Understanding Tolerances and Fits: How Restoric® Delivers Precision in Every Manufacturing Drawing

In mechanical design, fit matters—not just for functionality but for manufacturability, durability, and assembly ease. When two components come together, the amount of space—or lack of it—between them is defined by their fit class. Choosing the right fit ensures optimal performance, avoids costly machining errors, and improves product reliability.

At Restoric®, we specialise in high-precision CAD services and manufacturing drawings. Our team helps engineers and product designers apply the correct recommended fits, based on international standards like DIN 7157—to make sure that what’s designed digitally performs flawlessly in the real world.

Fits are a crucial part of the manufacturing process. They determine how two parts (such as a shaft into a hole) fit together. How they fit together depends on their function and requirements.

A shaft that turns in a bronze bushing needs to have a clearance fitting. How much clearance again depends on several factors. For example, one needs to make sure that once the shaft it at operating temperature and has expanded the clearance fit is still in existence.

If you need to accurately position a part using dowel pins, for example, the mating part will require an exact hole to not allow any movement, but still be capable of assembly without having to use a press.

If you want to secure a roller bearing in a housing, you need it to function as a bearing. Thus, the outer ring needs not to rotate, thus the outer ring needs to have an interference fit to the housing. The bearing will be provided with a tolerance. Based on the tolerance of the bearing, one can determine the tolerance requirement of the hole.

There are different fundamental deviations and tolerance grades which can be overwhelming when one has not studied or uses tolerances on a regular basis. Thus this post simply concentrates on recommended fits for some typical arrangements.

Fundamentals of Tolerances & Fits in simple English

A fit is described by 3 components.

1. Nominal Dimension
2. Fundamental Deviation
3. Tolerance Grade

The Nominal Dimension is the overall dimension you are aiming at. A Ø20mm shaft is the basic dimension. But when measured with a micrometre, what dimension is acceptable as it needs to fit into the Ø20mm hole without falling out.

The fundamental deviation determines the position of the tolerance zone with respect to the 0 line. The fundamental deviation ranges from A to S (a to s for shafts). For example, H is on the 0 line. So if the deviation or a H6 Hole is +13µm the tolerance starts with a lower limit of 0 and an upper at +13 µm.

The Tolerance grade states the number of the fundamental tolerance grade, e.g. 7 for the fundamental tolerance grade 7. Thus, the limit of the deviation. They range from IT1 to IT18. Basically, the lower the IT number the more accurate you need to machine. You should never state more accuracy than is required, as accuracy costs time and money. “As much as required no more than needed”

Deviation is the tolerance in µm or mm by which the part can be larger or smaller than the nominal size.

When stating tolerances and fits, always remember

“As much as required, no more than needed”.

As overstating costs time and money that is completely wasted.

 

The Basics: What Makes a Fit?

A fit is defined by three key components:

1. Nominal Dimension – The target size (e.g., Ø20mm).

2. Fundamental Deviation – Determines the location of the tolerance zone relative to the nominal (zero) line.

3. Tolerance Grade (IT Grade) – Indicates the acceptable variation or precision level (e.g., IT6, IT7).

⚠️ Tip: The lower the IT grade number, the higher the required machining accuracy—and the higher the manufacturing cost. Always specify only as much precision as necessary.

Common Fit Designations:

Fit: Planned joining condition between the hole and the shaft.

Why Tolerances and Fits Matter

When two components, like a shaft and a hole, are assembled, the “fit” between them defines how much clearance or interference exists. Choosing the correct fit is critical. It impacts:

• Performance and reliability

• Ease of assembly

• Manufacturing cost and accuracy

• Longevity of the product

Poorly selected tolerances can lead to excessive wear, assembly issues, or even part failure.

Restoric’s Approach: Precision Backed by Standards

At Restoric®, our team of engineers and designers ensures your manufacturing drawings include the correct fit classes, based on international standards like DIN 7157 and ISO fit systems. We help clients specify the right tolerance classes (e.g., H7/g6, H6/p6) based on how parts should behave—whether they should rotate freely, lock tightly, or slide with minimal resistance.

Real-World Fit Examples: Explained Simply

Let’s explore a few practical fit scenarios:

• Rotating Shaft in a Bronze Bushing
  This application requires a clearance fit, accounting for thermal expansion. Even at operating temperature, there must still be enough gap for the shaft to rotate freely.

• Dowel Pins for Alignment
  When positioning a part using dowel pins, you need a transition fit—tight enough to avoid movement, but not so tight that assembly requires a press.

• Bearing Housed in a Casing
  To prevent a bearing’s outer ring from rotating in its housing, an interference fit is used. The hole is machined smaller than the bearing’s outer diameter, based on the bearing’s tolerance spec.

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Fit Categories: Clearance, Transition, and Interference

1. Clearance Fit

Always leaves a gap between parts—ideal for moving parts like rotating shafts.

2. Transition Fit

May have a slight clearance or slight interference—used for parts that need to be located precisely but still removable.

3. Interference Fit

Parts are intentionally oversized—used for permanent assemblies where no relative movement is desired.

Each fit type comes with standard pairings like:

• H7/g6 – Common clearance fit

• H7/h6 – Tight transition fit

• H7/p6 – Standard interference fit

Below is a table of recommended fits commonly used. These application examples and the recommended tolerance classes.

You can download this table as a PDF from here: Possible tolerance fit example table

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How Fits are Applied to Manufacturing Drawings

Once the type of fist and thus the tolerance class has been specified. The nominal dimension and tolerance class need to be stated on the Manufacturing drawings. This is essential as the machinist will otherwise machine to General Tolerances.

If they are bought in parts, such as bushings, the exact bushing can now be specified. The machinist will use lookup tables or specified tooling (such as reamers for holes) to machine the part.

Below are example tables based on the basic hole and shaft system. Based on the tolerance class provided, the machining tolerances can be looked up.

Lets say on a vintage gearbox we have the gears on a layshaft rotating freely in a bronze bushing. The bronze bushing will need to be pressed into gear but have a clearance fit onto the layshaft.

Lets say is a 20mm shaft. The recommended fitting would be Ø20H7g6

Using the tables below we can determine that the

In summary, for instance, if a gear rotates on a bronze bushing in a vintage gearbox, you’d want:

• Bushing pressed into gear housing (interference fit)

• Layshaft rotating inside bushing (clearance fit)

In this case, a recommended fit for a 20mm shaft might be:

• Hole (bushing ID): Ø20H7 → 20.000mm to 20.021mm

• Shaft: Ø20g6 → 19.980mm to 19.993mm

These tolerances ensure smooth operation without binding or excessive play.

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Precision Engineering with Restoric®

At Restoric®, we take the guesswork out of tolerancing. Whether you need machining drawings, detailed CAD files, or fit recommendations, our experts guide you through:

• Selecting the right fit class

• Understanding ISO and DIN tolerancing systems

• Producing precise, production-ready drawings

We don’t just draft parts—we engineer solutions that perform.

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Need Help with Tolerancing or Fit Selection?

Let Restoric® handle the precision—so you can focus on innovation.
Contact us today for expert CAD services and manufacturing drawing support.