By DR. KEN WOODS, engineering manager for laser systems for Hypertherm Inc.

Retrofitted laser heads improve machine performance.

A laser's ability to cut tight tolerance parts with small features in an automated process make it a vital technology in today's industrial manufacturing environment.

Since 1990 there have been about 35,000 laser cutting machines installed worldwide and almost 9,000 of those are in the United States. During this period, the overall price of new laser systems has not changed substantially, but the available features and machine potential productivity have steadily increased. Most machines being installed in today's marketplace offer laser powers in the 3 kW to 6 kW range and include features such as high power CPU processors, high accelerations, intelligent nesting software, remote diagnostics and advanced material handling systems.

Improvement opportunities
One of the most common retrofits for older laser systems is a cutting head upgrade. These upgrades can be done by the machine manufacturer or by an independent service provider and can take anywhere from 4 hours to 2 days. There are several features that should be considered when selecting the best solution for a particular system.

Contact or non-contact height sensing
Many of the older systems have a roller ball or spoon stylus that regulates the distance between the cutting nozzle and the work piece. These systems tend to scratch the material surface and are prone to collisions with parts that have tipped up or are close to the cutting path. For this reason many of the cutting head retrofits are performed to convert to a non-contact sensing system based on a capacitive signal. The output signal from the height sensor can be matched to the existing signal to allow minimal changes to the CNC and achieve proper height following performance.

Available lens focal lengths and access
Several systems in the field employ a single focal length in the cutting head and often access to the lens for inspection or changes can be difficult. This can be a significant opportunity for improving the cutting performance and decreasing non-value-added operator time. Usually adding a second focal length to a machine can substantially improve performance. Longer focal lengths (7.5 in.) are typically used for thicker material (greater than 1/4-in.) while thinner material can usually be cut faster with a shorter focal length (5 in.) lens. Appropriate selection of the focal length will improve both cut quality and speed. Having the ability to quickly change the lens as new material is loaded on the machine is important to maximize cutting time. Additionally, the availability of low cost lens holders for the cutting head allows the operator to install a new lens into a spare holder while the machine is cutting.

Collision protection
Cutting head crashes can be a significant source of down time for many older systems. Some systems utilize plastic bolts that will shear in a collision, which requires the operator to stop the machine, remove the old screws, install new screws, and usually re-center the beam to the nozzle. Retrofits commonly use one of two systems for protection from this condition, magnetic and spring-loaded mounts. Magnetic systems have the advantages that the restoring force decreases with displacement and there are no limitations on the head displacement, but they cannot protect the head from a vertical crash and they require the operator to reset the head onto the mount. Spring systems are self-resetting when the collision condition is removed and will protect the head in a vertical crash condition while keeping the beam path enclosed, but damage can be done if the motion does not stop completely before the available travel is used. Regardless of the collision protection, another factor to consider is the potential cost of replacement parts after a collision occurs. Ceramic components on some cutting heads can be expensive and easily damaged.

Nozzle technology
A skilled operator has most likely developed a sense for relationships between cutting conditions and cut quality. These may involve changes to the equipment's cutting charts and utilization of different nozzle geometries. Many cutting head systems offer a wide range of nozzle sizes and styles to maintain or further enhance cutting performance. Special nozzles can be used for oxygen cutting of mild steel to improve cut speeds or quality, while other nozzles may give better results in high pressure nitrogen cutting of stainless. It is important that these geometries are explained to the operator during the installation and subsequent trials to allow optimum machine performance.

Other features
There are a number of other features that should be considered from an operator ease of use standpoint. The amount of time it takes for the lenses to be changed or inspected, the ease and repeatability of the lens centering process, the speed and accuracy of an operator changing the focal location for adjustments from stainless to mild steel cutting, and the ease and frequency of height sensor recalibrations are all issues that can reduce machine cutting time. Also, if the shop has several lasers a new cutting head might allow use of the same lenses in multiple machines. This can eliminate any confusion between a 7.5 in. effective focal length lens and a 7.5 in. working distance lens. Examine the overall construction of the cutting head and look for any components or cables that might be easily damaged by the cutting process.

Once these features have been evaluated it is also important to consider how the investment will improve machine productivity. Based on typical results from cutting head retrofits the following assumptions are made: The new cutting head will add a 5in. lens thereby increasing cutting speed on the thinner material from 180 to 200 IPM.

The new cutting head also offers cut speed improvements of 5 percent for the thicker material by either a new nozzle technology or better delivery of the assist gas.

Improvements in machine utilization from less down time due to crashes, nozzle centering, and lens inspection or changes are not included, but will most likely accelerate the payback period. 75 percnet of the machine time is for the thinner material, 25 percent of the time is for thicker material.

In one week of operation the system would have cut approximately 35 nests of the thinner material and 12 nests of the thicker material with the old parameters. Installation of the new equipment will expand capacity to produce an additional 2 nests in the thinner material and 1 nest of the thicker material per week, additional revenues of $506, and an return-oninvestment period of approximately 35 weeks.

While the cost of the upgrade is important, knowledge of the features available and the benefits of each are helpful in the decision process.