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Tire mold makers optimize manufacturing. (Process Machinery).

In reference to common tool and mold making, the production of molds for tire sidewalls shows a peculiarity: The workpieces are rotationally symmetrical. With the unconditional request to be able to have the tool positioned vertical to the surface, optimization for this purpose leads to a four-axis machine kinematic.

Experience has shown that a proccess optimized general concept of a highly productive machining system for tire sidewall molds consists of all its ingredients, including:

* Time optimized creation/generation of data;

* practical graphic output and graphical data modification at the machine;

* minimized disruptive contours of the spindle head;

* high machine dynamic and precision;

* automated working; and

* state-of-the-art safety precautions.

Special attention should be given to the data generating, that, as a rule, is a lot more time consuming and expensive with common CAD/CAM systems than necessary.

The machine

An examination of the LGT machine shows that its base is built with an enclosed frame made of solid steel profiles, welded together. Additionally stiffened, this design guarantees high inherent stability and the best damping quality, without the need for foundation work or footings.

This is important in order to maintain workpiece precision and to receive high quality surfaces. Besides that, absorption of forces due to high accelerations is improved

X- and Z-axis are on the transversal bar, C-axis turns the rotational symmetric workpiece and the B-axis facilitates the normal positioning of the tool everywhere on the surface (figure 1).


This construction does not need a fifth axis. In addition, compared to other machine concepts, for processing of 47" (1,200 mm) mold diameter it needs to travel only 19.7" (500 mm) in X direction. (LGR-S: 86" (2,200 mm)--molds with 43" (1,100 mm) travel in X).

A four-axis machine with its tool positioned vertical to each point of the mold surface consequently meets all requests for producing tire sidewalls. All conceivable contours, arcs and curves can be engraved with the required specifications and tolerances.

Superior workpieces contain complex engravings which demand very fine tools. To prevent tool breakage, maximum possible feed rate cannot nearly be used. So, in order to minimize processing time, acceleration ability is needed. Here is where the high-end AC servo control with highly dynamic motors comes in. The CNC integrated jolt limitation minimizes stimulation of machine vibrations and improves the surface quality noticeably. Absolute glass scales in the linear axes are compulsory in order to secure the necessary exactness of the workpiece. But what good does axis precision do if it is ruined by tolerances of the fixing device? In digitizing the fixed tire sidewall mold on the LGT-S, the system receives actual values of the surface, taking only about three minutes to accomplish.

One of the most modern PC-based CNC controls calculates the traverse path, so that a constant feed rate at the tool tip is achieved.

To validate the use of the high quality technique, a 24-fold tool changer facilitates the processing of an entire sidewall mold, typically over 24 hours, without manual support.

During data processing, one assigns tools with certain characters to objects or paths in the tool library. The system analyzes the output data at the start of the engraving operation and demands the corresponding tools. The tool life is taken into consideration, and it supports the multiple assembly of the same tool (sister tool). The integrated tool measuring makes the use of preadjusted tools unnecessary and tool breakage control possible.

Safety and enclosure

High spindle revolution (50-60 rpm) and machine dynamics are enough reason to consider operator safety. So, combined with the possibility of using intense cooling and lubrication, a machine frame with complete enclosure resulted. Large glass windows give clear views of the process and protect from engraving chips and potentially broken tools. Spacious opening doors allow crane loading of the work area. Coolant can be vacuumed, and the oil and chip sump can be equipped with a pump and filtering system. The extractable oil pan is constructed in such a way that even small amounts of oil are enough for adequate oil circulation.

Machine data summary

* High rapid speed: Up to 15.7 m/min., depending on axis (LGR-S);

* high axis-acceleration: Up to 2.0 m/[s.sup.2], depending on axis;

* maximum mold dimensions: LGT-S: 55" (1,400 mm) diameter, 15.7" (400 mm) height, 6,600 pounds (3t) weight; LGR-S: 86" (2,200 mm) diameter, 19.7" (500 mm) height, up to 11,000 pounds (5t) weight;

* maximum processing diameter: LGT-S: 1,200 mm; LGR-S: 2,200 mm;

* axes travel: LGT-S - X: 19.7" (500 mm); Z: 15.7" (400 mm); C: 360[degrees]; B: > [+ or -] 100[degrees]; LGR-S - X: 43.3" (1,100 mm); Z: 19.7 (500 mm); C: 360[degrees]; B: > [+ or -] 100[degrees];

* glass scales in the linear axes (5 [micro]m);

* powerful, water-cooled 2.6 kW spindle, for engraving and pockets;

* optional longer high frequency spindles for critical work piece contours;

* disruptive contours optimized B-axis; suitable for processing two-piece tire molds;

* constant feed rate at the tool tip;

* real surface actual values through digitizing the sidewall mold;

* twenty-four-fold active tool changer for typical 24 hour automatic operation (without reducing working area);

* tool measuring with automatic length correction and tool break control;

* intelligent tool management;

* enclosure suitable for wet processing;

* safety clutches for machine protection; and

* human-machine-interface with 12" TFT display.

The software

LTire is a universal 32 bit Windows program to produce tire sidewall molds of the highest quality. The Windows-concurring user interface guarantees easy operation, e.g., with online help, unlimited undo/redo functions and icon bars.

LTire is used for constructing the data, as well as for operating the machine. The uniform graphical user interface minimizes the costs for schooling and also allows the construction or correction of the design directly at the machine. Common CNC machines mostly offer only text editors for the handling. Here, the selection of the jobs to be engraved is very tedious and error-prone, especially if only a part of the job is to be engraved.

LTire software naturally is compatible with the company's other engraving software, used on their standard engraving machines.

To create the data for a complete typical tire sidewall mold takes about one-and-a-half hours. Afterwards, the engraving can be started immediately. No additional step of creating CAM-data for the machine (like in common CAD systems) is necessary. Besides, unlike other CAD systems, the created engraving program is not limited to one mold surface and can be used for any tire sidewall mold.

LTire-Design, the construction program

Using LTire, the complete design of the tire sidewall can be constructed very quickly and efficiently. It offers basic CAD functions, e.g.:

* Any graphical element can be created, edited, mirrored, distorted, rotated and aligned;

* multi-positioning;

* clipping;

* hatching, dotting;

* measuring functions;

* integrated calculator; and

* script language.

Additionally, LTire offers functions that can be found in other word programs:

* Usually, in other CAD systems, text data fit to one certain tire size. If the tire size changes, the text has to be manually adjusted and saved. The LTire text editor offers typical functions from a word program, e.g., several lines within text object, variable alignment (left, right, centered), fixed line length, text in arc, tilting, supporting Windows fonts.

* Free character spacing: Setting the distance, as well as using the kerning table of the font is possible.

* Personalized fonts can be created with the integrated font editor. Often, the contours of the characters are already available in another CAD system. It will only take about 30 minutes to have this new personalized font in LTire. The font editor also offers the possibility to create a kerning table. The distance between any letter combination can be set individually with the kerning table. In this way, the text gets a better optical view. Existing kerning tables from other fonts can also be used for a new or modified font.

In addition, LTire contains functions specially fitted to the construction of a tire sidewall design:

* Generally, the user interface allows one to enter the parameter values (radius, angle, etc.) that are on the drawing.

* The parameters for pockets (form, width, length, position, etc.) can be entered directly from the drawing in a special dialog.

* All elements (e.g., text graphics, pockets) are saved as separate objects with all their parameters (e.g., position, transformation mode, used tools). Changing the parameters is quickly done at any time. This way, an object can easily be newly positioned on the mold. The reference point for the position can refer to the upper edge, lower edge, left or right edge or to the center of the object.

* How each object of the sidewall should be transformed onto the tire mold is completely optional, e.g., polar, cartesian, rolled onto surface, projected. This guarantees the keeping of the given dimensions of each object.

* Data are often unsorted in other CAD systems. The start- and end-positions of alleged closed contours are not identical. LTire contains functions to sort and close contours. This is important for taking over data from other CAD systems, as well as reducing engraving time.

With simple objects, the contours directly represent the engraving path. For objects that have inside areas to be milled out, LTire automatically calculates the milling paths. This takes only a few seconds. Taking the geometry of the cutter into consideration, possible collisions are also prevented. The clearing strategy is optional, e.g., in straight lines of concentric from the inside to the outside. The automatic milling path calculation in LTire can be done in 3D, making clean corner contours possible with the use of conical cutters.

For every object, different tools can be defined, e.g., tools for rough cutting and fine cutting. The dynamic parameters of each tool can also be separately defined (e.g., feed, tool life, milling in layers).

It is possible to exchange data with other CAD systems (Catia, Mastercam, etc.), allowing the takeover of existing libraries, as well as data from special programs. This way, any free form surfaces/reliefs can be imported and engraved onto the sidewall mold. Data exchange is done using common formats (e.g., vector formats DXF, IGES, HPGL and several graphic formats like TIFF or BMP). Taking over 3D-surface models from CAD systems is not necessary. LTire gets the mold surface information either by digitizing or by loading the profile cross section line (from lathe data). The surface of each graphic element is created by automatic calculation of the cutter tracks, considering the cutter geometry.

LTire-M, the machine operating program

LTire-M as a machine operating program is based on the construction program LTire-Design, and therefore offers all its known operation functions, e.g., graphic view of cutter tracks, zoom, representation in any perspective.

Surface data acquisition of the sidewall mold is done with a probe scanner directly on the tire mold or by importing CAD data. On the whole profile curve, the surface data are interpolated using a spline function. This improves the engraving surface by reducing the surface roughness.

The tool direction (B-axis) for engraving can be varied. Engraving normal to the surface (e.g., for text), as well as engraving with a preset angle (e.g., for pockets) is possible.

The tool parameters (e.g., feed, tool life, milling in layers) that were set in the construction module can be changed quickly and easily at the machine, simply by changing entries in the library.

If necessary, other object parameters (e.g., position, changing text of serial number, activating the tool cooling) can also be altered at the machine right before milling starts. The graphical user interface allows the operator to select the single objects directly. Quickly selected on-screen single objects can be separately engraved, simplifying reworking a lot.

Multiple tool changing modes (manual, measuring point, tool changer) allow the optimal adaptation to customer requests. Using the ATC, complete automatic operation of the machine is possible with the tools automatically being changed and measured, not needing any manual intervention. The progam is also equipped with a tool breakage control.

Cutter length is determined at engraving time. Calculation of actual machine control data is carried out automatically during engraving. The time consuming manual generation of CAM data using a post-processor in the construction bureau (like in common CAD systems) is dropped. This also drastically minimizes the amount of data to be transferred to the machine.

All required mold tolerances can be achieved by setting the adequate output parameters.
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Article Details
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Comment:Tire mold makers optimize manufacturing. (Process Machinery).
Author:Engel, Gerson
Publication:Rubber World
Geographic Code:1USA
Date:Aug 1, 2002
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