Standard metric T-slots and T-nuts Engineering ToolBox - Resources, Tools and Basic Information for Engineering and Design of Technical Applications! - search is the most efficient way to navigate the Engineering ToolBox!

• T Slot Tolerances
• T Slots Tolerance

• Spacing of the T-slots The T-slot spacing of our standard products is 150, 200, 250 and 300 mm. If several tooling plates are combined to form plate fields, the distance to the plate joints should be half of the slot spacing distance in each instance.
• Width of tongue used with the T-Slots will be found in the complete standard, B5.1M. Throat dimensions are basic. When slots are intended to be used for holding only, tolerances can be 0.0 + 0.010 inch or H12 Metric (ISO/R286); when intended for location, tolerance can be 0.0 + 0.001 inch or H8 Metric.

Slots with draft
If you are designing a ramp for a dog to get onto and off from the couch or a bed, you may use plastic sections with slots to snap the sections together.
A slot in a plastic molded part may have draft. This complicates how it should be dimensioned. Below is a best practice for how to dimension slots with draft.
The figure below shows our slot. Not only does it have draft, but it also has fillets and rounds.

Below is our tolerancing scheme for our slot. It looks complicated, but it really is not. We will look at the tolerances one at a time.
We start with the goal of locating the the slot with a tolerance of position. Recall that a tolerance of position must be applied to a feature of size. That's easy if the slot has no draft. But our slot has draft. So we cannot define the entire slot as a feature of size. We need to pick a specific place in our slot to define as our feature of size. The top and bottom of the slot are not good places to define a feature of size because the top and bottom have corners that may or may not be sharp. This is why we go down a basic distance, a basic 10 in this case, from the top of the slot. This is where we will define our feature of size. Our size dimension says that at exactly 10 down from datum [A], the size must be 20 +/- 0.2.
Having established a feature of size, we can locate our feature of size with a tolerance of position. Our tolerance of position says that the center of our feature of size must fall within a 0.5 wide tolerance zone while holding the part on datum feature [A], and pushing it against datum features [B] and [C]. The slot is oriented to datums [A] and [B] and located to datum [C].
Everything we have looked at so far is specifically 10 mm away from datum [A]. Now we need to define the rest of the walls. We use a profile of a surface with a 0.1 wide tolerance zone. The profile of a surface does not locate the walls. They are located by the tolerance of position and the size tolerance. The profile of a surface controls the orientation and form. The profile tolerance zone will float within the size tolerance zone, and the profile tolerance zone will be exactly 99.5 degrees to datum [A] and exactly perpendicular to datum [B].
Finally we control the location, orientation, and form of the bottom of the slot with a profile of a surface that has a 0.5 wide tolerance zone. The tolerance zone is located and oriented to datum [A].

The figure below shows the tolerance zone for the tolerance of position. The tolerance zone is two parallel lines that are 0.5 apart. The lines are exactly 10 down from datum [A]. They are exactly parallel to datum [A], and exactly perpendicular to datum [B]. The center of the tolerance zone is exactly 100 from datum [C]. The center of the slot, exactly 10 down from datum [A], must fall within the tolerance zone.

The figure below shows the size tolerance zone and the profile tolerance zone for the walls. The size is established only at exactly 10 down from datum [A]. The size tolerance establishes two 0.2 wide tolerance zones. Remember that the size tolerance zones are located to datum [C] by the tolerance of position callout in the figure above.
The profile tolerance zones can float within the size tolerance zones. Each profile tolerance zone is independent of the other one. Each profile tolerance zone is two parallel planes 0.1 apart, exactly 99.5 degrees from [A] and exactly perpendicular to [B].

Finally we have the profile tolerance zone for the bottom of the slot. The tolerance zone is two parallel planes that are 0.5 apart. They are exactly parallel to datum [A], and the center of the tolerance zone is exactly 20 down from datum [A].

Our standard note says that the corners can range from sharp to 1 max radius, and now our slot is fully defined.

This is an EASY guide to GD&T. So if you want to learn and use Geometric Dimensioning and Tolerancing, you’ll love our easy step-by-step descriptions of how everything works.

What is GD&T?

GD&T, the abbreviation for Geometric Dimensioning and Tolerancing, is a set of standardized symbols and conventions that are used to describe parts in a way that makes it easier for customers, manufacturers, and other supply chain participants to successfully communicate. Parts that are manufactured in a shop must meet specific specifications. These specifications are shown on engineering drawings, which are normally produced by CAD software. GD&T is a particular set of conventions used on engineering drawings (often called “prints” from the older “blueprints”) that communicate how parts should fit together and how they function. This course will teach you the basics of how to understand GD&T symbols and their use.

GD&T is a standardized international system based on symbols that can be clearly understood by anyone that is familiar with the standard. It is a mathematical language, though it does not require you to be a math expert in order to understand and use the system. GD&T is a significant improvement over traditional older systems for describing dimensions that relate to the fit, form, and function of parts. It is very precise as well as concise relative to these older systems. Each work piece is described in terms of “zones of tolerance” that are relative to the Cartesian coordinate system. As you recall, Cartesian coordinates are just X, Y, and Z measurements from an origin. If you’re a CNC’er, you’re already quite familiar with them.

A simple drawing with GD&T symbols. Image Source

If GD&T seems complex, it is only because there is a long list of ways a physical part can deviate from the geometric ideal of what the designer had in mind. A shaft could be slightly bent, slightly tapered, slightly tilted, and slightly offset from where it was intended. When you consider all the different kinds of geometry found in machined parts, it’s no wonder that cataloging all the defects in that geometry gets a bit complicated. GD&T actually brings some order to that complexity and makes it systematic.

History and Background of Geometric Dimensioning and Tolerancing

Once upon a time there was a fellow named Stan Parker who was working for England on Torpedoes. As is often the case in wartime, they were not making them fast enough. Stan’s job was to find ways to produce them faster.
He got looking at parts that had failed the specified tolerances, but actually still fit. These were parts they’d discarded, but could have used. What better way to increase production than to quit wasting these parts.
As he delved into the problem, he discovered the reason the parts could fail but still fit was the shape of the tolerance zone. Imagine projecting onto the blueprint an outline that shows all the allowable positions for the center of a hole, for example. That shape is literally the tolerance zone.
With traditional +/- tolerancing, that shape of where the hole center can be is rectangular. After all, it’s +/- each dimension in X and Y.
Stan’s brilliant intuition, and the thing that launched a new way of thinking about tolerances, was to use a circular tolerance zone for hole centers instead of a rectangle.
Imagine–the difference between a circle and a rectangle made all the difference. To me, it even sounds like it must be a better idea to put a round peg into a round hole instead of a square one, LOL!
The rest, as they say, is history. More torpedos were made, and we now have a much improved way of thinking about tolerances. Stan’s innovation, by the way, came to be known by the nametrue position.

True Position and a bunch of other clever concepts together are what make up GD&T.

GD&T is an international system because the American Society of Mechanical Engineering and the International Organization for Standardization worked together to create it. The actual written standards they produced are ASME Y14.5M and ISO 1101. While the standards are relatively recent, the ideas behind GD&T go back to the 1940’s and 1950’s.

We should note that while some think of GD&T as defining a standard for inspection of parts, that was not its intent. Rather, it was designed to communicate what the designer intended about the dimensions and tolerances of the part. Such information can be very helpful to the inspector in determine what exactly needs to be inspected to ensure the part meets the designer’s original intent.

– Dimensioning describes the nominal or as-intended geometry.

– Tolerancing defines the allowable variation of the form and size of individual features, and the allowable variation in orientation and location between features.

An important goal of GD&T is to ensure that a part defined using GD&T and manufactured within the proscribed limits will fit and function with the largest possible tolerances. Proper use of GD&T can add quality and reduce cost at the same time since having tolerances that are too tight can drive up costs rapidly.

When and Why Use GD&T?

Let’s start in a very basic way–GD&T is designed to facilitate communication between entities that need to work together to manufacture something. If you don’t have to communicate with anyone, you may not need to know GD&T. CNC Hobbyists may have little use for it, for example. But, if you are a CNC Professional, it will be extremely rare that you work on something where you don’t need to communicate. Perhaps you need to make a part someone else designed. Or perhaps your part needs to fit with a part made by someone else entirely. In those cases, GD&T might save you considerable trouble. In modern CNC Shops, it becomes almost a pre-requisite that you know something about it.

Here are some of the key instances that call out for GD&T to be used:

– When part features are critical to function or interchangeability. For example, if you’re making a part that has to fit with a part made by someone else, you’d better call out all the features that will govern whether it fits properly.

– When functional gaging teachniques are desirable. Think of functional gages as measuring devices intended to verify a particular feature is within tolerance. For example, a “Go-NoGo” gage.

– When datum references are desirable to ensure consistency between manufacturing and gaging operations. A datum is used to show where a measurement is taken from. For example, CNC’ers are familiar with Part Zero being an important datum that measures where the motion of the CNC machine starts from for a particular part.

– When computers are being used in design and manufacture.

– When standard interpretations and tolerances are not already implied. There is no need to over-specify tolerances that are implied in some other way. For example, we might specify a thread without needing to provide a bunch of GD&T to lay out the dimensions and tolerances of every feature of the thread. Just saying which thread it is tells us most everything we need to know about its dimensions and tolerances.

Advantages and Disadvantages vs Old School Dimensions

The advantages of GD&T are pretty straightforward:

– It is very succinct and precise. With old-style dimensions and tolerances extensive non-standard notes had to be included to make sure everything was fully specified.

– It is a graphical international language.

– It expresses tolerances in ways that are often beneficial to manufacturing cost. For example, without GD&T, the tolerance on a hole center is often X and Y plus or minus some amount. That is a so-called “Square Tolerance.” WIth GD&T, the tolerance is expressed as a round area. This is a much more forgiving tolerance than a Square Tolerance if you think of drawing a circular whose edges just meet the corners of the square. That circle is quite a bit larger than the square.

Most GD&T devotees don’t recognize many disadvantages other than that there is a learning curve you will have to get through to learn GD&T. It isn’t hard, and fortunately you have already started on a free course.

T Slot Tolerances

GD&T Basic Concepts

Like almost anything else, GD&T basics are simple concepts that we can use to group all of the other information into. We’ll discuss the following key GD&T Basics:

– GD&T Symbols

– Basic Dimensions

– Feature Control Frames

– Datums and Features

– Material Condition Modifiers

If we think about engineering drawings, a basic drawing provides size and shape descriptions. If we add some tolerances, they affect the description of size. Adding GD&T affects the description of shape in a very quantitative way, and it also provides a more comprehensive way to think about tolerances. In fact, the specification of shape includes tolerance as well. For example, if we use GD&T to suggest a surface must be flat, that describes its shape. But we will also include a tolerance that describes exactly how flat the surface needs to be–tolerances for flatness, in other words.

Let’s take a more detailed look at each of GD&T’s basic concepts.

GD&T Symbols

The Symbols, or Geometric Symbols, are what most people think of first when they hear about GD&T. There’s a collection of symbols that are used to specify each shape characteristic or tolerance that GD&T can describe. Here is a typical example, the Concentricity Symbol:

GD&T Concentricity Symbol…

Like most GD&T graphical symbols, Concentricity looks like what it is–two circles are shown concentrically. This makes it easier to remember the symbols.

This course includes a detailed Quick Reference Chart for the GD&T symbols, and we’ll put a link to that chart at the bottom of every page for your convenience. You may find it convenient to keep the chart open on a separate browser window for reference when working with GD&T.

Basic Dimensions

When you see a number in a box, that’s a Basic Dimension.

A number in a box denotes a Basic Dimension. Image Source

Given that there is just a number in a box, Basic Dimensions have no tolerances.