Engineer's Guide To Screw Hole Sizes

Engineers Edge provides a comprehensive guide to screw hole sizes, with charts and calculations for both metric and imperial units. The website covers a range of screw types, including ISO metric, ANSI, and British Standard Whitworth. It also offers resources for specific applications, such as sheet metal and wood. For those new to engineering drawings, the website explains the symbols and dimensions used to represent different types of holes, such as simple, counterbored, countersunk, and tapped holes.

Screw thread tap drill size

The tap drill size is an important consideration when creating threaded holes for screws. The tap drill is the correct size of the drill bit to be used before tapping the hole for the screw. The tap drill size depends on the desired thread depth and the type of thread, such as coarse or fine.

For standard V threads, the tap drill size is typically chosen to be within a certain percentage of the major diameter of the screw. For coarse threads, a good tap drill is around 85% of the major diameter, while for fine threads, it is about 90%. These percentages can vary slightly, usually by about 2 percentage points. This rule of thumb works well for screw sizes between 1/4 and 1 inch in diameter.

For metric V threads, another method to determine the tap drill size is the "major minus pitch" technique. This involves subtracting the pitch of the threads (the distance between each thread) from the major diameter of the screw. This method also works for inch-based threads, but the pitch must first be converted from a fraction to a decimal.

It is important to note that the tap drill size is not the only factor to consider when creating threaded holes. The type of material being drilled and the specific application may also impact the recommended tap drill size. Additionally, there may be multiple possible tap drill sizes for a given screw size, as long as they yield the desired thread depth.

Charts and calculators are often used to determine the proper tap drill size for a specific screw thread size or the appropriate tap for an existing hole. These resources provide specific tap drill sizes in both imperial and metric units, taking into account the type of thread and the desired thread depth.

Sheet metal pilot hole sizes

When working with sheet metal, it is important to understand the correct pilot hole sizes for screws. Pilot holes are necessary to prevent splitting within the materials being fastened together. The size of the pilot hole will depend on the type and size of the screw being used.

Sheet metal screws can be either self-tapping or standard. Self-tapping screws have a notched tip that acts as a drill bit, allowing them to drive into the material and prevent denting. Standard screws, on the other hand, require a pilot hole to be drilled before insertion.

For standard sheet metal screws, the pilot hole size will vary based on the screw size. As a general guide, a size-4 screw should have a pilot hole of 3/32 inches, a size 6 screw requires 7/64 inches, and a size 8 screw needs a 1/8-inch pilot hole. Larger screws, such as size 10 and 12, require pilot holes of 9/64 inches and 5/32 inches, respectively. Even larger screws, such as size 14, will need a pilot hole of 3/16 inches, while a 5/16-inch screw will require a 1/4-inch pilot hole.

It is important to note that the pilot hole size may need to be adjusted to suit a particular application. Additionally, the type of screw used will impact the pilot hole size. For example, Type AB Self-Tapping Screws, Type B Self-Tapping Screws, and Type 25 Thread Cutting Screws all require the same pilot hole and drill bit sizes. However, Type A Self-Tapping Screws have their own specific recommendations for pilot hole diameters.

Screw thread design

Screw threads are a helical structure that can be wrapped around a cylinder or cone, with the former being called a straight thread and the latter a tapered thread. The design of screw threads is a complex mechanism that has been in development for centuries. The following is a guide to screw thread design.

History of Screw Threads

Basic screw forms were known to early Greek natural philosophers as early as the 4th century BC. For a thousand years, threaded shafts were used in wine and oil presses, and the screws designed by Archimedes are still used today for raising water. It wasn't until the 14th century that handmade threads and screws were used to hold two or more things together under mechanical tension and compression. During the Industrial Revolution, more objects were being bolted and screwed together than ever before. These early nuts, bolts, and screws were handmade and lacked common standards across industries and companies. It took many years and a lot of trial and error to figure out the best thread types for different applications and establish national and international standards.

Terms and Definitions for Screw Thread Design

It is essential that engineers understand the terminology associated with screw thread design to effectively communicate with their manufacturing partners. Here are some key terms:

• Crest: The high point of the groove.
• Root: The low point of the groove.
• Thread Angle: The angle between opposing flanks.
• Major Diameter: The largest diameter.
• Minor Diameter: The smallest diameter.
• Pitch: The distance between crests. For imperial threads, this is often stated as threads per inch or TPI.
• Pitch Line: An imaginary line, running parallel to the center line of the screw, lying halfway between the high and low projected intersections of the angled flanks.
• Pitch Diameter: Measured from pitch-line to pitch-line on opposite sides of the thread.

Types of Machine Threads

Most standards have versions of fine, medium, and coarse threads for different applications:

• Fine threads are typically found in smaller diameters and on precision instruments where small adjustments are necessary for alignment. However, they are more easily cross-threaded, stripped, or galled. Galling can be prevented with lubrication or an oxide coating, or by using threaded inserts.
• Medium threads are the most common for general-purpose assemblies.
• Coarse threads are the easiest to make and have the most resistance to pull-out. They are often found on heavy industrial machinery and provide the most clearance between threads for plating or coating.

Creating Machine Screw Threads

External threads are made with a lathe or by thread rolling, while internal threads are made by drilling and tapping. Hybrid cutting tools can now perform these operations in a single step, saving time and money.

Screw thread size and tolerance

The ISO metric screw thread family is a series of general-use, 60° straight threads. It is the most commonly used worldwide thread system. The ISO metric thread system consists of two series: the metric coarse thread series (MC) and the metric fine thread series (MF).

The ISO metric thread designation rules are as follows:

• The description always begins with a capital M, followed by the nominal thread diameter in millimetres. For example, M8 is a thread with a nominal diameter of 8 mm (0.315″).
• Pitch is designated by "X P". For example, M8 X 0.75 means an 8 mm (0.315″) thread with a pitch of 0.75 mm (0.03″ or 34 TPI). If the “X P” is omitted, the pitch is defined by the Coarse Pitch Series according to ISO-261.
• The number of starts is designated by "Ph". For example, M8 X Ph2.25P0.75. In this case, 2.25 mm is the lead, and 0.75 mm is the pitch.
• The class appears after the pitch and is preceded by a “-”. It consists of two or four characters. For example, M8 X 0.75 – 5g6g. Small letters represent external threads, while capital letters represent internal threads.
• The thread direction appears after the class and is preceded by a “-“. If omitted, the direction is right-hand. If “- LH” appears, it designates a left-hand thread. For example, M8 X 0.75 – 5g6g – LH is a left-hand thread designation.

Thread tolerance dictates how much a thread can deviate from the basic profile and still achieve a proper connection. Mass production of threaded fasteners requires efficiency, so perfectly sized threads are unlikely. However, for general purposes, a thread does not need to be perfectly sized to work correctly. A high thread tolerance indicates that the threaded fastener is for general use, while a low thread tolerance indicates that the thread is for a specialised use, such as in the aerospace industry, which requires high precision.

The format for thread tolerance designation follows these rules:

• Letters: Metric thread tolerance uses capital letters for internal threads and lowercase letters for external threads. These letters also designate tolerance position. For a UTS screw thread, A indicates external, and B indicates internal.
• Numbers: Numbers indicate the tolerance grade.
• Order: Pitch diameter tolerance comes before major diameter tolerance.

The letters A-H are in the metric tolerance system. Internal threads only use G and H, and external threads use a-h. H describes tolerance with zero deviation from the basic profile, and G describes tolerance with deviation above the basic profile. h describes tolerance with zero deviation from the basic profile, and g-a describe tolerance with deviation below the basic profile, which is a theoretical thread profile. An external thread’s dimensions must always be equal to or smaller than the basic profile. Internal thread dimensions must always be greater than or equal to the basic profile.

Metric threads use the numbers 3-9 to designate tolerance class. The higher the number, the looser the tolerance. There are four groups of tolerance grades:

• Internal thread minor diameter: 4, 5, 6, 7, 8
• External thread major diameter: 4, 6, 8
• Internal thread pitch diameter: 4, 5, 6, 7, 8
• External thread pitch diameter: 3, 4, 5, 6, 7, 8, 9

UTS has five classes (similar to ISO grades), but only three are regularly used:

• Class 1: Tolerance for loose-fit threads for easy use, even in dirty environments.
• Class 2: Class 2 tolerance is the most common and maximises strength.
• Class 3: Class 3 tolerance threads have a medium fit for high-quality work.

Screw thread symbols

60° Stub thread form

• Stub Acme thread form
• Acme thread - centralizing
• Acme thread, general purpose
• Aeronautical National Form taper pipe thread
• American Water Works Association
• British Association Standard thread form
• British Standard Fine thread series
• British Standard Pipe Parallel
• British Standard Pipe Taper
• British Standard Whitworth
• ISO Metric thread form
• American Buttress screw thread
• American National Coarse thread series
• American National Extra-Fine thread series
• American National Fine thread series
• American National gas outlet threads
• American National gas taper threads
• American National hose & fire hose coupling
• National Miniature thread series
• American Standard straight pipe thread
• American Standard straight pipe thread in couplings
• American Standard internal straight pipe thread (dry seal)
• American Standard straight pipe thread for hose couplings & nipples
• American Standard intermediate straight pipe thread (dry seal)
• American Standard straight pipe thread for locknuts
• American Standard straight pipe thread for mechanical joints
• American Standard taper pipe thread
• Dryseal American Standard taper pipe thread
• American Standard taper pipe thread for rail fittings
• Special threads of American National Form
• Dryseal SAE Short Taper pipe thread
• Special Gas Taper thread form
• Dryseal Short Taper Pipe thread
• Special Threads for Helical Coil Wire Screw Thread Inserts
• Unified and American thread series
• Unified and American Coarse thread series
• Unified and American Extra-Fine thread series
• Unified and American Fine thread series
• Unified Miniature Thread Series
• Unified Thread Series with Controlled Root Radius
• Unified Coarse Thread Series with Controlled Root Radius
• Unified Fine Thread Series with Controlled Root Radius
• Unified Constant Pitch thread series with Controlled Root Radius
• Unified Coarse Thread Series with Controlled Root Radius
• Unified Fine Thread Series with Controlled Root Radius
• Unified Miniature Thread Series
• Unified Constant Pitch Thread Series with Controlled Root Radius
• British Standard Whitworth Special Thread
• American National 8, 12 and 16 Thread Series (8N, 12N, 16N)
• American National Standard Straight Pipe Thread in Pipe Couplings (Tap marked NPS)
• American National Standard Fuel Internal Straight Pipe Thread (Dryseal)
• American National Standard Straight Pipe Thread for Hose Couplings
• American National Standard Intermediate Internal Straight Pipe Thread (Dryseal)
• American National Standard Straight Pipe Thread for Loose Fitting Mechanical Joints with Locknuts
• American National Standard Straight Pipe Threads for Free-Fitting Mechanical Joints for Fixtures (Tap marked NPS)
• American National Standard Taper Pipe Thread (See ANPT, NPTR)
• American National Standard Taper Pipe Thread (Dryseal)
• American National Standard Taper Pipe Thread for Railing Joints (Tap marked NPT)
• American National Thread-Special

These symbols provide a standardised way to communicate and identify specific screw thread types, ensuring consistency and accuracy in engineering and manufacturing industries.

Frequently asked questions