The K bevel is arguably the most challenging to
cut as it requires three torch passes: one to establish
the land and two more to cut the top and bottom
bevel, not necessarily in that order.
The large range of available bevel angles, material types, thicknesses, and amperage levels increase
the difficulty of determining the process parameters
for achieving a good plasma bevel. Even just one parameter change requires new process data.
Physics also plays a role since, as mentioned earlier, the plasma arc isn’t static. The consumables
wear with each cut, changing the arc. The variables
abound: the distance between the torch and workpiece, the molten metal flow path, the impact of
gravity on that path, and more.
All these factors determine the bevel-process
compensations required. It implies that the process
compensation data potentially needs continuous
tuning to maintain the desired cut quality, angle,
and dimensions over the life of the consumables.
This is true even with dedicated bevel cutting heads
that rotate to match the bevel angle being cut.
Beveling With Plasma
Better beveling with plasma could save fabricators
an enormous amount of time and money. It would
allow them to increase the number of parts cut in
the given time by replacing non-value-added activities with production. Today’s high-definition class
of plasma systems are more than capable of making
excellent beveled edges. The tools required for this
include a suitable torch, a dedicated bevel head,
and a CNC preloaded with the cutting parameters
through the plasma cutting software.
Bevel cutting requires a plasma system with
a high open-circuit voltage (OCV) and industrial
duty cycle. This ensures that the arc voltage during
piercing and cutting, especially at the higher bevel
angles (farther from perpendicular), will not exceed
the plasma systems’ capabilities, all while maintaining a reasonable clearance between the torch and
the plate. An adequate clearance is important for
avoiding damage to the consumables from molten
metal slag produced during cutting.
Most recommend a “pointy” plasma torch (see
Figure 2) because it provides the effective cut
height necessary to maintain a minimum clearance
for bevel cutting without posing any threat to the
consumables. “Pointy” is defined as a torch head
with a shield face diameter and included shield angle
that are as small as possible for any given amperage.
A clearance value of 0.08 to 0.1 inch is recommended.
While it is true that greater clearances would reduce the likelihood of torch and consumable damage, greater clearances also lead to higher effective
cut heights and higher cutting voltages, both of
which have their own set of problems. Higher cut
heights can lead to lower edge quality and require
higher angle compensations, while higher cutting
voltages could affect the plasma system’s duty cycle.
CNC cutting tables have two common types of
dedicated bevel heads: AC and ABXYZ (see Figure
3). The AC-type bevel heads tilt about the X axis and
rotate about the Z axis. The ABXYZ types tilt about
the X and Y axes and use XYZ linear-axes compensa-
tion to determine a virtual torch rotation point.
The bevel pivot point, or rotation point, is the
point in 3-D space that the bevel head tilts around.
This point is at the tip of the torch or the end of the
shield (see Figure 4). A bevel head uses validated
motion transformation equations to rotate about
the bevel pivot point.
Modern systems also minimize what’s known as
process shift, when the plasma arc moves across the
top of the plate as the torch tilts (see Figure 5). AC
heads accomplish this by optimizing the relationship between the bevel pivot point and the plasma
torch head. Knowing the size of that shift is necessary to calculate the eventual size of the beveled
part. That’s because the size of the shift determines
the amount of process compensation, or the level of
adjustment made to the torch path.
ABX YZ-type heads can maintain a virtual bevel pivot point at the top of the plate to maximize plate utilization. In this case, the process shift becomes zero.
Bevel head alignment, or calibration, also is important for achieving the correct bevel angles and
part sizes. Bevel head alignment is set by first determining the relationship between the bevel head
rotation point and the torch head, then by setting
the bevel head home position so that the torch is
square to the plate.
Bevel head manufacturers use either an automatic calibration routine through the CNC integration or
a set manual calibration procedure. Once the bevel
head is calibrated, the system can use the bevel cutting parameters to achieve the desired results.
By Madhura Mitra
An estimated 40 percent of parts cut on a plasma cutting table ultimately need a bev- eled edge, most often for weld preparation.
For the most part, these edges are cut with a secondary process. Fabricators cut parts to size using
plasma, then pick up and move the parts to another station to add the beveled edge. This not only
adds time and labor to most jobs, but also wastes
metal as the parts are cut a second time.
Using a bevel head on the plasma table itself can
eliminate the need for these secondary operations
and increase productivity, but one large obstacle
stands in the way: The physical behavior of the
plasma arc changes as the torch tilts and the consumables wear. The operator needs to compensate
constantly by adjusting the settings. Though not
exactly difficult, this task consumes a large amount
of time and material as it is very iterative, requiring
quite a bit of trial and error. For many, this impedes
productivity on the bevel cutting table, so much so
that they give up beveling with plasma altogether,
and instead let their expensive automated bevel
equipment sit idle.
Obstacles to a Good Bevel
Bevel cuts are described throughout the industry by
the English letter the cut resembles when looking at
a cross section of the metal. The types include V, A,
X, Y, and K (see Figure 1). The complexity increases
with the number of plasma torch passes required to
cut a bevel type.
The V and A bevels require only a single pass,
followed by Y (top and bottom) and X, which each
require two passes. For the Y bevel, one pass establishes the straight edge, or the land, and another
pass cuts the bevel. For the X bevel, two torch passes establish the top and bottom bevel angles.
Each bevel type, from the A bevel to the K bevel, has its
own unique plasma cutting challenges.
Bevel cutting with plasma usually calls for a “pointy”
cutting head with the smallest shield face diameter
and angle for any given amperage.