DRAFTING MANUAL. Fastener Selection for Sheet Metal Products and Other Materials GENIUM PUBLISHING. Section K4.17 Page 1 November 1989* Update 37

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1 Update 37 DRAFING MANUAL Section K4.17 Page General Screws are the most commonly used method of fastening or assembling a product, even with the advances in modern adhesive and welding techniques. hey are essential to a product s assembly and disassembly for maintenance and service. For a multitude of products, a business s design and manufacturing engineering function normally specifies which screws to select as fasteners. he product designer normally specifies screw size, head style, and length based on application, years of use, and experience. From this experience a fastener standard is normally documented for future product use. here are many types of screws, with thread types, recesses, and head designs for specific applications. 2.0 Fastener Selection 2.1 Application It is advantageous for low-volume screw users or those with noncritical applications to have a variety of materials in a general steel category to take advantage of improved availability and better cost breaks. An example is to specify a lowcarbon steel or 18-8 type stainless steel screw and not ask for a specific grade. igh-volume screw users or those with critical requirements can be very selective. 2.2 Material Low-carbon steel is commonly used to manufacture low-cost fasteners with a tensile strength less than 50,000 psi. Mediumcarbon steel, quenched and tempered, is widely used where applications require tensile strengths from 90,000 to 120,000 psi. Alloy steels are used in applications requiring tensile strengths of 115,000 to 180,000 psi, depending on the grade selected. 2.3 Environment - Corrosion Resistance Most fastener products can be plated for corrosion protection. he most commonly used plating materials are zinc and cadmium. Other corrosion protection finishes are nickel, chromium, copper, tin, and silver electroplating; electroless nickel and other immersion coatings; hot-dip galvanizing; and phosphate coatings. Where better corrosion resistance is required, use the 300 series and higher stainless steel. Where superior corrosion resistance is required, use materials such as brass, bronze, aluminum, or nickel. If tensile strength is unimportant, use plastics such as nylon in severe environment applications. 2.4 Drivability In selecting a screw fastener, consider the products assembly and disassembly process. Assembly line or unit assembly, hand or pneumatically driven, and head shape also affect drivability Drive Style he most common screw head styles for driving are the slotted, recessed, or hexagonal shapes. he hexagonal head is the preferred driving style Slotted recess is the least preferred driving style since it is used only when appearance and ease of assembly or disassembly with a common flatblade screwdriver is required. he slot style is limited by the amount of torque applicable. he slot can become inoperative after repeated assembly and disassembly if the edge of the slot wall wears or is damaged. Figure 1. Slotted Driving Recess Recessed head fasteners are used most frequently and are the designers first choice when appearance is required. here are many kinds of recesses, but the most common today are the Phillips and the Phillips POZIDRIV. he Phillips and Phillips POZIDRIV recessed styles have much the same advantages as the hexagonal head style discussed below. he POZIDRIV recess is slowly replacing the Phillips recess. he POZIDRIV recess is identified by the four radial lines centered between each recess slot. Its slots are a slight modification of the conventional Phillips. he changed slot design improves fit between the driver and the recess, minimizes spin-out from the driver, and enhances driver life. he POZIDRIV recess design is recommended for high-volume production requiring high-torque applications. A conventional Phillips screwdriver can be used to assemble or disassemble a POZIDRIV screw. A POZIDRIV driver is not recommended for installing or removing a conventional Phillips head screw. A POZIDRIV recess sells at a higher price than a conventional Phillips recess, although some suppliers provide it at the same price. Longer driver tool life and less product surface damage normally offset savings from the higher prices. he Frearson recess design differs somewhat from the Phillips recess s, and has the advantage of requiring one driving tool for all sizes from a #2 through a 3/8 screw size. Its disadvantages, however, are: 1. Limited availability. 2. he driver s sharp point can and often does damage a product s surface. 3. he recess s greater penetration tends to weaken the screw head. 4. Although one driver drives all sizes, sized drivers are recommended. Clutch head recess is a special-use item, primarily in the recreation vehicle industry. his recess requires a special driver for each screw size.

2 Section K4.17 Page 2 DRAFING MANUAL exagonal and fluted socket recesses are used less frequently. hey are available in expensive, high-strength alloys. Because of their unique outside diameter, they are useful on flanges, counterbored holes, and other locations with restricted clearances. Since the fluted socket recess is not readily available, consider it for very small sizes where a hexagonal key tends to round out the socket. his recess offers a spline design the key never slips in or causes excessive wear. Special applications for tamper-proof/resistant fasteners can be obtained from proprietary tamper-proof/resistant-type vendors. Figure 3. exagon Driving ead ype Pan head shape, the most widely used, is intended to replace the round, binding, and truss heads to force standardization. his head type is preferred since it presents the best combination of appearance with adequate head height to minimize weakness caused by the applicable recess s penetration depth. PILLIPS PILLIPS FREARSON POZIDRIV Figure 4. Round ead Shape Round head shape was used extensively, but has been delisted as an American National Standard. All new designs should give preference to the pan head design. FLUED EXAGONAL CLUC SOCKE SOCKE EAD RECESS Figure 2. Recessed Driving ead ypes exagonal head style is preferred for these reasons: 1. Least likely to spin out of a driver and damage the product s surface 2. Lowest initial cost 3. igher driving torque 4. Minimum worker fatigue 5 Adaptable to high-speed power drivers 6. No recess to weaken the head 7. No recess to clog with dirt and interfere with the screwdriver 8. Ease of assembly in difficult places 2.5 ead Shapes his head shape information applies to all recess and slotted head designs. For simplicity, only the slotted head design appears in the figures. Figure 5. Pan ead Shape he pan head s driving superiority shows the high edge of the pan head at its periphery where driving action is most effective, provides superior driver-slot engagement, and reduces the driver s tendency to spin-out and chew away the slot metal. Pan ead Round ead Figure 6. Drive-Slot Engagement Flat head shape is used when a flush surface is required. he countersunk portion aids in centering the screw.

3 DRAFING MANUAL Section K4.17 Page 3 Figure 7. Flat ead Shape Oval head shape is similar to the flat head s except that its low silhouette improves product appearance. Figure 11. Fillister ead Shape exagonal head shapes are available in eight variations. his shape s advantages are described in Section 2.4, Drivability. Its major disadvantage is appearance, which eliminates it from consideration for the front of many consumer products. he indented hex design (Figure 12A) is the lowest cost head shape since it is completely cold upset in a counterbore die and possesses an identifying depression in the head s top surface. Figure 8. Oval ead Shape russ head shape is similar to the round head s except that the shallower round head has a larger head diameter. his shape is used when extra bearing surface is required for holding power, when the clearance hole is oversized, or when the material is soft. his shape s low silhouette also improves product appearance. Figure 12A. Indented ex and Slotted Indented ex eads he indented hex washer design (Figure 12B) has a larger surface area to spread the bearing load. he washer is an integral part of the screw head that protects the product from wrench marks during final assembly or service disassembly. Figure 9. russ ead Shape Binding head shape is similar to the pan head s and is commonly used for electrical connections where an undercut is normally specified to bind and prevent frayed stranded wire gages. Figure 12B. Indented ex Washer and Slotted Indented ex Washer eads Figure 10. Binding ead Shape Fillister head shape has the smallest diameter for any given shank size. Its deep slot allows application of a greater torque. his head shape is neither readily available nor as widely used as some of the other head styles. he trimmed hex design (Figure 12C) requires an extra operation to produce clean, sharp corners with no indentation. his design improves appearance and eliminates the depression on top of the head which normally collects moisture and dirt. he slot design for Figures 12A through 12C is used mostly to facilitate product field service. he slot adds cost, weakens the head, and limits assembly and disassembly torque requirements.

4 Section K4.17 Page 4 DRAFING MANUAL nonferrous castings and any application listed under ype AB threads. Figure 12C. rimmed exagonal, Slotted rimmed ex, rimmed exagonal Washer, and Slotted rimmed ex Washer eads 2.6 Screw hread ypes Machine Screws Machine screws are designed to be driven into either tapped holes or a corresponding machined nut. A machine screw s threads are readily available in American National Standard Unified Inch Course and Fine hread Series. Machine screws are normally considered for joining applications where the material is too thick, weak, hard, or brittle to accept a tapped hole. he machine screw may well serve applications requiring no dirt or dust Self-apping Screws Many types of self-tapping screws are available in today s fastener market. All are available in the drive and head styles mentioned in Sections 2.4 and 2.5. he self-tapping type have five basic categories, three of which can create an internal thread when torqued into a pre-drilled or smooth hole. here are three types: thread-forming, thread-cutting, and threadrolling screws hread-forming screws. hread-forming screws form an internal thread by squeezing material. he screw head s pressure forces a mating thread into the workpiece. his thread normally joins sheet metal parts and is not recommended for joining plastic or brittle parts since the stresses the thread creates cause cracking. hese thread-forming screw types are commonly used: ypes A and AB - ype AB screw thread design has a spaced thread. Each thread is spaced further from its adjoining thread. his thread design has a gimlet point for ease of penetration in a pre-drilled hole. his screw type is primarily intended for sheet metal with a minimum thickness of in. (0.38 mm) and a maximum thickness of 0.05 in. (1.3 mm), impregnated plywood, natural wood, and asbestos compositions. ype AB thread design was introduced to replace the ype A thread with its slightly wider spacing between threads. ype AB thread s major advantage is that its threads are spaced exactly as the ype B thread design s. ype B - ypes B and AB screw thread designs have the same spacing. Instead of ype AB s gimlet point, though, ype B has a blunt point with incomplete threads. his point design is more suitable for thicker sheet metal up to in. (5 mm) and for threading in a blind hole. ype B can also be used in Figure 13. ype AB hread Figure 14. ype B hread ype C - ype C screw thread looks like ype B in form, but has a machine screw thread design. his type also has the blunt point shown in Figure 14. he ype C screw application is for metal thickness from in. (0.76 mm) to in. (2.54 mm). It may replace a machine screw in field service. When considering this thread type for applications involving hard materials or long-thread engagement, remember that this design type requires extreme driving torques hread-cutting screws. hread-cutting screws create their thread by actually removing material from the hole. he screw and cutting edge designs differ with each type. hreadcutting screws are used to replace the thread-forming type when disruptive internal stresses are undesirable or when highdriving torques are encountered. he most commonly used thread-cutting screws are: ype B (formerly known as ype 25) - ype B screw thread design has a spaced thread and blunt point similar to the ype B design. In addition, it has one cutting edge with a widechip cavity. Application of this design is primarily in very brittle plastics such as urea resin compositions, asbestos, and other similar compositions. Materials of these types require a larger space between threads to produce a satisfactory joint since they reduce the buildup of internal stresses which fracture brittle plastic a closer spaced screw thread design causes. he wide cutting slot creates a large cutting edge and permits rapid deflection of chips to produce clean mating threads. Best results are obtained when all holes are counterbored to prevent fracturing of the plastic. apped metal inserts normally required for use in plastic parts can be eliminated by using this thread design.

5 DRAFING MANUAL Section K4.17 Page 5 for all metal thicknesses and can be used interchangeably with a ype D screw in many applications. In tapping into cast iron, ype F is superior to ype D and permits a smaller pilot hole. Figure 15. ype B hread ype AB - ype AB and B thread designs are the same except that ype B has a gimlet point similar to ype AB thread design, a design not recognized as an American National Standard. Use this screw type only for high-volume applications (over 50,000 pieces of one size and type). For the same reasons listed for the ype B thread design, its primary application is in plastic. Figure 18. ype F hread ype (formerly known as ype 23) - ype thread design is similar to ypes D and F s except for an acute rake angle cutting edge. he cut in the screw s end eliminates a pocket that confines the chips. he slot s shape forces the chips ahead of the screw as it is driven. his design is recommended for plastics and other soft materials requiring a standard machine screw series. his screw design type is also recommended in place of ypes D and F when more chip room is required for deep penetration. Figure 16. ype AB hread ype D (formerly known as ype l) - ype D screw thread design has threads of machine screw thread diameter-pitch combinations approximating unified form with a blunt point and tapered entering threads. Also a slot is cut off-center, with one side on the screw shaft s center line. his radial side of the slot becomes the sharp, serrated cutting edge like a threadtapping tool. he slot leaves a thinner section on one side of the screw which collapses and helps pressure the cutting edge. his screw design is suitable for all metal thicknesses. Figure 17. ype D hread ype F - ype F screw thread design is identical to ype D s except that instead of just one slot, there are several cut at a slight angle to the thread s axis. his thread design is suitable Figure 19. ype hread ype BF - ype BF thread screw design is used in plastics. Although similar in looks to the ype F head (Figure 18), this thread type has fewer threads per inch. he wide pitch thread reduces the buildup of internal stresses which fracture brittle plastics when a smaller thread pitch is used. his design has a blunt point, tapered entering threads with several cutting edges, and a chip cavity that tends to relieve stress hread-rolling screws. he thread-rolling screw type forms an internal thread by flowing the metal. his design neither cuts through nor disrupts a material s grain flow lines as thread cutting screws do. his design compacts and work hardens the material, thereby creating strong, smoothly burnished threads. his design has the threads of machine screw diameter-pitch combinations. An ideal application is where cutting chips can short electrical equipment or jam a delicate mechanism. he lack of chip formation eliminates the

6 Section K4.17 Page 6 DRAFING MANUAL Figure 20. ype BF hread normal chip-cleaning requirements. he driving-to-stripping torque ratio is approximately 1 to 8 for a thread-rolling screw versus a 1 to 3 ratio for a conventional tapping screw. his higher drive ratio permits the driver torque release to be set well over the required driving torque and yet well below the stripping torque. he increased ratio also minimizes poor fastening due to stripped threads. Figure 21 illustrates three types of thread-rolling screws. Figure 21. hread-rolling Screw ypes Self-drilling and tapping screws. hese two screw types create their own opening before generating threads. he self-drilling and tapping screw thread design drills its own hole to form a mating thread, making a complete fastening in one operation. Eliminating pre-drilled pilot holes and alignment reduces labor for assembly. he process of drilling the hole must be completed before the screw thread can engage and the tapping process begin. o meet this requirement, the screw s unthreaded point length must equal or be greater than the material thickness to be drilled. Attention must be paid to the minimum and maximum material thickness which varies with screw size Self-extruding and tapping screws. he selfextruding and tapping screw thread design provides its own extrusion as it is driven into an inexpensively produced hole. he resulting extrusion height is several times the base material thickness. Application for this type screw is in thicknesses up to in. (1.2 mm). By increasing the thread engagement, these screws increase the differential between the driving and stripping torque and provide greater pullout strength. his thread design is excellent for grounding sheet metal for electrical connections since it does not produce chips. Figure 22. Self-Drilling and apping ype Figure 23. Self-Extruding ype 2.7 Designing Screw Fastener Selection Charts Creating your own fastener selection charts for local fastener selection is normally based on defining a minimum number of lengths and screw types for standardization purposes. For detailed specifics of each screw type, additional data may be required to completely define all the mechanical aspects of the fastener you require. Figure 24 is a sample screw hardware chart which displays only the critical dimensions for the application of the hardware and not the total design specifics. he finish requirements specified in this chart are general in nature. Reference additional material and finish data specifications if this type of chart is used to purchase fastener hardware. o start your own fastener standard, Figure 25 is a blank of Figure 24 s chart. 2.8 Suggested ole Size and Countersunk Data For Screw Fasteners he following chart (Figure 26) defines clearance hole requirements for screw fasteners and can be used as a standard for your application by adding a drawing or standards number. 2.9 Suggested Grip Guide for Fasteners he following chart (Figure 27) defines total grip dimensions for screw thread fasteners. Knowing the total material thicknesses to be gripped, you can derive the maximum screw fastener length. Once you know that length, use the fastener selection guide to choose the closest fastener length. his Grip Guide can be used as a standard for your application by adding a drawing or standards number.

7 DRAFING MANUAL Section K4.17 Page 7 DEFINES PYSICAL REQUIREMENS SCREW DESCRIPION MAERIAL REQUIRED MAERIAL FINIS REQUIREMENS DEFINES SCREW SIZE DEFINES SCREW DIAMEER All DIM IN INCES OL A B L D (MAX) SCREW, MACINE, FLA C'SK EAD, 82 CROSS RECESSED. MA'L LOW CARBON SEEL YOUR COMPANY SANDARD DWG. NO. FINIS NONE = UNPLAED C = CADIUM N = ZINC SIZE /4-20 5/ /8-16 MAX A B A B A B A B A B A B A B A B MAX D LG 1/ / / / / / / / / / / / / 3/ / / / NOE: LAS REE DIGIS OF DAS NUMBERS ARE IN 16ths OF AN INC. EXAMPLE: (YOUR CO. DRAWING NUMBER) C IDENIFIES A #4-40 x 3/4". PILLIPS CROSS RECESSED FLA EAD, SEEL, CADIUM PLAED. COMPANY DRAWING NUMBER LISS ONLY E PREFERED LOCAL LENG SANDARD OLERANCE OF SCREW LENG NOE EXPLAINING WA DAS NUMBERS MEAN. Figure 24. Sample Screw ardware Chart

8 Section K4.17 Page 8 DRAFING MANUAL ALL DIM IN INCES SCREW. YOUR COMPANY SANDARD MA'L: FINIS DWG. NO. OL SIZE MAX DIM DIA LG NOE: LAS REE DIGIS OF PAR NUMBERS ARE IN 16ths OF AN INC. Figure 25. Blank Sample Screw ardware Chart

9 DRAFING MANUAL Section K4.17 Page A ALL DIM IN INCES B CLEARANCE OLE FOR MOUNING B CLEARANCE OLE FOR FLA EAD SCREW MOUNING SCREW SIZE OLE DIAMEER B SINGLE MULIPLE OLE DIA. B COUNERSINK ANGLE DIA. A READ SIZE / UNC-2A / UNC-2A / UNC-2A / UNC-2A / UNF-2A 1/ /.516 1/4-20 UNC-2A 5/ /8-16 1/2-13 9/ / FLA EAD SCREWS ARE NO RECOMMENDED WEN E COUNERSUNK PLAE ICKNESS IS NO EQUAL O A LEAS 1-1/2 x E EAD EIG OF E COUNERSINK SCREW 5/16-18 UNC-2A 3/8-16 UNC-2A 1/2-13 UNC-2A 9/16-12 UNC-2A 5/8-11 UNC-2A Figure 26. Suggested ole Size and Countersink Data Guide for Screw Fasteners

10 Section K4.17 Page 10 DRAFING MANUAL ALL DIM IN INCES & A OAL ARDWARE ICKNESS G OAL ARDWARE ICKNESS G OAL ARDWARE ICKNESS G MAX. DIM. 1.5 D LENG 1.5 D LENG 1.5 D LENG SIZE FLA WASER, LOCK WASER SPL EXAGON NU & 1.5 READS G FLA WAS, LOCK WASER EXERNAL EXAGON NU & 1.5 READS G LOCK WASER SPLI, EXAGON NU & 1.5 READS G / / / / / Figure 27. Suggested Grip Guide for Fastener Length Selection