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1. Piercing 

Piercing is one of the main factors that affects the lifetime of a CNC oxy-fuel nozzle. There are two main methods of piercing, one is where the torch remains stationary but travels vertically up and down during the piercing process in combination with slow combusting gases (Natural gas, propane, etc.), this method is used predominantly on thinner sections of material (less than 15mm). The second method of piercing is for thicker materials, this involves the "creeping or flying" pierce, this is where the torch travels from the lead-in point towards the pierce point, during this action a trough is created where molten metal runs away from the pierce point whilst the pierce deepens. This method prevents consumables from melt coming from the piercing area that could damage to the nozzle. Review of the system manufacturers cut charts and CNC programming will indicate the relevant method and settings according to material thickness.

Torch speed during the piercing process
When the piercing process has achieved through thickness, the torch still continues with 100% cut speed along with lowering from piercing height distance to cut height (in combination with acetylene it stays in the same position)

Too low speed may lead to the nozzle contacting molten material from the piercing process, this can damage the nozzle and cause backfires or a flashback.

Thicker materials (above 100mm / 4`) – when cutting thicker materials, it may be necessary to increase the torch travel speed by up to 120% to avoid the issues mentioned above.

Torch height (nozzle distance from a workpiece)
During the piercing process, the torch has three positional heights, preheat, pierce, and cut, these heights govern the nozzle to workpiece distance through the three phases. Actual nozzle to workpiece heights can vary and are dependent upon the fuel gas used.

Fuel gas Acetylene combined with (99.5%) oxygen produces the highest flame temperature of around 3160°C, the hotter flame is more rapid in preheating materials. When the pierce has been established and the torch is travelling, the nozzle to workpiece height needs to remain constant, any increase in height will result in a temperature drop, this in turn requires a cutting speed reduction to compensate for heat loss.

Fuel gases: As mentioned above, Acetylene combined with (99.5%) oxygen produces the highest flame temperature, but as the oxy- fuel cutting process is not limited to the use of acetylene alone.

Other fuel gases are available with lower heat potential. For example, when combined with oxygen, the temperature can be: Propane 2828°C, MAPP 2896°C, Propylene 2976°C and natural gas 2770°C. Typically when using gases with a lower temperature of the primar flame, the torch height increases as soon as the cutting oxygen valve opens, this protects the nozzle from the build-up of slag during the piercing process. 

2. Preheat Period

 In the oxy-fuel cutting process prior to piercing, the material requires preheat, the preheating time depends upon material thickness, chemical composition (carbon content), ambient temperature and surface condition, material with a heavy mill scale deposit will require a longer preheating time to release the scale from the surface. Preheat temperatures are around 1200°C.

An indication of when the appropriate preheat temperature has been reached is the release of small sparks from the surface of the material, inadequate or excessive preheat is detrimental to the piercing process.

Too low preheat temperature leads to an incomplete pierce of the material. As the oxygen cutting stream flows into the preheated area, it will only partially penetrate the material thickness, this results in a "shower" of molten slag being thrown out of the pierce point, often this affects the nozzle, torch, hoses and machine, severe damage can be caused and can also lead to personal injury.

Too high preheat temperature causes the material to melt, when the oxygen cutting stream flows into the molten pool there is a violent reaction that sprays molten slag in all directions, a deep wide crater is formed causing damage to the material. The piercing process must be stopped to prevent affects mentioned above. 

3. Gas Delivery

 The amount of gas used in the oxy-fuel cutting process depends on the cutting speed – higher cutting speed, the smaller consumption of the gas (per one meter of the cut).

The lower flame temperature fuel gases do have their advantages, they are generally lower in cost and actual cutting speeds can be similar to acetylene, due to the flame shape, The flame of this type of fuel gas is longer. This is advantageous when cutting thicker materials.

Material thickness
Cutting thicker material demands a greater supply of oxygen, the consumption rate can be quite astronomic (up to 30m3 for 1 torch) this is mainly due to larger nozzle orifice and an especially important factor, oxygen to fuel gas ratio. The ratio of oxygen to acetylene is +/- 1.2:1 whereas propane is 4.3:1.

Where there is high gas consumption, the standard gas delivery through single cylinders will not suffice, the use of manifolds or cylinder banks (10 or more cylinders in a steel cage and all linked together) or more appropriately bulk storage vessels, the bulk storage option requires a considerable investment, installation is normally through the gas provider, regular inspection of the vessels is performed by the gas provider.

The stability of gas flow and pressure is crucial to the performance of an oxy-fuel system. The most common problem that is encountered with a cutting system is Gas Leakages, sometimes this can be difficult to establish as the cause of poor cut quality and high gas consumption. Low-pressure leaks in a torch, hoses or regulators can affect the thermal value of the flame, whereas a high-pressure leak can cause major problems in maintaining a cutting flame due to a venturi effect that draws air into the system. In both cases there is a risk of fire or explosion. Pressure drops and fluctuation in gas delivery to the torch can be caused by sticking or fluttering system actuator valves or defective gas pressure regulators that have a weakened diaphragm spring or perforated diaphragm, in most instances replacement is the best option.

It is essential that regular checks are performed to ensure the systems pressure gauges are functioning correctly, checks for leaks, broken lens, sticking indicator needle or damaged body, the accuracy of the gauge reading affects the thermal value of the flame.

Manufacturers cut charts contain gas flow values relating to a specific nozzle size and material thickness, but as a material composition changes from 0.20% carbon content to 0.25% carbon content, flame adjustment may be required to achieve the optimum cut quality, cut charts are a guide, the values are not binding. 

There is no substitute for operator experience, reliance upon the accuracy of pressure gauges alone will not guarantee that the flame exiting the nozzle is suitable for the purpose intended, correct flame adjustment draws on the operator's ability to identify by sound and vision the optimum setting.

4. Oxy-fuel cutting torch variants

Two variants of cutting torch are used in the oxy-fuel process, they are identified as a type which works with a nozzle mix system or an "Injector" type cutting torch, they both achieve the same cutting quality but the "Injector" type reaches greater cutting speed. Their construction differs as do their respective nozzles.

Injector type torch relies on high pressure oxygen drawing the fuel gas into the mixing chamber the torch is suitable for use where there are large pressure drops. Thermacut® manufactures a wide range of nozzles suited to the Injector type cutting torch. The nozzles are available in various fitting forms, threaded, bayonet and push-fit, they are produced to replace the following examples: 

Fast cutting nozzles suitable for model: *

  • Messer® Vadura® 1215,1217,9215
  • Messer® Gricut® 1230,1232,9230
  • Harris® 6290-VVC
  • Harris® 6290-VAX

High performance nozzles suitable for model: *

Torch which works with a nozzle mix system
With this torch the mixing of gases takes place at the nozzle. Nowadays, this type of the torch is considered as an absolete one, Thermacut® will upon request manufacture this nozzle type that is suitable for Koike® and Tanaka® oxy-fuel systems.

5. Safety

Safety during the oxy-fuel cutting process is paramount, even with the correct fit-up and system settings, loose scale or slag can cause issues at the nozzle that in turn can create a Flashback, this violent and dangerous event could lead to cylinder fire or explosion, therefore preventative measures are fitted to the torch and gas regulators. The devices are called "Flashback Arrestors", there are two types available, single use and resettable, single use are replaced after a flashback whereas the resettable type as the name suggests can be used again. An oxy-fuel system should never be used without flashback arrestors being fitted.

The cutting torch, gas regulators, flashback arrestors and hoses should be checked on a frequent basis (some countries have a minimum requirement for this process), check for leakage, physical damage, cracking or deep burns to the hoses. Do not operate a defective system. 

6. Nozzle care

Oxy-fuel nozzles are a consumable part, they have an unmeasurable life expectancy, this is because of the issues mentioned earlier in this article but also the manufactured quality and not to forget the way the operator cares for them. Clean well cared for nozzles will return good service standards and have a longer life expectancy than nozzles that are dirty or damaged.

Not all nozzles are the same, some are bare brass, chrome plated, or even hard chrome plated, it follows that a chrome plated nozzle would be expected to give longer service than a bare brass type, this is mainly due to bare brass not having the same capability as the chrome plated nozzle to prevent molten material sticking to the nozzle and causing it damage, likewise hard chrome plated nozzles have more resilience to molten material.

Nozzle cleaning can be performed by an operator; it is not difficult to do but care must be taken to ensure that the nozzle shape and form are not changed in any way as this will affect cut quality and performance. 

  There are do's and don'ts

  • Do use the correct type of nozzle cleaning kit with its sized needles, combination brass brush
  • Do work on a clean dry surface.
  • Do not use other types of file.
  • Do not enlarge or change the shape of the nozzle orifices.
  • Do not use a steel wire brush, use a soft brass brush.
  • Do not reduce the size or change the shape of the flutes and lastly when using a two-piece nozzle do not reduce the length of the inner nozzle. 

Tip! To extend the life of a nozzle, try edge starting whenever possible, use "piercing speed" to avoid the slag coming from the piercing area. In the other words, set up the right piercing process which increases the lifetime of the consumables in the greatest extent.

Did you know that THERMACUT® developed an original EX-TRAFLAME® torch for oxy-fuel cutting? Download your brochure right here!

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*THERMACUT® is in no way affiliated with ESAB®, Messer®, GCE® or Harris®, Koike® or Tanaka®.

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