Powder molding device for forming spiral bevel gear

Details Powder molding device for forming spiral bevel gear Powder molding device for forming spiral bevel gear

1999-11-19

2001-06-19

Davis, Robert (Department: 1722)

Plastic article or earthenware shaping or treating: apparatus

Means for molding powdered metal

C425S441000, C425SDIG005, C249S059000

Reexamination Certificate

active

06247912

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a method of forming a spiral bevel gear, more particularly a method which makes it possible to form a spiral bevel gear having a small pitch angle (e.g. 45° or less) by powder metallurgy, and a powder molding device.
Conventional sintered spiral bevel gears molded from powder had a large pitch angle. This is for the following reasons. As shown in
FIG. 3
, in the case of spiral bevel gears having a pitch angle &thgr; not less than 45°, the entire tooth flanks appear in a plan view as seen from the apex side of the cone of spiral bevel gear, so that compressed molded articles can be punched out of dies without encountering any problem even on an ordinary powder molding device having its upper and lower punches and die nonrotatably supported.
But in the case of a spiral bevel gear having a pitch angle &thgr;<45° as shown in
FIG. 4
, the left tooth flank LF or right tooth flank RF are hidden as viewed in plan. Thus, if one tries to simply punch a molded article out of the die, the article would not come out because the molded spiral teeth interfere with the mold (having teeth for molding).
Thus, conventional sintered spiral bevel gears made by powder metallurgy were limited to ones having a pitch angle &thgr;>45°, while conventional spiral bevel gears with &thgr;<45° were all made by cutting.
Powder metallurgical method is higher in mass-productivity and lower in the manufacturing cost than cutting method. Thus, it is desired to manufacture spiral bevel gears having tooth flanks not seen in plan by powder metallurgy.
An object of this invention is to provide a method which solves this problem.
SUMMARY OF THE INVENTION
According to this invention, there is provided a method of manufacturing a spiral bevel gear of the type in which at least one of right tooth flanks and left tooth flanks thereof are not seen in plan as viewed from the apex side of the cone of the spiral bevel gear by means of a powder molding device having an upper punch and a lower punch and a die with teeth for molding, the method comprising the step of punching a compression-molded article out of the die while rotating the article relative to the die or the upper punch with a lead L given by the following formula:
 L
min
<L<L
max
L
max
=2&pgr;/&agr;(r′−r″)tan(&thgr;−&thgr;″)
L
min
=2&pgr;/&bgr;(r′−r″)tan(&thgr;−&thgr;″)
wherein:
&agr;: central angle between A and B of
FIG. 5
{A: corner of left tooth flank (which is right tooth flank if the teeth are twisted leftwardly) on the tip circle. B: intersection of the bottom side of the left tooth flank (which is right tooth flank if the teeth are twisted leftwardly) and the tip circle on plan view.}
&bgr;: central angle between C and D of
FIG. 5
{C: corner of right tooth flank (which is left tooth flank if the teeth are twisted leftwardly) on the tip circle. D: intersection of the bottom side of the right tooth flank (left tooth flank if the teeth are twisted leftwardly) and the tip circle on plan view.}
r′: radius of the tip circle at the small end face
r″: radius of the root circle at the small end face
&thgr;: pitch cone angle
&thgr;″: root angle
FIG. 5
shows, on an enlarged scale, a portion of the spiral bevel gear of FIG.
4
. In
FIG. 5
, in order to punch out the corner A of the left tooth flank LF on the tip circle
5
without interfering with the point B on the mold, it is necessary to cause the relative rotation between the molded article (spiral bevel gear) and the molding teeth on the die (not shown) in mesh with the teeth of the molded article by at least angle &agr; while they are relatively moved axially by distance Z (
FIG. 4B
) relative to each other. The maximum lead L
max
(
FIG. 6
) that can create the relative rotation by the angle &agr; is given by the following formula (1).
L
max
=2&pgr;/&agr;(r′−r″)tan(&thgr;−&thgr;″)  (1)
On the other hand, if the relative rotation angle is too large, the corner C of the right tooth flank RF on the tip circle would interfere with the mold. Thus, it is necessary to limit the maximum relative rotation angle while the molded article and the molding teeth are axially moved relative to each other by distance Z to a value smaller than the central angle &bgr; between B and D. The minimum lead L
min
(
FIG. 6
) that creates the rotation by angle &bgr; is given by the following formula (2).
L
min
=2&pgr;/&bgr;(r′−r″)tan(&thgr;−&thgr;″)  (2)
Thus, by rotating the molded article and the die with the molding teeth relative to each other with a lead L that satisfy the formula
2&pgr;/&bgr;(r′−r″)tan(&thgr;−&thgr;″)<L<2&pgr;/&agr;(r′−r″)tan(&thgr;−&thgr;″)
it is possible to punch the article out of the die.
The optimum lead L
opt
(
FIG. 6
) for relative rotation will be: ½(L
max
+L
min
)
∴L
opt
=&pgr;(1/&agr;+1/&bgr;)(r′−r″)tan(&thgr;−&thgr;″)
The powder molding device embodying this invention includes a rotatable die having teeth for molding. When a molded article is punched out, the die is rotated with a lead L guided by a fixed guide means. Thus, the molded article can be removed without interfering with the die and without rotating. The molded article can thus be punched out of the die without any damage.
The same effects are expectable with a structure in which an upper punch having teeth for molding is firstly pulled out of the die guided by a fixed guide means while rotating with a lead L, and then the molded article is punched out of the die by pushing it up with a lower die. But this structure has a problem in that a twisting force is applied to the molded article during compression molding while the upper punch is pushed into the die while rotating with a lead L. The device in which not the upper punch but the die is rotated is free of this problem, and thus the molding is more stable.
Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which:


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patent: 5366363 (1994-11-01), Good et al.
patent: 5946963 (1999-09-01), Suzuki et al.
patent: 5996226 (1999-12-01), Yang

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