Training Articles
Torch Height Control for Plasma Cutting
By Dave Cook, Centricut Technical Services
Director
Article originally appeared in "Welding Design & Fabrication"
- March 2000
The automatic torch height control or (THC) is one of
the most important elements of the mechanized shape cutter. But
it is also the piece of equipment on the cutting machine most
likely to be misused, nonfunctional or altogether missing. In
many fabrication shops you will see the torch operator manually
driving the torch up and down while cutting-his eye on the torch
and his thumb on a toggle switch. The THC is either not working
properly or the operator has not been trained on how to use the
equipment. In either case the shop is not getting the most out
of its cutting operation.
There are a variety of torch height controls or "standoff controls"
on the market. ESAB®, Thermal Dynamics® and Hypertherm® manufacture
torch height controls for use with their plasma cutting equipment.
Innerlogic® Inc. specializes in manufacture of THC's. Each of
these automatic THC's has a variety of functions and features.
But all use the same basic elements to control position of the
torch relative to the material being cut. This "torch to work"
distance or "standoff" is critical to both cut quality and parts
life.
Elements of the THC
- Motor-driven torch positioner. This device moves
the torch up and down in response to signals from the control
console. Screw drive, rack and pinion, and belt driven mechanisms
are used to translate motion.
- Control console. This is the brain of the system.
It receives input commands from the remote control, sends
output commands to the torch positioner and monitors position
feedback and voltage signals.
- Plasma system interface. This device usually consists
of a voltage divider card that is mounted inside the plasma
power supply. The voltage divider card monitors power supply
voltage during cutting. It "divides" the voltage signal and
sends a smaller signal voltage to the control console.
- Remote control. The remote control is the HMI or
human machine interface. This is where the operator sets two
critical cutting parameters: initial height and arc voltage.
It may be located inside the CNC control or in a separate
remote box.
The difference between IHS (initial height sensing) and
voltage setting is very important to understand and is often
confused. IHS is an adjustable parameter that determines the
height at which the torch will fire and begin piercing through
the plate. In order to set the initial height the THC must first
find the position of the plate. A variety of schemes are used
for sensing the plate from simple mechanical devices, to motor
stall technology, proximity sensors, and electrical circuits
that sense nozzle or shield contact. Regardless of the method
used, the most important thing is that the torch finds the plate
and accurately indexes to the proper pierce height whether on
thick plate or thin, rusty plate or clean, above water or below.
Correct pierce height is essential. If the torch pierces too
low the consumable parts will be damaged. (The number one cause
of premature parts and torch failures.) See figure 1. If it
attempts to pierce too high, the arc will not transfer causing
a misfire. The rule of thumb is to pierce at 150-200% of the
cut height. Pierce high; Cut low.
Figure 1
Torch to Work Distance or standoff during the cutting
process is adjusted by monitoring and controlling the arc voltage.
(Note: arc voltage is the same thing as power supply output
voltage.) Plasma power supplies are a current source-they generate
a very steady operating current when the torch is cutting. Voltage
on the other hand varies depending on the distance between the
cathode (the electrode in the torch) and the anode (the material
being cut). Voltage is directly proportional to resistance (Ohms
law V=I*R) The resistance in the arc is a function of the distance.
When the torch-to-work distance increases the voltage goes up;
when the torch-to-work decreases the voltage goes down.
The torch height control uses arc voltage to maintain a consistent
distance from the plate while the torch is cutting. This allows
the system to maintain proper torch height regardless of variations
in the material, or flatness of the cutting bed. The operator
sets the arc voltage on the remote control according to the
cut charts in the plasma system's operations manual. This voltage
setting is usually between 100 and 200 VDC.
After the torch height control has completed initial height
sensing and the torch has pierced the plate, motion of the cutting
machine is enabled and the torch begins to move. Once the cutting
begins, the THC starts sampling arc voltage from the power supply
and comparing it to the target voltage set by the operator.
It adjusts the torch up or down to maintain that target voltage.
Each voltage setting corresponds to a specific height that
optimizes the arc characteristics for a clean, straight cut.
Torch height has the greatest affect on bevel angle of the plasma
cut piece. Setting the voltage too high results in more material
being removed from the top of the kerf than the bottom. This
causes excessive top rounding and positive bevel (See figure
2). Setting the voltage too low results in too much material
being removed from the bottom of the plate. This causes undercutting
or negative bevel (See figure 3).
Figure 2
Figure 3
Today's torch height controls have many features and capabilities.
But the two most important to understand are initial height
sensing and arc voltage control. Piercing at the right height
will save you money on parts. Cutting at the right height will
ensure good cut quality and minimize expensive rework operations.
When properly used a torch height control pays for itself in
a short time.
Correct Height:
Here are three recommendations for torch height controls:
- If your THC is broken fix it; if it is ancient, retrofit
to a new one
- If you don't have a THC on your plasma profiling machine
get one.
- If you are buying a new machine, get the best THC you
can afford. To get the most out of your investment train
your operators to use all of the THC functions.
A good THC with a well-trained operator at the controls will
pay for itself in improved parts life, enhanced cut quality,
and decreased downtime.
