7 October 2003

P.Comand; F.Martecchini - ATOMAT Group

Published on Iron & Steelmaker - may 2002

ABSTRACT

Identification marks for reinforcing bars are today required by several international standards. Marks must be rolled on one side of the bar to denote the Producer's mill designation, bar size, type of steel and minimum yield designation, etc. Those marks must then be indented on the notched master rolls. Up today several techniques have been utilized for this operation. This paper will briefly report an overview on these techniques and will introduce the new Integrated Marking Device developed by ATOMAT Group for its AT820 E CNC notch milling machine. Benefits of this marking system, specifically designed to meet the demand of flexibility, high quality and low cost will be analyzed. In particular the paper will report performance data and examples of this new marking technology both on Cast Iron and Tungsten Carbide rolls.

0. INTRODUCTION

In the past Identification marks for rebar were a prerogative of some countries where standardized norms required a clear identification of the producer and/or of the quality of the product. Today, with the exception of marginal markets, the identification mark is required for the large majority of worldwide rebar production. Also, more steel plants are oriented for export production and are then required to meet different standards. Identification marks are different for each norm, but the requirement of letter and number combination is growing quickly. This is also true in areas where the mark was traditionally provided by modifying the notching geometry. In Europe, where according to the German Institute for Standardization (DIN), the mark is a combination of skipped and enlarged ribs, marks with letters or symbols are becoming popular. Considering the growing demand of sophisticated marks, rebar producers are looking for equipment able to reach performance and the quality requested by the market at minimal cost to maintain competitiveness. A new marking philosophy applied to a modern CNC notching machine seems to reach the goal of a low cost, accurate and flexible marking tool.

1. OVERVIEW OF MARKING TECHNOLOGIES

Today, hand marking is still used in some plants, carried out by using grinding wheels on turbines with the help of mechanical pantographs. However, the most popular marking equipment is based on the principle of electro-discharge machining. In reality, the majority of the machines are electro-arc metal disintegrators that run as marking machines. Many years ago, electric metal disintegrators were developed to remove broken drills, taps and punches to salvage expensive components. An arc metal disintegrator works by creating a sequence of intermittent electric arcs to break the metal into minute particles. The hollow branding electrode, fixed at the head of the machine, vibrates during operation, while a coolant is injected through the electrode for cleaning the powdered metal and to cool the working area. The hole created by the arc assumes the same shape as the electrode shell. Through a proper combination of electrodes, a mark can then be produced.
Modern disintegrators are equipped with programmable repeatable positioning to speed positioning of the electrodes into the grooves along the sequence of grooves in the roll. Since size and shape of the burned area is the same as that of the electrode shell, for each rebar size and for each letter/number, a specific set of electrodes is necessary. Burning time for one letter approximately 0.6 mm deep ranges from less than one minute in cast iron, up to approximately two minutes in tungsten carbide. In the overall marking time, several other time-consuming manual operations must be considered:

1. Roll handling from notching to marking machine
   
   
2. Electrode centering on the groove and positioning for each letter
   
   
3. Electrode change for each letter
   

In addition, accuracy of the mark surface generated by an electric disintegrator is limited by the characteristics of this technology. With the growing demand of quality, this can limit the application.

Together with electro-arc metal disintegrators, also sink die electrical discharge machining (EDM) devices are frequently used. In this case, the electrode is solid and works sunken in a dielectric fluid.

The operating principle is almost the same. During operation, the electrode is moved toward the work piece until the space between them is such that the voltage in the gap can ionize the dielectric fluid and permit an electrical discharge (spark) to pass from the electrode to the work piece. The amount of material removed from the work piece with each pulse is directly proportional to the energy it contains. An accurate mark profile can be realized with this machining if proper parameters are used. Because of this, the application seems to more closely meet the quality demand of today's market.

In general, EMD machines are more accurate and sophisticated (2 axis CNC) and can work with an electrode shaped with the full mark logo to reduce operating time. These solid electrodes have the shaped of the full mark, and their reference plane is on a radius that must be equal to the radius of the roll. In this way, indenting more accurate marks in a shorter time is possible. A limit of the flexibility of this method is that different electrodes must be used depending on roll wear (and then on its outside diameter). In fact, to fit the groove, the electrode must have the same radius. This changes with the roll wear.

Today, burning tungsten carbide with disintegrators and EDM machines is still a risky operation because of rapid heating and cooling of the roll electrode surface. Depending on machining conditions used, cracks and micro-craters can appear on the surface layer. Such heat-affected zones (HAZ) cause cracks of the tensile stress in a brittle material such as the tungsten carbide.

2. THE NEW IMD

In 2000, Atomat Group developed a new approach to roll marking. Integration of the two operations (notching and marking) in the same machine has been requested by roll shop people for many years. Recently, this demand has become more urgent due to the growing requirement of accurate marks in international standards. Several difficulties have postponed the integration of a marking device in the notching machine.

The original approach was to install the EDM device in the notching machine, but several obstacles were encountered. First, the EDM process generates electrical currents that could affect the electronic of the new sophisticated notching machines, as well as cause local galvanic corrosion on moving components of the machines. Second, the EDM process requires the use of a liquid media.

This implies the redesign of part of the machines to collect and recycle this media. A new approach to the problem was then investigated with the following goals:

1. Make the new device as easy as possible; provide the opportunity for installation even in machines previously manufactured and already in operation
   
   
2. Improve quality and consistency of the mark
   
   
3. Provide flexibility to meet different mark types requested by the market
   
   
4. Minimize time required by the operation, in particular, the operator's costs
   

The prototype of such a device was realized and tested in early 2001. Now the IMD is regularly installed in all new AT820 E CNC notch milling machines. For other AT820 machines already operating, the IMD is installed upon request. The IMD works by machining with special tools rotating at medium speed. The rotation is provided by a electrical brushless digitally controlled Siemens motor.

The device is installed on the notching head of the AT820 E CNC machine. The CNC control used is the same for both notching machine and IMD. The software for IMD is fully integrated on the same platform of the At820 E notching machine. The new visual interface for AT820 E machine includes all functions for easy and flexible use of the marking device.

2.1 EASY AND FLEXIBLE PROGRAMMING

With the new IMD, the mark can be easily integrated in the notching programs. In particular, the operator will fix the letter parameters:

1. Positioning of letters: between notches or in place of skipped notches
   
   
2. Choice of fonts
   
   
3. Depth of letters
   
   
4. Height and width of letters as percentage of the groove dimension
   
   
5. Angle of the letters
   
   
6. Tool parameters ( tool feed and rotation speed)
   

Parameters requested for the mark dimension are not absolute values. Instead, they are a percentage of the groove dimensions. In this way, the mark is defined only one time and is valid for the whole size range of production. Size change does not require changing the mark program .

Tool positioning and marking operations are performed automatically at the end of the notching cycle without intervention of the operator.

Sometimes, an indented mark that is independent of the notching operation can be requested. For example, this can happen when a logo must be marked on rolls already notched or in a free area without any link to the notch dimensions. In this case, a special program enables the operator to define new marks. Also, the user-friendly interface makes this programming simple.

2.2 IMD TOOLING

The IMD was studied to be used in both tungsten carbide and cast iron rolls. The new IMD can be safely used in marking tungsten carbide rolls since the mechanical milling action does not affect the tungsten carbide structure. The machining of hard carbides in this accurate application requires specific tools with PCD inserts. For cast iron tungsten carbide K01 grade tools are used.

With this system, the same tool can perform all required letters or the logo. Marking speed is related to mark characteristics. In general, the parameters in Table 1 can be considered standard for cast iron and tungsten carbide.

The mark in was performed on tungsten carbide 30 percent binder roll, 16mm re-bar . Total marking time was approximately 12 minutes with 0.8 mm letter depth. Accuracy and consistency of the mark is assured by machining technology and computerized control.

2.3 SPECIAL AND COMPOSITE LOGOS

One problem related to marking programming is that it is impossible to foresee all possible logos requested by a customer. In fact, sometimes the mark is not a simple combination of letters and numbers or the company's logo.

In this case, this problem can be solved using software that can translate the ISO file (g-code) into a parametric file that can be used by the IMD interface. In other words, the procedure is as follows.

The logo can be designed on computer aided design (CAD) and translated by computer-aided manufacturing (CAM) into an ISO file. A special software converts this ISO file in a parametric file that is usable by the interface. Once the logo is designed and translated, the logo dimensions can be defined according to the specific rebar size. Design several logos (one for each size) is not necessary. Instead, the dimensions can be input, and the software will automatically redesign it with the new dimensions.

3. CONCLUSIONS

Marking is an important characteristic when evaluating overall quality of rebar. Producers are looking for a marking technology that can assure the following characteristics:

1. Accuracy
   
   
2. Consistency
   
   
3. Flexibility
   
   
4. Low operating costs

Developed with the goal to overcome the constrains of traditional marking methods, the new IMD for AT820 E CNC notch milling machine seems able to provide a solution to these new demands.