Magnets used for handling plates / cut-outs in a CNC plasma / oxyflame cutting machine

The fabrication industry is using extensively plasma/ CNC (oxy flame) profile cutting of steel plates. Generally plates of size 6,300 x 1,500 mm to 12,500 x 3,500 mm of varying thickness are loaded onto these plasma/ oxy flame cutting machines. The machine is fed with a program and the cutting commences.

These machines are pretty fast and cutting for a standard profile for a 6,300 x 2,500 mm plate would take anywhere between 2 to 6 hours depending on the size of profile being cut and thickness.

The unproductive time consumed is to load a full plate and remove the cut-outs when the machine either sits idle, or one has to order for bigger beds to load multiple plates thus partially reducing the downtime. The challenge is to load a full plate and unload the cut-outs in quick time to get maximum productivity.

The plates generally available in bundles are stacked near the machine. To load a single plate onto the machine, it takes a minimum of 2-4 helpers along with crane operator and about 20 minutes to load the plate. The plate is to be shifted a little and lifted using a side clamp or a shovel and a wooden dunnage is inserted in between the top plate and the next plate. Then a chain is passed in between the two plates. Similar process is repeated in the other side. Once we have chain from both the sides of the plate, they are hooked to the crane and a plate is lifted. To speed up the process sometimes plate clamps are also used. This process has inherent problems. Firstly adding wooden dunnage takes a lot of time and space. Since the plate is to be physically moved it requires space around the stack, time and certain amount of skill. Unskilled workers may cause accidents.

Also, as the wooden dunnage is added in between plates, it makes the plate to bend. The thinner the plate, the bendiness is more and at times these bend become permanent. Alternate methods have to be adopted to straighten out the bend after cutting.

Thirdly lifting with chains etc. adds a lot of stress to the plate and at times even damage them.

An electro permanent magnetic plate handling system can be used to load full length plates on to the machine. The beam/ magnets are designed according to the plate size. The number of magnet/ distance between magnet and magnet capacity depends on the load. As the thickness of the plate decreases, the number of magnets increases and the distance between the magnets also reduces. Since thin plates have a tendency of bending/ bowing down, the magnets have to the placed accordingly.

  • Size of plates to be handled i.e. length and width.
  • Thickness of plates. Thinner plates require more number of magnets as distance between them has to be reduced to ensure safe lifting. Thinner plates sag more! The magnets have a number of safety features making the equipment fool proof for handling of load.
    • The lifted plate will not drop even during power failure.
      The magnet design is electro permanent is nature. If houses 2 sets of permanent magnets, called reversible magnets (which are switched electrically and undergo a change in magnetic polarity every time a lift operation is commenced) and non-reversible magnet (which maintain their magnetic polarity throughout the lift of the magnet). When the two permanent magnets (NRM and RM) are opposing each other, there is an internal magnetic short circuit. When these magnets are aligned in the same direction, the magnetic power is thrown out of the body and any ferrous object near the vicinity of the magnet would be held on to it. Electricity is required only during switching ON/ OFF the magnet. Once switched, because of the permanent nature of the magnet inside, no external electrical force is required to keep the job clamped. This ensures that the lifted job will not fall even during power failure.
    • Ensure that the magnet lifts only one plate.
      Magnet selection is done to lift thickest plates. However while handling thin plates (say 5-8mm), more than one plate may get lifted. But for cutting, we need only one plate at a time. The second/third plate, if lifted, be dropped using an inching button provided which reduces the power gradually. Press the inching button till the second/third plates are dropped and only one plate is held by the magnet. This is a unique design of our systems.
    • 30% more magnetic power given in transportation stage.
      The magnetisation process is divided into two parts. In the first pick up stage, magnetic power can be varied depending on the thickness of the plate. A minimum of 25% to a maximum of 70% magnetic power is given during first magnetisation process. Once done, we check and insure that only one plate is lifted. If more than one plate is lifted, the inching process is done (as explained above) to make sure that the only one plate is lifted. Once it is ascertained that only one plate is lifted, the second magnetisation process is initiated. During second magnetisation, irrespective of the selection, 100% magnetic power is given. Hence the magnet power is increased by a minimum of 30% prior to transportation stage. Once done, we are certain that the clamping power is to its maximum level and the lifted job can be transported.
    • Accidental Demagnetisation Prevention Mechanism (ADPREM).
      This is a safety devise added to the beam. It ensures that demagnetisation is disabled while the load is in air. Even if the demagnetisation button is pressed, the load will not drop till the beam is rested on a surface and the chain is slackened.
    • No wooden dunnage is required between plates.
      Since the magnets are in built with power selection and inching facility, no wooden dunnage is required. Plates of similar thicknesses can be stacked one on top of the other. This prevents any bending of plates and also reduces cycle time.
    • 97% electrical power saving when compares to similar electromagnet.
      Due to the electro permanent nature of the EPM, when compared to a conventional electromagnet, there is a 97% saving in electrical power.

Once the plate is loaded on to the cutting machine, the program cuts the plates. The cut plates can be of various shapes and sizes. Since the above mentioned magnetic plate handling system was designed for loading of full plates only, one cannot use the same for unload of cut-outs.

Generally the cut-outs are handled using conventional methods or using smaller magnets.

We manufacture e-lift, permanent magnetic lifter which is lever operated. Generally the capacity used for handling of cut-outs is 300 kg or 500 kg versions because they are very light weight and handy. One can easily operate the same on cut-out to be lifted.

The only down side is that these magnets are rated for a maximum temperature of 80°C. Hence, after cutting, the plates have to be allowed to be cooled down before the magnet could be used. If the e-lift is placed on a hot object whose temperature is >80°C, it will cause permanent damage the magnets from inside.

To cater to hot handling, we also supply “HOT E-LIFT” permanent magnetic lifter. HOT E-LIFT has capability handle cut-outs up to 200°C. The cut-outs can be lifted sooner as they need not cool down very much. These are useful at places where time is limited and fast cleaning of table is required.

With use of small magnets, we still have a problem of unloading one piece at a time. This is ok if the cut pieces are large and in 10-15 lifts the machine table is cleaned up.

If the cut items are small, say an average of 300 x 300 mm, it takes a lot of time to clean the complete table, more so with size and geometry becoming very complex. Also, since the cutting operations have become faster with modernised machines, there are times, when the cutting is completed. The machine waits for the table to be cleaned for next plate to be loaded for cutting. Since the cost of running the machines are pretty high, this downtime increases the cost of cutting.

For fast cleaning of cut-out and skeleton we have developed the SARDA CUT-OUT HANDLING SYSTEM. The system is designed according to the customer requirement depending on the minimum job size which the customer wants to lift. The system constitutes of multiple magnets on a spreader beam placed strategically such that the smallest job size would be lifted. Since the number of magnets add a considerable weight to the beam, it is recommended that the equipment should be designed taking into account the crane capacity. For one of our customer we designed a telescopic cut-out handling system. The customer wanted to load a 16 meter long plate onto the machine. The minimum cut-out size was 300 x 300 mm. The maximum plate weight was 18,000 kg. The crane capacity was 25 Ton.

To keep the total weight of beam and magnet under 7T, we proposed a telescopic cut-out handling system. Using the equipment a full plate of 16,000 x 2,500 mm approx. 18,000 kg maximum could be loaded onto the machine.

Once the cutting is completed, cut-out in the range of 6,000 x 2,500 mm can be unloaded in one lift. The beam can be compressed prior to the lift so that it takes less space. Hence in 3 lifts, the complete table can be cleaned and the next plate could be loaded.

Thus use of the SARDA CUT-OUT HANDLING SYSTEM enabled our customer to cut 15 – 25% more steel per day.

The use of SARDA CUT-OUT HANDLING SYSTEM not only helps cut more steel, it also saves time, reduces cost of cutting, reduces accidents and saves space. LOTS OF SAVINGS!.

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