By D. Cook and J. Start
Article originally appeared in "Practical Welding Today " - May/June 2002
Using Consumables Until They "Blow"
Look in your used parts bucket and you will probably see parts that have been run to failure. Using severely worn consumables can not only ruin a good piece of metal, it can cause expensive torch failures and unnecessary down time. Running the parts to failure is a pitfall that is easily avoided. There are several signs of worn consumables; the experienced operator can often tell by sound or color of the arc or subtle changes in torch height that indicate parts wear. However, the best way to judge the condition of the torch parts is to periodically check the cut edge quality of the metal, and check the torch parts when the cut begins to deteriorate. Keep a record of the average parts life over time (in number of starts or arc-on time) and develop guidelines for expected parts life based on the amperage, material and thickness. Once average parts life is established the operator will know when to check and or replace the parts--preventing a catastrophic failure.
Using the Wrong Parameters & Parts for the Job
Consumable selection depends on the material and thickness being cut, the amperage and plasma gas used and other cutting parameters. The operator's manual will define which consumables are appropriate for various types of cutting. Using incorrect consumables can lead to shortened parts life and reduced cut quality. It is particularly important to run parts at the correct amperage. The best cut quality and parts life is usually achieved when the amperage is set to 95% of the nozzle's rating. If the amperage is too low, the cut will be sloppy; if it is too high, the nozzle life will be poor.
Assembling the Torch Incorrectly
The torch should be assembled so that the parts are aligned correctly and fit together snugly. This ensures good electrical contact and the correct flow of gas and coolant through the torch. When changing parts, keep consumables on a clean shop rag to prevent dirt or metal dust from contaminating the torch. Cleanliness during torch assembly is very important and often neglected. When applying o-ring lube, use just enough to put a shine on the o-ring. Too much lubrication can cause clogging of the gas swirler and metal dust contamination in the torch. This can lead to uncontrolled arcing in the plasma chamber and ultimately torch failure. Grease should never be applied to torches--it can cause destructive arcing and burning within the torch.
Neglecting Routine Maintenance
Torches can last for months or even years with proper care. Torch threads must be kept clean and seating areas should be checked for contamination or mechanical damage. Any dirt, metal dust or excess O-ring lubricant should be cleaned out of the torch. To clean the torch, use a cotton swab and electrical contact cleaner or hydrogen peroxide.
Not checking gas and coolant flow
The flow and pressure of gas and coolant should be checked every day. If the flow is insufficient, consumables will not be cooled properly and parts life will be reduced. Inadequate flow of cooling water due to worn pumps clogged filters, low coolant level etc. is a common cause of parts and torch failure. Constant gas pressure is important to maintaining the cutting arc. Excess gas pressure is a common cause of "hard starting," a situation in which the torch fails to initiate an arc when all other conditions for normal operation are correct. Too much gas pressure will also cause rapid deterioration of electrodes. Likewise, plasma gas must be kept clean to prevent short consumable and torch life. Compressed air systems are especially prone to oil, moisture, and particulate contamination.
Piercing too low
Standoff, the distance between the workpiece and the tip of the torch, is critical to both cut quality and parts life. Even slight variations in torch height can affect the angularity of the cut surface. The height of the torch during piercing is particularly important. One common error is to pierce too low. This causes molten metal to spatter the front of the nozzle and shield causing damage to the parts and sub-sequent cut quality problems. Arc "snuffing" can even occur if the torch pierces when touching the metal, or drags along the surface while cutting. If the arc is "snuffed", the electrode, nozzle, gas swirler, and sometimes the torch are destroyed. Piercing at a height of 1.5-2X the recommended cut height protects the torch and parts from damage.
Cutting Too Fast or Too Slow
Cutting too fast or slow will cause cut quality problems. If the speed is too slow the cut pieces will develop "low speed dross" a large bubbly accumulation of dross along the bottom edge. Slow speeds may also cause a widening of the kerf and excessive amounts of top spatter. If the speed is too fast the arc will lag backward in the kerf causing a beveled edge, a narrow kerf and a small hard bead of dross along the bottom edge of the cut piece. High speed dross is difficult to remove. The correct cutting speed will produce minimal dross--the result will be a clean edge that needs little rework before the next step in the manufacturing process. "Stretching" the arc. Arc stretching can occur at the beginning and end of the cut if the arc has to "stretch" (deviate from a straight, perpendicular path) to find metal. Arc stretching can cause the arc to cut into the side wall of the nozzle. When doing an edge start, the plasma arc should be started with the nozzle orifice directly centered over the edge of the work piece. This is important to remember in punch press/plasma operations where the arc is started off of a punched hole. In this application, the arc should be started off the edge not the center of the punched hole. Arc stretching can also occur at the end of the cut if the torch is programmed to run off the plate with the arc on, or if the "lead out" follows the kerf of previously cut metal. Timing of the arc off signal and programming of the lead out can minimize this effect.
Crashing the Torch
"Tip-ups" and crashes can irreparably damage your torch. Torch collisions with the workpiece can be prevented by programming the shape cutting system to travel around (rather than over) cut parts. Torch height sensors also offer protection from torch crashes by correcting for variations in the material. However, voltage regulated height controls can fail to protect the torch. For example "torch diving" often occurs at the end of a cut if the torch follows the kerf for too long. (The torch height control dives to compensate for increased voltage as the arc stretches). Careful programming of the lead out and torch height control function can minimize this. Finally, breakawaytorch mounting devices can help prevent damage to the torch if a collision does occur.
Ten Common Mistakes to Avoid
- Using Consumable Parts Until They Blow
- Changing Consumable Parts Too Frequently
- Using the Wrong Parts and Parameters for the Job
- Assembling the Torch Incorrectly
- Neglecting Routine Maintenance
- Not checking gas and coolant flow
- Piercing Too Low
- Cutting Too Fast...
- ...or Too Slow
- "Crashing" the Torch
A good operator with well-maintained equipment can save a shop countless hours of downtime and thousands of dollars in operating expenses. These savings will result in greater profits for your cutting operations and your company