Improved Machine for Grinding Parabolic Surfaces.
Year: 1892

page_text: Date of Application, 26th May, 1892-Accepted, 25th June, 1892
COMPLETE SPECIFICATION.
Improved Machine for Grinding Parabolic 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 and ascertained in and by the following
statement :-
This invention relates to a grinding apparatus for producing parabolic surfaces
such as looking glass or other glass surfaces based on the well known construction
of the parabola which can be demonstrated.
This construction is characterised by the attribute of the parabola that the
distance of a parabolic point from the focus on the one hand and from the axis of
an ordinate on the other hand will give a constant result. An explanation of
the construction on which this apparatus is based is given at the end of this
Specification.
Our novel apparatus the characteristics of which are simplicity and the absence
of guide rails is specially adapted for producing concave parabolic surfaces.
Fig. 1 is a side view of the apparatus partly in section.
Fig. 2 is a horizontal section through the upper part of the apparatus.
Fig. 3 is an end view of the apparatus some of the parts being in section on the
line in Fig. 2.
Fig. 4 is a detailed of Fig. 3 on the linea in Fig. 2 but on an enlarged scale.
Fig. 5 is a diagramm representing the construction of the parabolic curve.
The grinding apparatus consists of the frame or bow L and base plate L' with
step bearing L2 for the vertical shaft S carrying the strap drum c for rotating the
wooden disc a and glass d fixed to the same by plaster of Paris, pitch or in other
suitable manner.
In consequence of the rotation of the disc a around the parabolic axis it is only
necessary that the grinding tool e describe the one part A P of the parabola for
determining the parabolic profile which depends on the central way A¹ P¹ (Fig. 1
& 5).
The grinding or other disc is guided in the parabolic way A' P¹ in the following
manner:
The disc e is arranged in the bifurcated end of a guide rod which runs through
a rotary sleeve or muff on a pivot on the tranverse bar I (said pivot representing
the focul point) and so that the said rod i can glide freely through the said sleeve
(Fig. 2 & 4).
A chain, wire or the like of predetermined length is attached to the pin or pivot !
or to a roller on the same and runs around a friction roller or chain or cord
drum e' of the disc e to a movable point or to the winding roll n¹ on the pivot n said
pivot being arranged in a slide m caused to move horizontally by means of a
screw spindler when the drum q is operated.
If the grinding disc e with the roller e' remains perfectly vertical or normally
under the suspension point n and is moved radially to the disc a the parabolic.
profile will be produced by the edge of the disc, as in any case the sum of
distances and e, n remains constant.
lg
In order that the roll n' during its longitudinal movement can pass the roller /
fixed in the direction of and above the vertex of the parabola A' PI to be produced
both of these rollers n' l' and also the roller e' near the grinding disc are arranged
in different planes (Fig. 2 & 3).
The standards J, K serve to carry the guide spindle holder I.
The wire, chain or the like is continuously held under tension by the weight of

page_text: the grinding disc e and guide rod i so that the said disc lias the tendency to adjust
itself normally beneath the pivot n the distance from the points of attachment on
the edge of the disc n' being determined by the radius of the discs at e¹ in' said
discs being uniform in size. This arrangement of rollers n¹ e¹ of like size with
chain wire or the like running around same gives the guide for the central points P
in respect to the fixed point and the movable point n moving in the direction of
the ordinate vertical to P' so that these three points will always form rectangular
triangles the sum of the distances l, e and e, n remaining unchanged. Under these
circumstances the point Pl will describe a parabolic curve as represented by the
diagramm Fig. 5. If p (Fig. ) is a point of a parabola to be constructed, with
the vertex A and focus o the further points founded on the well known properties
of the parabola will result. If a line 05. is drawn through the centre o rectangular
to the line o A¹ and a vertical line p, s from the line o, e to p, and the connecting
line p, o the sum s, p and p, o will determine all the other parts of the parabola and
can be represented by a cord of unvarying length one end of which is fixed at o the
other ends being guided on the line o, a. By putting the cord under tension and
always retaining a rectangle at o a lower rectangular point will be formed which
runs through the parabola when the instance the intermediate point a has the
proper proportion 9. and .g osp and p o.
It will be evident that if any desired line for the cord parallel to the cord o, x as
shown by the dotted lines in Fig. 5 is taken a parabola will under the given
conditions be produced as pl and
9
o are = s¹ p and p, 0.
The circumference of the grinding disc e the centre of which is guided on this
principle in the parabolic line A' P will run on an equidestant curve A, P which
practically so nearly corresponds with the original parabolic line that the same can
be used as profile curve for the paraboloid to be ground.
Having now particularly described and ascertained the nature of this invention
and the manner in which the same is to be performed, we declare that what we
claim is :
1) An apparatus for producing parabolic surfaces consisting of the grinding disce
suspended on the one hand from the focul point of the parabola by a chain, wire,
cord or such like of predetermined length guided horizontally by means of a
screw spindle and on the other hand so guided by the rod i movably arranged at
the focul point that the centre of the centre of the disc will when adjusted in
vertical direction so move with the suspension point that the same will describe a
parabolic curve, substantially as described and shown in the drawings.
2) An improved apparatus for producing parabolically curved surfaces substan-
tially as described and shown in the drawings.
Dated the 26th day of May 1892.
PHILIP M. JUSTICE, Fel. Chart. Inst. P. A.,
55 & 56, Chancery Lane, London, Filing Agent.