ESAB Cutting System’s Expert Motion Plasma Bevel System is the culmination of over 25 years of continuous improvement in product development. Below is a little bit of what you need to know about plasma bevel cutting.

The edges of a part can be cut at an angle to allow for welding or special fit-up with other parts. A plasma bevel system can be used to make top bevels or bottom bevels. A "top" or "bottom" bevel requires only one pass around the part.

A top bevel creates a bevel facing the top of the part. This requires that the remnant plate be lifted from the table first, in order to retrieve the part.

A bottom bevel places the beveled face on the bottom of the part, allowing the part to be removed from the table prior to the remnant plate.

Generally, top bevel cuts produce a better quality cut part than bottom bevels. On bottom bevel cuts, burnback occurs on the top of the part, and irregularities result on the face of the cut.

Plasma bevel systems can also be used to cut bevels with a "land" or "nose" on the finished part. A bevel cut with a land will require two cut passes. When cutting multiple passes, it is important to always cut the bottom cut, or the longest cut first. Therefore, a bottom bevel with nose would require the bevel be cut first. The nose, or straight cut would occur on the second pass.

A top bevel with nose would require the nose, or straight cut, to be cut first. The bevel cut would occur on the second pass.

Three pass cuts are possible under limited circumstances. However, due to the complexity of programming, and the limitations of the process, three pass beveling is not recommended for production cut parts.

Due to plasma cutting torch limitations, the maximum thickness that can be beveled depends on the bevel angle to be cut. When bevel cutting at 45°, the actual cut thickness is 1.4 times the plate thickness. The actual cutting thickness for a given plate thickness and bevel angle is determined using this formula:

Actual Cut Thickness = Plate Thickness / cos(θ)

For a programmer trying to determine the feasibility of a specific application, it is more useful to solve for the maximum bevel angle that can be cut on a specific thickness of material. There are several different torches and consumable combinations that can yield different cutting capabilities, so it is necessary to know the maximum thickness that can be cut with a given setup. When the plate thickness and torch maximum cutting capability are known, the maximum bevel angle can be determined by this formula:

Max Bevel Angle θ = InvCos (Plate Thickness / Max Cut Thickness)

For plasma bevel cutting, there many parameters that must be considered. Even if a torch is tilted to an exact programmed angle, the resultant cut angle may be different.

An offset must be programmed to allow for accurate resultant cut angles. The CNC should automatically correct for the difference between actual torch angle and resulting cut angle depending on material type and thickness.

Also, as the torch tilts and begins cutting through thicker material, the kerf (or material removed as a result of the cut) is increased slightly.

This also must be accounted for to provide accuracy and repeatability in plasma bevel cutting. Again, the CNC must automatically accounts for this change as a factor of material type and thickness.

In addition to the additional kerf offset for cutting through thicker material while beveling, there must be consideration for a geometric offset dependent upon whether cutting the top of the part to programmed dimension, or the bottom of the part to programmed dimension.

If the bevel cut is as shown above, the torch must offset from the programmed path by the tangent of the cut angle times the material thickness. This offset should also be automatically accounted for in the CNC control based on material type and thickness.

As a plasma torch is tilted to a bevel angle, the torch moves closer to the material. An elevation offset must be programmed to ensure the torch does not crash into the plate on tilt. Also, the material thickness is increased while cutting a bevel, and this requires an additional height offset to provide for quality cutting. Again, this offset must be automatically compensated for based on material type and thickness.

The advantage of automatic accomplishment of all of these parameters and offsets is obvious. ESAB's Vision CNC fully automates all of the above geometric compensations. With other systems, some or all of these offsets must be accounted for in programmatic changes or with setup tables in offline software.

The greatest obstacle to accurate plasma beveling, particularly when attempting to cut plasma bevel with a land or nose, is the inability of arc voltage to accurately maintain the correct elevation of the torch above the plate.

One of the reasons arc voltage is not a reliable method for maintaining height control during bevel cutting is that plasma consumables wear during operation. This wear creates a difference in elevation even though the commanded arc voltage is constant.

When cutting vertical, this change in torch height has a negligable effect on the part size, and only minor effect on edge bevel angle. When bevel cutting, the effect is much more dramatic.

In the diagram above, it is apparent that even a small degree of change in torch elevation during cutting will result in significant changes of the dimension of the finished part. This makes the ability to cut beveled edges with a land or nose virtually impossible on a consistent basis.

This problem of using arc voltage as the method for maintaining height during plasma bevel cutting is significantly worse during dual pass cutting to produce bevels with lands. In the diagram below, the effects of consumable wear or speed changes during bevel with land cutting are shown in detail.

But even if the all of the problems with arc voltage height control regarding speed and consumable wear could be overcome, there is an additional problem when considering multi-pass bevel cutting. During this style of cutting, the plasma torch cuts over the same cutting path two times.

During the first cut, the part is separated from the adjacent scrap metal. Because of the effects of thermal cutting, this scrap does not stay in position after the cut. The actual arc voltage measured during cutting is the result of the difference in potential, or voltage, measured from the torch electrode to a cross section of the material being cut. If the plasma arc attaches to more metal sections, the arc voltage is artificially high. This would cause a machine using arc voltage control to lower the torch elevation automatically and result in inaccurate cuts.

Because there is no way to ensure where that scrap element will be located in relation to the part, there is no possible method of using arc voltage height control alone during multi-pass cutting and obtain accurate and repeatable results.

For this reason, ESAB uses a tactile sensing height control system that ensures that the elevation of the plasma torch above the material being cut within +/-0.012" regardless of consumable condition, cutting speed, or position of the scrap.

A number of things make a torch well suited for beveling, including accuracy, concentricity, ruggedness, and nozzle design. The PT-36's torch body and consumable production is held to a higher standard of accuracy and concentricity. This leads to more consistent bevel angles and part size.

And when compared to competitive torches, the PT-36 is much more robust. With heavier, thicker nozzles, cups, shields, and retainers, the PT-36 holds up better in demanding beveling applications.

Finally, with its long, pointed nozzle, the PT-36 allows the front end of the torch to stay closer to the plate when beveling, while the side of the torch has more clearance.

This means shorter arc length and therefore more control over the arc for more accurate cutting. It also means fewer crashes as the torch will more easily clear any slag or debris on the plate surface.

One of the most important requirements when plasma bevel cutting is the ability to throw a long arc. When bevel cutting at 45 degrees through 1.25" thick plate, you are actually cutting through more than 1.75" of material. Add to that the increased standoff that naturally occurs when the torch tilts, and you need a very long arc. To create and maintain an arc that long, your plasma power source must have the capacity to supply a high voltage while supplying full cutting amperage. The EPP family of power supplies are designed to provide operating voltages in the range of 200 to 300 volts at full output current.  Some competitive systems top out at 175 volts, meaning they can't throw an arc as far, and won't provide the beveling performance you expect.

Also consider under water beveling. Unlike competitive systems, only the PT-36 can be equipped with an Air Curtain assembly for effective under water plasma beveling, up to about 30 degrees. This technique reduces arc glare and noise, while capturing the vast majority of the plasma fumes, and keeping the plate cool.

ESAB offers two options for plasma bevel cutting systems: