The process of selecting a mechanized plasma cutting system (PAC) is more complicated than it was 15-20 years ago, when these equipment offered little variety and were extremely expensive.
The offer began to expand in this new 21st century, when manufacturers introduced cnc plasma cutting low-cost devices that could run on a personal computer. Later, hardware and software developers made enormous contributions in the form of new, affordable, and easy-to-use products.
Today, the potential buyer is faced with an increasingly wide range of models and prices on the market. To this end, knowing in greater depth the different mechanized plasma cutting tables available, particularly those that include cnc control, will simplify the arduous task of making the most appropriate decision.
In a previous article we started with the first step of selecting a PAC table, that is, the details to take into account when purchasing a plasma cutting system. Now we are going to continue with the analysis of the mechanical system that makes up the equipment, providing a series of elements that we must especially consider.
The mechanical system of a team of mechanized plasma cutting includes three main components:
- The cutting table
- The gantry or gantry optionally provided with a torch height control mechanism
- The broadcast
None of these components are easily upgradeable, so it’s important to choose them wisely.
Let’s go by part stopping briefly with each one.
The precision and speed of our cutting operation are based on the robustness of the equipment used. Cheap and flimsy tools compromise both accuracy and production. Therefore, the table must be resistant and heavy, even when we do not plan to cut thick plates. The robustness characteristics do not concern only the support base, but also the inertial base that will absorb and attenuate vibrations. Therefore, the table we choose:
- It has to rigidly support the weight of the material to be cut, as well as the cutting equipment.
- It should not flex or lose accuracy during normal operation.
- At the end of a rapid movement of the gantry it must not shake or move from its place.
Cutting tables are typically offered in two styles: either the gantry is fixed and the table moves below it, or the table is fixed and the gantry moves above it. There is a great debate between the manufacturers about which of the two models is more rigid, and this depends, in large part, on their sales volume. From a practical point of view, however, the decision comes down to which element is heavier and more difficult to move: the gantry or the table, although the most common systems, at least for plasma cutting, are those of fixed table and mobile gantry. In turn, tables that offer fully welded and compact gantries with the cutting table will provide high cutting speed without vibration.
After considering the style, we must examine the table construction material. Options are generally steel or aluminum. Steel is stronger than aluminum, so steel tables require less material and will be lighter, but will have the appearance of brittleness. Aluminum tables look larger, but give the appearance of strength. Whether they are made of one material or another, the tables will have more than enough resistance, even for large-scale jobs.
In order to speed up the handling of materials, the table must also be easy to load and unload. The gantry must be parked outside the cutting surface, in order to allow enough space for the entrance of a “clark” or forklift destined to transport the thick plates.
On the other hand, the table must be able to handle a full plate without any obstructions and, preferably, its grids must be at the same height as the frame or even slightly higher, which facilitates the unloading of heavy plates.
Porch or “gantry”
The portico is that oversized heavy bar that holds the cutting tool and moves along and up the table. We see an example of portico not welded to the table in the figure below.
Since its accuracy also depends on force and weight, the gantry must not be so brittle as to bend or twist under shear pressure. Therefore, to maintain accuracy, it must be designed to be rigid and minimize torque. Its size must also be evaluated, since if it is too heavy it will make it difficult to control the torch when working at high speed and precision. A lightweight gantry, on the other hand, can produce surprisingly accurate results at high speeds.
On most equipment mechanized plasma cutting, the X axis and the Axis y define the plane of the cutting table. Some manufacturers use the longest dimension of the table as the X axis and the shortest dimension as the Y axis, although this can vary depending on the orientation of the table. For his part, the Z axis it always represents the ascending and descending movement of the torch, as we see in the figure below.
The frame moves in each of these three dimensions on a structure of linear guides. The accuracy of the gantry depends on the straightness and rigidity of these guides. It is difficult to recommend a specific type of guide, but when we think about purchasing them we should ask ourselves the following questions:
- What is the expected life of the bearings and guides?
- How are they replaced?
- How much does it cost to replace them?
- Are they protected from damage?
- What maintenance do they require?
With regard to these last two questions, it is necessary to point out that the plasma cutting It generates a very fine metal powder that is extremely harmful to mechanical and electronic components. Not all linear guides are suitable for working in these conditions, so it is advisable to look for guides capable of withstanding this working environment, that is, that are well sealed against dust.
Sealed guides are better protected from dust and slag, although they do not remove them. Eventually the dust will mix with the oil and create a gritty sludge that will ruin the parts. If we opt for sealed guides, we must bear in mind that, when the time comes, we will have to replace them completely.
Unsealed guides, on the other hand, are much simpler. For example, an unsealed guideway bearing can be completely cleaned in a very short time and if it needs to be replaced, there is no need to purchase the entire guideway. However, unsealed guides are directly exposed to cutting sparks, as well as the effects of dust and slag.
Torch height control
The rigidity that must characterize a good frame also responds to the fact that the frame is also responsible for the movement on the Z axis, that is, the ascending and descending movement of the torch that, at the time, represents the torch height.
In applications of small-medium size it is common to observe that the plasma cutting operator Manually directs torch up and down during cut. However, this is not currently a good practice on an industrial scale and/or if a high quality of cut is desired.
The advances in plasma cutting technique have shown that the distance between the torch and the workpiece is a critical element of the process, as it not only improves the angularity and speed of the cut, but also reduces dross and extends the life of the consumables. This observation has given rise to a whole technological development for the torch height control (THC, for its acronym in English) during the last 15-20 years, to the point that in the current market there are no industrial grade machines that do not have THC control.
Although a variety of THC controls are available, each with different functions and features, they all use the same basic elements to control the position of the torch relative to the material to be cut, and are basically of two types:
- automatic THC: The THC system detects the workpiece and maintains a set distance between the torch and the workpiece.
- programmable THC: CNC control adjusts different heights for drilling and cutting.
Most THC-providing systems employ the arc voltage to control the physical distance between the torch and the workpiece during cutting. The arc voltage gives an accurate indication of the length of the arc, that is, the distance between the torch and the work piece. In this way, the systems maintain a constant distance between the torch and the plate by reading the voltage delivered by the plasma and keeping it constant in its trajectory.
Some companies, as we see in the figure above, use the THC system either in a stand-alone configuration or with an integrated CNC, which continuously samples the arc voltage and automatically adjusts it, without operator intervention, to obtain the correct torch height. during the life of the consumables. This translates into optimal cut quality, better productivity and lower operating costs in plasma cutting applications, there are fully automated technologies designed for carbon steel sheet, which produce significantly better hole quality than previously achieved in plasma cutting.
An important addition to THC system It is an anti-collision device, composed of a mobile part that holds the torch and a fixed part provided with sensors to ensure that the part that holds the torch is seated correctly. If for some reason there is a collision between pieces that have already been cut, for example, this device sends a signal to the CNC to stop the process and the movements.
It is clear that although not essential, the THC control It is highly recommended for our mechanized plasma cutting equipment, especially in medium-large scale workshops. If we decide to acquire it, and regardless of the type we choose, we must ensure that it has the following characteristics:
- Precise detection of the workpiece surface before each cutting cycle. This must be done at high speed to minimize cut-to-cut cycle times and should work accurately on both thin and thick plates.
- Ability to quickly travel to recommended pierce height and then start torch motion.
- Quick travel to recommended cutting height after drilling is complete.
- Activation of the AVC (arc voltage control) as soon as the machine reaches the programmed cutting speed.
- Ability to recognize the proximity of the cutting groove in order to minimize sinking.
- Ability to communicate with the CNC/part program to eliminate sag on curves or slowdown on intricate parts.
The movements of the mechanical system of a mechanized plasma cutting equipment are governed by motors, which also present their diversity and design. This set makes up the transmission of a team, where terms such as rack and pinion drive system, stepper motors Y servo motors, which allow the gantry to move on the linear guides, directing all movements on the table.
Given the extension and importance of the elements that make up the transmission system of a mechanized plasma cutting equipment, we will return to this point in more detail in a future article.