Plasma Cutting Guidelines
The purpose of this document is to promote
- Longer Consumable Life
- Greater Productivity
- Enhanced Cut Quality
- Performance Benefits
Do Not Use Consumables Until They "Blow"
Consumables should not be used until they fail. There are several ways to identify worn consumables. The experienced operator can often tell by the sound or colour of the arc but the best way to judge the condition of consumables is to monitor the cut edge quality of the material and check the torch components when the cut edge starts to deteriorate. A record can be kept of average part life over time which can then be used to establish guidelines for checking the consumables.
Too Frequent Changing of Consumables
When parts are changed, the operator needs to know what signs of wear to look for.
-A nozzle should only be replaced if it is gouged on the inside or outside, or if the hole has worn out of round. Hold the nozzle up to light and observe the shape of the aperture
-The pit of the electrode is spent when it is deeper than 1.0mm for the hafnium insert in HD Plasma. The feeler gauge can be used to determine depth of erosion.
-Gas swirlers should only be changed when there is dirt in the holes or they are cracked and excessively worn or seals damaged (HD)
-Shields should only be replaced if they show signs of physical damage. They can often be cleaned of spatter and reused
Use The Correct Consumables/Parameters When Tooling The Job
The correct consumables must be used, with the appropriate pierce delay, height, arc voltage etc. It is particularly important to cut at the correct amperage. If amperage is set too low the cut will be sloppy, if too high the nozzle life will be limited. The tables provided by Hyperthem are aimed at high cut quality. If finish is not important then quality can be traded against speed by increasing the cutrate.
Assemble the Torch Correctly
Torch parts should be aligned and fit together snugly. Consumables should be kept clean when being changed to prevent torch contamination. The o-ring lube should be sparingly applied with only enough used to put a shine on the o-ring. No other lubricants should be used on the o-ring. The o-ring lube is supplied with the consumables when the machine is first commissioned. Take care with the removal of the electrode that the accompanying swirl ring is not damaged. The ease of the electrode removal will be directly proportional to how well lubricated the o-rings are.
Routine Maintenance
Torches can last for months and 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. O-rings should periodically be checked and lubricated.
Coolant Flow
Coolant levels must be monitored. Inadequate flow of cooling fluid is a common cause of parts and torch failure.
Pierce Height
If pierce height is set too low this can cause molten metal to spatter the front of the nozzle and occlude the aperture. Recommended pierce heights are set in the Hypertherm book and as a rule equate to 1.5-2X the recommended cut height.
Cutting Speed
’Low speed dross’ is a large bubbly accumulation along the bottom edge. Too slow a speed can also result in a wider kerf and excessive top spatter. Too high a feedrate can be identified by a backwards lagging kerf on the cut edge, a narrower kerf than expected and small beads of hard to remove dross along the bottom edge of the piece (high speed dross).
Arc Stretching
’Arc Stretching’ is when an arc has to deviate from perpendicular in order to ground and find metal. This can cause the arc to deviate into the side of a circle, for example, as the cut terminates and intersects with the leadin. At this point because of the deviation from perpendicular it can also cause the arc to cut into the nozzle orifice.
Pierce Points
When doing an edge start, the plasma arc should be initiated with the nozzle directly centred over the edge of the work piece. Likewise piercing in a drilled or punched hole should also be started directly over the edge of the hole, not in the centre.
Factors Affecting Consumable Life
-Number of Pierces: stresses the electrode during arc initiation and termination causing hafnium erosion
-Cut Duration: excess heat developed through long cuts erodes the hafnium
-Material Thickness: thicker materials require higher current which causes more hafnium erosion.
-Cut Termination: when an arc terminates abruptly because it runs off the plate, an excessive amount of hafnium is ejected. This loss translates to a reducing between 10 and 15 of arc starts for the electrode
-Gas Supply and Purity
-Adherence to defined cut parameters (Hypertherm manual)
Gas Supply
A common cause of excessive consumable wear. Leaks can both create low flow conditions or introduce impurities into the plasma gas supply. Gas flow restrictions can lead to nozzle destruction when the pilot arc remains attached to the inside of the nozzle orifice for too long. Gas contamination is indicated by a blackened surface on the electrode. Compressed air must be kept clear of oil, moisture and particulate contamination.
Work Cable Connection
A good electrical connection from the workpiece back to the plasma power supply is required. Without a good connection arc transfer will be inhibited and excessive pilot arcing will result. It can happen that slag buildup on the slats can impede proper current flow. If required a jumper cable can be connected from the table directly via a clamp to the workpiece.
Divots and Bumps
There are two common defects ‘divots’ and ‘bumps’. A divot occurs when the arc removes too much material at the end of a cut. This can happen when at the end of a cut the arc comes into contact with the void of material that it has already removed, and transfers to the saved part. The hole is therefore out of round. A bump occurs if the arc terminates before it comes into contact with the void of material it has already removed. Different types of leadin and leadouts, as well as under and overburn, can be experimented with to minimise these defects.
Kerf Width
Cut (kerf) width is set in Toolshop when the toolpath is applied to the nest/dxf. Specific combinations of material and consumable will have a specific kerf width, this being the width of material that is removed by the plasma torch in its cutting action. Problems occur when the defined and actual kerf widths are different, leading to undersized or oversized parts. Cut variables that cause the arc column to shrink will narrow the kerf, causing oversize parts and vice versa with undersize parts when the arc column grows.
Bevel Angle
This depends primarily on cut height and speed. Positive bevel is when the part tapers outwards at the bottom and is often caused by cut height being too high and sometimes accompanied by high speed dross. Negative bevel is often caused by cut height being too low and often accompanied by low speed dross. Bevel angle is also affected by the clockwise swirling of gases from the plasma torch – this produces a ‘good’ and ‘bad’ side of the cut. External (male) cuts are always clockwise and internal (female) cuts anticlockwise to take this into account. An irregular bevel angle may indicate that the torch is out of square or nozzle misaligned or severely worn.
Automatic Torch Height Control
The distance between the torch and the top of the material is kept constant by maintaining a constant voltage reading between the two. The controller adjusts the z axis up and down to keep the voltage constant. This is particularly useful for thinner gauge materials that often do not lie flat.
Poor CNC Programming = Poor Cut Quality
The programmer must understand the fundamentals of plasma theory and utilize the Hypertherm literature to correctly program pierce height, arc voltage, pierce delay etc. As good as the operator is, if these are not set up correctly an inferior cut will come off the machine.
Lag Lines
For Mild Steel the correct angle for the lag lines is 10-15deg. If cut speed is too slow then the lag lines will be vertical, if too fast they will be at greater than 15deg. Lag lines for cutting Stainless are ideally at approximately 30deg.
Maintenance Checks - Daily
-Verify inlet gas pressures
-Check gas flow settings on control panel
-Check coolant pressure and temperature
-Inspect torch
-Inspect and replace consumables as required
Factors Affecting Consumable Life
-Clean power supply
-Verify fan operation
-Clean torch threads and current ring with cotton bud
-Verify coolant level