Britforge3D

Understanding Tolerances in 3D Printing

One of the most important concepts to understand in 3D printing is that printed parts are never exact copies of their digital designs. This can be surprising at first, especially for people coming from traditional manufacturing or CAD backgrounds. On screen, a model appears perfect and precise. In reality, the printed result is always an approximation — and that’s completely normal.

The way we manage this approximation is through tolerances. A tolerance is simply an accepted range of variation between parts that are meant to fit together. Rather than expecting absolute accuracy, designers allow for small differences so components can still assemble and function correctly.

In traditional manufacturing methods such as machining, tolerances can be extremely tight because material is removed in a controlled, predictable way. In 3D printing, parts are built layer by layer from molten plastic. This introduces more variability, which means tolerances must be wider to remain realistic and reliable.

Several factors contribute to this variation. Each layer is placed individually, material flow can vary slightly from one moment to the next, plastic contracts as it cools, and even a well-maintained machine has mechanical limits. Because of this, designing without tolerance allowances is one of the most common causes of failed assemblies in 3D printing.

For most FDM printers, a realistic expectation is accuracy within about two tenths of a millimetre for general-purpose parts. Well-tuned machines can sometimes achieve tighter results, but designing around ideal conditions rather than typical ones often leads to frustration. Good design is about predictability, not perfection.

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Why Holes Almost Always Print Smaller

A classic example of tolerance issues appears with holes. If you design a hole at a specific diameter, it will almost always print slightly smaller than expected. This behaviour is not a printer fault — it’s a natural outcome of the printing process.

As the nozzle moves in a circular path, molten plastic tends to curve inward rather than forming a perfect circle. Layer stacking adds a stepped edge to what should be a smooth surface, and cooling causes the material to contract slightly. When these effects combine, the final hole ends up undersized.

Because this behaviour is consistent, it can be designed around. A practical approach is to increase hole diameters in the design stage, typically by two to four tenths of a millimetre. For example, a hole intended for a 5 mm bolt may be designed closer to 5.3 or 5.4 mm to ensure reliable fit after printing.

For applications where precision really matters, post-processing is often the best solution. Drilling or reaming printed holes removes variability and produces clean, accurate results that are difficult to achieve through printing alone.

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Shrinkage: Why Parts Change Size as They Cool

Shrinkage is another unavoidable aspect of 3D printing. When plastic is heated and then allowed to cool, it contracts. Different materials shrink at different rates, which is why the same design can behave very differently depending on the filament used.

PLA is one of the most dimensionally stable materials and tends to shrink very little, which is why it prints so predictably. PETG shrinks slightly more but compensates by being more flexible, reducing the risk of cracking. ABS, ASA, and nylon shrink significantly more and are far more sensitive to cooling conditions, which is why enclosures and controlled environments are often required.

Shrinkage affects more than just overall size. It influences flatness, causes warping, and distorts holes and mating surfaces. This is why material choice plays such a critical role in dimensional accuracy. Choosing the wrong material can introduce accuracy issues that no amount of slicer tuning will fully correct.

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How Print Orientation Affects Accuracy

Accuracy in 3D printing is not uniform in all directions. The printer controls movement in the horizontal directions using motors and belts, which are generally very precise. Vertical movement, however, is governed by layer height, which limits resolution in that direction.

As a result, parts tend to be more accurate in the horizontal plane than in the vertical one. Vertical holes are often less accurate than horizontal holes. Tall, thin parts can drift slightly as layers accumulate, and changes in layer height directly affect vertical resolution.

For parts where accuracy is critical, orienting important surfaces in the horizontal plane often produces better results. This does not eliminate the need for tolerances, but it does reduce variability.

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Layer Height and Dimensional Control

Layer height plays an important role in how accurately features are reproduced, particularly in the vertical direction. Smaller layer heights improve resolution and produce smoother mating surfaces, while larger layer heights exaggerate the stepped nature of the print.

For parts where fit matters, consistency is more important than ultra-fine detail. Moderate layer heights tend to produce more predictable results, especially when combined with sufficient top and bottom layers. Adaptive layer height can be useful, but only after careful testing, as it introduces additional variability.

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Calibration: The Hidden Foundation of Accuracy

No amount of clever design can compensate for a poorly calibrated printer. Dimensional accuracy begins with consistent extrusion, stable motion, and reliable bed positioning.

Extrusion accuracy ensures that the correct amount of material is deposited. Belt tension affects positional accuracy in the horizontal plane. Bed leveling and Z-offset determine how accurately the first layer sets the foundation for the entire print. Advanced features such as pressure advance help manage material flow during acceleration and deceleration.

At BritForge3D, printers are routinely calibrated and verified to ensure that parts behave predictably across multiple prints. This consistency is essential for professional prototyping and repeat production.

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Designing for Fit: Clearance as a Design Tool

Rather than trying to eliminate variation, experienced designers use clearance intentionally. Clearance is the space between mating parts that allows for assembly after printing.

Loose fits allow parts to move freely. Sliding fits enable controlled movement. Snug fits hold parts in place without force. Press fits require testing and careful iteration. These categories are not fixed rules, but starting points that must be validated through prototypes.

Fit-critical parts should always be tested before final production. Small changes in clearance can dramatically change how a part behaves.

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Threads, Inserts, and Reliable Fastening

Printed threads can work, but they have limitations. Coarse threads perform better than fine ones, and orientation matters greatly for strength. For assemblies that will be taken apart repeatedly, printed threads are rarely the best solution.

Heat-set inserts provide a far more durable alternative. Installed correctly, they create strong metal threads within printed parts, allowing repeated assembly without wear. Designing for inserts requires careful attention to hole size and installation temperature, but the improvement in reliability is significant.

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When Precision Truly Matters

Some projects demand tighter control than most home printers can reliably provide. Mechanical assemblies, snap-fit enclosures, replacement parts, and engineering prototypes often fall into this category.

This is where a professional print service adds real value. At BritForge3D, tolerances are adjusted per material, critical components are test-fit, and print orientation is chosen specifically to maximise accuracy. The result is consistent, dependable production for customers across the UK.

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Final Thoughts

Dimensional accuracy in 3D printing is not about achieving perfection. It is about achieving predictability. Once you understand how tolerances, shrinkage, orientation, and calibration interact, you can design parts that fit first time and perform exactly as intended.

In the next technical guide, we will explore heat resistance, load, and long-term durability — essential knowledge for parts designed to survive real-world use.

Guide 2 Summary – Layer Height, Infill, and Strength

In this guide, we explored how strength in 3D printing is created internally rather than through surface appearance alone. You learned why thicker walls often matter more than high infill, why thinner layers are not always stronger, and how print orientation can dramatically affect performance.

The key takeaway is that strength comes from balance. Smart combinations of layer height, wall thickness, infill, and orientation produce better results than extreme settings in any single area.

The next guide focuses on one of the most common challenges in functional printing: understanding tolerances, shrinkage, and why parts do not always fit together as expected.

Heat Resistance, Load, and Long-Term Durability in 3D Printed Parts

A common assumption in 3D printing is that if a part feels strong when it comes off the printer, it will remain strong in use. In reality, many printed parts fail not because they were overloaded, but because they were exposed to heat or stress over time. Understanding durability means thinking beyond initial strength and considering how materials behave in real-world conditions.

Heat plays a critical role in plastic performance. Most plastics do not need to reach their melting point to fail. Instead, they gradually soften as they approach their heat deflection temperature. At this point, a part may still look intact but can no longer support load without deforming.

PLA is a good example. While it is stiff and strong at room temperature, it softens at relatively low heat. This is why PLA parts often warp inside cars, near electronics, or in warm environments. The part has not melted; it has simply lost its ability to resist deformation.

Load introduces another dimension to durability. When a part carries weight continuously, it experiences stress even if that stress is well below its breaking strength. Over time, this can lead to creep, which is slow, permanent deformation. PLA is particularly prone to creep, which is why it is unsuitable for long-term load-bearing parts.

PETG behaves differently. Its slight flexibility allows it to absorb stress rather than concentrating it in one place. This makes PETG far more resistant to cracking and long-term deformation under moderate loads. However, this flexibility can also be a disadvantage where rigidity is required.

ABS and ASA are designed for harsher conditions. They maintain their shape at higher temperatures and resist impact better than PLA or PETG. ASA adds UV resistance, making it suitable for outdoor use where sunlight would otherwise degrade the material.

Nylon takes durability further still. It excels under repeated stress, vibration, and impact. Nylon parts often bend rather than break, making them ideal for mechanical components. However, nylon’s performance depends heavily on moisture control, as absorbed water changes its mechanical behaviour.

Durability is also influenced by geometry. Sharp corners concentrate stress, while smooth transitions distribute it. Thin sections heat and cool faster than thick ones, which can introduce internal stress. These factors all affect how a part ages over time.

Print orientation plays a major role as well. Loads applied across layer lines exploit the weakest direction in FDM printing. Orienting parts so that stress runs along the layers dramatically improves durability without changing material or settings.

Environmental exposure must also be considered. UV light, moisture, chemicals, and temperature cycling all degrade plastics differently. A part designed for indoor use may fail quickly outdoors if material choice is not adjusted.

Durability, therefore, is not a single property. It is the result of material choice, geometry, orientation, and environment working together. Ignoring any one of these factors invites failure.

Professional printing workflows consider durability from the start. At BritForge3D, parts are evaluated not just for print success, but for how they will perform weeks, months, and years after installation.

Understanding long-term behaviour allows designers to avoid surprises and create parts that remain reliable throughout their service life.

Guide 3 Summary – Tolerances and Dimensional Accuracy

This guide explained why printed parts are never exact replicas of their digital designs and why this is not a flaw, but a characteristic of the process. By understanding tolerances, shrinkage, and orientation-related accuracy, you can design parts that fit reliably instead of relying on trial and error.

You learned that dimensional accuracy is about predictability, not perfection. Once variation is understood and planned for, assembly issues become far less common.

In the next guide, we move beyond fit and focus on how printed parts behave over time, exploring heat resistance, load, and long-term durability.

3D printed parts are anisotropic, meaning their strength varies depending on direction. Understanding this concept is critical for engineering-grade parts.

What Is Anisotropy?

Unlike injection-moulded parts, 3D prints have directional strength:

• X/Y plane: strongest
• Z axis: weakest (layer adhesion)

Ignoring anisotropy leads to unexpected failures.

Designing With the Load Path

Always align your print so the load path runs parallel to layers, not across them.

Examples:

• Brackets should lie flat
• Hooks should print sideways
• Shafts should not be printed upright

Using Geometry to Improve Strength

Strength isn’t just about settings geometry matters.

Effective design features:

• Fillets instead of sharp corners
• Ribs instead of solid blocks
• Gradual transitions in thickness
• Hollow structures with thick walls

These reduce stress concentration and improve performance.

Support vs Strength Trade-Offs

Reorienting for strength sometimes requires more support material.

Tips:

• Accept supports if strength improves
• Remove supports carefully to avoid scars
• Consider splitting and bonding parts

Professional Print Strategy

At BritForge3D, orientation is selected based on:

• Load direction
• Surface finish requirements
• Post-processing needs
• Material behaviour

Get expert orientation applied automatically when you upload your file.

As projects grow, parts must fit together reliably. Designing for assembly is where many 3D prints fail.

Snap Fits in 3D Printing

Snap fits rely on controlled flex.

Best practices:

• Use PETG or nylon (not PLA)
• Avoid sharp internal corners
• Design generous lead-in angles
• Test multiple tolerances

Printed snap fits should flex once not repeatedly.

Press Fits and Interference

Press fits require extremely careful tolerancing.

Guidelines:

• Start with 0.1–0.2 mm interference
• Test with small calibration prints
• Account for material shrinkage

Never assume theoretical dimensions will work first time.

Modular Design Benefits

Breaking designs into modules allows:

• Easier printing
• Better orientation
• Reduced support
• Improved repairability

Fastening options include:

• Screws with heat-set inserts
• Pins and dowels
• Adhesives (epoxy, CA glue)

Tolerance Stacking

Multiple small inaccuracies add up.

Avoid:

• Long chains of mating parts
• Tight tolerances across many components
• Single-point failure designs

Instead, design adjustment points.

Professional Assembly Support

BritForge3D regularly prints:

• Multi-part assemblies
• Mechanical enclosures
• Functional prototypes

We can advise on fit, tolerance, and assembly strategy before printing.

Upload your project at BritForge 3D

Understanding 3D Printing File Types: STL, 3MF, OBJ and G-code Explained

When a customer uploads a file for 3D printing, the first question we consider is not how large the part is or what material it requires. The first question is much simpler: what file format are we working with?

File type plays a significant role in print reliability, geometry accuracy, material settings, and even colour information. Many users treat file formats as interchangeable, but they are not. Each format exists for a reason, and choosing the correct one can improve workflow efficiency and reduce print errors.

In this lesson, we will explore the four most common file types used in 3D printing: STL, 3MF, OBJ, and G-code. More importantly, we will examine when to use each one and why.

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Why File Format Matters in 3D Printing

A 3D printer does not understand design software files such as SolidWorks, Fusion, or Blender files directly. Instead, those designs must be exported into a format that slicing software can interpret.

The file you export determines:

• How geometry is stored
• Whether units are preserved
• Whether colour and material data are included
• Whether slicing settings travel with the model
• How easily the file can be repaired

Using the wrong format can introduce scaling issues, lost metadata, corrupted geometry, or unnecessary workflow steps.

Understanding file formats allows you to control your process rather than react to problems later.

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STL – The Industry Standard

The STL file (Standard Tessellation Language or StereoLithography) is the oldest and most widely recognised format in 3D printing.

STL files describe geometry using triangles. Every curved surface is approximated by a mesh of small triangular faces. The smaller and more numerous the triangles, the smoother the surface appears.

However, STL files contain only geometry. They do not store:

• Units (mm or inches)
• Colour
• Material data
• Print settings

This simplicity is both a strength and a limitation.

Benefits of STL

STL is universally supported. Nearly every slicer, CAD package, and 3D printer workflow accepts STL files. Because the format is simple, files are generally stable and predictable.

For single-material functional parts, STL is often perfectly adequate.

Limitations of STL

Because STL does not store units, scale errors are common. A part designed in millimetres may open as inches in another system.

STL also cannot store multiple bodies intelligently, colour data, or advanced metadata.

For simple prints, STL works well. For more complex workflows, it begins to show its age.

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3MF – The Modern Alternative

3MF (3D Manufacturing Format) was developed to solve the limitations of STL.

Unlike STL, 3MF files store:

• Units
• Multiple bodies
• Colour information
• Material assignments
• Print settings (when exported from slicers)

This makes 3MF far more powerful for modern workflows.

Why 3MF Is Often Better for 3D Printing

Because 3MF preserves units, scaling errors are eliminated. What you design is what opens in the slicer.

3MF also allows multi-material and multi-colour information to travel with the file. If you are using a multi-material printer, this is critical.

Additionally, some slicers store support placement and slicing profiles inside the 3MF file. This makes collaboration easier because another user can open the file exactly as intended.

When to Choose 3MF

If you are working in a modern slicer and want to preserve configuration, or if you are printing multi-material parts, 3MF is usually the better choice.

For professional workflows, 3MF is increasingly becoming the preferred format.

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OBJ – Geometry with Visual Data

OBJ files are commonly used in animation and 3D modelling rather than manufacturing, but they also have applications in 3D printing.

Like STL, OBJ files describe geometry through mesh data. However, OBJ files can also store:

• Colour data
• Texture references
• Surface information

This makes OBJ useful for full-colour printing systems.

When OBJ Makes Sense

If you are printing artistic models, figurines, or textured parts that require colour mapping, OBJ can be valuable.

However, OBJ files can be larger and more complex than STL. They may also include unnecessary data for simple functional prints.

For most engineering parts, OBJ is not required.

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G-code – The Machine Instruction File

G-code is very different from STL, 3MF, or OBJ.

While those formats describe geometry, G-code describes movement instructions for the printer.

Once a file is sliced, the slicer converts the model into G-code. This file contains:

• Exact nozzle movements
• Extrusion rates
• Temperature commands
• Fan speeds
• Layer heights

G-code is printer-specific. It is generated for a particular machine, with specific settings.

Why You Don’t Share G-code as a Design File

Because G-code contains machine instructions, it is not portable in the same way geometry files are.

A G-code file created for one printer may not work correctly on another due to:

• Different bed sizes
• Firmware differences
• Extruder configurations
• Temperature limits

For this reason, when sending a file to a professional 3D printing service, you should always send geometry files (STL or 3MF), not G-code.

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Choosing the Right File Type

So how do you decide?

If you are printing a simple single-material part and need universal compatibility, STL is safe and reliable.

If you want to preserve units, materials, and advanced data, 3MF is the superior modern option.

If you are working with colour textures or artistic models, OBJ may be appropriate.

If you are printing directly from your own slicer to your own machine, G-code is the final production file — but not something to share externally.

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Professional Workflow Recommendation

At BritForge3D, we recommend:

• 3MF for most modern workflows
• STL for compatibility and simple parts
• Avoid sending G-code unless specifically requested

We re-slice all files to ensure compatibility with calibrated machines and optimised material profiles.

Sending geometry files instead of machine code allows us to ensure print strength, dimensional accuracy, and material suitability.

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Common Mistakes to Avoid

One of the most common mistakes is exporting STL with extremely low mesh resolution. This results in visible flat facets on curved surfaces.

Another mistake is forgetting to check units before export.

Finally, some users send G-code assuming it guarantees identical results. In reality, it can introduce compatibility risks.

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Final Thoughts

Understanding file formats is not just about software preference. It is about workflow control.

Choosing the correct file format improves reliability, reduces errors, and supports better collaboration between designers and print services.

As 3D printing evolves, 3MF is increasingly replacing STL for professional workflows. However, STL remains deeply embedded in the ecosystem.

In the next guide, we will explore how to prepare files properly before upload — including mesh repair, manifold checks, and design validation.

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