16 May 2001

Steel Times International - January 2001

A recent improvement in control on CNC machines for cutting the notches in the grooves of rolls for the production of ribbed reinforcing bar, has resulted in a 20-30% reduction in machining time and given the ability to notch harder WC grade roll material. Production of reinforcing bar has been greatly affected by significant changes that have taken place in recent years in the steel market. Unlike in the past, when steel production was generally confined to a regional market, steelmakers now have to compete on a global scale. In particular, for reinforcing bar production, this means that now several different quality Standards have to be met, while in the past, only the regional Standard was normally required. To compete in the global market, each individual reinforcing bar must be identified with the manufacturer's mark, which usually consists of a combination of skipped notches and enlarged notches on the ribbed keying surface of the deformed bar. Tough international competition also means that the quality requirement is growing, while production cost must be lowered at every step. To meet these demands, it is necessary to have the proper equipment tailored to production requirements. The ribs and notches on the surface of deformed rebar are produced by roll indention. The notching machines that cut the pattern into the grooves on the rolls which result in the ribs on the rolled bar, require the following characteristics:

1. High flexibility to meet all present and future market requirements in notch shape, notch marking and notch enlarging;
   
   
2. High accuracy to guarantee consistency and quality in the final product;
   
   
3. Maximum automation to reduce manpower costs in operation;
   
   
4. Short working time to avoid the risks of lost production time.
   
   

NEW GENERATION OF NOTCH MILLING MACHINES

In the second half of the 90s, a new generation of notch milling machines came onto the market. These machines, characterised by the use of Computerised Numeric Control, have quickly replaced the old notching machines based on the mechanical transmission of the notch pattern to a cutting head.
The benefits of the CNC machines are related to the high level of automation of the process as well as to their computerised control.
In particular:

1. In the old mechanical machines, setup fur every rib type or size was a long and demanding job requiring the changing of gears. The most advanced CNC machines are provided with easy to use and customised interfaces for a quick and safe choice of the rib type and parameters. Some interfaces are also provided with an automatic check to avoid any possible mistake by the operator.
   
   
2. CNC machines can usually store information to cut a large number of different groove patterns. This means that in large rolls containing several grooves, the machine can operate continuously using all the grooves available without the need for an operator, and even when grooves are at difference distances apart.
   
   
3. High constrains are inherent in gear controlled machines and not all the rib types are possible. For example, it was not possible to notch grooves when the number of ribs was a multiple of the number of principles. These constrains have been eliminated by the new CNC machines.
   
   
4. One of the most important advantages of the new CNC machine is that they can guarantee all the special performances requested by the new manufacturer identification code marks, such as rib skipping and notch enlarging.

The CNC notching machines usually operate in accordance with the procedure shown in. The notches are produced on the indenting roll by a rotating cutting tool: the combined rotation of the cutting tool and of the mandrel carrying the roll generates a helix of notches, the number of which, and their pitch angle, depending on the ratio between tool and mandrel rotation speed. Generally, the CNC control works on four axes: the tool rotation, the mandrel rotation and the two horizontal axes. With such a control system, the machine can automatically recognise the number of principles and, if the number of notches is divisible by them, the tool is automatically positioned one notch over for each roll rotation. Thanks to the longitudinal axes control, even a difference in the distance between each groove centreline can be managed with ease. Thus, all the possible combinations of notch enlarging and skipping for trademarks are readily achieved. In CNC machines, the tool usually has exactly the same shape as the final rib. During the working process, the tool is moved along the radial axes at a constant speed, ie with identical feed rates for each roll rotation. With such a constant feed rate, the depth of material removed is constant for each pass. One of the minus points of this way of operating is that the maximum volume of material has to be removed by the tool in the final step ie when the root of the notch is being cut, as material is removed from the full wall face of the notch at each pass of the tool. But this final step is also the step that defines the final surface finish of the notch; thus the tool has to work in the worst condition exactly when it is necessary to have the best surface quality. In operation, the surface finish requirement limits the feed rate of the tool at all passes as the feed rate has to be the same for each step, even although, in earlier passes when the surface finish is not critical, material could be removed at a faster rate. It is clear that these advanced machines have the potential to fulfill the demand of the new market, but there is still room for improvements and further optimisations. These developments can be achieved only if supported by a proper strategy based on technical exchange or, better, a real partnership between the machine manufacturer and the customer to better understand the key points in the related operations and to accept the challenges of new and sophisticate requests. It was the result of one such challenging requests – for a large diameter 80mm rebar – and close collaboration with the machine operators that ATOMAT developed a new cutting technology; the AL-CUT® SYSTEM.

NOTCHING OPTIMISATION

The new AL-CUT® sistem was developed during an upgrade of Atomat’s AT 820E CNC notch milling machine, which was upgraded to notch grooves to roll 80mm threaded, re-bar. The traditional approach to this problem would be to focus on stronger components, slower cutting speeds, etc. But in particular, this last point was analyzed in the light of feedback and technical discussions with the machine operators. In particular, when producing concentric ribs, the notching time is always very long due to the fact that the total distance to be covered by the tool is longer than in the crescent rib profile . he new AL-CUT® SYSTEM is based on two principles: Alternate cutting of one side of the notch and then the other; and progressive feed speed of the tool. In the new system, the cutting process moves the tool axially at each pass in order to have it working only on the front and on one of the two sides of the notch. The tools operate alternately on the right and left side of the notch to reduce the shearing stress and resultant vibrations. This is achieved by changing the mutual position between roll and tool at each pass under software control. In addition, the tool feed speed can be changed progressively from the start to the end of the notch cut. In this way the tool feed speed will be the most suitable for each notching step: ie a maximum when the tool starts and the cutting surface is minimum, but lower at the end when the volume of material removed is highest as the notch is enlarged and the maximum accuracy is required. This is particularly useful for concentric ribs where the working times are longer and the volume removed at the start of the operation is small. In summary, the benefits expected with the new AL-CUT SYSTEM are:

1. Lower cutting stress for the same volume removed;
   
   
2. Dramatic reduction in tool vibration;
   
   
3. Consistency in the volume of material removed;
   
   
4. Reduction of the number of passes required for the same notch geometry with consequential reduction in operating time;
   
   
5. Improvements in the final surface quality; -Longer tool life.

EXPERIMENTAL RESULTS

The AL-CUT® SYSTEM was tested both on cast iron rolls and tungsten carbide rolls On cast iron rolls, the new system gave excellent results for large diameter grooves, with time reductions of 25-30%, and produced an excellent finish to the notch surface. The same system was tested on the tungsten carbide (WC) rolls and confirmed the time reduction expected. In addition, the notching of harder grade WC was tested with unexpected performance. Generally, for re-bar production, the tungsten carbide roll grade is 70% WC, i.e. a 'soft' quality. This quality which has a hardness of around 8lHRa, must be used since notch milling machines are unable to notch harder material. This means an increase in the wear rate of the rolls in use with resulting costs for rolls and roll maintenance. With the new AL-CUT® SYSTEM, the cutting stresses are reduced thanks to the reduction in the cutting surface. Thus it becomes possible to machine harder material and so improve the roll life in the stands. The machine was tested on tungsten carbide rings grade 85% WC, the most wear resistant grade normally used in the last stand of wire rod production. With one single tool, four grooves were notched with excellent quality of finish.