FOCUS ON PLASMA CUTTING
3 . Tube and section plasma cutting This is used for the processing of tube , pipe or any form of long section . The plasma cutting head usually remains stationary whilst the workpiece is fed through and rotated around its longitudinal axis . There are some configurations where , as with threedimensional plasma cutting , the cutting head can tilt and rotate . This allows angled cuts to be made through the thickness of the tube or section , commonly used in the fabrication of process pipework where cut pipe can be provided with a weld preparation in place of a straight edge .
Cutting with gas
“ Plasma gases are usually argon , argon / hydrogen or nitrogen . These inert gases can be replaced by air but this requires a special electrode of hafnium or zirconium . Use of compressed air makes this variant of the plasma process highly competitive with the oxy-fuel process for cutting carbon-manganese and stainless steels of up to 20mm thick . Inert gases are preferred for high-quality cuts in reactive alloys ,” commented welding specialist , TWI .
“ So why use oxygen in a plasma torch ? The same reason that you use oxygen in an acetylene torch – the oxygen in the plasma stream reacts with mild steel . That is why pure oxygen is only used when cutting mild steel , or ‘ carbon steel ’. The chemical reaction between the oxygen in the plasma gas and the base metal helps to speed up the cutting process and improve the edge quality . But since oxygen doesn ’ t have the same reaction with stainless steel or aluminium , less expensive gases can be used for those metals , like nitrogen or compressed air ,” added ESAB .
“ Other specialty gases are sometimes used for other purposes . Argon gas is used when plasma marking . A mixture of argon and hydrogen is often used when cutting thicker stainless steel or aluminium . Some use a mixture of hydrogen and nitrogen , or methane and nitrogen when cutting thinner stainless steel . Each mixture has its advantages ( improved cut quality ) and its disadvantages ( cost & handling ),” added the Swedish specialist .
Six tips for improving plasma cut quality
Plasma cutting specialist , Linde , has outlined five tips below for helping to improve plasma cutting efficiency .
“ While plasma cutting systems are generally user-friendly machines , experience will show that there ’ s more to it than meets the eye . After learning the basics of plasma cutting , there is still more to learn – such as improving cut quality , prolonging consumable life and improving efficiency ,” it explained .
The best practice tips and suggestions below are designed to increase plasmacutting efficiency and precision as well as prolong the life of plasma cutting systems and consumables .
Tip 1 : Choose the right plasma cutter . There are various factors to consider when purchasing a plasma cutting system including cut capacity , duty cycle , purchase cost and operating costs . Think about the jobs you will do most often when selecting your machine and choose one that suits your needs . Ask yourself questions such as : Do you plan to cut by hand or on a table ? Do you need the unit to be portable ? How often will you be using your plasma cutter ? What kind of material do you plan to cut ? What ’ s your budget ? Will you use a generator or local power outlets to power your machine ?
Tip 2 : Set up your machine properly . Because the cutting process is based on an electrical charge that melts the surface of the metal , a ground clamp is needed to protect the user and the plasma cutter unit . When setting up your plasma cutting machine , secure the ground clamp to clean metal only and grind off rust or paint which can disrupt the flow of electricity . It is also essential to place the clamp as close to the cut area as possible .
It is also important to frequently check worn spots and cables for loose connections or anything that might inhibit the electrical flow . Finally , for efficient cutting results , it is important to use the correct tip for amperage setting . Using the right tip will help you to save time and money .
Tip 3 : Practice your plasma cutting . The first cut is important for any cutting project . Without pulling the trigger , trace the path you plan to cut . Practice your movements to make sure that you have enough freedom of movement to make one continuous cut , as stopping and starting again in the same spot can create irregularities in the cut edge .
You can also make a sample cut on the same type of material with which you ’ ll be working . This helps to ensure that you are using the right settings and travel speed .
Tip 4 : Watch the travel speed . The faster the travel speed ( especially on aluminium ), the cleaner the cut . If during your practice cut , or during the actual work , you see sparks shooting along the top of the metal , you are going too fast and need to slow down . When the travel speed is correct , you will see the arc coming out at about a 20-degree angle , opposite the direction of the torch ’ s travel path .
Travelling at the right speed and using the right amount of heat will produce a very clean cut with less dross on the bottom of the cut and little or no distortion to the metal .
Tip 5 : Brace yourself . Use a drag shield if your machine is equipped with one . This allows you to rest the torch on the workpiece while maintaining optimal standoff without touching the tip to the metal , which can adversely affect cut quality and consumable life .
If you ’ re not using a drag shield , or are trying to make very precise cuts , place your noncutting hand down and use it as a guide for the other hand . This rest hand position allows freedom of movement in all directions while helping you steady your hand and maintain a constant standoff .
Tip 6 : Check your consumables . Using worn consumables is a common mistake when it comes to plasma cutting . It not only ruins a good piece of metal but can also cause torch failure , resulting in unnecessary downtime and reduced productivity .
There are several ways to identify worn consumables for plasma cutting operations . For example , a skilled operator can identify this by noting the subtle changes in the torch height or the sound of the arc . It will also help if you maintain a record of average part life over time . This will allow you to establish a guideline for checking and replacing the consumables . n
ISMR December 2021 / January 2022 | sheetmetalplus . com | 33