Hypertherm HPR400XD, HPR800XD User Manual

Thick metal cutting techniques

For HPR400XD™ and HPR800XD plasma cutting systems

White paper

Introduction

Using plasma to successfully cut thick metal requires more
skill and technique than using plasma on thinner metal. The
thick metal cutting techniques described in this document
may be needed from the beginning of the cut with an edge
start all the way to finishing the cut with a completely
severed part.

Note: Unless otherwise specified, for the purposes of this document,

thick metal consists of stainless steel and aluminum from 5 inches to

6.25 inches (125 mm to 160 mm) thick. The techniques detailed in this
document were developed using 304L stainless steel. Materials used for the
development of this white paper were based on U.S. customary units
(inches). Metric conversions are provided for reference.

This document describes thick metal cutting techniques
developed for the HPR800XD that can help manage the
large plasma lag angles associated with thick metal cutting.
It also describes the timing and sequencing needed to be
successful in piercing up to 4 inches (100 mm) stainless
steel and 3 inches (75 mm) aluminum. This document is
broken into four sections:

• Thick metal cutting techniques overview on page 2 is an
overview of different lag angle management techniques for
thick metal cutting that covers a plasma cut from the
beginning to completion.

• Dogleg lead-out details for thick stainless steel on page 4
covers the details of a special lead-out technique (known as
the dogleg or acute angle lead-out) that can allow you to
completely sever a stainless steel part up to 6.25 inches
(160 mm) thick.

• Stationary piercing (up to 3-inch stainless steel and
aluminum) on page 8 describes the timing and sequence to
be followed to perform stationary piercing on 3 inch
(75 mm) stainless steel and aluminum.

• Moving pierce technique (up to 4-inch stainless steel) on
page 10 describes a moving pierce technique for thick
stainless steel that can be used with both the HPR800XD
and the HPR400XD. This technique, combined with
PowerPierce
the HPR800XD to 4 inches (100 mm) and for the
HPR400XD to 3inches (75mm).

®

technology, extends the pierce capacity for

Thick metal cutting techniques overview

Approximately 0.25 inches (6 mm)

Edge start

Positioning

Proper positioning of the torch is important to allow the
molten metal (or melt) to carry down the majority of the
thickness (especially when starting on a rough edge). Set
the height of the torch to the cut height listed in the cut
chart, which you can find in the Operation section of the
HPR800XD Instruction Manual (806500 [Auto Gas] or
806490 [Manual Gas]). Place the torch center-line about

0.25 inches (6 mm) from the workpiece edge. The edge of
workpiece should be approximately lined up with the shield
face diameter, as shown in Figure 1.

Figure 1 – Edge start positioning

Initial cut speed (lead-in speed)

A reduced cut speed should be used for at least the first
1 inch (25 mm) of the cut before traveling at the full cut
speed. The recommended initial cut speed should be 75%
of the full cut speed.

Cornering

Special cornering considerations may be necessary when
working with thick metal due to the extreme lag of the tail
(bottom portion) of the arc. If no technique is used, the cut
edge may lose its form, especially near the bottom of the
cut. Use one of the following methods:

• Rounding corners

•Corner delay

• Corner slow-down

Rounding corners

One method to maintain edge form is to round off corners of
90 degrees or less. In general, the radius should be equal to
or greater than the kerf value (larger is better).

Motion (or pierce) delay

An adequate motion delay must be used to allow enough
time for the arc to melt the majority of the edge prior to
motion being initiated. Suggested motion delays for 800 A
thick metal cutting are listed in the cut charts in the
HPR800XD Instruction Manual. These times may need to
be adjusted based on your application.

Corner delay

Allow the motion to dwell in the corner for approximately one
second to allow the arc tail to “catch up.”

Corner slow-down

Approximately 1 inch (25 mm) before entering the corner,
slow down the cut speed to 75% of the full cut speed.
Maintain 75% of the cut speed for approximately 1 inch
(25 mm) after the corner before resuming the full cut speed.

2

Completing the cut

Dogleg lead-out region

Lead-in

Part

One of the following techniques may be necessary on metal
5 inches (125 mm) thick or greater to fully complete the cut.
Otherwise, the arc may jump the very bottom portion of the
cut as it exits the edge of the metal or enters the kerf,
resulting in an incomplete sever of the cut piece.

Exiting the edge of material (lead-out speed)

For cuts that involve the arc exiting the edge of the material
(as shown in Figure 2), a reduced cut speed should be used
for the last 1 inch (25 mm) of the cut. The recommended
final cut speed should be 75% of the full cut speed. The

cutting table must continue motion beyond the edge of
the plate.

Figure 2 – Lead-out exiting the edge of the material

External contour part cut (dogleg or acute angle lead-out)

The dogleg (or acute angle) lead-out technique can be used
to redirect the tail of the arc and the molten metal flow into
the remaining portion of the cut (or “tab”), thereby allowing
for a complete cut (see Figure 3 and Figure 4).

1 Follow the edge start, lead-in, and cornering

recommendations previously listed.

2 Cut the external contour of the part and approach lead-out.

3 Just as the arc breaks through to kerf, change the cut

direction by approximately 120 degrees into the skeleton at
115% of the cut speed.

4 Continue the lead-out segment for approximately

1.25 inches (32 mm) – the molten metal flow from cutting
into the skeleton melts the tab, which completes the cut and
allows the part to “drop.”

Dogleg lead-out details are included in the Dogleg lead-out
details for thick stainless steel section in this document.

Figure 3 – Example of dogleg lead-out programmed cut path Figure 4 – 6 inch (150 mm) “dropped” external

contour cut

3

Dogleg lead-out details for thick stainless steel

Location of tab

Proper lead-out for thick material is critical to completely
sever a part; otherwise, a small tab may keep the part
attached to the skeleton at the point where the lead-out
enters the lead-in (as shown in Figure 5).

Figure 5 – Example of a “tab” in a thick stainless steel contour cut

This tab is due to the extreme lagging tail of the arc, the lack
of molten material flowing through the kerf, and the
insufficient voltage to maintain the arc attachment at this
distance from the torch. Crossing the kerf for the thickest
materials may not be possible for the same voltage limitation
reason, and even if the arc does transfer to the opposite
side of the kerf, the arc tail is likely to jump over the tab.

The dogleg method for stainless steel takes advantage of
this lagging arc by focusing it onto the tab section of the
cut. At the point where the leading kerf edge breaks into the
lead-in edge (and before the voltage reaches the critical
value of the transformer), the cut path changes direction into
an acute angle (60 degrees works well) toward the skeleton
(see Figure 6). This allows the arc to transfer to the skeleton
material, which reduces the voltage while driving the molten
material down towards the tab and subsequently melting it
off.

4

Overshoot

Lead-in

Leading kerf edge

Overshoot distance

Kerf width (K)

Lead-in edge

α

Programmed path

1

2

α

2

---





tan

------------------------ -

1

2

---

– Correction+











In order for the leading kerf edge to enter the lead-in edge
(with kerf compensation active), the programmed path must
overshoot by some distance (see Figure 6).

Figure 6 – Overshoot definition

This overshoot distance can be calculated using the
following equation:

Overshoot = K

Tabl e 1 – Correction factors.

Thickness Kerf Correction Factor

5 inches (125 mm) 0.530 inches (13.43 mm) 0.30

6 inches (150 mm) 0.680 inches (17.27 mm) 0.25

6.25 inches (160 mm) 0.700 inches (17.78 mm) 0.25

As an example, if α = 60° for a thickness of 6 inches, the
overshoot value is:

where K is kerf, α is angle, and Correction is an additional
factor necessary to ensure adequate penetration of the arc
into the lead-in section. The Correction factor values for
5–6.25 inches (125–160 mm) are shown in Ta bl e 1 .

K(0.866-0.5+0.25) = 0.68(0.616) = 0.419 inches

5