A burr can be both too small to see with the naked eye and large enough to compromise a gear’s performance.
This is particularly true in micro gears.
In most cases, however, the burr itself is not the real problem—it is simply a sign that something upstream in the manufacturing process is no longer performing as it should, whether that be the cutting tool, machining parameters, the mold, or another process variable.
Let’s start with scale, because everything changes depending on the size of the gear.
A 20-micron burr on a module 3 gear tooth is generally insignificant.
The very same burr on a module 0.3 tooth, however, is anything but negligible.
On a tooth that small, a burr of that size is large enough to shift the point where the mating teeth come into contact.
And when the contact point changes, the way torque is transmitted from one gear to the other changes as well.
How does a burr form?
Burr formation is an inherent consequence of every machining process.
No cutting edge, regardless of how sharp it is, ends in a perfectly sharp point—it always has a small radius.
As the tool removes material, that rounded edge cannot shear the material completely at the very end of the cut.
Instead, a small amount of material is plastically deformed and pushed toward the edge rather than being cleanly cut away.
That is how a burr is created.
On a large gear tooth, the burr is tiny relative to the geometry and generally has little influence on performance.
On a micro gear tooth, the same burr becomes proportionally much more significant.
This explains why
a burr should primarily be viewed as a diagnostic indicator.
As the cutting tool wears, its edge radius increases.
The material is pushed rather than cut, causing the burr to grow progressively larger.
Toward the end of tool life, this increase often becomes quite sudden.
For most of the tool’s service life, burr height remains relatively small and stable.
Once wear reaches a critical point, however, burr size can increase dramatically—from just a few microns to well over 100 microns within only a few meters of cutting.
Monitoring burr height during production therefore provides a simple and effective way of determining when it is time to replace the cutting tool.
There is another effect that is often overlooked.
A burr can also deceive the person trying to inspect it.
Even if it is invisible to the naked eye, a burr may still protrude beyond the tooth profile along its edge.
One might assume this would distort measurement results.
In reality, it does not.
Modern gear measuring machines—including contact measurement systems—automatically exclude the tooth edges from their evaluation algorithms.
They detect the geometry at the edge but do not include it when calculating the gear profile.
For very small gears, manufacturers increasingly rely on non-contact optical measurement, although the primary reason is different: avoiding the risk of the stylus touching a delicate tooth flank and potentially damaging its surface.
So when should a burr actually be removed?
The answer depends largely on the workpiece material, and the choice of deburring process is more important than many people realize.
With soft, ductile materials such as copper or engineering plastics, mechanical deburring is often ineffective.
Instead of breaking away, the material tends to deform, and removing one burr often causes another to form nearby.
With hardened steels, the opposite occurs.
The burr fractures cleanly, leaving the tooth surface much cleaner.
Thermal deburring processes expose the component to temperatures of several thousand degrees for just a few milliseconds.
They are highly effective for certain geometries but may pose unnecessary risks for extremely thin gear teeth.
Electrochemical deburring takes the opposite approach.
Instead of applying mechanical force or heat, it dissolves the burr through an electrochemical reaction.
Because the process is contact-free and generates virtually no thermal load, it produces no secondary burrs and does not induce mechanical stress in the component.
For delicate precision parts, it is often the preferred solution.
There is, however, one practical limitation.
The process takes place in a liquid electrolyte, requiring each component to be individually held, suspended, and electrically connected.
When the parts become extremely small, handling them one by one is not always practical.
Ultimately, there is one principle that changes the entire perspective:
A completely burr-free machining process does not exist.
The realistic objective is not to eliminate burrs entirely, but to determine the maximum burr size that can be tolerated without affecting the component’s function—and to keep the process consistently within that limit.
There is even an international standard dedicated specifically to this subject.
ISO 13715 allows designers to specify directly on engineering drawings the permissible size of burrs and undefined edges, ensuring that functional requirements are clearly defined from the outset.
