## What is a blind hole?

A blind hole is a hole that does not go all the way through a part. A blind hole goes to a specified depth and stops.

## How to dimension a blind hole

To specify a blind hole requires listing the diameter of the hole and a dimension to control the depth. The depth can be controlled by directly specifying the depth of the hole or by identifying the amount of material that will remain.

## Blind hole vs thru hole

A thru hole, sometimes called a through hole, goes completely through a part. It has two open ends whereas a blind hole has one open end and does not break through to the opposite side. In the image above, the three holes on the left are all blind holes. The hole on the right is a thru hole.

## Blind hole symbol

There is no GD&T symbol for a blind hole. A blind hole will be specified with a diameter and a depth specification or remaining amount of material. In the example below, the blind holes have a diameter of 0.25 and go to a depth of 0.40.

## Can you make a flat-bottomed blind hole?

You can make a flat bottom blind hole, but it can be difficult depending upon what type of material is being drilled. A modified drill bit or an end mill can work.  This video explains some tips.

## Blind hole example

The example below has three blind holes. They all have a diameter of 0.500 but they have different depths. From left to right, the depths are 0.500, 0.250 and 0.100.

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!

## What is a limit tolerance?

A limit tolerance is a form of dimensional tolerancing that specifies a tolerance range for a specific feature. Limit tolerances are also known as limit dimensioning and are an effective way to specify requirements on a blueprint. They clearly identify the tolerance range without requiring additional calculations by the blueprint reader.

## Limit tolerance symbol

There is no GD&T symbol for a limit tolerance. Per ASME Y14.5, the notation for a limit tolerance is to callout the upper and lower tolerance boundaries for a dimension. The upper end value of the tolerance range goes on top of the lower end value as shown in the examples below.

## Limit tolerance vs unilateral tolerance

A unilateral tolerance lists a nominal value along with a plus or minus tolerance. Using these two values allows the blueprint reader to calculate the upper and lower ends of the tolerance range. A limit tolerance skips the calculation step and directly specifies the upper and lower end of the tolerance range. If the unilateral tolerance above was instead specified as a limit tolerance it would be 5.8-6.0.

## Limit tolerance vs bilateral tolerance

A bilateral tolerance lists a nominal value along with a plus/minus tolerance. Using these two values allows the blueprint reader to calculate the upper and lower ends of the tolerance range. A limit tolerance skips the calculation step and directly specifies the upper and lower end of the tolerance range. If the bilateral tolerance above were instead specified as a limit tolerance it would be 16.5-17.5.

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!

## What is least count in measurement?

Least count is the smallest possible measurement that a tool or measuring instrument can resolve. For example, if a measuring tape has graduations of 1/8” then the least count would be 1/8”. As the least count of a measuring tool gets smaller, it is capable of taking more accurate and precise measurements. A measuring tool such as a micrometer or digital caliper will have a smaller least count than a standard ruler. This means it is able to measure in smaller increments and therefore is more accurate.

## What is the formula for calculating least count?

Least count is calculated by taking a known interval and dividing by the number of divisions.

Least count = known interval / number of division

In the case of the ruler above, in each 1 inch interval there are eight divisions. This means that the least count of the inch ruler is 1/8”.

For the mm side of the ruler, in each 1 mm interval there are ten divisions. This means that the least count of the mm ruler is 0.1mm.

## Micrometer least count formula

The formula for calculating the least count of a micrometer is:

Least count of micrometer = least count of main scale / number of divisions on secondary scale

This is slightly different than our ruler example above. This is because a micrometer usually will have a secondary scale. The primary scale is along the sleeve of the micrometer while the secondary scale is around the thimble.

For most inch micrometers, the least count of the main scale is 0.025” and the number of divisions on the secondary scale is 25 therefore the least count would be 0.001”. Some micrometers have a tertiary scale around the sleeve. This third scale has 10 divisions. This allows the secondary scale to be read with greater accuracy. The least count of the secondary scale (0.001”) / number of divisions on tertiary scale (10) = 0.0001”.

For most metric micrometers, the least count of the main scale is 0.5mm and the number of divisions on the secondary scale is 50 therefore the least count would be 0.01mm.

## Least count of vernier caliper

The formula for calculating the least count of a micrometer is:

Least count of vernier caliper = least count of main scale / number of divisions on secondary scale

For most inch vernier calipers, the least count of the main scale is .025” and the number of divisions on the secondary scale is 25 therefore the least count would be .001”.

For most metric vernier calipers, the least count of the main scale is 1mm and the number of divisions on the secondary scale is 50 therefore the least count would be .02mm.

## Least count of common length measuring instruments

 Measuring instrument Least count in inches Least count in mm Micrometer .0001" .001mm Digital Micrometer .0001" .001mm Dial Caliper .001" .01mm Digital Caliper .0005" .01mm Vernier Caliper .001" .02mm Ruler .100" .1mm Measuring tape 1/16" 1mm Height gauge .001" .01mm Dial indicator .001"/.0001" .01mm/.001mm Spherometer .0001" .001mm Micrometer screw .0001" .001mm

## What is a spotface?

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.

## 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

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

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.

## Spotface example

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!

## 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

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

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.

## 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

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

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.

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!

## 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.

## 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.

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

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.

## 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

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

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.

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!

## 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.

## 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.

## Other names for a tolerance block

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

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!

## 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.

## 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.

## 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.

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.

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!

## Related Articles

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.

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!

## 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.

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.

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!

## Engineering, Manufacturing and Quality Abbreviations and Acronyms

OMG why are there so many acronyms and abbreviations? FYI here is a huge list of common meanings. Hopefully it helps decipher some of the jargon and gibberish.

8D – method of problem solving commonly used in the automotive industry.

AQL – acceptance quality limit, previously acceptable quality limit

APQP – advanced quality product planning

AS9100 – aerospace quality standard

ATP – acceptance test procedure

BOM – bill of materials

CAGE – Commercial and Government Entity code

CAM – computer aided manufacturing

CAPA – corrective and preventive action

CAR – corrective action report

CFE – customer furnished equipment

CFM – customer furnished material

CNC – computer numerical control

COA/C of A – certificate of analysis

COC/C of C – certificate of conformance or certificate of compliance

COTS – commercial off the shelf

CSM – customer supplied material

CT – center thickness

DFARS – Defense Federal Acquisition Regulation Supplement

DFMEA – design failure mode effects analysis

DPA – destructive physical analysis

DPAS – Defense Property Accountability System

DPPM – defective parts per million

DSS – data summary sheet

ECN – engineering change notice

EIDP – end item data package

ERP – enterprise resource planning

ESD – electrostatic discharge

ETV – edge thickness variation

FAI – first article inspection

FAIR – first article inspection report

FMEA – failure mode and effects analysis

FOB – free on board

FOD – foreign object damage

FW – face width

GFE – government furnished equipment

GFM – government furnished material

GFP – government furnished property

GIDEP – government industry data exchange program

GMIP – government mandatory inspection points

GSI – government source inspection

GSS – government source surveillance

HIC – humidity indicator card

IAQG – International Aerospace Quality Group

IAW – in accordance with

IOT – internet of things

ISO – International Organization for Standardization

ITAR – International Traffic in Arms Regulation

JIT – just in time

KPI – key performance indicator

MBB – moisture barrier bag

MRB – material review board

MSDS – material safety data sheet

NADCAP – National Aerospace and Defense Contractors Accreditation Program

NC – non conformance

NCM – non conforming material

NDT – non destructive testing

NIST – National Institute of Standards and Technology

OCM – original component manufacturer

OD – outside diameter

ODM – original design manufacturer

OEM – original equipment manufacturer

OTD – on-time delivery

PCB – printed circuit board

PDCA – plan, do, check, act

PEM – plastic encapsulated microcircuits

PFMEA – process failure mode and effects analysis

PID – product identification document

PL – parts list

PM – preventative maintenance

PPAP – Production Part Approval Process

PO – purchase order

QA – quality assurance

QAPP – quality assurance program plan

QC – quality control

QML – qualified manufacturers list

QPL – qualified product list

QTP – qualification test plan

QTR – qualification test report

RCA – root cause analysis

REACH – registration, evaluation, authorization and restriction of chemicals

RFI – request for information

RFQ – request for quote

RMA – return material authorization

RoHS – Restriction of Hazardous Substances

SCAR – supplier corrective action request

SDS – safety data sheets

SMP – supplier management process

SOP – standard operating procedure

SOW – scope of work

SPC – statistical process control

TDP – technical data package

TIR – total indicator runout

WI – work instruction

WIP – work in progress

XRF – x-ray fluorescence