Chemistry: electrical and wave energy – Processes and products – Electrostatic field or electrical discharge
Patent
1987-04-02
1988-09-20
Valentine, Donald R.
Chemistry: electrical and wave energy
Processes and products
Electrostatic field or electrical discharge
2041295, 20412955, 2041297, 204224M, 204225, 204DIG9, 204228, 204212, 219 69V, B23H 718, B23H 720
Patent
active
047723688
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention refers to a process for spark erosion or electro-chemical machining, in particular of tapered or bevel gears of hypoid tooth profile out of one piece, whereby the workpiece to be machined is immersed into an electrode of a machine operating on the spark erosion or electrochemical principle by means of a movement that is helicoidal relative to the electrode, said electrode having the negative shape of the workpiece.
BACKGROUND OF THE INVENTION
Tapered or bevel gears of hypoid tooth profile are primarily used in gears for automotive vehicles and helicopters. They are prepared from high-alloy steels in a soft state on special gear cutting machines having profiled cutting tools, and the cutting is carried out by the rolling or hobbing method. Afterwards the gears are hardened.
Due to the inavoidable hardening distortions they loose their original accuracy. In order to make them suitable again for their use in vehicle gears, they are lapped in pairs on special lapping machines. During lapping also the most favorable mounting position for each wheel pair is determined. During mounting of the gear the mounting of the wheel pairs must be adjusted to this present position by means of calibrated washers and/or particular bearing bushes.
One would prefer to avoid this labor-intense handling and also desire to have higher accuracies as well as surfaces of "grinding quality". Unfortunately bevel gears of hypoid tooth profile have a very complex tooth geometry. The flank lines in the rolling plane may be spirals, cycloids, hypoids, circular arcs or involutes. The narrow curvatures of the teeth and the narrow tooth gaps permit only the use of grinding tools of the size approximately of a pencil tip or, exceptionally, also thin-walled cup wheels. These grinding tools exclude already the possibility of grinding economically precut or preforged bevel gears having dimensions that are increasd by the so-called grinding allowance.
From tool making it's known to machine complex three-dimensional workpieces by means of spark erosion and/or by the electrochemical method. as generally known, the subsequent hardening and therefore the undesired hardening distortions are avoided. The most recent state of the art on spark erosion is described in leaflet EDM 3374, Robofil/Roboform, of Charmilles Technologies S.A., 109 rue de Lyon, Geneva (Switzerland), dated May 1984. On pages 12 and 13 the possible working forms of the immersion erroding machine are represented and briefly described.
For machining bevel gears of hypoid tooth profile by means of spark erosion, the "spiral-shaped machining of threads and spiral-shaped recesses" comes closest to it. On this kind of machining the workpiece is immersed, with a relative helical or screwmotion between it and the electrode, into the later which has the negative shape of the workpiece. With this kind only such workpieces can be machined which correspond to the following conditions: tecnhologically necessary working gap width.
With this kind of mahcining a cylindrical or tapered thread e.g. can be made only if the thread pitch is constant. If the thread pitch varies more along the entire spiral than does the technologically necessary width of the working gap between the electrode and the workpiece, interlocking occurs during immersion of the workpiece into the electrode. If the thread pitch along the entire thread spiral varies as little as the technologically necessary working gap width, the thread will be partially damaged during immersion of the workpiece into the electrode. It is to be noted that in all three cases the workpiece and the electrode are equidistant in their final position.
Gear wheels of hypoid tooth profile are complex three-dimensional bodies. The pitch of the teeth, the tooth width and the flank normals vary along the tooth width according to higher orders. Therefore they will be briefly described in a practical manner with the pitch angle .delta., with the cone distance R, with the tooth width b, the segment angle in function of th
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Dubno Herbert
Valentine Donald R.
Werkzeugmaschinenfabrik Oerlikon Buhrle AG
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