Plano Convex Lenses for a CO2 Laser Cutter

In an earlier article we explained about the two main types of Zinc Selenide (ZnSe) lenses used within a laser cutting machine and that the most commonly used of these is a Plano (flat) Convex (curved outwards) lens

Aside from physical dimensions this type of lens has three defining characteristics:

1. Focal Length (F/L): the point from the lower (flat) surface to the point of beam convergence
2. Focal point: the point of beam convergence
3. Depth of focus: a range measured between a point above and below the focal point in which the lens delivers a desirable result.

The focal range is application specific and will vary depending on the material being processed and whether the application is laser cutting or laser engraving.

During manufacture, by variation of the convex surface the point of convergence can be shortened or lengthened and in turn the size of the focal point and depth of focus will vary too

As with photography, selecting a specific lens for certain applications can enhance significantly the output quality; for example, a close-up lens or a telephoto lens. The same is true for lenses within a laser cutting or laser engraving machine. 

Lenses in a laser cutter are interchangeable.

By increasing the focal length of the lens, the truncation of the beam becomes narrower and the focal point becomes larger. This is particularly useful for laser cutting through materials of a greater thickness or for laser engraving surfaces that are not uniformly flat because longer focal length lenses have a greater depth of focus.

Some manufacturers refer to long focal length lens as a 'cutting lens' and a short focal length lens as an 'engraving lens'. Technically, this is not accurate.

Longer focal length lenses are more useful when engraving high debris materials, such as wood for example, where the greater distance between the lens and the material provides for a lower risk of lens contamination.

Using a lens with a short focal length will have a very wide truncation with a very narrow depth of focus, so the uses for such a lens are very limited, however the resulting very small focal point provides for the highest possible resolution and also for maximum energy density of the laser; good for removing more material in a single pass. A good use for this type of lens would be marking photographic detail or engraving rubber stamps

The short focal length lens is commonly misunderstood as being only for laser engraving. Actually, it can also be useful for some laser cutting applications. For example, some materials are very heat sensitive and a lens with a very small focal point is less likely to draw heat away from the cutting point into the material causing adverse effects, such as material warping.

In a typical laser cutting or laser engraving machine the range of Plano Convex ZnSe lenses will be as follows:

1. 38.1mm (1.5”) F/L: high resolution engraving, <1mm material thickness cutting
2. 50.2mm (2”) F/L: good resolution engraving, <5mmmaterial thickness cutting – this is the best 'all-round' or 'general purpose' lens
3. 63.5mm (2.5”) F/L: lower resolution engraving, 3-10mm material cutting
4. 101.6mm (4”) F/L: low resolution engraving, 10-20mm material cutting

Lenses of longer focal length are available but only used for cutting and will not generally be suitable for smaller format machines.

At Lotus Laser Systems we manufacture a wide range laser, marking and engraving solutions with a variety of lens options. Our experts would be happy to advise you which configuration would suit your application best.

Types of Air Assist for a Laser Cutter

Most laser cutting machines will incorporate some form of air or gas assist the function of which is three-fold:

• To improve the efficiency of the laser cutting process. Generally, this works by the positive air keeping vaporised debris away from the path of the beam in order that all of the laser energy is applied to the material.
• To improve the quality of output. This works by the positive air helping to keep vaporised debris away from the surface or the edge of the material being laser cut so that contamination of the material is minimal.
• To reduce the requirement for maintenance (cleaning) and prolongue the life of optical elements. This works by positively pressurising air at the focus carriage (the laser head) so that contaminants are kept off of the sensitive surfaces of the optical elements.

Well designed laser cutting machines will provide for all three of the above functions.

Some machines do not incorporate a nosecone. These are not good for laser cutting but are sometimes better for laser engraving because without the nosecone the laser head is lighter in weight, so it can move faster with lower vibration. If these machines have air assist then it’s usually delivered via a small bent metal tube delivered at an angle to the work surface at the focal point. The angled air assist helps to dislodge debris, pushing it into the flow of the extraction across the material surface.

It’s important to note that unless the laser fume extraction is very well specified then this type of directional air assist can cause premature contamination of the lens. At Lotus we do not favour this design of air assist.

For best results when laser cutting the laser head will incorporate a nosecone, which will sit a few millimetres away from the surface of the material. At the end of the nosecone will be a small hole through which both the beam and air assist are delivered. If the bore of the hole is too big and/or the distance of the nosecone from the material is too great then the whole process of air assist becomes very inefficient.

Nosecones with smaller holes work better as they direct the air more locally through the cut instead of around the cut. The smaller hole also helps to increase air pressure and allows for a smaller air pump to be used. In case where a gas is used instead of air, such as nitrogen for example, a small nosecone is essential to minimise the waste of a potentially expensive gas.

An often overlooked aspect to the design of air assist is the hose bore (diameter) through which the air flows from the pump to the nosecone. Many machines use a tube with a bore that is too small. This greatly reduces the volume of air that can flow through it.

Using a larger bore air assist delivery tube reduces the speed at which the motion system can run but greatly increases the efficiency of the air assist process. With some models of Lotus Laser Systems it’s common for customers that process high debris materials like laser cutting wood or laser cutting card for many hundreds of hours without the need to clean the optics. Conversely, many machines on the market with poor air assist designs will require almost daily cleaning of optical elements.

One important point to note is that when a nosecone is fitted air assist MUST always be used otherwise the process can work in reverse and the lens can quickly suffer contamination and even catastrophic failure.

Lastly, it is very important for the laser cutting machine to be able to activate the air assist through its control software.

At Lotus Laser Systems we manufacture a wide range laser, marking and engraving solutions with a variety of air assist options. Our experts would be happy to advise you which configuration would suit your application best.

Efficient Extraction From a Laser Cutting Machine

Most people when evaluating which laser cutting machine to purchase perform a simple tick-box exercise listing the various functions and features as listed in the manufactures sales brochure. Few actually analyse those features in detail to see how well designed they are and even fewer test those features to see how well or efficient they work.

Inefficient laser fume extraction is very costly in terms of increased maintenance, decreased throughput, poor quality output (wasted materials) and reduced component life. Bad extraction will be hazardous to the health of the operator and is the biggest cause of flaming (fires) in Laser Cuttes.

One of the most misunderstood and overlooked aspects of a laser cutter is the efficiency of the laser fume extraction design. In the majority of plotter format machines, extraction is taken from a vent positioned at the rear of the worktable. This causes fumes to be drawn over the surface of the material.

This type of extraction is generally OK for laser engraving but not at all good for laser cutting because the rising fumes block the path of the beam, dropping laser efficiency. The direction of air can cause lightweight materials to move on the worktable and this design of extraction often allows contaminants to contact with the laser head. The platen used in almost all such machines will quickly contaminate, causing a restriction with air-flow.

Some of the larger format machines utilise extraction from the laser head by way of a long, flexi hose. Again, this only works for engraving but it has many inherent faults. Firstly, the hose contaminates fast, restricting flow and dropping efficiency and with constant flexing it will perforate over time. Because of the narrow hose diameter the vacuum pressure needs to be very high and the volume of air that can be taken in this way is very low. It’s therefore not possible to use an extract to atmosphere system with this type of local extraction. This type of extraction cannot be used when cutting small parts and/or lightweight materials as they can be pulled into the path of the beam by the air-flow.

By far the most efficient way to extract from a laser cutter is from underneath the material. We call this Underside Extraction or UE for short. This causes a vacuum underneath the material, which if the blower is strong enough can hold slightly warped materials flat to the work-table. This is especially useful for laser cutting paper, laser cutting leather or laser cutting textiles. When combined with a well designed air assisted nosecone UE will pull the laser fumes down through the cut line virtually eliminating any chance of those fumes contacting the laser head. This will often increase edge quality, especially for high debris cutting applications like laser cutting wood and reduces maintenance in some cases to near zero.

Well designed UE will provide for an even pressure and flow of air across the whole worktable so the best machines incorporate some kind of funnel underneath the worktable with the point of extraction placed centrally to the funnel.

The best laser cutting machines will incorporate dual extraction ports: one to draw over the material and another from the underside.

At Lotus Laser Systems we manufacture a wide range laser, marking and engraving solutions and can supply a wide range of extraction solutions. Our experts would be happy to advise you which configuration would suit your application best.

Extract to Atmosphere for a Laser Cutter

In an earlier article we discussed two main types of laser fume extraction for a laser cutting machine one of which was laser fume filtration and the other Extract to Atmosphere (ETA). Here in this article we take a more detailed look at the correct design of an extract to atmosphere system.

This is a highly specialised subject. Most laser cutting machine and laser engraving machine suppliers as well as many fume extraction suppliers do not truly understand what is required to provide for an efficient, well performing extract to atmosphere set-up for a laser cutting machine.

By some standards ETA is merely a small turbine with enough flexi-hose to hang out of the nearest window. Nothing could be further from the truth or in fact nothing could be more inadequate and potentially dangerous than this type of set-up.

For an ETA system to perform well and most importantly for it to be safe, the contaminated air within the laser cutting machine must be quickly removed and expelled well beyond roof height where it will quickly dissipate into the atmosphere safely.

This requires moving very large volumes of air, a by-product of which is the generation of high levels of noise. This is the main reason why some sites, such as those in inner cities, simply are not suitable for ETA.

Start at the laser machine by connecting a 1m flexible hose from the extraction port to some kind of gate. The gate will be used to control flow as well as shut-off the machine from cold/damp air that may enter from outside when the system is not in use.

The type of flexible hose is critical; under no circumstances use cloth or foil types of flexi hose as these are designed for very low pressure extraction, such as tumble-dryers, and they will collapse and perforate.

The only type of flexi hose to use is made from PU, sometimes treated to be ant-static and with very strong reinforcing wires. Use a transparent version as you can then monitor the level of contamination and change it when necessary

After the gate the duct must be made from galvanised steel with at least 150mm diameter. At each join it must be riveted and sealed using a suitable mastic. This duct will widen to match the diameter of the fan inlet. From the fan it will be a constant diameter to match the fan outlet.

The fan MUST be positioned OUTSIDE of the building so that the duct from the laser cutter is under vacuum. This is safer because if the duct is compromised fumes will not escape from it into the work environment. If circumstances allow, site the fan on the floor as it will be easier to service.

The performance of the fan is critical to the efficiency of the machine and should be specified by a professional making careful calculations for the size of the laser machine and the length/design/resistance of the duct.

To reduce noise, just before and just after the fan fit a pair of attenuators. These act similar to the baffle box on a car exhaust.

Beyond the fan outlet attenuator the duct should run in a straight line upwards to a point 1m beyond the roof line or apex of the building. The duct should terminate with a jet cowl and NOT any other type of duct cap.

Careful consideration should be given to the discharge point to allow for maximum dissipation of the expelled air and minimal nuisance to neighbours.

This type of set-up will provide for many, many years of use with next to no deterioration in performance and little to no required maintenance.

At Lotus Laser Systems we manufacture a wide range laser, marking and engraving solutions and can supply custom designed ETA solutions. Our experts would be happy to advise you which configuration would suit your application best.

Laser Cutting on a Lamella Table

In an earlier article we discussed two types of table used in a laser cutting machine one of which was a lamella table. Here in this article we take a more detailed look at this type of cutting table.

A lamella cutting table is essentially a series of thin bars laid on edge onto which the material to be laser cut is placed. These bars can be manufactured from sacrificial material such as acrylic or more commonly they are made from anodised aluminium.

If made from aluminium the shape of the bar is critical to its effectiveness. A well designed lamella will be shaped like an arrow head with a slightly rounded tip, which is the point where the material comes in to contact with the bar. Such a shape will deflect any laser beam that passes through the material away from the underside thereby minimising or even eliminating any backward reflection.

This is particularly useful for transparent materials such as laser cut clear acrylic, heat sensitive materials such as laser cut mirrored acrylic or materials that contaminate very easily because of the high amount of debris they produce, such as laser cutting wood.

A well designed laser cutting machine with a lamella table will allow for reasonably thick and therefore heavy sheet materials to be placed upon it without any distortion.

The best machines incorporate a design where the lamellas are easily removable. This allows for the removal of bars that are not required, reducing the surface area that the material is held by to a bare minimum and thereby creating a total void beneath the material.

Well made lamellas will last for many years and unlike the alternative, more commonly found honeycomb table, lamella bars can be cleaned, will not distort and so are not a consumable item. Should the bars ever require replacement then they can be replaced just a few at a time for a negligible cost.

For processing sheet materials like laser cutting acrylic or laser cutting plywood a lamella cutting table will produce results that far surpass that of any other form of cutting 

When combined with a well specified laser fume extraction system, especially when extraction is from the underside of the material, cleaning of the laser cutting machine will be massively reduced in frequency.

At Lotus Laser Systems we manufacture a wide range laser, marking and engraving solutions with many types of cutting table. Our experts would be happy to advise you which configuration would suit your application best.

Laser Cutting on a Honeycomb Table

In an earlier article we discussed two types of table used in a laser cutting machine one of which was a Honeycomb table. Here in this article we take a more detailed look at this type of cutting table.

The honeycomb cell structure is made from what are essentially very thin strips of aluminium that are in some places glued together. This glued section is then extruded, concertina fashion, to form a sheet of honeycomb shaped cell structures.

This honeycomb material is actually designed to be laminated between two sheet materials, for example MDF, where it's used primarily in the construction industry. The resulting 'sandwich' board is very light weight and very strong, however, without this lamination the opposite is true; the honeycomb is incredibly weak.

The idea behind using this honeycomb is to minimise the occurrence of backward laser beam reflection; where the laser beam passes through the material into the honeycomb cell there is a void that allows the beam to pass well out of focus and so not reflect back into the material.

In some respect this works and a honeycomb table can be good for cutting non-rigid materials such as laser cutting textiles or when the requirement is to laser cut very small parts, however, because of the weakness of the structure it deteriorates very fast. It is also impossible to clean this type of table and problems occur when placing heavy materials upon it and/or when using extraction from below the material.

Therefore, this type of cutting table is truly a consumable item and frequent replacement can be costly over time.

Common faults with a honeycomb are that the aluminium sheet deforms, which actually can cause the very backward reflection that this type of table is trying to avoid. When processing heavy material, such as laser cutting acrylic, it can have a tendency to sag, casing for beam defocus. Once dirty it can cause debris to stick to new materials and using it with higher power CO2 lasers can actually cause the cell structure to separate.

Furthermore, there are limitations with regard to the overall size of a honeycomb cutting table.

In short using a honeycomb cutting table often has more limitations than benefits so unless it is your only available option or your application is one of the few where a honeycomb table works well it is far better to use a lamella type table when laser cutting sheet materials.

At Lotus Laser Systems we manufacture a wide range laser, marking and engraving solutions with many types of cutting table. Our experts would be happy to advise you which configuration would suit your application best.

Cutting Tables For CO2 Plotter Lasers

For low power (<100w) CO2 cutting lasers one element of the machine that is often overlooked and misunderstood is the worktable.

The design and type of worktable can have a significant difference on the performance of the laser machine, the working life of its components and the quality of its output.

Most systems that are commonly available on the market today will consist of one or a combination of the following worktables:

• A stainless steel plate: This type of worktable is usually found in machines designed primarily as a laser engraver. For most laser cutting applications this type of table produces very undesirable results and requires significant maintenance (cleaning) even after the smallest of jobs.
• Honeycomb table: This type of worktable is manufactured from very thin aluminium that is glued and extruded to form a honeycomb structure onto which materials are cut. The design of this table is to minimise backward reflection and to allow hot gases and particulates to pass away from the underside of the material.
• Lamella table: This type of worktable incorporates thin bars that are usually made from anodised aluminium and indicates that the system has truly been designed as a laser cutting machine.

Most users would have had experience of a honeycomb table because this is the type of table supplied by most of the major EU and US laser machine manufacturers. Such tables are not very robust as the honeycomb material was never intended to be used in this way and will quickly deteriorate by way of bending and impact damage as well as contaminating very fast.

Therefore a honeycomb table is deemed a consumable item because it cannot be cleaned or repaired and with frequent use will require replacement in a number of months if it is to perform correctly. This is not a cheap item so it is therefore no wonder why equipment manufacturers like to incorporate this table in their machines!

It is however a good solution for laser cutting textiles and similar non-rigid materials.

For laser cutting acrylic, laser cutting wood and other rigid sheet materials a lamella table is a far more robust solution. In a good machine the lamella bars are very strong, allowing materials of a greater size/thickness/weight to be laser cut and the bars can be easily and quickly cleaned. It’s therefore common for these bars to last for several years of heavy use and replacement bars are often very low cost.

For best results when laser cutting rigid materials a high performance laser fume extraction unit should be combined with a lamella table as this combination delivers the ultimate solution in almost every aspect.