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Plasma Technology

Automatic torch height control: No longer a luxury

By Eric Smith

Above: Photos of two nozzles after 450 pierces. The nozzle that pierced at the lower cut height (right) has significantly more metal spatter than the nozzle that pierced at a higher height (left). The nozzle on the right will no longer provide cut quality cuts and is ready to be discarded. The nozzle on the left is still usable.

April 2015 - While people sometimes assume that a quality automated torch height control (THC) isn’t a critical component for cutting systems, the reality may be otherwise. Companies without it typically spend money every day on more consumables than needed, more time on secondary operations and produce fewer parts per shift. 

Plasma cutting requires constant adjustment and a torch height control is designed to hold your torch at an optimum height relative to the plate. Additional plasma cutting advances—such as tools to create bolt-ready holes, improve edge quality and lengthen consumable life—require faster and more accurate THC to take advantage of improved plasma performance.

Height controls are either manual or automatic. Manual THC systems use a mechanical device to maintain a single fixed torch-to-plate distance and are set by the operator. Automated height controls are more varied. A semi-automated control uses a motor-driven lifter mechanism with timed position changes made by the CNC. Semi-automatic devices are not as common due to the cost of lifter components without the added benefits of a fully automated system. More common is a fully automated THC system. 

A plasma arc is not perfectly straight, much like the flame of a candle. The top and bottom of the plasma arc is slightly curved or tapered. If the torch is positioned with the arc too high or low at the plate, your cut is going to be tapered as it will follow the shape of the arc. This is referred to as having a cut with a positive (when the torch is too far) or negative (too close) bevel. To avoid this, it is important your torch position is set so that the middle, or straightest part of the arc, is positioned at the part edge (Figure 1). 

FFJ-0415-plasma-image1

Torch position impacts consumable life and performance. The electrode found in all consumable sets contains a small amount of hafnium inserted into the tip. This hafnium serves as the electrical contact for the arc to the plate. When the torch is fired, this tip softens as heat moves through it. When the arc is turned off, most of the hafnium cools and solidifies, however, some of the element escapes through the end of the nozzle. The hafnium tip is getting smaller and the torch, which maintains its height based on arc voltage, moves closer to the plate. Eventually the torch will hit the plate, unless the operator compensates by adjusting the arc voltage setting—something most operators don’t do. Instead, they stop and replace the half-used consumables when they see a degradation of cut quality. 

An improper torch position can impact consumable life even before you start cutting. While cutting, molten metal from the plate tends to fall underneath it, but during the pierce process, that metal has nowhere to go. It can splash up toward the torch, hitting and sticking to consumables. To reduce build-up of this spatter, set the pierce height as high as possible. Generally, the recommended height is 1.5 to two times the cut height (Figure 2). 

FFJ-0415-plasma-image2

Done repeatedly, piercing at cut height will significantly reduce consumable life. In contrast, torch height controls with programmable pierce and cut heights (found in most THCs with arc voltage control) maximize consumable life. Hypertherm, Hanover, New Hampshire, a manufacturer of plasma, laser, and waterjet cutting systems, conducted internal testing that confirms this fact. The company made hundreds of pierces in a controlled laboratory setting. Half the pierces were made at cut height, meaning the torch to work piece distance was not increased. The remaining pierces were performed using an automated THC in which the pierce height was 1.5 to two times the cut height. 

The cutting process

How exactly does THC work? During the cutting process, the height control is responsible for several key tasks. Here’s a look at what the torch is doing during an average cutting job.

Initial height sensing: The first step for THC is to find the plate, called Initial Height Sensing (IHS). There are two primary IHS methods: stall force and ohmic contact (an electrical junction between two conductors that has a linear current-voltage curve as with Ohm’s law). With stall force, the torch lowers down to the plate until a stall threshold is met. This method works well with thicker plate. When cutting thinner material, the ohmic contact method is preferred as it demonstrates greater sensitivity.

Piercing: This generally lasts between 0.1 and 3 seconds and is programmable, either through a connecting CNC or directly on the THC. The duration of a pierce depends on a number of factors, such as material type, thickness and cutting current. As mentioned above, pierce height is 1.5 to two times greater than the cut height. 

Transition to arc voltage control (AVC): After the pierce, the torch drops down to the desired cut height and the gantry moves the torch along the desired cut path commanded by the CNC’s X-Y controller. Cut height is set to achieve optimal cut quality. This value, included in cut charts from the plasma system manufacturer, is usually between 4 mm (0.15 in.) and 6 mm (0.25 in.). 

AVC: As the torch is no longer touching the plate, another method is needed to ensure the torch is kept at the desired height. The most common feedback method used is called arc voltage control, which measures the value of the arc and then compares it to an ideal value. This value varies depending on the distance between the work piece (plate) and the electrode emitter. If the arc voltage is higher than the ideal, the torch is too far from the plate. If lower, then the torch is too close. In either case, the lifter will raise or lower the torch until the values match. 

Selecting a torch height control

The following features should be considered when shopping for a THC: programmable height positions: cut, pierce, transfer, and puddle jump height. Cut height is the torch’s distance from the plate while cutting, while pierce height is the position of the torch while piercing. Transfer height is used when piercing thick plate. Puddle jump height, also used on thick plate, allows the THC to maintain the higher pierce height for a short period of time as the torch moves forward. 

Collision detection and torch protection: In the event of a collision with scrap or tipped-up parts, some THCs allow the torch to break away from its holder, minimizing any damage to the torch. A sensor, typically included on better THCs, can detect the breakaway, quickly stopping the operation.

THC speeds: Basic THC controls typically have a speed of 5,000 mm per minute (200 ipm). More advanced systems, used for high-definition cutting, travel at a minimum of twice that speed.

IHS speed: Plate detection (IHS) requires slow speed positioning for accuracy. Advanced height controls allow the torch to lower quickly to a position just above the plate, then reduce speeds for accurate plate detection, minimizing total process time.

Custom mechanics: Most THC mechanics (lifter) are matched to the motor. However, some systems allow for custom lifter mechanics. 

Automatic kerf crossing detection: During cutting, the torch sometimes crosses over an existing cut path, meaning the arc will cross an open area where there is no metal. This can cause the arc voltage to drop, causing the torch to quickly drop toward the plate. Automatic kerf crossing detection tells the torch to temporarily remain at its previously set height.

S arc voltage: The sample arc voltage feature automatically establishes ideal arc voltage based on the value present shortly after the pierce. This generates a new arc voltage set-point for tracking during operation, maintaining the ideal height which in turn optimizes consumable life. 

Advanced parameter input: Connectivity to the CNC allows for the transfer of advanced THC parameters including embedded process codes in the part program for greater performance. 

Skip initial plate detection: Used on jobs with multiple pierces, the torch will use IHS at the start of the job to find the plate and set a pierce height. It will then pierce the plate, move into the cut position and cut out the first part. For the second part, the THC will skip the initial plate detection feature and move directly to the earlier pierce height reducing cut-to-cut cycle time. 

While many factors impact the performance of any plasma cutting operation, a good quality automatic torch height control is a critical component for any plate cutting shop interested in maximizing productivity and profitability. This is true whether you are purchasing a new cutting system or simply updating an older system. Improvements in cut quality, consumable life and productivity will occur regardless. FFJ

Eric Smith is a senior product specialist at Hypertherm Inc.

 

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