is screwed to 
the holding portion 2 on the tool side and extends through the housing 11. 
Fixedly arranged on the rod 14 is a disc-shaped actuator 15 for the two 
compression springs 7 and 8. The constructional unit comprising the rod 14 
and compression springs 7,8 may be easily replaced. 
The carrier device 4 has rollers 16 which are perpendicular to the axis and 
are mounted in the holding portion on the workpiece side and engage in 
axial guides 17. The guides 17 are defined by a sleeve 18 which is secured 
by adhesive to the holding portion 1 on the machine side. A collar 19 
engages over the sleeve 18 with the interposition of an oil sealing ring 
20. The collar 19 is screwed to the holding portion 2 on the tool side. 
The coupling 6 has a coupling disc 21 which is in sliding surface contact 
not only with the holding portion 2 on the tool side but also with the 
tool receiving sleeve 5. It is provided with four radial slots which are 
angularly spaced from one another by 90.degree.. Pegs 22 on the holding 
portion 2 on the tool side engage in a pair of opposing slots with the 
interposition of rollers 23, see the upper portion of FIG. 1. Pegs 24 on 
the tool receiving sleeve 5 engage in the other pair of opposing slots, 
again with the interposition of rollers 25. This is shown in the lower 
portion of FIG. 1. In this connection, the lower portion of FIG. 1 is on a 
sectional plane which is offset by 90.degree. from the plane of the 
drawing. The coupling disc 21, which comprises plastics material, may be 
highly loaded and is easily movable with respect to the holding portion 2 
on the tool side and the tool receiving sleeve 5. It permits the desired 
radial movements of the latter for the purpose of compensating for an 
axial offset. 
The illustrated tool is a thread cutting tool 26. The tool holder in 
accordance with the invention may however be used with the same advantage 
in countersinking and milling and also in boring and reaming fixtures. 
FIG. 2 shows the rod 24 with the compression springs 7 and 8 and the 
disc-shaped acutator 15 between them. However, in this case the 
compression springs 7 and 8 do not comprise lens-shaped elements 9 but 
elements 9' with plane parallel sides. The interposition of spacer discs 
10 can be omitted. To improve the spring characteristic each element 9' is 
provided with a circular peripheral notch 27. The elements 9' have a 
circular periphery, as do the elements 9. 
Modifications are possible within the scope of the invention. Thus the 
elements 9 and 9' can without difficulty have, for instance, a polygonal 
periphery. The same applies to the spacer discs 10, whereby their 
thickness can also differ. System 3 can also be reversed in such a way 
that the actuator 15 is secured to the holding portion 1 on the machine 
side and that the housing 11 is arranged in the holding portion 2 on the 
tool side. Furthermore, with an appropriate different arrangement of the 
abutments for the two compression springs 7 and 8 the housing 11 can be 
omitted. The coupling 6 can be provided at any desired position on the 
tool holder, that is to say even in the holding portion 1 on the machine 
side. Additionally, a lubricant reservoir can be provided for the coupling 
6.