Dimensioning & Tolerancing to Control Fit, Function, and Assembly

Why Dimensions Exist: Geometry Isn't Manufacturable Without Numbers

A drawing or blueprint of a part can provide a designer or manufacturer with an idea of what the final product will look like. However, to manufacture the part, more specific information is required. This is where dimensioning comes into play.

Dimensions tell you in detail about the size of an object, the space between components, and where components are located on the object.

A fundamental rule of draughting is that the dimensions or details are the authority and should be read in preference to the drawings. The drawings are there to provide context for the dimensions but once the dimensions have been read from the drawings, the drawings become secondary to the values themselves. In consequence, when manufacturing from drawings, you manufacture to the values noted from the dimensions and not the shape as seen on the drawing itself. Therefore, with the dimension "50mm length" and the drawing showing a length of 48mm, the part would be manufactured to a length of 50mm. Always.

What a Dimension Controls: Size vs Location vs Geometry

The dimensions (i.e., what you are controlling) are not immediately self-explanatory. It is necessary to read them in a specific order to understand what you are controlling.

First question: what parameter is that dimension controlling? Is it a size (width of a block)? Or a location (location of a hole from edge of the block)? Same number, different meaning.

My next question is: is this a size dimension or a location dimension? Another one of my rants – 50mm callouts can mean completely different things depending on where the dimension arrow is pointing. So here we have a 50mm callout – is this the width of the feature in question, or is it the distance from the reference edge? Look at the dimension arrows, that's the only way to know for sure.

Third: never assume symmetry. Even if a part looks symmetric in your drawing, do not make the assumption that it is. Just because something looks centrally located does not mean that it actually is. Remember: if it is not explicitly stated in the drawings to be "centered" or symmetrical, it should be manufactured to the dimensions that are referenced, not to how it looks. If there is a dimension callout and a centerline with both values being equal, then manufacture precisely to that. If there is only a single dimension callout and a centerline, manufacture exactly to the dimension callout.

Dimensions trump drawing scale. Bottom line: look at the real dimensions for accurate information.

Dimensioning Rules: One Source of Truth, No Ambiguity

Dimensioning has very specific rules which, if not followed can cause problems.

Rule #1: All dimensions must appear once. Not repeating dimensions is far easier than allowing their redundancy and then trying to manage the consequences of possibly erroneous revisions. Redundancy breeds ambiguity.

Rule 2: Do not overcluster. Dimension lines that are too dense can obscure features and make the drawing difficult to read. Dimension lines to one feature should not overlap with dimension lines to a different feature. Keep dimensions spaced out or utilize different views when necessary, but keep the drawing clean.

The third rule is: Dimensions should be legible from the bottom or right side of the drawing. So try to avoid going against the grain of standard drafting practices that make life easier for everyone.

We all have our likes and dislikes when it comes to how to draw things; some of these will be determined by the kind of work that we do, some are a matter of personal preference, but all can have serious implications if not followed correctly. Violating at least some of these "rules" can produce manufacturing errors including misreading of dimensions, conflicting specifications and ambiguous intent; as the complexity of the drawing increases, the severity of careless dimensioning increases.

Why Tolerances Exist: Manufacturing Variation Is Inevitable

Many are unaware that all products have tolerance limits, and it's thanks to these that things remain useable even after wear and tear.

Manufactured parts are not perfect and, as such, always contain a degree of error. Machined dimensions cannot be infinite precise and therefore parts print with a degree of tolerance integrated into the model geometry. When a part geometry is defined within a 3D modeling package, the dimensional attribute of an architectural dowel of 50mm, for example, might be expected to print exactly to 50.000000mm; in reality it will print to something very close to this value.

The tolerances dictate the allowable variation. This means that all parts to be considered acceptable must fall within a specific range of measurement. Parts measured outside of this range are classified as scrap.

Tolerancing involves a delicate balancing act between three attributes: performance, manufacturability, and cost. Performance (also known as precision) dictates that closer tolerances are better. However, the actual limits of material and process variability and the cost of special features or processes must also be taken into consideration.

Setting a tolerance that is too close will over-specify the part requiring very expensive precision machining that a simple mill could have produced, whereas setting it too loose can result in assemblies that don't fit together properly or don't function as intended. The key is to understand where very close tolerances are necessary to meet part performance requirements.

Reading a Tolerance: Limits, Allowable Variation, Worst Case

This section details how to understand the limits that are established for components within a blueprint. It explains how those tolerance limits work and discusses the difference between allowable variation and worst case scenarios.

What are the biggest tolerances? What are the tightest tolerances? What are the least important tolerances?

1. What's the nominal (target) dimension? This is what the machinist is trying to achieve.

2. What is the variation allowed for the dimension? This specification can be given as a plus/minus value (±0.1mm) or as limit dimensions (49.9—50.1mm).

3. Will it work at extremes. This is the most critical question. If your part is at the Upper Limit of tolerance, will it still work. What about the lower limit of tolerance. If the answer is no then your tolerance is wrong.

I don't want to see tolerance calls in engineering specs other than as hard requirements – that means the manufacturing process can meet them, not that the spec calls for somewhat looser or looser tolerances. Manufacturers have to hit these things. If you don't think your shop can repeatedly make parts to ±0.01mm, then either get a better shop, or remake the spec calls with looser tolerances.

Fits Between Parts: Clearance, Interference, Transition

When two components are to be assembled together, such as a shaft and a hole or a pin and a bore or a bearing and a hub, the tolerances of the parts involved must be carefully considered. If wrong, the consequences can be either that the assembly fails to come together or, having been assembled, fails in service.

Clearance fit is a type of fit where the hole has a greater dimension than the shaft. The worst tolerance position of the clearance fit always is greater than the worst tolerance position of the shaft. There is room for sliding, rotating, or movement of the shaft inside the hole for applications such as rotating, sliding, or movement.

Interference fit: In this category, the shaft is always larger than the hole, and the tolerance on the hole is always less than the tolerance on the shaft. Force is required to assemble the shaft into the hole, and this can be achieved by using a press fit, by heating the hole, or by cooling the shaft. Once assembled, interference fit components are very difficult to disassemble and are typically used for permanent assemblies such as bearings.

This spec leaves to chance how the transition fit will be slight clearance or slight interference. You will get the correct accumulation of fit throughout the parts length, but where the slight fit falls within the tolerances is intentionally vague. This spec requires accurate positioning, but does not require no clearance or interference.

When a part maker selects an inappropriate fit type, rotating parts become difficult to turn, non-rotating parts do not stay locked in place, and loose parts come loose at inopportune times. Fitting becomes an essential part of design engineering when parts need to be assembled together. Understanding fits and their effects during design enables the part maker to select the proper fits so that assemblies will function as desired.

DATUMS. Part 7 DATUMS - Defining the Inspection and Assembly Reference Frame.

Datums are references such as points, lines or surfaces that are used to measure other objects and features. In many ways datums act as an "origin" for many objects; in many co-ordinate systems the origin is defined as (0,0).

This post is the second in a series that discusses why we need datums. Why is this important? Simply put, when we describe a dimension such as "this hole is 50mm from the edge", we don't know what we have said, because we don't know which edge we have referred to. Moreover, in almost every situation, at least one of the edges being referred to won't be flat, or the faces at right angles to it won't be square.

The datum of a feature defines three properties of the feature surface; Orientation, Alignment and Positional relationship with other feature surfaces.

On drawings, datums are designated with letters inside of triangular symbols – (Datum A, Datum B, Datum C, etc.). These surfaces are crucial to the manufacturing and inspection of the part and are typically the first surfaces that the part is clamped against during machining operations or referenced off of during inspection.

Datums give dimensions a starting point. "50mm from edge" means nothing, "50mm from Datum A" means everything. Without datums dimensions are useless.

Section 8 Why This Matters: Bad Tolerances Break Good Geometry¶

This section includes essential information on why tolerancing must be addressed properly to avoid construction problems, especially as manufacturing grows in scale and complexity.

Engineering failures don't come from poor geometry. They come from poor dimensioning and tolerancing. The part has the right shape, but the wrong size, or the wrong tolerances. Or the fit-up is incorrect, leading to parts that do not slide together smoothly when they are supposed to, or parts that bind when they are supposed to fit freely.

In this session, we look at how drawings can manage 3 of the most important aspects of design, and thereby directly affect the form of your buildings and landscapes.

Function Parts work, because the tolerances guarantee sufficient variation limits; a bearing rotates smoothly, because the correct clearance fit was specified.

All Parts fit together as designed. The dimensions and fits were chosen to allow for variation in manufacturing, and to be as realistic as possible. All components mate properly, holes line up, shafts slide in.

Standardisation: Any part designated on the drawing can be a substitute for any other part. This is what enables mass production and not individual assemblies to be customised to particular specifications.

It's hard to make parts if you don't know how to read dimensioning and tolerancing. One cannot understand the purpose of design criteria without studying it. If you do not have a good grasp of this, you will approve parts that don't fit, reject parts that would work, and make very expensive drawings that are hard to make.

How Level 3 Builds on Levels 1 and 2

In Levels 1 and 2, we covered how to read shapes and interpret what the part looks like, learning to translate 2D to 3D as well as reading individual features.

Level 3 adds control to that shape. Now you know not only what the part looks like, but also how large it is, where key features are located, the acceptable amount of variation and how it interfaces with other parts.

Studying tolerances readies you for the critical task of reading assembly drawings, where parts with widely different tolerances have to fit together, for understanding manufacturing feasibility, quality control procedures, and for exploring more advanced topics like Geometric Dimensioning and Tolerancing (GD&T).

Task: Interpreting Dimensions, Tolerances, and Fits

Scenario: I am reviewing a hand drawn draft detail of a shaft and housing assembly. The part consists of a shaft, housing, snap ring, and seal.

• Shaft: Ø25.00 ± 0.02 mm, length 100 mm
• Housing bore: Ø25.10 ± 0.03 mm, depth 100 mm
• Rotational fitting: The shaft should rotate smoothly inside the bore of the housing.

Questions:

1. What is the maximum possible shaft diameter?
2. 65mm.
3. What is the minimum recommended clearance between the shaft rotational circumference and the housing bore?
4. The fit is typically a sliding fit where the shaft will not necessarily rotate freely.

Key takeaway: If we want parts to fit together then tolerances on the components must be dimensioned appropriately, and a worst case analysis performed to ensure parts will always fit together. However for free movement ie: a shaft fitting through a hole, the fit is known as a clearance fit where the diameter of the shaft is always less than the diameter of the hole, no matter how the variation in the tolerance specifies. Proper dimensioning and tolerancing of components is crucial to assembling parts correctly, and to ensure parts will not fail due to assembly. However, as mentioned earlier, complete variation removal is not possible and therefore we must design around the variation that does exist.