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At present, several international standards are demanding identification marks on reinforcing bars. Marks have to be rolled onto one side of the bar to denote specifications like the bar size, type of steel, manufacturer’s mill designation, and minimum yield designation.
The identification marks vary for each specification. However, a combination of letters and numbers is typically required these days. This method substitutes the conventional method of identification by changing the geometry of the notches rolled on the bar. Considering the growing demand for advanced marks, rebar manufacturers are looking for equipment that can fulfill the performance and the quality required by the market at a cheap cost.
How can These be Applied?
A new marking philosophy using a machining head connected to a contemporary CNC notching machine achieves this target of a precise, cost-effective, and flexible marking tool.
Hand marking is still being used in some plants (which is typically performed using grinding wheels, and sometimes, with the help of a mechanical pantograph). If this method is excluded, the most popular marking—based on the principle of electro discharge machining—can be used to mark the roll and thus imprint the resulting rebar.
Marking Using Arc Metal Dischargers
To achieve the action of an arc metal discharger, a sequence of intermittent electric arcs is produced to break the metal into minute particles. During the time of the operation, a hollow branding electrode fixed to the machine head vibrates when a coolant is injected via the electrode to cleanse the powered metal and to cool the functioning area.
The hole created by the arc assumes the shape of the electrode shell. Thus, the accurate mark can be formed through a sequence of appropriately shaped electrodes.
Modern Arc Machines
Contemporary arc machines are equipped with programmable repeatable positioning to speed up the electrode’s location along the sequence of furrows in the roll. Since the shape and size of the burned area are same as that of the electrode shell, a certain set of electrodes should be used for each rebar size and for each number/letter.
Time to Mark Rolls
The time needed to burn in each letter of about 0.6 mm deep ranges from less than one minute in cast iron rolls to several minutes in tungsten carbide-coated rolls. The overall marking time is relatively longer than this, since many other time-demanding manual operations have to be performed. These include roll handling from marking machine to notching machine, electrode switching for each letter, and electrode centering on the furrow and positioning for each letter.
Accuracy of Arc Machines
Additionally, the technology’s characteristics limit the precision of the surface finish of the mark formed using this method. With the growing demand for quality, this can become a constraint in its application.
Electrical Discharge Machining
Sink die electrical discharge machining (EDM) is mostly used as an alternative to such electro arc disintegrators. In this case, the electrode is solid and the workpiece is soaked in a dielectric fluid.
The operating principle is more or less the same as before. During operation, the electrode is moved toward the workpiece until the space is such that the voltage in the gap can ionize the dielectric fluid and allow an electrical discharge (spark) to travel from the electrode to the workpiece.
The amount of material removed from the workpiece with each pulse is directly proportional to the energy applied. This makes it possible to acquire highly precise mark profiling if the parameters are suitably set. From the quality aspect, EDM complies with existing specifications.
Benefits of EDM Machines
Commonly, EDM machines are more advanced and accurate, and use two axes CNC to direct the head. They can also work with shaped electrodes, for example, shaped to a logo, which reduces the time needed for indenting. On the contrary, the electrode’s reference plane is on a radius that should correspond to the radius of the roll. This helps in creating more precise marks in a shorter overall time, in contrast to disintegrator machines.
One limitation in the versatility of this method is that various electrodes have to be used, based on the radius of the roll groove which differs with roll wear and product size.
Processing Tungsten Carbide
Moreover, burning marks into tungsten carbide with EDM machines and disintegrators is still an unsafe operation, as the rapid heating and cooling of the roll surface can result in micro-craters and cracks on the roll surface based on the machining settings used. Such heat-affected zones (HAZ) can cause cracks triggered by tensile stress in a fragile material, such as tungsten carbide.
Integrating Notching and Marking
The integration of the two processes of notching and marking into the same machine has been a continuous demand from roll shop people for a number of years. Recently, this demand has turned out to be more pressing because of the growing requirement in global standards for precise marking.
Problems Integrating EDM
Many issues had deferred the combination of a marking device within the roll notching machine. The original plan was to arrange an EDM in the notching machine, but this solution encountered many problems.
Primarily, the EDM process yields electrical currents that could have an impact on the electronics of the new advanced notching machines, and also cause local galvanic corrosion on the moving components. Secondly, the EDM process uses a liquid dielectric. This made it necessary to restructure the notching machine to receive and recycle this liquid.
A New Approach
A novel solution was needed to match the following specifications:
- Build a new device as simple as possible so that it can be fitted into the prevailing machines
- Improve the consistency and quality of marking
- Provide the versatility to adhere to different kinds of marks required by the market
- Decrease the time of operation and, particularly, the operator’s costs
How It Works
A prototype of that device was created and tested in the first half of 2001. The IMD is currently fitted as standard on all the latest AT820 E CNC notch milling machines, and can be altered to any AT820 already in service.
The IMD’s working principle requires machining with cutting tools rotating at a reasonable speed. This rotation is provided by an electric brushless digitally controlled motor.
The device is positioned on the notching head of the machine. The CNC control is standard for both the notching head and the IMD.
The software meant for the IMD is fully integrated into the same platform as the notching machine. A new graphics interface displays all the functions to use the notching machine and the marking head.
The IMD marking head can be easily integrated into the notching programs. The operator screen helps in choosing the following factors:
- Choice of fonts
- Depth of letters
- The angle of the letters
- Height and width of letters as a percentage of the groove dimension
- The positioning of letters (between notches or in the place of skipped notches)
- Tool parameters (tool feed, rotation speed, etc.)
The factors required for the dimension of the mark, are not absolute values but rather percentages of the dimensions of the groove. This way, the mark should be established only once for the entire size range of production. Size variation does not require a variation of mark program.
Tool positioning as well as marking operation is automatically performed towards the end of the notching cycle, without any operator intervention.
Sometimes, it is essential to notch a mark independent of the notching operation. This can occur, for example, when a logo has to be marked on already notched rolls or in a free area without any reference to the notch’s dimensions. Here, a special program offers full freedom to the operator to establish new marks. The user-friendly graphical interface makes this programming simple to handle.
The IMD was engineered for use on both tungsten carbide and cast iron rolls. It can safely be used on tungsten carbide rolls because the mechanical milling action does not impact its structure. Machining of hard carbides in this precise application requires tools with PCD inserts, while WC (K01 grade) tools are sufficient for cast iron rolls.
The same tool can create all the logos or letters required. The marking speed is related to its characteristics. The parameter stated in Table 1 is standard for cast iron and tungsten carbide.
Table 1. Typical operating speeds for different materials
|Tool Rotation (rpm)
|Feed Speed (mm/min)
The ATOMAT mark is carved into the furrow of a tungsten carbide (30% binder) roll. The time needed to create the mark was approximately 12 minutes with the letters carved 0.8 mm deep. Accuracy and consistency are ensured by the computerized control and machining technology.
One difficulty involved in any marking programming is that, all potential logos that may be demanded by a customer cannot be predicted. Essentially, the mark is typically not a basic combination of numbers and letters but comprises a company’s logo.
This problem is overcome by developing a software that can convert the ISO file (geode) into a parametric file, that can be used by the IMD interface. The logo can be created on CAD and translated by CAM into an ISO file. Subsequently, the software converts this ISO file into a parametric file that could be utilized by the interface.
In this way, as soon as the logo is made and translated, its dimensions can be established based on the specific rebar size. Thus, it is not necessary to design many logos of each size. Users have to simply input the dimensions and the software will re-scale the logo to the new dimensions automatically.