Screw Design Guide

Introduction

The most popular thread used in fastening is the machine screw thread. The machine screw thread has a 60 degree thread angle made in a continual running helix pattern. All fastener threads have five defining dimensions, they are the major diameter, minor diameter, pitch diameter, flank angle and pitch.

Feature Male Thread Female Thread
Major Diameter This is the diameter across the crest of the thread. It is easily checked with calipers. This is the diameter across the root of a female thread. This diameter is very hard to measure and is best checked by a thread gage.
Minor Diameter This is the diameter across the root of the thread. The easiest way to check this simnsion is with an optical comparator. This is the siameter of the through hole of the female thread. It can be easily checked with a pin gauge.
Pitch Diameter This is the mid point between the crest and root of the thread. It is check by using a pitch micrometer. This is the mid point between the crest and root of the thread. It is check with the use of a thread gague
Flank Angle This is the angle made by the intersection of the two thread flanks. It is best checked with an optical comparator. This is the angle made by the intersection of the two thread flanks. It is best checked with a thread gauge.
Pitch This is the distance between two consecutive thread crests. It is best checked with an optical comparator. This is the distance between two consective thread crests. It is best checked with a thread gauge.

Tolerances

A fastener with a machine screw thread can have one of three thread tolerances, they are “Class 1”, “Class 2”, and “Class 3”. The tolerance is called “Class of Fit”. To indicate that a thread is a male thread an “A“ is placed behind the Class of Fit number for example 2A. To indicate that a thread is a female thread a “B” is used, for example, 3B.

Class 1
Is a very loose fit. It is rarely used in today’s market place.
Class 2
Is the most widely used tolerance class by the fastener industry. All industrial applications use a Class 2 Fit.
Class 3
Is thought of as the, “Aerospace” tolerance. This class of fit is used on more critical applications. Dimensions for all thread sizes and their corresponding class of fit can be found in the ASME B1.1 Specification.

Thread Nomenclature

The fastener industry has developed a very strong nomenclature to identify a thread. Here is how to call out a fastener thread. In this website, the thread nomenclature has been shortened to use only the thread size and threads per inch. A Class 2 tolerance is assumed, and the internal / external thread profile can be determined from the rest of the description.

Thread Nomenclature

Machine Screw Threads

A machine screw thread is defined by starting with the size of the thread, then the number of threads in one inch, followed by the thread form call out, “UN” for Unified National then the letter “C” for coarse, “F” for fine, the Class of Fit, and an “A” for an external thread or “B” for an internal thread.

Sheet Metal Screw Threads

In sheet metal screw threads there is no Class of Fit. The nomenclature used calls out the size and the type of thread for example, #6 Type B. See “Threads” on page 9 to see all the various sheet metal threads.


Features

A screw can be broken down into various components (Figure 4). These components have been dimensionally detailed by ANSI standards. A complete set of fastener standards can be found in the Industrial Fasteners Institute (IFI) Fasteners Standards book.

Screw Design Features

A machine screw thread is defined by starting with the size of the thread, then the number of threads in one inch, followed by the thread form call out, “UN” for Unified National then the letter “C” for coarse, “F” for fine, the Class of Fit, and an “A” for an external thread or “B” for an internal thread. In addition to the nomenclature used to describe a thread, there is also a standard terminology to define the rest of a fastener. This is accomplished by describing the features of a fastener. Other features can also be added to a screw (Figure 5). The various combinations of each of these components can produce an endless number of possible screw options. This guide will try to detail many of these options and what their unique values are. Terminology used to call out a fastener:

  • 4-40 x 5/16 PH-PN-MS-STL-Z Phillips pan head machine screw steel zinc.
  • 6-32 NY-IN-LK-NU-SS 6-32 nylon insert locknut stainless steel.

Drives

Pencom offers a wide variety of drives to choose from to fit your application. Below is a list of common drives Pencom manufactures. If you have any questions about the type of drive you should use, please contact your local sales representative for assistance.

Type 1 Cross Recessed

The most popular drive found in small diameter screws. It is often referred to as a Phillips ® drive after the firm that developed it many years ago. Tools are readily available to install this screw. Its ability to keep the screwdriver bit in place is good and it transfers torque to the screw well. The Type 1 cross recess can be found in almost all applications and offers the best “off the shelf” availability.

Slotted Drive

The oldest of screw drives dating back to the industrial revolution. It is still very popular where home applications are involved. Tools are readily available almost anywhere. If a slotted screwdriver is not available, other items (knife or coin) have been known to stand-in. The slotted drive offers a poor ability to hold the driver bit in the drive, preventing the full transfer of torque to the screw.

6 Lobe Recess

Developed by Camcar ® in the 1970’s to address the limitations of the popular drives of the time. They called this drive the Torx ®. The 6 lobe recess offers excellent drive to Screwdriver bit engagement. The bit nests into the drive in a way that, during use, all torsional forces are transferred to the screw. This allows for maximum loading of the fastener. As driver bits have become more popular, the 6 lobe recess has worked its way into more consumer products. It is very popular for high volume assemblies.

Tamperproof 6 Lobe Recess

Allows a high volume manufacture to get some level of tamper resistance into their product. This is often used on disc drives and other computer products to convey the message that the general consumer is not welcomed. All the benefits of the 6 lobe recess are still in force. Driver bits are difficult to get if not in the trade.

Pozi ® Drive

Also developed as an improvement on the Phillips ® drive. Officially called a Type 1A cross recess, the design produces more grain flow in the drive area. This is designed to increase the toughness of the drive and reduce the stripping problem inherent with the Phillips ® drive. Though not very popular in the USA, the pozi drive has become very popular in Europe. Drive to bit engagement is slightly better that a “Phillips”. Transfer of torque from the screwdriver to the screw is good. Drive bit availability in the USA is poor.

Combination Drive

Developed to get the production benefits of the Type 1 Cross Recess. It has good drive to bit engagement and thus good torque transfer to the screw. This drive is often used when there is product assembly done by the consumer. Either a standard Phillips ® bit or a slotted bit can be used for assembly.

Slotted 6 Lobe Recess

A Developed to make the 6 lobe more convenient to the end user. This drive still offers all the high-speed assembly benefits of the 6 lobe recess to the manufacture. It also allows the end user to disassemble the screw with a slotted driver. This style recess has become very popular where customer disassembly is required. Many high tech consumer products have made this their standard (computers, prints, disc drives, etc.)

Square Drive

Square Drive has become popular in the construction industry. The drive to bit engagement is excellent, and the torque transfer to the screw is also very good. Square drive bits are readily available from construction supply stores, but are more difficult to obtain for consumers. This produces a mildly tamper resistant condition with the screw.

Hex Recess

Used primarily on socket products. The points on the hex bit produces a high-pressure point against the contact location of the hex recess during assembly. This produces a higher drive bit wear then a 6 lobe recess. To protect the hex recess, all steel screws are heat-treated. Recess to screwdriver bit engagement is excellent. Torque transfer from the bit to screw is excellent. Driver bits have excessive wear. Typical applications are high-end instrumentation, medical and test equipment.

Tamperproof Hex Recess

Tamperproof Hex Recess is a way to make a standard hex recess tamper resistant. During the manufacturing process, a small shaft of material is formed that sticks up from the bottom of the hex recess, this prevents any standard hex bits from engaging the recess. Since most hex bits are hardened, it becomes a difficult task to modify. The driver to bit engagement and the torque transfer is the same as a hex recess. Typical application is when simple tampering needs to be prevented.

Torx Plus ®

Patented drive from the Camcar Division of Textron. Its’ design comes from its predecessor, the 6 lobe recess. The benefits of this Drive according to Camcar is high drive torques. To date, most application needs can be solved with the 6 Lobe Drive. There is a cost premium when using the Torx Plus ®.

Anti Cam-out Recess

(ACR Phillips II®) has vertical ridges running down into the recess. These ribs engage with horizontal ribs on the driver bit to lock the driver to the screw. The drive was developed to address the inherient stripping problems of the type Cross Recess. It was also designed to take back some of the 6-Lobe market. There is a cost premeium when using the ACR ®.

One Way Drive

Falls into the family of tamper resistant drives. It is based on a slotted drive, which has had the back flanks removed. This allows the drive to be installed but not removed. The one-way drive is a very user-friendly drive since a slotted screwdriver is readily available. This drive has become very popular in public areas where field assembly will take place. The process to remove a one way screw is very difficult.

Spanner Drive

Spanner Drive has two small holes on the top of the drive. A screwdriver with two male pins is inserted into the mating holes on top of the screw for assembly. The spanner drive can be easily removed with the proper driver bit. The drive to bit engagement is good, but the drive pins can be easily overloaded, so the torque transfer is low. This drive has become very popular in public areas where tamper resistance and field repair are required.

Tri-Wing

Developed as a drive for the aerospace industry. Its design puts more of the installation force vector onto a larger drive flank. It enables the drive bit to transfer more torque into screw load. However, what is gained in installation is lost in removal. The tri wing has also been used as a tamper resistant drive.

Phillips Square

Designed to meet the demanding needs of manufacturing engineers. The Phillips Square Drive combines the cross recess and the square recess. The result: a highly efficient and functional “multi-driver serviceable recess.


Heads

Pencom offers a wide variety of head types to choose from. Below is a list of common head types Pencom manufactures. If you have any questions about the type of head you should use, please contact your local sales representative for assistance.

Pan Head

General purpose head used by all industries. It has a good bearing area to spread load into the mating part.

Fillister Head

Fillister Head has a smaller outside diameter then the pan head. It is excellent for use in a counter bored hole. The higher head allows for a deeper drive. The bearing area is small and damage around the mating hole is possible.

Flat Head

The general purpose head that is used when a flush condition is required. The standard flat head has an 82º head.

Oval Head

Used to add a finished look to the surface of a product. The head angle is 82º.

100 Deg. Flat Head

Used in thin materials like those found in the aerospace industry.

Undercut Oval Head

Used to add a finished look to the surface of a product when the materials will not allow a standard oval head. The head angle is 82º.

Undercut Flat Head

Used in thin materials where the deep shape of the standard flat head is undesirable. The head angle is 82º.

Trimmed Oval Head

Similar to the oval head machine screw except that the size of head for a given size screw is one or two sizes smaller than the oval standard sizes. The head angle is 82º.

Trimmed Flat Head

Trimmed Flat Head has a smaller diameter head than the standard flat head. It is used when a smaller diameter head is needed. The head angle is 82º.

Truss Head

Lower in height and has a larger bearing area then the pan head. Truss head screws offer a pleasing look on the surface of a product.

Indented Hex Washer Head

Offers excellent torque transfer into the screw when driven from the hex body. The washer increases the bearing area of the screw.

Cheese Head

Cheese Head is a specialized head that is used by design engineers for its particular look.

Indented Hex Head

Excellent torque to load transfer. In most cases a design engineer will choose the Indented hex washer head instead.

Socket Head Cap Screw

Standard for all of the high strength hex recess driven screws.

Binder Head

Specialized style that is chosen for its look by design engineers.

Wafer Head

Low profile head style that has a very low height. It does not require a countersink in the mating material.

Button Head

Used for producing a hi-tech look on a product, generally mated with either a hex or a torx recess.


Threads

Pencom offers a wide variety of thread types to choose from. Below is a list of common thread types Pencom manufactures. If you have any questions about the type of thread you should use, please contact your local sales representative for assistance.

Machine Screws

Machine Screw

Threads are the most common of all thread profiles. It has a 60 degree flank angle.

Type 1

Thread cutting screw with a machine screw thread pitch, blunt point, tapered entering threads and a single cutting edge.

Type C

Threads are machine screw threads with a tapered point that will form a thread.

Type 23

Threads are machine screw threads that have a milled section removed from the shaft. This larger space is helpful when there is paint or other materials that could build up during the cutting process.

Trilobular

Threads are machine screw threads with a thread forming feature. Trilobular screws will form their own threads in the mating materials. They will form threads in all soft ductile metals (Aluminum, Low Carbon Steel, Brass, and Copper).

Type F

Threads are machine screw threads that have two grooves for cutting a thread into the mating material. This thread style has been replaced by the better performing TA style.

Sheet Metal Screws

Type A

Sheet metal screws have a 60 degree flank angle and a space between the threads. The Type A is still available but most applications have been switched to the more available Type AB and Type B.

Type 25

Sheet metal screws have a Type B thread shaft with a blunt point. A milled groove is added to the thread making it a thread cutting version of the Type B.

Type AB

Threads are standard sheet metal screw threads. They have a 60 degree flank angle thread and can form a thread in soft materials. The Type AB has the same point as the Type A.

Type TEK2

Sheet metal screws have the same thread as a Type B with a drill feature for a point. Type TK screws are very popular in applications where a screw must drill a hole in the part.

Type B

Sheet metal screws have the same thread as the Type AB. The end of the screw is blunt, offering more thread engagement in short length applications. The Type B is the most popular thread in the sheet metal family.

Threads for Plastic Applications

Type PF

Type PF is a thread designed to be used in soft materials. It has a 30 degree flank angle thread that slices through the plastic without inducing high stress levels. It has the smallest minor diameter of the screws for plastics, and offers the highest pull performance.

Type PLAST 48/2

Trilobular thread for plastics. It has a 48 degree flank angle that produces less stress on a plastic boss than a 60 degree thread. The minor diameter is also smaller than that of the Type B which gives it a better pull out.

Type HL

Thread forming screw with a double-lead, consisting of a high and low thread. The lower thread varies in height from 1/3 to 1/2 that of the higher thread, which is sharper and flatter than a standard thread. Type HL thread is for use in plastic, nylon or other low-density materials.

Type PLAST 60/1

Trilobular thread rolling screw desinged for high performance fastening in a wide range of thermoplastics. The 60 Deg profile on the type 60/1 screw penetrates deeply into the work piece, absorbing higher torques without stripping and resisting pullout forces.


Points

Pencom offers a wide variety of points to choose from. Below is a list of common points Pencom manufactures. If you have any questions about the type of point you should use, please contact your local sales representative for assistance.

Machine Screws Points

Cut-off Point

The standard shape when a screw is processed without any special attention. The Cut-off point produces a very abrupt transition between the point and the thread.

Header Point

Produced during the forming operation. There is a small chamfer that is added to the screw blank. When the thread is added to the screw blank. This small chamfer produces a smooth transition between the point and the thread.

Dog Point

Unthreaded minor diameter shank of a screw. It is helpful for the alignment into hard to see holes. Here it is used with a machine screw thread.

Sheet Metal Screws Points

BP Points

Cone point on a Type B thread. It enables the screw to locate and then pry or force its way into holes that are not aligned. Generally when this feature is needed the designer will opt for the more readily available Type AB.

Dog Point

Unthreaded minor diameter shank of a screw. It is helpful for the alignment into a hard to see hole. Here it is used with a Type B thread.

Needle Point

Sharp long point that allows the thread to extrude a female thread in the mating material.


Attached Washers

Pencom offers a wide variety of attached washer methods to choose from. Below is a list of common attached washers Pencom manufactures. If you have any questions about the type of attached washer you should use, please contact your local sales representative for assistance.

Attached Washers

Internal Tooth Lock Washer Sems

Used when it is desirable to have the locking teeth near the mounting hole. They are also popular when there is high volume assembly. The teeth do not come in contact with the assembler’s fingers as is the case of the external tooth washer.

Split Ring Lock Washer Sems

Very popular locking device. However, a quick review of the tension provided by the split ring will show that it is a very small percent of the total load on a fastener. Its true benefit is that it provides a hardened bearing surface for the fastener to load against. This improves the torque / tension curve and provides the best joint loading.

Square Cone Lock Washer Sems

Designed to provide an additional spring component in the fastened joint. As the joint flexes, the square cone washer sems is able to respond and keep the joint in tension. The square cone washer sems also has a bearing surface that is not part of the typical conical spring washer.

Flat Washer Sems

A great way to eliminate the need to handle two components. Any size and shape washer can be assembled on to a screw. The flat washer will increase the bearing area of the screw.

External Tooth Lock Washer Sems

Takes the standard external tooth lock washer sems and permanently attaches it to the screw. This eliminates the need to handle two parts. This is a good locking device, but it’s real strength is in the ability of the teeth to dig into surfaces. This is very effective in sizes up to #8. Sizes above a #8, the load gets high enough to flatten the teeth and render them useless.


Threads Per Inch

The importance of thread ramp angle is the fewer number of threads, the higher the ramp angle of the thread. As a screw is tightened, a load (Force) is applied to the flanks of the threads. This load is held in place by the friction between the two threads and the friction between the bearing surface of the screw and the mating material. The higher the ramp angle of the thread the easier it is for the load to slide down the thread.

Ramp angle for a 6-20 Type B Sheet Metal Thread

Ramp angle for a 6-32 Machine Screw Thread


Intergrated Locking

When fasteners are used in a vibration environment, there are many solutions that can be used to keep the assembly tight. Locking features can be added to the fastener to improve its resistance to vibration.

Integrated Locking Features

Patch Lock

Process make threaded fasteners of all sizes and configurations self-locking and self-sealing. Patch lock can be used with both male and female threads and leaves fasteners fully adjustable. Patch Lock is completely dry, fused to the fastener, and requires no curing time after installation.

Epoxy Lock

Powerful locking / sealing fastener thread adhesive that is pre-applied to either male or female threaded fasteners. Though the adhesive surface is dry to the touch, the forces of engagement crush the surface skin, mix the separate epoxy components, and initiate a chemical reaction that fixtures in minutes. Curing continues after installation for 72 hours.

Strip Lock

Tough bar of Nylon automatically inserted into a milled slot in a threaded fastener. The result is a fastener that is self-locking, self-sealing, and fully adjustable. Strip lock performs well in both high and low temperature extremes.

Serrated Teeth

Intagrated formed teeth on there bearing surfaces which are shaped and directed to “bite” into the surface of the member being joined when tightened. This “biting” mechanism creates a ratchet-type resistance to loosening without the sloppiness or variation of a multi-piece bolted joint.

Pellet Lock

Pellet Lock is a special, tough nylon plug, inserted into a drilled hole, which makes any threaded fastener self-locking and self-sealing, while leaving it fully adjustable. In addition to nylon, KEL-F® and Vespel3® are available for extremely high temperature applications.


Socket Screws

Socket screws can be placed into a sub group of all machine screws. They have a machine screw thread, but this is where the similarity stops. Socket screws are made primarily in two materials, steel and stainless steel. The steel socket screws are made from an alloy steel and are heat treated to high tensile strengths. The stainless steel socket screws are generally made from 302 or 304 and are not heat treated. Socket screws are also made from 17-7 Stainless Steel when a heat treated part is required, these are expensive and should be avoided if possible.

Socket Screws Features

Socket Head Cap Screw

The most popular in the series and can be found in stock in a full range of sizes (#2 thru 1”). Generally supplied in a black finish, they can also be zinc plated. Tensile strength 180,000psi (steel), 80,000 – 100,000psi (Stainless Steel). Strength will vary slightly depending on size.

Flat Head Cap Screw

Used when a flush assembly is required. The socket head style is chosen over the standard phillips or slotted drive when a higher tech look is desired. Tensile strength 137,000 – 150,000psi (steel), 80,000 – 120,000psi (Stainless Steel). Strength will vary slightly depending on size.

Button Head Cap Screw

Button Head Cap Screw has become the screw of chose on small electronic devices. It offers a low profile look and high performance strength. Tensile strength 137,000 – 150,000psi(steel), 80,000 – 120,000psi (Stainless Steel). Strength will vary slightly depending on size.


Shoulder Screws

Pencom offers two styles of shoulder screws. One is Socket Head Shoulder Screw (cold headed) and the other is precision shoulder screw (screw machined).

Shoulder Screws Features

Socket Head Cap Shoulder Screws

Socket Head Cap Shoulder Screws has a +/- 0.001 tolerance shoulder diameter starting at ¼” thru 2”. The tolerance on the shoulder length is +/- 0.005. All Socket Shoulder Screws have a hex recess for a drive. They are made from alloy, steel, are through hardened to Rc 32 – 43 and are similar to a grade 8 fastener. Parts are generally supplied in a black condition and then the shoulder is ground to a silver appearance.

Socket Head Precision Shoulder Screw

Socket Head Precision Shoulder Screw has a +0.000 / -0.001 tolerance shoulder diameter starting at 1/8 thru 1/2”. The tolerance on the shoulder length is +0.002 / -0.000. Precision shoulder screws are made in three materials, Brass, 303 Stainless Steel, and 12L14 Steel case hardened. Their strength level is similar to a grade 2 fastener. Phillips, slotted, and hex sockets are the available drive.


How Sems Screws are made

A screw blank is made, looking exactly like a finished screw without the threads. The shaft of the screw blank is at the pitch diameter. The pitch diameter is midway between the minor diameter and the major diameter.
A washer is manufactured with an inner diameter that is slightly larger that the pitch diameter of the screw blank, but smaller than the major diameter of the finished screw.
The washer is assembled on to the screw blank. The Threads are made in a process called thread rolling. The thread rolling operation forces the material into the minor diameter and forces material up to create the major diameter. The diameter of the thread is larger than the washer’s inner diameter. The washer will never be able to be removed again.

Materials

Fasteners are made from many materials but most design applications can be handled with either carbon steel or stainless steel. The common carbon steel grades used for fasteners are AISI 1010 thru 1022. The common stainless steel grades are, 303 and 304. They are part of the 18-8 family (18% Chrome, 8% Nickel).

Material Application Strength Corrosion Resistance
Low Carbon Steel All Low Must be plated
Medium Carbon Steel Automotive Medium Must be plated
Alloy Steel Critical Joints, Socket Head Products High Fair
Titanium Aerospace High Excellent
Monel Marine High Excellent
Inconel High Temperature High Excellent
Brass Marine / Cosmetic Low Good
Stainless Steel 300 Series All Medium Excellent
Stainless Steel 400 Series Thread Cutting / Thread Forming Medium / High Good
Aluminum Light Weight Low Excellent
Phosphorus Bronze Electrical Medium Good
Nylon Chemical Very Low Excellent
Ultem Chemical Low Exellent
Copper Electrical Low Good