Conventional oxygen cutting uses low pressure and volume of a relatively inexpensive
gas and is, therefore, less costly than laser cutting with high-pressure nitrogen.
A 9.5mm stainless steel part can be cut using oxygen at a low pressure of approximately
3.5bar (50 psi) and a flow rate less than 0.06 m3/hr. Cutting the same part
using high-pressure nitrogen requires a pressure of 21bar (300 psi), at 34 m3/hr.
There are alternatives!
Laser cutting using high-pressure nitrogen is traditionally more costly because
of the higher volume and pressure required and because nitrogen in Multi-Cylinder-Packs
(MCP’s) is generally more expensive than oxygen. Using the same 9.5mm
stainless steel work piece as an example, assist gas accounts for 40% of part
processing cost if oxygen is used. In contrast, when nitrogen cutting using
MCP’s, the assist gas portion accounts for 90% of the cost. Based on a
2.2kw laser system, the cost to process a 9.5mm stainless part is approximately
£4 to £5 per hour using oxygen and about £15 to £16
per hour using nitrogen MCP’s. This is where on-site Nitrogen generation
comes into its own. By producing the required nitrogen via the tried and tested
domnick hunter nitrogen system the operating costs are comparable to those using
oxygen. Therefore users gain all the additional benefits that nitrogen assist
gas brings but without the increased expense.
For an average 2.2kw system, the total hourly operating cost for oxygen cutting,
including all cutting and non-cutting consumables, will be between £5
and £6 per hour. The total hourly operating cost, including all cutting
and non-cutting consumables for nitrogen cutting ranges from £8 to £20
per hour, based on material thickness and using MCP’s. The non-assist
gas part of the cutting consumables cost (external optics, lens, and nozzle)
remains the same for high-pressure cutting. Moreover, because of the increased
cost of nitrogen cylinders versus oxygen, and the typically higher pressure
used in nitrogen cutting ~ 9bar to 21bar (125 - 300 psi) compared to oxygen's
1bar to 4bar (15 - 50 psi) non-assist gas cutting expenses represent a smaller
hourly percentage of the total laser operating cost. Approximately 90 to 99%
of cutting cost associated with bought-in cylinder nitrogen is assist gas related,
depending on material type and thickness. Moving to a domnick hunter on-site
nitrogen generator can reduce operating costs by up to 90% when compared against
traditional bought-in MCP’s.
Cut speeds or Feed-rates in thicker stainless for oxygen cutting are comparable
to nitrogen gas cutting, so parts are processed at about the same speed. As
recently as five years ago, high-pressure nitrogen cutting could be accomplished
only at slower cut speeds. The introduction of higher-power laser systems has
improved nitrogen assist-gas cutting speeds so there is no significant feed-rate
advantage with conventional oxygen cutting 4.8, 6.4, and 9.5 mm stainless steel
materials.
Looking Down the Production Line
To accurately calculate cutting costs and weigh the value of high-pressure
cutting, it is important to look beyond the laser cutting process to operations
further down the production line. While high-pressure nitrogen cutting using
bought-in cylinders is more expensive than conventional laser cutting, the milled
edge quality produced by this process eliminates all secondary operations and
often reduces welding time. The cost of high-pressure cutting using nitrogen
must be weighed against the decreased throughput time due to elimination of
secondary operations.
High-pressure nitrogen cutting adds value to the component by producing a
burr-free, cosmetically appealing part. The benefits of high-pressure nitrogen
cutting are realised outside the laser system in reducing labour-intensive grinding
and other post-processing operations, such as welding. Cut edge quality produced
using high-pressure nitrogen generally requires less tolerance on welding fixtures
and less time spent welding.
The best candidates for using high-pressure nitrogen are those who work primarily
with stainless steel or are required to produce parts that are cosmetically
appealing. Increasingly, manufacturers are required to produce parts that meet
total quality requirements for form, function, and appearance.
