Spotfaces – All About

What is a spotface?

spotface example on part

A spotface is a machined section of a part that allows a fastener to sit flat. This is usually a bolt head or washer but can be other fasteners. A spotface is generally very shallow and removes just enough material to create the clean, even, flat surface. Spotfaces are most often used when machining castings or forgings. Spotfacing is done using a manual or CNC milling machines.

Spotface vs counterbore

A spotface is functionally no different than a counterbore. A counterbore usually references a feature that is deeper than a spotface. While a spotface creates a flat mounting surface, a counterbore acts to recess the fastener. It would be safe to call a spotface a counterbore but not the other way around.

cutaway examples of countersink and counterbore

Spotface vs countersink

The primary difference between a countersink and a spotface is that the countersink has an angled bottom whereas a spotface has a flat bottom.

Spotface symbol

Spotface Blueprint GD&T Symbol SF in a u
Spotface symbol
Counterbore Blueprint GD&T Symbol u shape

The symbol used to callout a spotface is the counterbore symbol with the letters SF in the middle. This is per the engineering drawing standard ASME Y14.5. At times, a blueprint may indicate a spotface feature simply through the use of a counterbore symbol. Additionally, older drawings and blueprints may reference a spotface as SF or SFACE instead of using the symbol.

How to dimension a spotface

spotface blueprint example

A spotface is dimensioned by specifying its diameter and depth. At times the amount of remaining material may be specified instead of the depth. The symbols for diameter and depth are shown below.

Diameter Blueprint GD&T Symbol o with line through it
Depth Blueprint GD&T Symbol line with arrow pointing down

Spotface example

spotface cutaway example

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Countersinks – All About

What is a countersink?

A countersink is an angled taper applied to a hole that allows a fastener (usually a flat head screw or similar) to sit even with, or below the surface which has been countersunk. Occasionally, a countersink is used simply as a method of chamfering or deburring a hole.

Countersink symbol

Countersink Blueprint GD&T Symbol two lines pointing down

The GD&T callout for a countersink is shown below. Some older blueprints may use the notation CSK to identify a countersink dimension.

If you want to type the ⌵ symbol, hold the ALT key and press 9013. See this list for other common keyboard shortcuts for GD&T and blueprint symbols.

How to dimension a countersink

countersink blueprint example

A countersink is dimensioned by specifying the diameter of the countersink where it meets the surface and the included angle. In the above example, the part has a 0.5 thru hole and a countersink with a diameter of 0.7 and an included angle of 82°.

How to measure a countersink

Countersinks can be measured by many different gauges. The easiest tool to use, assuming the tolerances aren’t too tight, is a pocket comparator with a reticle. Optical comparators and CMMs are regularly used to measure countersinks with very tight tolerances.

What does a countersink look like?

countersink example on part

Countersink vs chamfer

A countersink and a chamfer are very similar. A countersink is basically no different than a chamfer on a hole.

The main difference is that a chamfer is normally thought of as being at 45 degrees (though the angle can vary). A countersink is usually one of many different standard angle sizes.

The most common countersink angles are 82°, 90° or 100°.

Note that in the case of the 90° countersink, this callout is the same as a 45° chamfer because the countersink angle takes both sides into account, so it is twice the chamfer angle.

Countersink vs counterbore

cutaway examples of countersink and counterbore

The difference between a countersink and a counterbore is that a countersink has an angled bottom and a counterbore has a flat bottom. Countersinks are often used to recess a flat head screw. Counterbores are used to recess bolts, washers and other fasteners.

Countersink vs spotface

spotface example on part

A spotface has a flat bottom like a counterbore while a countersink is angled. A spotface is used to create a flat area in a specific location to allow a fastener such as a screw or bolt to sit squarely.

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Counterbores – All About

What is a counterbore?

A counterbore is a circular hole with a flat bottom which coincides with another hole. The counterbored section allows a bolt head or other fastener to be recessed.

What does a counterbore look like?

counterbore on metal part
An example of a counterbore in a piece of metal

Counterbore symbol

The GD&T callout for a counterbore is shown below. The counterbore symbol will often be used together with the diameter symbol and the depth symbol. Older blueprints may specify a counterbore with the notation CBORE instead of the counterbore symbol.

Counterbore Blueprint GD&T Symbol u shape

If you want to type the ⌴ symbol, hold the ALT key and press 9012. See this list for other common keyboard shortcuts for GD&T and blueprint symbols.

How to dimension a counterbore

Diameter Blueprint GD&T Symbol o with line through it
Depth Blueprint GD&T Symbol line with arrow pointing down

A counterbore is dimensioned by including the diameter of the counterbore along with specifying the depth. The two ways to specify the depth are to specify how deep the counterbore is or the thickness of the remaining material. Both methods are acceptable and commonly seen.

In the example below, the part has a .250 hole and a .500 diameter counterbore to a depth of .100.

counterbore blueprint example

How to measure a counterbore

Counterbores can be measured with many different types of gauges. The simplest inspection tool to use, assuming the tolerances aren’t too tight, would be a caliper.

Pocket comparators, gage pins, and depth micrometers are other types of measuring equipment that are frequently used to measure counterbores.

Counterbore vs countersink

cutaway examples of countersink and counterbore

The difference between a countersink and a counterbore is that a countersink has an angled bottom and a counterbore has a flat bottom. The angle of the countersink can vary with many different angles used such as 82°, 90° and 100°.

Counterbore vs spotface

spotface example on part

A counterbore and a spotface are very similar. A counterbore is used to recess a fastener while a spotface is used to create a flat surface located allow a fastener to be used. A spotface is used to let a fastener sit flat and in a specific location. A spotface is basically a shallow counterbore.

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Tolerance Blocks – All About

What is a tolerance block?

The definition of a tolerance block is a section on an engineering drawing or blueprint that identifies tolerances for a dimension that aren’t specifically called out on the print. Notice how the 17.5 dimension below has a tolerance directly associated with it.

Now look at the 12.5 dimension and notice that no tolerance is specified. Because there is no tolerance called out, the tolerance in a tolerance block (general tolerances) would be applied to this specific dimension.

Note: The contents of the tolerance block are often referred to as the general tolerances. Depending on the units used on the blueprint, the tolerance block can be specified in metric or imperial (inch) units.

Tolerance block examples

tolerance block example

How to read a tolerance block

Most tolerance blocks are identified based on the number of decimal places of the feature on the blueprint. 

Using our previous examples again, notice that the 12.5 dimension has one number after the decimal place. Based on the tolerance block, this would assign the +/- 1 mm tolerance to the dimension. 

If the dimension was instead 12.54 then the tolerance assigned would be +/- .5mm. Angular dimensions often are specified in the same way. In our example, all unspecified angular tolerances would be assigned the =/- .5° tolerance.

tolerance block example

Other names for a tolerance block

  • Default tolerances
  • General tolerances
  • Standard tolerances
  • Title block tolerances

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Full Radius – All About

What is a full radius?

A full radius is a radius that smoothly blends into another surface. Full radius is most often specified in a rounded slot feature or a feature that mimics a rounded slot.

A full radius sometimes noted as a true radius or full R is outdated language and not part of the current revision of the drawing standard ASME Y14.5. The full radius callout is referencing a smooth transition from the radius to an adjacent surface.

full radius blueprint example

How to measure a full radius

Because no reference standard documents the requirements of a full radius, there are no specific requirements for the callout.

When a full radius, true radius or full R is called out on the drawing, the blueprint drafter is attempting to control the blend into and out of the specified radius.

What is a full radius?

A full radius is a radius that smoothly blends into another surface. Full radius is most often specified in a rounded slot feature or a feature that mimics a rounded slot.

A full radius sometimes noted as a true radius or full R is outdated language and not part of the current revision of the drawing standard ASME Y14.5. The full radius callout is referencing a smooth transition from the radius to an adjacent surface.

full radius blueprint example

How to measure a full radius

Because no reference standard documents the requirements of a full radius, there are no specific requirements for the callout.

When a full radius, true radius or full R is called out on the drawing, the blueprint drafter is attempting to control the blend into and out of the specified radius.

Full radius vs radius

There is no difference between the drawing callouts of full radius, true radius and radius. Because there are no specific requirements for a full radius referenced by any drawing or GD&T standards, there is no difference in the requirements of a full radius or full R vs a radius or R. A full radius does not have a tolerance. A radius if drawn correctly will have some form of a +/- tolerance or be controlled through a GD&T requirement such as profile or cylindricity.

There is no difference between the requirements of the example below or the previous one.

full radius blueprint example

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Full radius vs radius

There is no difference between the drawing callouts of full radius, true radius and radius. Because there are no specific requirements for a full radius referenced by any drawing or GD&T standards, there is no difference in the requirements of a full radius or full R vs a radius or R. A full radius does not have a tolerance. A radius if drawn correctly will have some form of a +/- tolerance or be controlled through a GD&T requirement such as profile or cylindricity.

There is no difference between the requirements of the example below or the previous one.

full radius blueprint example

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TYP & Typical on Blueprints [What They Mean & How to Inspect Them]

What does typical mean on a blueprint?

Typical on an engineering drawing identifies a repeated feature. This is identical to a feature which is identified as 2x or 5x. 

A typical dimension callout will occasionally be followed by a 2x, 5x or similar, to specify the quantity of features which are tolerance the same. 

The typical callout will most often be used as part of a repeating pattern such as a bolt hole circle, to identify the hole sizes or angle between the holes. 

Another common application is to identify a common chamfer size on a component. It should be noted that the notation of “typical” is not a part of the current revision of the ASME Y14.5 standard and therefore not a recommended notation for use on an engineering drawing. There are however countless blueprints in the wild which may already use this language.

What is the symbol for a typical dimension?

There is no GD&T symbol for a typical dimension. A typical dimension callout is identified with either TYP. or TYPICAL. In the example below, the typical notation is used to reference that the slot on both sides of the part is to be machined to the same depth.

typical callout blueprint example

A better way to identify the same dimension would be as shown below. It is best to not leave anything to the imagination of the person interpreting the blueprint.

slot depth blueprint example

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Ballooning/Numbering A Blueprint

What is a ballooned or numbered blueprint?

ballooned drawing example

Commonly referred to by many different names including ballooned drawing, bubble drawing, numbered print, etc. A numbered drawing or blueprint is a way to identify individual attributes of a part or assembly as depicted in an engineering drawing. The numbers and balloons or bubbles are ordinarily done in red ink or a red font as seen above.

What are ballooned drawings used for?

Numbered drawings are a common component in the inspection process. They can be a part of your in house inspection procedure or a requirement which you provide to your customers. Ballooned drawings are also frequently used as part of a first article inspection report package. They allow the reader to connect an individual measurement to its location on the blueprint. This is especially handy when multiple attributes with the same nominal values are present.

How to number a drawing

Numbering or ballooning a blueprint is a process that has some flexibility to the order in which attributes are labeled. The most important aspect is that all attributes are assigned a number including all notes and general tolerances as needed. When in doubt, number it. Where to start is a matter of preference but make sure that your numbering sequence is easy to follow. Generally the person numbering the drawing will start in the top left view and work their way clockwise assigning numbers to attributes in that particular view. This process is repeated for all views present on the drawing in a top to bottom, left to right manner similar to the way you read a book. Some users will list attributes in the notes first such as material, but this is simply a matter of preference. Just make sure to number all the relevant attributes in the notes. If you work in a logical manner, it will be much easier for the customer or reader to follow along.

Example of a fully numbered drawing

What to include in a numbered drawing

Assign a number to every attribute on the print including all notes and applicable general tolerances. Some notes may include more than one attribute in a single note which requires a numbered attribute.

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Datums – All About

What is a datum?

A datum is a reference point for measurements. A datum is a theoretically perfect feature. It is often a main surface of a part called a plane but can also be a point such as the center of a diameter or an axis such as the center of a cylinder.

How are datums used?

Datums are used to orient measurements. In the case of something simple such as a perpendicularity specification, the datum is the surface against which the toleranced feature is checked. For a positional tolerance, features are located relative to a datum.

Symbol for a datum

Datum Blueprint GD&T Symbol a in a box with arrow

Datum names

Datum names are identified with a capital letter. The most important reference feature is usually identified with a capital A. In order of importance datums will be assigned consecutive letters (B, C, etc).

How is a datum used in a feature control frame?

feature control frame description with parts identified

Datums are placed at the end of a feature control frame.

Primary datums

primary datum identifier

A primary datum is the main locating surface used for alignment.

Think of a box set on a table. In this case the bottom of the box is the primary datum.

The table would be used to simulate the primary datum. In this example measurements could be taken from the table to other features of the box to verify they meet the required dimensions.

The primary datum controls one axis of freedom. The box can move back and forth or spin in place on the table but it can not turn end over end. This is similar to the way a granite surface plate is used in machine shops all over the world.

Secondary datums

secondary datum identifier

A secondary datum is the secondary alignment feature. In the box on a table example, if the table and the box were pushed up against a wall then the wall would be the secondary datum.

The box would contact the wall at a minimum of two points. Contacting the wall would constrain the movement in another axis.

The table controls end over end movement. The wall stops the box from spinning. It is however still able to move back and forth along the wall.

Tertiary datums

tertiary datum identifier
A tertiary datum is the third alignment feature. In the box on a table example, if the box on a table which is now against a wall was pushed into a corner then the new wall would be the tertiary datum. This new datum works to control the last remaining axis of movement.

What is a datum target?

A datum target is a specified location that is used to measure a part. It identifies the point(s) from which measurements will be taken.

The use of datum targets or points provides a level of control over the datum surface. Some surfaces are incapable of being used like a normal datum.

A very large datum surface or a surface that has a lot of variation in its form are two common reasons why datum targets are used.

Datum targets are also used to mimic real world use such as contact points in an assembly.

Datum target symbol

Datum Target Blueprint GD&T Symbol circle with diameter 3 in top and A1 in bottom half

The top half of the datum target symbol is often empty. The upper half lists the area of the datum target when specified. The lower half of the datum target symbol lists the datum target name. 

Is a datum real or theoretical?

Both really. The real datum is the actual surface referenced. This surface is imperfect and will have variation to it.

No surface is perfectly true.

In practice though a datum is used as a theoretically perfect surface.

A good example would be placing a datum surface down on a surface plate which is known to be very flat. The imperfect actual surface of the part will come to rest on the high points sitting on the surface plate. Measurements can be taken from the surface plate as if it was the datum surface itself.

Examples

true position callout

True position callout referencing datums A & B

perpendicularity callout example with feature control frame

Perpendicularity callout referencing datum A

circular runout callout

Circular runout referencing datum A

Datum dimensioning vs chain dimensioning

Chain dimensioning is the process of dimensioning features of one another in a row. Datum dimensioning is used when features are referenced from a common point. Take a look at the differences in the examples below.

datum dimensioning blueprint example
Datum Dimensioning
chain dimensioning blueprint example
Chain Dimensioning

A disadvantage of chain dimensioning is that the tolerances stack up. This means that the location of the far right hole can actually vary by as much as +/- 0.040”.

The example which uses datum dimensioning maintains the +/- 0.010” tolerance for each location. Chain dimensioning can be a perfectly acceptable way to tolerance your parts, just make sure you have taken into account the additional tolerance that can apply to the feature.

Plural of datum

Nothing too strange. More than one datum would be multiple datums.

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Feature Control Frames – All About

What is a feature control frame?

A key component of geometric dimensioning and tolerancing (commonly referred to as GD&T). On engineering blueprints, the feature control frame consists of a symbol to identify the type of tolerance, the amount of tolerance and reference datums if applicable.

How to read a feature control frame

A feature control frame is read from left to right. It reads “Type of control” of “Tolerance” to Datum. It should be noted that if a diameter symbol is present before “Tolerance” then it indicates the shape of the tolerance zone is cylindrical.

Examples

true position callout

True position of 0.2 to datums A and B

perpendicularity callout example with feature control frame

Perpendicularity of 0.001 to datum A

cylindricity callout

Cylindricity of 0.001

circular runout callout

Circular runout of 0.010 to datum A

Composite feature control frame

A composite feature control frame controls both a pattern on a part and the location of individual items in the pattern. 

The upper section of a composite feature control frame specifies the tolerance for the pattern to the overall part. 

The lower section specifies the tolerance for individual features to the pattern. In the example of a bolt hole circle, the upper section controls the tolerance for the location of the bolt hole circle on the part. The lower section would control how closely the individual holes must follow the pattern.

composite feature control frame

Feature control frame symbols

gd&t symbols
gd&t symbols

For more information see our GD&T Symbols Quick Reference

Basic dimensions

Basic Dimension Blueprint GD&T Symbol dimension in a box

Basic dimensions are identified by a rectangular frame around the dimension. 

They are dimensions that are theoretically exact. They do not have a tolerance themselves (general blueprint tolerances do not apply). 

Instead they are controlled by another characteristic. This is often seen with positional tolerances such as the true position of a hole. The hole location will be specified as basic dimensions. 

A true position tolerance will then be assigned to the hole which will control how far off the nominal location the hole can be.

Want to learn more?

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GD&T Symbols Quick Reference

A cheat sheet type reference for the most common GD&T symbols.  

See also our GD&T Font – GD&T Keyboard Shortcuts List

Symbol

Name

Description

Straightness

Straightness is how close to a straight line a feature is.

Flatness

Flatness is how flat a feature is. All points on the feature must lie within two parallel planes that are spaced the tolerance width apart.

Circularity

Often called roundness. Circularity refers to how close to a perfect circle a single location is. Circularity is at one location. This can be thought of as a single circle on a cylinder. Usually circularity would be checked at multiple locations along the cylinder. This cylinder can be the inside of a hole, the outside of a shaft or various other features.

Cylindricity

Cylindricity is the same as circularity (often called roundness) with the exception that the requirement applies across the whole surface instead of at a single location. Cylindricity works to control taper whereas circularity does not.

Parallelism

Parallelism refers to how close to 180 degrees two surfaces are.

Perpendicularity

Perpendicularity is how close to 90 degrees two features are. This can be any combination of planes or axes.

Angularity

Angularity is the same as perpendicularity with the exception that the two features are not at 90 degrees to one another but instead at a different specified angle.

Concentricity

Concentricity is how close the axes of two features run together.

True Position

True position is a theoretically exact location of a feature.

Symmetry

Symmetry is the same as concentricity but is applied to features that aren’t round. This means that the axes or centers of two features must run together.

Profile of a Line

Profile of a line controls the shape of a cross section of a feature. It can control size, form and location.

Profile of a Surface

Profile of a surface is similar to the profile of a line tolerance but it controls the entire surface instead of a single cross section.

Circular Runout

Circular runout controls the runout in a single location of a circular feature such as a cylinder.

Total Runout

Total runout controls the runout of an entire surface of a circular feature instead of at a single location. When compared to circular runout, total runout would check the entire cylinder.

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A cheat sheet type reference for the most common GD&T symbols.  

See also our GD&T Font – GD&T Keyboard Shortcuts List

Symbol

Name

Description

Straightness

Straightness is how close to a straight line a feature is.

Flatness

Flatness is how flat a feature is. All points on the feature must lie within two parallel planes that are spaced the tolerance width apart.

Circularity

Often called roundness. Circularity refers to how close to a perfect circle a single location is. Circularity is at one location. This can be thought of as a single circle on a cylinder. Usually circularity would be checked at multiple locations along the cylinder. This cylinder can be the inside of a hole, the outside of a shaft or various other features.

Cylindricity

Cylindricity is the same as circularity (often called roundness) with the exception that the requirement applies across the whole surface instead of at a single location. Cylindricity works to control taper whereas circularity does not.

Parallelism

Parallelism refers to how close to 180 degrees two surfaces are.

Perpendicularity

Perpendicularity is how close to 90 degrees two features are. This can be any combination of planes or axes.

Angularity

Angularity is the same as perpendicularity with the exception that the two features are not at 90 degrees to one another but instead at a different specified angle.

Concentricity

Concentricity is how close the axes of two features run together.

True Position

True position is a theoretically exact location of a feature.

Symmetry

Symmetry is the same as concentricity but is applied to features that aren’t round. This means that the axes or centers of two features must run together.

Profile of a Line

Profile of a line controls the shape of a cross section of a feature. It can control size, form and location.

Profile of a Surface

Profile of a surface is similar to the profile of a line tolerance but it controls the entire surface instead of a single cross section.

Circular Runout

Circular runout controls the runout in a single location of a circular feature such as a cylinder.

Total Runout

Total runout controls the runout of an entire surface of a circular feature instead of at a single location. When compared to circular runout, total runout would check the entire cylinder.

Want to learn more?

GD&T is a complicated subject and understanding it correctly can be the difference between a perfect part and scrap.

The best way to learn GD&T is from experienced teachers who can break down the material into manageable pieces.

Luckily, we know someone.

And MachinistGuides.com readers get an exclusive discount on training!

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