Apparatus for machining a hollow cylinder to produce a pattern drum

A hollow cylinder, generally a foil, for use in screen printing, is patterned by laser beam machining. The foil is held at its ends and rotated about its axis while a carriage is shiftable along a guide extending on the bed along the cylinder and carrying the laser optics for training a laser beam against the cylinder. Support rollers engage the wall of the hollow cylinder from the exterior on the carriage adjacent the laser impingement location.

FIELD OF THE INVENTION 
My present invention relates to an apparatus for machining a hollow 
cylinder by means of a laser to produce a pattern drum, i.e. a cylindrical 
template for screen printing and the like. 
More particularly, the invention relates to an apparatus for the removal of 
material from a hollow cylinder by means of a laser and wherein the 
apparatus comprises a support for rotatably journaling the hollow cylinder 
about its axis, i.e. a guide extending parallel to the hollow cylinder, a 
carriage displaceable on that slide and provided with laser optics for 
training a laser beam at a site upon the hollow cylinder at which material 
is to be vaporized by the laser, and a drive for the slide. 
BACKGROUND OF THE INVENTION 
In the production of round pattern drums or templates it is known to apply 
a photosensitive lacquer to a perforated foil and to expose the 
photosensitive lacquer through a desired mask so that, by development of 
the photolacquer, desired perforations of the foil are exposed to enable 
screen printing to be effected therethrough. 
It is also known to produce a screen printing drum of the aforedescribed 
type to coat a perforated foil with a material which can be evaporated or 
destroyed by a laser beam and then to expose the perforations in the 
desired pattern by focusing the laser beam at which this coating material 
is to be removed. 
In this system, the silk screen pattern blank is drawn over a 
circumferentially expandable support cylinder which, in the expanded 
state, runs perfectly round or to mount the silk screen blank over a shaft 
having support bodies therealong, the movement of the support bodies which 
are shiftable along the shaft and the laser head being coordinated with 
one another to ensure effective bracing at the site of the blank upon 
which the laser beam is trained. In devices of the latter type the 
positioning of the screen printing pattern blank cannot be automated. 
OBJECTS OF THE INVENTION 
It is, therefore, the principal object of the invention to provide an 
apparatus for machining a hollow cylinder to produce a pattern drum by 
laser ablation of a cover lacquer or the like of a perforated foil or for 
directly forming perforations in the foil which will permit automation of 
the blank mounting and overcome other drawbacks of prior art systems. 
Another object of my invention is to provide an improved laser machining 
apparatus which does not require drawing the cylinder or drum blank over a 
shaft or support cylinder while nevertheless affording the requisite 
degree of bracing of the cylinder particularly in a region thereof at 
which machining by laser occurs. 
SUMMARY OF THE INVENTION 
These objects and others which will become apparent hereinafter are 
attained, in accordance with the invention by providing the apparatus with 
supporting cones engaging opposite ends of the hollow cylinder and one of 
which is shiftable along the axis of the hollow cylinder, i.e. in the 
axial direction thereof. Since only one of the support cones requires 
shifting in the direction of the blank axis, it is simple to provide for 
the displacement of this support cone and positioning of the pattern drum 
blank by automatic means. 
According to a feature of the invention support rollers engage the surface 
of the cylinder and serve, in addition to bracing of the surrface, to 
clean the engraving location, i.e. the region at which the laser beam is 
trained upon the drum. The rearmost roll or wheel serves to remove 
residues and cinders of the machining process while the leading roll or 
wheel removes particles which might be detrimental to the machining action 
of the laser beam at the point of impingement of the beam upon the drum. 
More particularly, an apparatus for machining a hollow cylinder to produce 
the pattern drum comprises: 
a support defining an axis for a hollow cylinder to be machined and 
receiving the hollow cylinder for rotating the hollow cylinder about the 
axis; 
a guide extending parallel to the axis; 
a carriage displaceable along the guide relative to the hollow cylinder; 
a drive operatively connected to the carriage for displacing same along the 
guide; 
a machining laser generating a beam adapted to remove material from the 
hollow cylinder; 
laser optics on the carriage training the beam against the hollow cylinder; 
and 
a plurality of roller elements bearing frictionally against an external 
surface of the hollow cylinder and rollingly engaging same in an axial and 
in a circumferential direction. 
According to a feature of the invention the roller elements are mounted 
upon the carriage to engage the external surface of the hollow cylinder in 
a region thereof proximal to a machining location at which the beam 
impinges upon the external surface. 
Advantageously, suction means are provided for applying suction to the 
external surface in a region of the location between two of the roller 
elements. 
The roller elements can be rollers having axes inclined to the hollow 
cylinder. The rollers have concave peripheries engaging the external 
surface or peripheries with generatrices coinciding with generatrices of 
the external surface of the hollow cylinder. As noted, the roller elements 
can be disposed forwardly and rearwardly of the laser impingement location 
with respect to a direction of displacement of the carriage along the 
guide. 
The roller elements can be mounted for oscillating movement in a direction 
of the axis of the hollow cylinder and/or can be swingable mounted on the 
carriage. 
According to a feature of the invention, the suction means include a vacuum 
housing shiftable with respect to the laser optics in a direction of the 
axis of a laser beam and independently thereof. The suction means can also 
include a vacuum chamber and a suction air duct connected thereof with a 
direct inflow technique, the suction air duct engaging around a lower half 
of the hollow cylinder. 
The roller elements can be driven by a respective motor and, where they 
have concave peripheries, these peripheries may conform in cross section 
to a segment of an ellipse or hyperbola.

SPECIFIC DESCRIPTION 
The apparatus as shown in FIG. 1 comprises a torsion-stiff and 
bending-stiff bed 1 provided with a tail stock bearing 2 and a head 
bearing 3 for supporting a pattern cylinder 4. The latter may comprise a 
perforated foil coated with a material to be ablated by the laser beam or 
may be composed of a material to be perforated by the laser beam in the 
desired pattern. 
Parallel guide rails 5 extend parallel to the axis of the drum or cylinder 
4 and carry a carriage 6. 
The head 3 contains a drive for rotating the hollow cylinder 4. A 
longitudinal slide drive 10 is provided, for example in the form of a 
stepping motor 20 whose stepping increment can be adjusted so that the 
advance of the longitudinal carriage 6 can be in increments of about 10 
micrometers. The stepping motor 10 is connected to a lead screw 18 coupled 
with the longitudinal slide 6. 
A laser unit 11 provides at the output side 12 of the laser, a laser beam 
13 reflected at two reflector stations 14 to a mirror 15 representing the 
laser optics on the longitudinal slide 6. The longitudinal slide 6 carries 
a cross slide 16 driven by the cross feed spindle 16' to permit adjustment 
of the laser optics along the axis of the laser beam. The mirror 15 severs 
to deflect the laser beam 13 parallel to the axis of the cylinder in a 
direction perpendicular to the outer surface of the hollow cylinder. 
The pattern to be created on the drum or hollow cylinder 4 is converted 
into control pulses and can be generated by sensing a master or which can 
be stored in memory in the electronic control system shown 
diagrammatically at 100 and coupled to the apparatus described. These 
control pulses pulse the laser beam and step the slide 6 as well as the 
rotation of the hollow cylinder 4, thereby removing the sheathing of the 
foil so as to expose the openings thereof and produce the screen printing 
pattern. 
In this manner a point by point generation of the desired pattern on the 
drum can be ensured with complete programming of the laser 11, the 
movement of the cylinder 4 and the movement of the slide 6. Obviously 
repetitive pattern reproductions on the pattern drum can be achieved in a 
simple manner. 
As will be apparent from FIG. 2, moreover, support cones 7 and 8 are 
provided to engage in the ends of the hollow cylinder 4. One of the two 
support cones, for example the support cone 8, can be movable in the 
direction of the axis of the cylinder 4 manually or automatically The 
support cone 7 and 8 have profile surfaces which can form steps, for 
example the steps 23, on which a cylinder 4 of the appropriate size can be 
received. 
The support cone 7 and 8 are each journaled for rotation about the axis of 
the cylinder and can be driven with the cylinder. The means 101 
illustrated diagrammatically in FIG. 2 can include a hand wheel and 
spindle for axially displacing the support cone 8 A stub shaft 9 carrying 
the support cone 8 may be connected to the motor within the head stock 3. 
As can be seen from FIGS. 3 and 4, rollers 19 and 20 engage the cylinder 4 
in the region of the laser beam impingement upon the surface of the hollow 
cylinder and have axes 21 and 22 which are inclined to the cylinder axis 
102, these rollers having a curved outer peripheral contour. The rollers 
are provided in the engraving region 17 shown diagrammatically, i.e. the 
location at which the laser beam 13 impinges upon the cylinder The rollers 
have only been illustrated by the diagrammatic ellipses 19' and 20' in 
FIG. 1 and will be understood to be mounted upon the carriage 6 for 
movement with the laser beam. 
Because of the inclined axes, as these rollers 19 and 20 roll on the 
surface of the cylinder 4, an axial sliding component is provided between 
the surface of each roller and the outer surface of the cylinder to effect 
an intensive cleaning of the cylinder from adherent cinders and particles 
formed by the burn off of the cover material. One of the rollers serves 
primarily to remove the cinders while the other removes particles which 
might have been detrimental to the laser beam. 
The rollers 19 and 20 are not fixedly positioned with respect to the laser 
head 15, etc. but can additionally oscillate or swing in the direction of 
the cylinder axis to reinforce the cleaning action. 
In addition, they can be swingable about an axis intersecting not only the 
cylinder axis but also the respective roller axes at right angles to 
permit the contract curve between the pattern cylinder and the rollers to 
be accommodated to various sizes of template of pattern radii. 
The rollers 19 and 20 can have surface layers of plastic or textile 
material to promote the cleaning action. 
Furthermore, the rollers 19 and 20 provide a bracing of the region 17 so 
that the focus of the laser beam 13 can always lie precisely on the 
surface of the cylinder 4. 
Furthermore, the system has the significant advantage that, upon a change 
of the pattern cylinder 4, the cylinder need not be drawn over a mandrel 
or the like and for the change the rollers 19 and 20 need only be swung 
out of the way and need not be replaced. 
Since only the swinging of the rollers 19 and 20 out of the way and the 
horizontal positioning of one of the support cones is necessary for 
replacement of a pattern drum, the described system can be easily 
automated. 
For further reinforcement of the cleaning action, independently of the 
pressing of the rolls against the cylinders and vice versa, a vacuum can 
be generated in the region of the engraving location 17. This suction 
applied to the region between the rollers 19 and 20 and around them 
ensures that the foil of the pattern drum will remain in contact with the 
rollers and thereby counters any tendency of the foil to move away from 
the rollers. In addition, the vacuum or suction removes particles which 
may have been only loosened by the relative movement of the rollers 19 and 
20 against the cylinder 4, whereby such loosened particles will be 
entrained in the air stream and carried away. 
The particles and vapors produced by the laser beam impingement on the 
cover material can also be carried away in this flow. 
In FIGS. 5 to 7 I have shown another embodiment of the invention. Here the 
two rollers are two bevelled rollers or conical rollers 25 and 26 which 
bear against the foil 4 such that their generatrices correspond to the 
generatrix 30 of the hollow cylinder 4. 
The frustoconical support rollers 25 and 26 are journaled by bearings 31 on 
a support housing 32 carried by the cross slide. 
The laser beam 13 is here focused within a laser cutting head 27 by an 
appropriate objective lens system and is trained through a small bore 33' 
in the mouthpiece 33 of this head onto the hollow cylinder 4. 
Ahead of the laser cutting head 27 is a vacuum housing 28 so constructed 
that it encloses the mouthpiece 33 and defines a vacuum chamber 29 
therearound. The vacuum chamber 29 communicates via a gap 34 with the 
region around the engraving region 17. A vacuum pipe 35 draws air from the 
vacuum chamber 29 and air flows into this vacuum chamber through the gap 
34. This flow creates a pressure differential between the atmosphere in 
the working space and the vacuum chamber and ensures that the template 
will be forced against the frustoconical support rollers 25 and 26. Thus 
even if the cylinder is composed of a comparatively thin foil, it will not 
tend to move away from the frustoconical support rollers 25 and 26. 
The frustoconical support rollers 25 and 26 are mounted on the housing 32 
by means of clamping sleeves 36 which can be tightened by clamping screws 
39. The pins 37 extending from the bearings of the frustoconical support 
rollers 25 and 26 are threaded and can be positioned by a counternut 38 
after they are inserted in the smooth surfaced bores of the respective 
split clamping sleeve 36 which can retain the pins 37 when the screws 39 
are tightened. This allows the support rollers 25 and 26 to be readily 
dismounted upon loosening of the screws 39 and to be reinserted without 
loss of the adjustment of the positions of the rollers 25 and 26 and 
without requiring them to be readjusted in position. 
The vacuum housing 28 is provided with a half round cover 40 (FIG. 7) which 
affords rapid and convenient access to the mouthpiece 33. The vacuum 
housing is shiftable relative to the laser cutting head 33 along the laser 
beam axis 48 so that the focus of the laser beam can be set independently 
of the gap 34. A very narrowly dimensioned gap 49 seals the rear wall of 
the vacuum housing 28 against the laser cutting head 27. As can be seen 
from FIG. 8, the last mentioned embodiment can be varied by offsetting the 
axes 42 and 43 so that they lie in two different planes from the plane of 
the axis 48 of the cylinder 4. In this case, the roller 25 engages the 
cylinder 4 approximately along its generatrix 44 while the roller 46 
engages the cylinder along its generatrix 45 The result is a contourtrue 
bracing of the foil cylinder. 
In FIG. 9 I have shown a modified arrangement of the ends of the hollow 
cylinder 4 requiring different support devices for these ends. Instead of 
the supporting cones 7 and 8 which are utilized in FIG. 2 to support and 
center the ends of the hollow cylinder, here, for the same purpose, two to 
three-jaw chucks 46 are provided as are will known from the lathe arts. 
These three jaw chucks are able to engage rigid end pieces 47 of the 
cylinder with sufficient accuracy. 
In this embodiment as well one of the sets of rollers 19, 20 or 25, 26 are 
used to brace the pattern cylinder wall in the region of the laser 
machining. 
In the embodiment of FIG. 10, it can be seen that an annular suction 
passage 51 can communicate with the vacuum chamber 29, this suction 
passage extending below the lower half of the cylinder 4. 
Over the region of this suction passage, therefore, a subatmospheric 
pressure is generated in relation to the air pressure in the work room and 
the thin wall of the hollow cylinder is held in shape because of the 
annular gap provided by the suction air passage also during its 
circumferential travel. An uncontrolled shifting of the foil wall of the 
cylinder which is normally superimposed upon the desired rotation can here 
be avoided and thus the small deviations from the cylindrical shape which 
can be detrimental to precision removal of material by the laser beam can 
be avoided.