April 30, 2013
A tube is a tube, as most in the industry would declare. But getting that tube from point A to point B isn't a simple proposition. That's why fabricators need to keep in mind key factors when looking to set up a material handling system for a tube cutting operation.
Fabricated tubes are processed and used for a variety of structural, plumbing, and manufacturing applications. Steel often is used for construction tubes over other metals like aluminum when extra strength is necessary. Steel tubes also are used in automotive applications and as parts of furniture or other manufactured consumer products. Aluminum and other alloys are used widely for various tube applications because of their unique properties that lend additional functionality to the equipment or process that each part is intended to be used with.
Fabricators are generally somewhat familiar with tube cutoff machines (see Figure 1). Cutoff machines are in wide use around the world and achieve their purpose: a clean cut and accurate length without any unintended crimping or deformation of the tube end. To process a tube quickly and efficiently, fabricators need to make the end cut very quickly. However, the process of getting the tube to the cutting device, accurately measuring the tube length before the cut is achieved, and cutting the tube so that the diameter, or the “roundness,” of the tube remains true can be much more difficult than it seems. Moving a tube into a tube cutoff machine or perhaps into other manufacturing processes is not as straightforward as sheet handling. Sheet metal is not likely to roll away or shift out of position.
Fabricators should keep five primary questions in mind when they are specifying and designing an optimal automatic material handling system for any tube cutoff operation.
If the fabricator is working with a single tube length, the material handling system usually is much simpler than when working with multiple tube lengths. The tube bundle, as packaged from the raw material supplier, is placed in a loading device, usually a nylon belt or sling that allows the tubes to flow freely within the cradle (see Figure 2). The bundle is raised on one side to align the tubes with an inclined magazine table, which allows one tube at a time to gravity-feed in single-row orientation into the ready position and to be fed into the cutoff apparatus.
The cutoff machine moves the tube forward until it hits the tube stop (see Figure 3), which has been preset to the desired cut length. Contact with the tube stop triggers the cutoff machine to engage the shear or saw blade, which makes the cut in the precise length the process requires.
At this stage the cut tube is conveyed out of the cut position to a collection point. It is then ready to move to the next step of the process, at which point another tube moves into the cutoff position and the process is repeated.
This type of material handling scenario is most efficient for high-volume applications in which the same length will be cut hundreds of times, and it allows for extremely high material handling speeds, increasing overall efficiency. In the meantime, many modern fabricators are embracing the trend toward lower-volume tube processing in which many different lengths are processed. This trend, often referred to as just-in-time manufacturing, requires a cutoff machine with more flexible feed options and the ability to pre-program a variety of part lengths to be cut.
Servo hitch feeds offer this flexibility for modern tube cutoff machines. Instead of relying on the tube stop to determine cut length, the machine operator can program various cut lengths before the cradle is loaded with material. When the tubes are indexed into the machine, a servo-controlled linear rail first securely clamps the tube to be cut, then moves the material forward to the precise length programmed by the operator, achieving an exact cut length.
Finished tube length should help the process designer determine what type of transfer system is needed, but the process designer also needs to consider the next steps after the tube is cut to the proper length. The secondary operations—deburring, testing, washing, packaging, or a combination of these—greatly influence what type of material transfer system is needed.
The process designer also should consider the probable length range of the processed tubes. To cut short lengths, it may be necessary to integrate a hopper feeding system that accepts randomly distributed raw lengths and reorients the unprocessed tubes into a predetermined pattern for feeding into the cutoff machine. Short lengths can be efficiently transferred by a stair-step-style elevating system that indexes parts one flight at a time, up and into a magazine loading rack. Long tube lengths are controlled easily through a simple drop box system (see Figure 4) or a conveyor kick-off device that converts the tubes from an end-to-end orientation to a side-by-side orientation. The tubes then can be indexed one at a time into the next operation.
Any material handling system needs to be designed to withstand the maximum probable stress. If the system is intended to process small-diameter or light material such as aluminum tubes, weight should not be of too much concern. These materials are typically light enough that off-the-shelf aluminum frame conveyors can be easily integrated into the system design.
Larger-diameter and heavy-wall tube, because of the increased weight, presents much more of a challenge to the system designer. It is important that any system be built to withstand the potential day-to-day abuse that is inherent when handling larger and heavier material. These more rigorous systems require custom design and fabrication work; off-the-shelf systems won’t stand up to the larger, heavier materials.
Most simple processes, those with the least number of steps, can rely on gravity as an efficient means of moving work from one station to the next. The best option may be to simply move the tube in-process to a chute and let it drop into place for the next step.
For applications in which multiple stations are required and a greater degree of control is needed, gravity-fed systems may not be the best solution. For more complex processes, a transfer system such as a walking beam or pick-and-place gripper system is often the best option.
A gripper system is often the best choice when shorter-length material is to be processed. The overhead gripper picks each individual piece of work-in-progress and moves it to the next station. The walking beam method relies on an integrated cam system to lift and move the in-process tubes from one station to the next. A typical process for which the walking beam system (see Figure 5) is a good choice is a double-end chamfering operation with length checking.
This process starts with a tube already cut to the desired length in the cutoff station, then feeds into a station where both ends of the tube are machined. The walking beam rotates and retrieves the newly machined tube and moves it to the next station where it is fed into a measurement unit to verify the length of the tube. The tube is then moved, again by the walking beam, to a third station where a blower removes the chips and debris left in the tube interior by the machining process. The process is repeated for each tube as it is deposited into a collection area to await the next step of the manufacturing process.
Gravity is the most typical and usually the most efficient conveyance method for finished tubes. This method works best when large numbers of the same length of material are processed. The material typically rolls down a short incline into a collection rack or bin and is ready to be conveyed to the next step in the manufacturing process.
If the manufacturer plans to cut multiple lengths within the same setup, it may want to invest in equipment capable of handling the varied lengths. By relying on the data used to program the variable-length tube cuts made in the first step of the process, the operator can program the collection system to trigger exit locations from the conveyor collection system into individual collection points for each tube length. When those containers are filled, they are moved to specific production lines, and a new collection bin continues to collect the specified length of finished tubes.
An important consideration is the potential for damage to processed tubes at the collection point. If the tubes are collected in a parallel manner (side-by-side), they typically “cushion” themselves as they are dropped from the collection incline into the collection bin. The potential for damage is greatest when a tube is dropped on its end; this often distorts the end of the tube and potentially mars the exterior.
If the tube to be processed requires a surface-sensitive finish, nylon gripping surfaces are typically used to eliminate the potential for marring the material during transfer between stations.
To avoid possible damage that can occur with gravity-fed collection and transfer systems, fabricators can use magnets or vacuum cups to transfer materials.
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