TPT January 2008

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133 January 2008 ›
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.
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:
• Cage forming in the forming section of the tube mill
• Rotary sizing in the sizing section (limited flexibility)
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
› Figure 2: Various cutoff machine types
fi Figure 3: Examples of quick change tooling