Process for eliminating errors in prepositioning pieces to be manufactured by electro-erosion

An improved machining process using electro-erosion to form integral hinges in rods in making flexible and/or electric suspension blades for various industrial devices. The machining process includes repeatedly eroding a rod alternately on two opposite sides to form two curved indentations, one on each side of the rod at a given point along its length, using a curved cylindrical electrode, so that the rod is progressively narrowed at this point until the required thickness of hinge is obtained. The process may also include, using as reference planes, to position the eroding electrode accurately, surfaces of a frame to which the rods have been attached. In this way, the rods are accurately prepositioned and thin hinges are produced with a repeatable high accuracy. Products made by this process include a pendulum-type accelerometer and an elastic support for a gyroscope useful in aeronautical applications.

BACKGROUND OF THE INVENTION 
The present invention is related to a process of machining parts by 
electro-erosion and to various industrial products, such as accelerometers 
or gyroscopes, made by practicing this process. 
More particularly, the present invention is related to a process for 
obtaining flexible and/or elastic suspension blades processing an 
extremely thin linear section. 
The invention further relates to a procedure for accurately prepositioning 
the parts, which are to be machined. 
One example of a precision-made product machined according to a process 
described above is a pendulum type accelerometer which contains two blades 
formed from two parallel rods with aligned integral hinges, having a 
thickness of a few microns in their central sections. 
A second example is the elastic suspension for a gyroscope, where rotation 
in two perpendicular axes is controlled by two pairs of rods, each pair 
having aligned integral hinges, having a thickness of a few microns in the 
central sections of the rods. 
SUMMARY AND OBJECTS OF THE INVENTION 
In particular, the invention relates to a process for electro-erosion 
machining a rod to form an integral hinge at a point along its length by 
repeatedly eroding the rod alternately on two opposite sides to form two 
curved indentations, one on each side of the rod at the said point, using 
a curved electrode so that the rod is progressively narrowed at this point 
until the required thickness of hinge is obtained. 
The invention also relates to a process for electro-erosion machining a 
rod, one end of which is attached rigidly to a frame, by positioning the 
eroding electrode using as a reference positioning plane one surface of 
the said frame, and thus avoiding errors in locating that part of the rod 
to be machined. 
An advantage gained by using a reference plane as described above is that 
the parts to be machined on a given assembly, containing one or more rods, 
may be located with consistently good accuracy. For example, if integral 
hinges are to be machined in an assembly using a process according to the 
invention, the axes of the hinges may be aligned accurately, or made 
accurately perpendicular to each other as required. By choosing a 
reference plane, cumulative errors in positioning successive hinges are 
avoided by this process, and no undue mechanical stresses are introduced 
when parts of the assembly are caused to rotate about the hinges. 
Furthermore, the process may be extended to mass production. 
A further advantage gained by using a process according to the invention 
for making hinges in precision apparatus such as accelerometers and 
gyroscope supports, is that the finishing process on the hinges is the 
last operation performed in constructing the apparatus; therefore there 
are no subsequent mechanical procedures which would risk damage to the 
fragile parts. Considering the brittleness and high cost of the parts 
involved in such apparatus, this is of great benefit to the industry.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1 to 4 illustrate the processes according to the invention, and a 
pendular-type accelerometer machined according to these processes. A thin 
hinge 5 is formed in a rod 2 by machining indentations of arcuate 
cross-section in opposite faces of the central section of the rod, using a 
cylindrical electrode (not shown). Electro-erosion is performed 
successively on one side and on the opposite side of the rod, the rod 
becoming narrower in the ZZ' axis. Hinges of only a few microns thickness 
are possible by this method. 
The hinges are machined after the assembly is complete, so as to avoid the 
risk of damage to the hinges or distortion of the apparatus as a result of 
mechanical processes during assembly. The accelerometer assembly shown in 
FIGS. 1 to 4 comprises a pendular weight 6 suspended from a support frame 
1 by two rods 2 with central hinges 5 to allow flexion along the XX' axis. 
The rods are rigidly mounted in recesses in a section 3 of the support 
frame and in recesses 4 in the pendular weight 6. The rods are shown as 
having cylindrical cross-section, but obviously they could equally well be 
square, rectangular or polygonal with an even number of faces. 
The reference planes used for positioning the eroding electrode accurately 
are faces 7, 8 and 9 on the support frame. 
FIGS. 5 and 6 show an example of an elastic suspension for a gyroscope, 
where movement in two degrees of freedom is achieved with two pairs of 
hinged rods. As before, the final machining of the hinges is left as the 
last process after the assembly of the parts. 
The gyroscope platform disc 14 is supported by two hinged rods 15 and 16 
which are attached at their upper ends to a cylindrical support frame 12. 
The platform disc 14 is free to rotate about an axis AB. The cylindrical 
support frame 12 is itself supported by two hinged rods 17, 18 attached at 
their upper ends to a rigid support 13. The support 12 is free to rotate 
about an axis CD which is perpendicular to AB, thus defining a second 
degree of freedom of the gyroscope platform disc. 
The rods and support are assembled by conventional processes, holes being 
bored in the support to receive the rods. When all the rough-machining is 
completed and the parts assembled, the electro-erosion process according 
to the invention is employed to make the four hinges with great accuracy. 
The tuned elastic suspension of gyroscope 11, as shown in FIGS. 5 and 6 has 
three annular elements 12, 13, 14 interconnected two-by-two: 13 with 12, 
and 14 with 12 using two rods respectively 17 and 18 for annular elements 
12 and 13, and 15 and 16 for annular elements 12 and 14. 
The converging position of axes CD and AB of the said rods is highlighted 
in these figures. 
The preliminary positioning of rods, linking to one another a fixed element 
and a moving element, is effected according to a method known to the prior 
art. 
In the case of the pendulum (FIG. 3), two parallel vertical holes are 
simultaneously bored in weight 6 of the pendulum and the corresponding 
section 3 of the support; holes intended for receiving the ends of the two 
solid rods 2a; or rough machined. 
In the case of the tuned elastic suspension of a two-degree-of-freedom 
gyroscope, alignment of the holes in the prolongation of one another, 
two-by-two, is also effected according to a method known to the prior art. 
Two methods of a electro-erosion already known in the prior art are 
suitable for practicing this process. The first is electrolytic erosion, 
where a steady current is passed between an electrode and the workpiece 
with an electrolyte flowing between the two. The second is 
electro-sparking where ions are removed from the workpiece as a result of 
sparking between electrode and workpiece, with a coolant fluid passing 
between the two. 
This invention provides a significant improvement in the production of 
precision made apparatus, in that a repeatable accuracy may be obtained, 
and fewer products wasted through damage.