Grinding or laser ablation: when does dimensional accuracy decide?

With precise functional surfaces, however, the question changes. What then counts is not only whether a troublesome layer is removed. It is also relevant how strongly the base material, the edge or the existing surface structure is affected.

Laser-based ablation can be an alternative here – but not for every surface, and not automatically more cost-effective.

Why grinding is so attractive in day-to-day operations

Grinding is one of the most well-established mechanical processing methods.

The equipment required is widely available. Staff know how to work with it. Consumables are obtainable, and many tasks can be started directly without complex process development.

This simplicity has considerable economic value.

Not every corroded bracket needs a sophisticated laser system.

If an employee brings a robust surface into the required condition in a few minutes with an angle grinder, that is a benchmark to be taken seriously.

DGUV Information 209-002 “Schleifen” accordingly covers a wide range of established grinding tools and typical working methods.

A sound technical assessment should therefore not attempt to present grinding as an outdated process in general.

The question is rather:

Where does the mechanical contact create disadvantages that are relevant for the specific component?

What mechanical removal does to the surface

An abrasive removes material mechanically.

In doing so, it makes no physical distinction between “unwanted coating” and “valuable base material”.

As long as the abrasive grain is in contact and sufficient force is applied, material can be removed.

In practice, the operator controls the process through:

• Grit
• Tool
• Contact pressure
• Angle
• Movement
• Processing time

An experienced employee can work very precisely with these.

Nevertheless, the process remains mechanical in principle.

Once the troublesome top layer has gone, further movement can also affect the substrate.

On robust surfaces, that may be completely irrelevant.

On a mating surface or a thin edge, it can become decisive.

Laser-based ablation, by contrast, involves no direct mechanical tool contact.

The technical background to this is described in the article How does laser ablation work when cleaning metal?.

When dimensional accuracy becomes relevant

Not every component has tight tolerances.

A steel girder may well tolerate a few tenths of a millimetre of surface change without any problem.

With other components, the picture is different.

Dimensional accuracy can be relevant, for example, with:

• Mating surfaces
• Sealing faces
• precise tool contours
• thin-walled components
• defined edges
• Repair areas
• geometrically critical components

Here it should not only be checked how quickly a coating is removed.

It should also be assessed:

How much base material is altered in the process?

A process that works five minutes faster can still be worse cost-effectively if a functional surface then has to be reworked.

That is precisely why rework belongs in every process comparison.

Edges and thin components

Edges present a particular situation in manual mechanical processing.

If a grinding tool is guided over an edge, the local contact pressure can change.

Depending on the tool and the working method, an originally sharp geometry can be rounded off.

For many tasks, that is of no consequence.

With restored components, fits or visible design edges, however, it can be undesirable.

The same applies to thin sheet metal.

Here, alongside material removal, heat and mechanical action can also become relevant.

Laser processes are likewise not automatically free of influence.

With unsuitable parameter settings, laser energy can alter the substrate.

The difference therefore does not lie between:

Grinding alters – a laser never alters.

It lies between two different ways of introducing energy or mechanical action into the surface.

For sensitive applications, the final condition actually achievable should be compared.

Tool wear as a process variable

An abrasive changes during use.

Grains become blunter, discs wear and the effective geometry can change.

This means the processing behaviour can also change over the course of the process.

In manual work, an experienced operator often compensates for this automatically.

In standardised processes, however, the condition of the tool should be taken into account.

Consumables additionally create:

• Purchasing costs
• Storage requirements
• Changeover times
• Disposal

This does not mean that these costs have to be particularly high.

They simply belong in the process calculation.

With a laser there is no abrasive body that is mechanically consumed during removal.

Optical protective components and filters have to be taken into account instead.

So here too, “consumption versus no consumption” is not the right comparison.

The types of consumption differ.

Grinding marks and intended roughness

Mechanical influence can be expressly desirable.

If a surface is to be roughened for a subsequent coating or bonding process, a grinding process can clean it and create the required topography at the same time.

That is an advantage.

A surface left as unchanged as possible is not the goal for every subsequent process.

For surface pre-treatment, Fraunhofer IFAM therefore works with both abrasive and laser-based processes.

The desired final condition determines the right technology.

It is also worth noting that different mechanical processes can produce different surface structures despite similar roughness values.

Studies on austenitic steels, for example, show differences between ground and glass bead blasted surfaces.

It follows that:

A single roughness value does not necessarily describe the complete surface condition.

For technical joining processes, it should therefore be defined which surface properties are actually required.

Local areas instead of complete surfaces

Grinding can also be used locally.

An employee can work on a small spot in a targeted way.

With very precise, geometrically limited areas, however, the effort increases.

If, for example, a narrow contact area is to be exposed from a coating, adjacent material may have to be protected or the work carried out with particular care.

For laser-based coating removal, Fraunhofer IFAM highlights the sharply delimited local processing.

With certain components, that can be an advantage.

Typical examples would be:

• electrical contacts
• Earthing points
• defined bonding surfaces
• local repair areas
• narrow joining zones

The smaller and more precise the area, the more cost-effectively relevant the ability to process a locally limited area can become.

Processing speed

For large, simple surfaces, grinding can be extremely cost-effective.

A powerful tool and an experienced operator achieve high removal rates.

A laser therefore has to offer a specific additional benefit for an investment to be justified.

When measuring speed, however, the same final condition should be compared.

If grinding removes material faster but an additional finishing step is necessary afterwards, that time has to be taken into account.

If the laser needs several slow passes, that too belongs fully in the calculation.

What matters is:

Time until the component is released.

Not merely the speed of the first working step.

Accessibility and geometry

A grinding tool needs mechanical access.

The disc, brush or abrasive belt has to be able to reach the surface.

Very narrow grooves or complicated internal areas can be problematic.

A laser, in turn, needs optical accessibility.

It, too, cannot process concealed undercuts through the material.

Both technologies therefore have geometric limits.

The difference lies in the type of access required.

With a specific component, the following should therefore be checked:

Can a tool be applied mechanically?

Can a laser be directed optically onto the surface?

Which method reaches more of the areas that actually need to be treated?

Automation

Grinding can be automated.

Robot-assisted grinding processes are well established in industrial production.

Tool wear, contact force and component tolerances have to be controlled accordingly, however.

Laser processes can likewise be automated.

Here, beam movement and processing head are guided by scanners, axis systems or robots.

With an optical process, the mechanical contact force between tool and workpiece is eliminated.

That can simplify path planning for certain tasks.

Conversely, a stable process window for the energy input has to be found with a laser.

Here, too, neither technology automatically has the simpler automation process.

The requirements are simply different.

Dust, extraction and occupational safety

With grinding, dust, particles, sparks, noise and hazards from rotating tools can be relevant.

DGUV Information 209-002 describes typical accident and health hazards as well as suitable protective measures for grinding work.

Laser-based ablation likewise produces particles and process emissions.

In addition, with open, high-power systems the laser radiation itself has to be taken into account.

DGUV FBHM-139 covers class 4 laser cleaning machines and the protective measures required.

Safety may therefore not be generalised as an advantage of one side.

Both processes require a properly planned workstation.

The types of hazard differ.

How the comparison is carried out on the component

A split reference surface is suitable for a sound decision.

A comparable component, or two identical areas, are brought to the same target condition with both processes.

At least the following variables are then assessed:

• Processing time
• The surface condition
• Dimensional change
• Roughness, if relevant
• Rework
• Consumables
• Set-up time
• Operator effort

With critical functional surfaces, additional measurements can be necessary.

A mere before-and-after look may not be enough there.

When grinding clearly remains sensible

Grinding is particularly attractive when:

• a robust surface is involved,
• mechanical material removal is permitted,
• Roughening is desired,
• a simple tool is sufficient,
• only occasional work is carried out,
• Investment costs are to be kept to a minimum.

In such cases, the mechanical process can be the most cost-effective solution.

There is no reason to replace it merely because a newer technology exists.

When laser ablation should be examined

A laser-based process becomes particularly interesting when:

• Base material is to be treated in as controlled a manner as possible,
• precise partial areas are exposed,
• no mechanical contact force is wanted,
• recurring local areas are automated,
• Edges or functional surfaces are relevant,
• Abrasives in the surrounding area would be problematic.

Whether that actually results in an economic advantage has to be measured on the specific process.

Conclusion: dimensional accuracy can matter more than maximum removal rate

Grinding or laser ablation: when does dimensional accuracy decide?

As soon as the surface has a technical function.

For numerous robust tasks, grinding remains a fast, simple and cost-effectively strong method.

At the same time, however, mechanical contact can affect the surface structure and, where removal is correspondingly deep, the geometry as well.

Laser-based ablation offers a different approach without mechanical tool contact.

That does not automatically produce a better result, however.

The decision should answer the following questions:

1. May base material be removed?
2. Is roughening desired?
3. How sensitive are edges and functional surfaces?
4. How large is the area to be processed?
5. What rework arises?
6. Which process reaches the target condition faster?

Only then is it possible to judge whether grinding or a laser-based process is more cost-effective for the specific task.

Sources

• DGUV Information 209-002 “Schleifen” — Typical hazards and protective measures in grinding work.
• Fraunhofer IFAM – Surface decoating by laser — Local and selective coating removal.
• Fraunhofer IFAM – Surface Technology — Comparison and development of mechanical, abrasive and laser-based pre-treatment processes.

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