An Improved Apparatus for Grinding Parabolical Surfaces.
Year: 1892

page_text: Date of Application, 19th Dec., 1892-Accepted, 1st July, 1893
COMPLETE SPECIFICATION.
An Improved Apparatus for Grinding Parabolical Surfaces.
We, KONRAD SCHWARZ and GEORG SCHWARZ, trading under the firm of
N. Wiederer & Co. of Fürth in the Kingdom of Bavaria, Manufacturers, do
hereby declare the nature of this invention and the manner in which the same
is to be performed to be particularly described in and by the following.
statement :-
This present invention relates to an apparatus for grinding parabolical surfaces
such as mirror or glass surfaces and is based on a special geometric-(or so-called
secant) construction of the parabola. Suppose, for instance, that, in this
construction, the axis of the parabola and an auxiliary line are drawn perpen-
dicularly and parallel to each other, the perpendicular distances or divisions on said
auxiliary line which are formed by the intersecting points of the lines or radii
vectares from the vertex or axis, will always be changed in the same ratio as the
horizontal distances between the points extending outwards from the vertex are
increased.
A detailed demonstration of this geometric principle (Figure 4) will be found
at the end of the Specification.
Figure 1 is a side view.
Figure 2 a top view of the apparatus.
Figure 3 is a part end view partly in elevation partly in section on an enlarged
scale.
Figure is a diagram representing the parabolic construction.
The disc a which carries the glass d to be treated, is arranged to rotate with the
vertical shaft b which carries the strap drum c. The axis of these parts forms also
the central axis of the parabola of which the vertex A and point. P are given.
Now when the vertex A and the point P are determined upon, a so-called guide
parabola A' P' (in which the centre of the grinding tool is guided) can be fixed
so as to describe the given parabola.
As will be evident from Figure 1 the grinding tool e is mounted in a vertical
slide or carrier f, being held in same by a bifurcated guide rod i which adjusts
itself automatically on a pivot or antifriction roller mounted on the vertically
moving slide m whilst a peg or stud of the guide rod i is guided in a curved
slot running in the direction towards the vertex A' of the parabola. This para-
bolic curved slot z also serves the purpose of keeping the rod constantly pointed
to the vertex A¹. If the vertical slide f is now moved from the vertex position to
the right as far as it can be moved horizontally by means of its sliding block w
and if, at the same time, a vertical falling movement is given to the pivot which
supports the pronged rod i, the grinding tool, being subjected to the double move-
ment, takes a parabolic course and describes the part parabola A B which represents
the profile for the parabolical rotary surface which is produced by the rotation of
the glass d around the axis of the vertex. The mechanical device for obtaining
the movement mentioned is connected to the guide n and horizontal frame J
forming the frame-work of the apparatus.
The slide w is moved to and fro from the centre towards the edge of the surface
to be operated on by means of the screw spindler (arranged parallel with the
supporting frame J) which is driven by means of fast pulleys. A proportionately
(but in a fixed ratio) changed motion is thus transferred to a second slide v
(similar to slide w) on the screw spindle by the connection of the latter with the
spindle by means of the gear wheels q q'. "A band m³ is attached to this slide "
and runs over the pulleys o p being attached to a slide m moving in a vertical guide
way n. By this means the rising and falling movement of the guide rod i will be
jo

page_text: in the same ratio as the lateral sliding movement of the slide w beneath which the
grinding tool e will always be maintained vertically it having a vertically sliding
movement in the block w for this purpose.
From the foregoing it will be evident that when the pulley s is driven and the
grinding disc simultaneously rotates, a successive grinding of the surface to a
parabolic curve will be effected. The grinding disc can of course be replaced by a
polishing disc or other tool.
Although the aforesaid device is only shown as applied for the production of
convex parabolical surfaces, it will be evident that the same can be modified so as
to produce concave parabolic surfaces. In Figure 4 which represents the con-
structive diagram for the grinding machine for producing parabolic surfaces as
before described the distances between a, a, and a commencing from the axis
are uniform as also the distances or divisions between y¹ y² y³ y¹ on the vertical line
commencing from o.
If now the points y' y² y³ y' are connected by lines with the vertex A! of the
parabola and parallel lines rectangular to A', o corresponding to 21, x², ³ ³ are
drawn, the intersecting will give the points of the parabola A¹, P' as shown in the
diagram when the corresponding perpendicular line (23) is moved in a direction
rectangular to the axis and proportionally to the distance or division y³ which is
subdivided by rays of parallel lines, running to the vertex of the parabola at
uniform distances on the line o, y. These intersecting points give the parabolic
curve described by the axis of the grinding tool. In order to ensure the correct
movement of the grinding tool a special guide way for the point ¹ is employed,
which, as is evident from Figure 4, must describe a curve ¹² characterised as a
parabolic conchoide. This curve provides for the accurate arrangement of the
bifurcated guide to the vertex.
The axis of the grinding disc describes during its movement an equidistant
curve A. P. sufficiently near to the parabola A' P' for all practical purposes so that
the same can be considered as the profile for the paraboloide.
Having now particularly described and ascertained the nature of this inven-
tion and the manner in which the same is to be performed, we declare that what
we claim is-
An apparatus for grinding parabolic surfaces substantially as described and
illustrated in the drawings.
Dated the 19th day of December 1892.
PHILIP M. JUSTICE,
Associate Agent.