TPT January 2008

The secret here is to eliminate all those activities that do not add value. One example is stopping the mill to cut the strip when using an accumulator for the next roll change. The mill has to stop, the strip cut, the mill restarted and the strip run out of the mill. This consumes a considerable amount of time and produces a lot of scrap. Adding a flying strip shear to the start of the mill can solve this problem. When the end of the coil approaches the entry of the mill, the flying strip shear is activated to cut the strip and ensure the continuation of the mill. The welder is switched off when the strip reaches the weld point and the tube is then run out to the end of the sizing section. The mill has not stopped and the only scrap is the last one or two metres of tube. This ensures a saving in both time and scrap. In addition to removing all the non value added work, what remains needs to be reviewed and where possible converted to outside work. That is, work that can be carried out while the mill is operating. The classic here is the roll change itself. This is traditionally where two sets of stands exist and the tooling is changed in one set while the other set is in the mill and making tube. There are many methods of achieving this. 1. The first is not to change the tooling but use flexible tooling. Examples of this are: 2. Cassette tooling where the tooling is changed over in cassettes but the stands remain. This is applicable to large pipe mills where changing over the stands is impractical or excessively expensive. 3. Sub base system where the mills stands are mounted on sub bases and changed over using cranes or specially designed changeover cars. • In the traditional method both the front and back stands are on the sub base. • In the H set design the back stand stays on the mill and only the front stand and the tooling is on the sub base. This reduces cost and the number of components. 4. Turret mill design. This concept has the mill stands mounted on a four sided turret with stands mounted on two or four of the • Cage forming in the forming section of the tube mill • Rotary sizing in the sizing section (limited flexibility)

› Figure 2 : Various cutoff machine types

The issue of getting tube or pipe away from the mill is complicated by the need to decide if post processing is to be carried out in line or as a batch process off line. When the tube comes off the mill each individual tube is under control. If any post tube mill processing is required then now is the ideal time to do it. If the tube is bundled and then post processing carried out, the space requirements increase dramatically and the stock levels also increase rapidly. Space and stock require a lot of money for no return. It is better to spend the money on processing the tube as it is made. It is also important to note that the selection of the cutoff process can have a large bearing on the post processing operations. If clean cut ends are required then the milling type cutoff or the cutoff saw may be the best option. If the line is a very high speed then the nick and shear process may be required. If minimum distortion is required for end-facing and chamfering operations or no cutting swarf is a necessity then maybe a rotary disc cutoff is required. There are many options available – some off-the-shelf and some that have to be engineered. In terms of post processing, the major pieces of equipment that are now available are internal swarf removal, end-facing and chamfering, hydrostatic pressure testing, packing and stacking and other processes. In some cases, the mill output speeds may exceed the throughput speeds of some of the post processing equipment. Decisions will have to be made on the most efficient method of processing. Downtime at the mill When it comes to the tube mill the major reason for the mill to stop should be roll changes and job changes. From a financial point of view there is a balancing act required between holding less stock and minimising the mill downtime and scrap due to roll changes. From an engineering and management point of view the aim should be to reduce the rolling cycle – this is the time between rollings of any given size with a parallel reduction of the time lost and scrap produced due to roll changes and job changes. A highly trained and motivated work force can achieve amazing times on a full roll change on a very basic mill; however, this is rarely consistently achievable.

fi Figure 3 : Examples of quick change tooling

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J anuary 2008

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