Gear cutting – milling – or planing – Milling – With cutter holder
Reexamination Certificate
2000-03-14
2002-03-05
Briggs, William (Department: 3722)
Gear cutting, milling, or planing
Milling
With cutter holder
C279S103000, C408S23900A
Reexamination Certificate
active
06352395
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a tool holder to be removably attached to a spindle of a machine tool, as well as to a tool holder attachment mechanism.
2. Description of Related Art
A conventional tool holder has a structure in which a shank portion is tapered such that diameter decreases rearward (herein, the term “rear” or “rearward” refers to a direction toward a machine tool, and the term “front” or “frontward” refers to a direction away from the machine tool). A taper hole is formed in a front-end portion of a spindle of a machine tool such that diameter decreases rearward. The shank portion of the tool holder is inserted into the taper hole and drawn rearward by means of a draw member, whereby the tool holder is fixedly attached to the spindle.
However, the shank portion is restrained only through contact between the tapered outer circumferential surface of the shank portion and the wall surface of the taper hole formed in the spindle, while a gap is maintained between the front-end face of the spindle and the rear-end face of a flange portion of the tool holder located at the front side of the shank portion. Accordingly, the shank portion cannot be held with a sufficiently large force for coping with high-speed cutting or low-speed, heavy-duty cutting.
In an attempt to solve the problem, a force for restraining the shank portion of the tool holder has been increased. Specifically, restraint is effected through abutment between the front-end face of the spindle and the rear-end face of the flange portion of the tool holder, in addition to the restraint effected by means of the tapered outer circumferential surface of the shank portion and the wall surface of the taper hole.
However, in order to effect such restraint, relevant manufacturing tolerance must be rendered more severe, causing an increase in cost. Also, after long-term use of the spindle, need to repolish the taper hole arises. Since the size of the taper hole increases as a result of repolishing, restraint of the tool holder is effected simply by means of the end face of the flange portion of the tool holder and that of the spindle. Since no contact is established between the tapered shank portion and the taper hole, a tool shakes, disabling continuation of machining. Also, when the spindle rotates at high speed, a front-end portion of the taper hole formed in the spindle expands as a result of action of a centrifugal force, causing reduction in restraint effected by the taper hole. As a result, a tool held by the tool holder vibrates, causing impairment in machining accuracy.
In order to cope with the problem, a shank portion of a holder body of a flanged tool holder is formed to have a straight portion, and a taper cone is fitted to the straight portion. The taper cone is oriented such that diameter decreases rearward. The taper cone is preloaded by means of Belleville springs (disc springs) which are interposed between the taper cone and the shank portion, such that the outer circumferential surface of the taper cone is pressed against the wall surface of the taper hole. Application of the preload and rearward drawing of the holder body solve the problem to a certain extent, and manufacture is relatively easy. However, a force for drawing the holder body must be increased; otherwise, the taper cone is not sufficiently restrained by means of the wall surface of the taper hole.
In another tool holder, a shank portion of a holder body includes a straight larger-diameter end portion and a straight smaller-diameter end portion. A taper cone includes a corresponding larger-diameter straight hole and a corresponding smaller-diameter straight hole. However, as a result of employment of the straight holes in the taper cone, the holder body must be drawn by a considerably large force in order to effect sufficient restraint of the tool holder.
FIG. 17
shows a tool holder
30
which is fixedly attached to a tool machine, such as a machining center, by means of a conventional tool holder attachment mechanism. As shown in
FIG. 17
, the tool holder
30
includes a taper shank portion
31
; a flange portion
32
to be gripped located at the larger-diameter side of the taper shank portion
31
; and a cylindrical tool attachment portion
33
extending from an end face of the flange portion
32
in opposition to and in alignment with the taper shank
31
.
In attachment of the tool holder
30
to a spindle
40
of a machine tool, the taper shank portion
31
is inserted into a taper hole
41
formed in the spindle
40
, and a pull-stud draw mechanism disposed within the spindle
40
is engaged with an inserted end of the taper shank portion
31
. The inserted end of the taper shank portion
31
is then drawn rearward by means of the pull-stud draw mechanism so as to closely fit the taper shank portion
31
into the taper hole
41
, thereby attaching the tool holder
30
to the spindle
40
.
According to JIS or ISO standard, in order to closely fit the taper shank portion
31
of the tool holder
30
into the taper hole
41
formed in the spindle
40
, a predetermined gap D (about 2 mm or 3 mm) must be provided between an end face
401
of the spindle
40
and a corresponding end face
311
of the flange portion
32
of the tool holder
30
. The standard prescribes that the tolerance in manufacture of the taper shank portion
31
with respect to the gap D be ±0.4 mm.
Through establishment of the gap D between the end face
401
of the spindle
40
and the end face
311
of the flange portion
32
of the tool holder
30
, the taper shank portion
31
of the tool holder
30
can be closely fitted into the taper hole
41
formed in the spindle
40
. However, close contact cannot be established between the end face
311
of the flange portion
32
of the tool holder
30
and the end face
401
of the spindle
40
. As a result, a cutting load is concentratedly imposed on the taper shank portion
31
of the tool holder
30
, accelerating tendency toward fretting corrosion of the contact surface between the taper hole
41
and the taper shank portion
31
. Further, coupling rigidity between the tool holder
30
and the spindle
40
is impaired, resulting in a failure to perform heavy-duty cutting.
In order to cope with the problem, Japanese Utility Model Application Laid-Open (kokai) No. 15947/1994 proposes a tool holder attachment-mechanism in which a pair of horseshoe-like spacers
34
(
FIG. 17
) are screwed on the end face of a flange portion of a tool holder so as to fill the gap between the end face of a spindle and that of the flange portion.
In the conventional tool holder attachment mechanism shown in
FIG. 17
, the end face
311
of the flange portion
32
of the tool holder
30
can be brought into close contact with the end face
401
of the spindle
40
via the spacer
34
, which is screwed on the end face
311
of the flange portion
32
of the tool holder
30
. As a result, high coupling rigidity is established between the tool holder
30
and the spindle
40
, thereby enabling heavy-duty cutting. However, since the spacer
34
having a thickness corresponding to the gap D must be custom-made for each model of tool holder, manufacture and management of the spacers
34
are troublesome and involve increased cost. The spacers
34
may be attached to the end face of the spindle. However, this causes the following problem. In the case of a newly manufactured machine tool, threaded holes can be formed in the end face of a spindle for use in screwing the spacers
34
into place. However, in the case of an existing machine tool, cutting such threaded holes raises a problem in machining accuracy, since, among components of the machine tool, the spindle involves the highest required accuracy.
SUMMARY OF THE INVENTION
The present invention has been accomplished in view of the above-mentioned problems. A first object of the present invention is to provide a tool holder which can be fitted into a taper hole formed in a spindle even when a force for drawing a holder b
Matsumoto Masakazu
Taguchi Masahiro
Yamamoto Yusaku
Briggs William
Ladas & Parry
Nikken Kosakusho Works Ltd.
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