Explanatory Information (NOT PART OF ANSI STANDARD)

Transcription

1 4 Safety guards 4.1 General requirements All abrasive wheels shall be used only on machines provided with Safety Guards. Exceptions: This requirement shall not apply to the following classes of wheels and conditions. (1) Wheels used for internal grinding while advancing, retracting or within the work (see illustration 45). (2) Mounted wheels, 2 and smaller in diameter used in portable operations (see definition, section , page 25). (3) Types 16, 17, 18, 18R and 19 cones and plugs and threaded hole pot balls where the work offers protection or where the size does not exceed 3 in diameter by 5 long. (4) Metal centered diamond lapidary wheels (see section , page 13) either notched, segmented or continuous rim used with a coolant deflector, when operated at speeds up to 3,500 SFPM. (5) Type 1 wheels not larger than 2 in diameter and not more than 1/2 thick, operating at peripheral speeds less than 1,800 SFPM when mounted on mandrels driven by portable drills. (6) Type 1 reinforced wheels not more than 3 in diameter and 1/4 in thickness, operating at peripheral speeds not exceeding 9,500 SFPM, provided that safety glasses and face shield protection are worn. (7) Valve seat grinding wheels. (8) Machines that are remotely operated, and with an enclosure sufficient to retain the pieces of a wheel should there be a breakage, shall not require a safety guard at the wheel. Lock-outs and administrative procedures shall be used to assure that no personnel are in the enclosure while the machine is in operation. NOTE: For additional forms of operator protection, see annex A. Explanatory Information (NOT PART OF ANSI STANDARD) E 4.1 General requirements Exceptions: (1 through 7) On difficult to guard applications, the exceptions to the use of safety guards are based on the fact that the operator can be protected by personal safety equipment since these classes of wheels have low mass. Also, in many instances the work acts as a guard. Adequate face protection is particularly important when using this class of wheel. Illustration 45 An example of an internal grinding operation where the piece being ground serves as the wheel guard 50

2 4.2 Portable grinding Design of safety guards The wheel guard shall be mounted so as to maintain alignment with the wheel. The guard shall be positioned so that pieces of an accidentally broken wheel shall be deflected away from the operator. The combined strength of the fasteners shall maintain the guard in a safe position in the event of wheel breakage. Wheel guards shall be designed so that if they become damaged or distorted in the event of wheel breakage, they will not become a hazard Safety guards for specific wheel types Safety guards for specific wheel types shall meet the following specifications Type 1 grinding wheels, cutting-off wheels, and tuck pointing wheels A safety guard for Type 1 wheels shall enclose the top and both sides of the grinding wheel through at least 180 of the wheel (see figure 11). Enclosure of the spindle end, nut and the outer flange is not required for cutting off wheels and tuck pointing wheels. E Design of safety guards The safety guard should allow the wheel to be used in the manner for which it was designed so as not to discourage use of a guard. E Safety guards for specific wheel types The safety guard should be designed so that the wheel can be removed and mounted without removing the guard from the grinder. It is permissible to remove a guard member to change wheels. Care must be taken to insure that the removable section of the guard can be easily replaced and will ensure the original safety of the guard after the removable section has been replaced. E Type 1 grinding wheels, cuttingoff wheels, and tuck pointing wheels An essential safety feature of a guard for a Type 1 wheel is that it encloses the upper half of the wheel. In case of accidental breakage of the wheel, the guard will deflect the pieces away from the operator (see illustration 46). Illustration 46 Examples of typical guarding of Type 1 wheel operations Figure 11 Typical drawn steel guard for straight wheels Type 1 51

3 Vertical and angle grinders, Types 6 and 11 cup wheels A safety guard for vertical or angle grinders using Type 6 or Type 11 cup wheels shall cover the wheel s plane of rotation toward the operator for at least 180 ; shall cover the side of the wheel toward the driving flange for at least 180 ; and shall have a skirt which is adjustable to within 1/8 of the plane of the surface of the wheel. (See figure 12.) NOTE: a. Cup Back Bushings under no circumstances shall be considered a substitute for a guard, and wheels so manufactured shall not be used without a guard. b. So-Called Revolving Cup Guards shall not be used, and under no circumstances shall be considered a substitute for a guard. E Vertical and angle grinders, Types 6 and 11 cup wheels Cup Wheels are available with either a threaded or unthreaded hole. Guards are available for each of these types. The following illustrations, 47 and 48, show the typical types. Each is shown mounted in conjunction with a guard. NOTE: a. Some Type 11 cup wheels and Type 6 cup wheels are manufactured with a steel bushing that covers the complete back of the wheel and comes up the side of the wheel about 1/2. These are commonly called Cup Back Bushings. b. A Revolving Cup Guard is a formed steel cup which covers the full back of the wheel and comes a short distance down the side of the wheel (approx. 9/16 ). The cup has a small clearance between the side of the cup and the wheel (approx. 1/16 ). It is sandwiched between the wheel and the grinder and rotates with the wheel. This design is not acceptable and shall not be used as a substitute for a guard. Grinding surface Figure 12 Typical drawn steel guard for cup wheels Types 6 and 11 Guard slotted for adjustment for wheel wear Illustration 47 Type 6 cup wheel showing band type guard Illustration 48 A band-type guard used with Type 11 cup wheel 52

4 Vertical and angle grinders, Types 27, 28 and 29 depressed center wheels A safety guard for vertical or angle grinders using Types 27, 28 and 29 wheels shall cover the wheel s plane of rotation toward the operator for at least 180 ; shall cover the side of the wheel toward the driving flange for at least 180 ; and shall have a lip on the outer edge which curls inward to deflect wheel fragments and to provide necessary strength. The lip shall extend beyond the surface of the wheel throughout the 180 coverage (see figure 13 and illustration 49). E Vertical and angle grinders, Types 27, 28 and 29 depressed center wheels Illustration 49 An angle grinder equipped with a Type 27 wheel and guard Figure 13 Typical drawn steel guard for depressed center wheels Types 27, 28 and Maintenance of safety guards for portable machines Safety guards shall be inspected periodically. Safety guards shall be in good functional condition. Damaged, bent and severely worn guards shall be replaced. A guard which has been subject to a wheel breakage is likely to be weakened internally and shall not be reused Strength of safety guards Portable electric grinders Safety guards shall conform to the materials and dimensions of tables 4, 5, 6 or 7 as applicable, or satisfactorily meet the performance criteria prescribed in section

5 An electric grinding machine shall be marked with the maximum unloaded speed in RPM. Figure 14 Typical drawn steel guard for Type 1 and cutting-off wheels Table 4 (see figure 14) Materials and minimum thickness of drawn steel guards for Type 1 wheels used on portable electric grinders Material Used in Construction of Guard Sheet Steel Min. Tensile Strength 60,000 psi Maximum Thickness of Wheel Wheel Diameter inches 2 through 5 Over 5 through 8 Minimum Thickness Minimum Thickness For Speeds Up Through (SFPM) inches A B A B 9, /16 1/16 3/32 1/16 12, /32 1/16 3/32 3/32 The recommendations listed in the above table are guides for the conditions stated. Other material, designs or dimensions affording equal or superior protection are also acceptable. Table 5 (see figure 14) Minimum basic thicknesses for peripheral and side members for safety guards for use with cutting-off wheels on portable electric grinders Material Used in Construction of Guard Structural Steel (minimum tensile strength 60,000 psi) Maximum Thickness of Cutting-Off Wheel Speed not to Exceed Cutting-off wheel diameters inches 6 through 11 inches Over 11 through 20 inches A B A B 1/4 inch or less 14,200 SFPM 1/16 1/16 3/32 3/32 1/4 inch or less 16,000 SFPM 3/32 1/8 1/8 1/8 The recommendations listed in the above table are guides for the conditions stated. Other material, designs or dimensions affording equal or superior protection are also acceptable. 54

6 Table 6 (see figure 12) Material thickness guide for construction of drawn steel guards used on portable electric grinders with cup wheels, Types 6 and 11. Operating speed not to exceed 9,500 SFPM. B C D Wheel Diameter Washer Back Plate Thickness Hood Skirt inches inches inches inches 4 3/32 1/16 1/8 5 3/32 3/32 1/8 6 3/32 3/32 1/8 The recommendations listed in the above table are guides for the conditions stated and are based on 50,000 psi minimum tensile strength. Other material, designs or dimensions affording equal or superior protection are also acceptable. Table 7 (see figure 13) Guide for construction of drawn steel guard used on portable electric grinders with Types 27, 28 and 29 wheels. Material Used in Construction of Guard Sheet Steel Min. Tensile Strength 60,000 psi Thickness of Wheel Wheel Diameter A B For Speeds Up Through (SFPM) inches inches inches /8 inches or less 3 through The recommendations listed in the above table are guides for the conditions stated. Other material, designs or dimensions affording equal or superior protection are also acceptable Performance criteria for guard design for portable electric grinders Procedure: 1. Guard shall be mounted on a firm structure, using the part of the housing which is normally used to mount the guard to the grinder. 2. Test speed shall be at least 110% of the marked no-load speed of the grinder. 3. Grinding wheel shall be the maximum diameter and thickness recommended by the manufacturer for use on the grinder. The grinding wheel is to be broken by a sharp impact, notching, or other suitable means and broken into major pieces while running at the test speed. Major pieces being defined as two (2) or more pieces, each being at least 25% of the wheel mass. Acceptance Criteria: 1. All fragments shall be contained or deflected through an arc of 180 in the plane or planes of wheel rotation. See figure 15. Exception: Small granules are not considered fragments. E Performance criteria for guard design for portable electric grinders With the advent of ever changing technology and improved materials, it becomes imperative that there be some performance criteria for the design of machine elements. By designating the important parameters it allows the designer freedom to use his ingenuity to design a better product to do the job. The design criteria in is designed to provide reasonable protection to the operator and still allow him to grind efficiently. 55

7 Figure 15 This figure illustrates the guard test procedure. Note the orientation of the guard relative to the no fragment zone is optional 2. The guard shall not be separated from the tool. No fasteners or mounting hardware may enter the NO FRAGMENTS zone. Deformation of the guard and/or movement on the mounting is acceptable. Three guards shall be tested. If all three satisfy acceptance criteria 1 & 2 above, the design is acceptable. If two of the three fail either 1 or 2, the guard design is not acceptable. If one of the three fails, then three more guards shall be tested. If all three of the second set satisfy 1 & 2 above, the guard design is acceptable Portable air grinders Safety guards shall be designed and constructed to meet the acceptance criteria of section should an abrasive wheel breakage occur at the grinder s maximum attainable speed. Maximum attainable speed is defined as the maximum speed which the tool can achieve under the most adverse conditions of misadjustment or malfunction of any speed control devices, when supplied with the higher of the rated pressure or 90 psi (6.2 bar) air pressure Gasoline-powered cut-off machines For design and testing requirements for guards on high-speed, portable, gasoline-powered cutoff machines, see ANSI B , Portable, Handheld, Internal Combustion Engine Driven Cut-off Machines Safety Requirements. E Portable air grinders Since air grinders may have higher attainable speeds than electric grinders due to possible governor malfunction, improper adjustment or excessive air supply pressure, guards conforming to tables 4, 5, 6, and 7, may not provide the prescribed protection. Guards are developed to be compatible with the characteristics of the specific grinder, and tested in a manner similar to that prescribed in section With the proper choice of test speed the portable electric test procedure can be used with portable air grinders. 56

8 4.3 Stationary grinding Guard exposure angles The maximum exposure angles are specified in the following paragraphs. The angle of exposure shall be measured with the vertex at the center of the spindle and extended to the guard periphery. The maximum exposure angles shall not be exceeded. Visors or other accessory equipment shall not be included as a part of the guard when measuring the guard opening, unless such equipment has strength equal to that of the guard Bench and pedestal grinders The angular exposure of the abrasive wheel periphery and sides for safety guards used on machines known as bench and pedestal grinders shall not exceed 90 or one-fourth of the periphery. This exposure shall begin at a point not more than 65 above the horizontal plane of the wheel spindle (see figure 16 and illustration 50). E Guard exposure angles Maximum guard openings are based on the fact that the line of flight of broken wheel pieces will be tangential in the direction of rotation of the wheel. Note that all exposure angles are measured from the center of the machine spindle. E Bench and pedestal grinders Figure 16 Exception: Wherever the nature of the work requires contact with the wheel below the horizontal plane of the spindle, the exposure shall not exceed 125. (See figure 17.) Illustration 50 An example of a well-designed guard for a bench grinder. Note that exposure does not exceed the 90 maximum stipulated Figure 17 57

9 Exposure adjustment, bench and pedestal grinders Safety guards of the types described in section where the operator stands in front of the opening, shall be constructed so that the maximum angular exposure above the horizontal plane of the wheel spindle shall never be exceeded. Safety guards shall be constructed so that the peripheral member or tongue guard can be adjusted to the constantly decreasing diameter of the wheel. The distance between the wheel periphery and the peripheral member or tongue guard shall never exceed 1/4. The peripheral member or tongue guard are an integral part of the wheel guard system and shall be constructed and/or attached to meet the strength requirements for the rest of the guard. Exception: Bench grinders with wheels not greater than 5 in diameter operating at peripheral speeds less than 5,000 SFPM with rated outputs not greater than 1/10 HP need not conform to spark arrestor (also called adjustable tongue) requirements. E Exposure adjustment, bench and pedestal grinders Figure 18 shows a satisfactory method of accomplishing exposure adjustment. Other methods that agree with the basic rule are also acceptable. Note that the exposure angle is always measured from the center of the wheel spindle. Figure 18 Correct The adjustable tongue gives required angular protection for all sizes of wheels used. 58

10 4.3.3 Floorstand grinders The angular exposure of the abrasive wheel periphery and sides for safety guards used on machines known as floor stands shall not exceed 90 or 1/4 of the periphery. This exposure shall begin at a point not more than 65 above the horizontal plane of the wheel spindle. (See figure 19, and section 4.4, page 67.) Exception: Wherever the nature of the work requires contact with the wheel below the horizontal plane of the spindle, the exposure shall not exceed 125 (see figure 20). Figure 19 Figure Floorstand grinders exposure adjustment Safety guards of the types where the operator stands in front of the opening shall be constructed so that the maximum angular exposure above the horizontal plane of the wheel spindle shall never be exceeded. The distance between the wheel periphery and the adjustable tongue or the end of the peripheral member at the top of the guard opening shall never exceed 1/4. (See figures 21, 22 and 23.) E Floorstand grinders exposure adjustment Figures 21 and 22 show two satisfactory methods of accomplishing exposure adjustment. These sketches are for purposes of illustration only. Other methods that agree with the basic rule are also acceptable. Figure 23 shows a condition that does not comply with the requirements. Note that the exposure angle is always measured from the center of the wheel spindle. Figure 21 Correct Showing adjustable tongue giving required angular protection for all sizes of wheels used. 59

11 Figure 22 Correct Showing movable guard with opening small enough to give required protection for small size wheel used. Figure 23 Incorrect Showing movable guard with size of opening correct for full size wheel but too large for smaller wheels Self-closing guard In order to protect all personnel who are in the plane of rotation of the grinding wheel, all floor stand grinders designed to be used with 24 diameter wheels and larger, shall be equipped with a guard which closes automatically at the occurrence of a wheel breakage (see illustration 51). Other guard designs which provide equivalent protection to the personnel in the plane of rotation of the wheel may be used. E Self-closing guard The engineering concept of the self-closing guard is very simple to understand. In the event of a wheel breakage, the wheel pieces fly tangentially outward. They strike, at an angle, one or more paddle-like surfaces which are fixed to a moveable gate located within the primary guard. The kinetic energy of the wheel is transferred to this gate, causing it to instantly swing shut so as to prevent broken Illustration 51 Two examples of floor stand grinders with automatically closing guards 60

12 pieces from escaping from inside the primary machine guard. High speed photography shows the effectiveness of this guard design. When a wheel breakage occurs, the piece of wheel that actually leaves the opening of the guard was not the piece facing the operator at the instantaneous moment of breakage. The piece was one well within the guard, but headed for the guard opening due to tangential rotational forces imparted to it just prior to breakage Inspection and maintenance of self-closing guards The self closing guard shall be inspected every time a wheel is changed. The inspection shall include the following checks: 1. Check the paddle-like underlying surfaces which cause the guard member to rotate when struck by broken wheel pieces. Correct for any wear or damage according to the machine builders recommendation. 2. Check the spark arrester, (the longer tab, also called tongue, located at the leading edge of the rotating guard member), for wear. Repair or replace if worn. This tab, on some grinders, is part of the speed control system and it is important that it be maintained properly. The self closing guard shall be mechanically operated every six (6) months. The shear pin shall be removed and the internal guard shall be moved through its full travel. The shear pin shall be replaced with a new shear pin of equal mechanical properties. E Inspection and maintenance of self closing guards The self closing guard is the last protective device the operator has in case of an accidental wheel breakage. It is important that it work properly at the time of a wheel breakage. Since wheel breakages are very rare, maintenance of this device can be easily neglected. Only by checking the guard mechanism before each wheel is mounted can you be assured that the self-closing guard will function properly. Shear pins must be replaced with the same mechanical strength pin that the guard was originally designed for. Higher strength pins may not shear, resulting in subsequent failure of the guard Design criteria for a self-closing guard 1. The guard shall be designed to retain all pieces of the wheel that are large enough to cause serious bodily injury to the operator. 2. The guard shall be designed to operate properly if the largest wheel the grinder is designed to use breaks at a speed of 17,000 SFPM. E Design criteria for a self closing guard Overspeeding is a major cause of breakages for these types of wheels. It is also the breakage condition that would subject the guard to its maximum loads. 61

13 4.3.4 Cylindrical grinders The maximum angular exposure of the abrasive wheel periphery and sides for safety guards used on cylindrical grinding machines shall not exceed 180. This exposure shall begin at a point not more than 65 above the horizontal plane of the wheel spindle. (See figure 24 and illustration 52, and section 4.4, page 67.) E Cylindrical grinders Figure 24 Illustration 52 A cylindrical grinding machine employing a well designed guard Surface grinders The maximum angular exposure of the abrasive wheel periphery and sides for safety guards used on surface grinding machines where grinding is done on the wheel periphery shall not exceed 150. This exposure shall begin at a point not less than 15 below the horizontal plane of the wheel spindle (see figure 25 and illustration 53). For surface grinders using cup wheels, see section , band type guards, page 66. E Surface grinders Illustration 53 This surface grinder has a welldesigned guard conforming to basic requirements. The guard is designed to allow easy access to the wheel Figure 25 62

14 4.3.6 Swing frame grinders Swing frame grinders using straight wheels The maximum angular exposure of the grinding wheel periphery and sides for safety guards used on machines known as swing frame grinding machines shall not exceed 180, and the top half of the wheel shall be enclosed at all times. (See figure 26 and illustration 54.) E Swing frame grinders using straight wheels Swing frame grinders with cup wheels Swing frame grinders made specifically for cup wheels shall have wheel guard coverage of 180 on the side toward the operator. Illustration 54 This swing frame grinder has an excellent guard. Note that the guard encloses at least 180 of the wheel Semiautomatic snagging machines The maximum angular exposure of the abrasive wheel periphery and sides for safety guards used on grinders known as semiautomatic snagging machines shall not exceed 180 and the top half of the wheel shall be enclosed at all times. (See figure 26 and illustration 55.) E Semiautomatic snagging machines Top grinding Figure 26 Where the work is applied to the wheel above the horizontal center line, the exposure of the abrasive wheel periphery shall be as small as possible and shall not exceed 60. (See figure 27.) Illustration 55 An automatic snagging machine grinding carbon steel Figure 27 63

15 4.3.9 Masonry saws On machines specifically designed and used for masonry sawing (see section , page 13), the maximum angular exposure of the cuttingoff wheel periphery for safety guards shall not exceed 180. (See figures 28, 29 and illustration 56). E Masonry saws 180ϒMAXIMUM EXPOSURE 180ϒMAXIMUM EXPOSURE Figure 28 Acceptable configuration Figure 29 Preferred configuration (clockwise direction of rotation) Guards for these machines specifically designed for masonry sawing, using only steel centered diamond cutting-off wheels not exceeding 16,000 SFPM, shall use steel guards of at least 12 gauge (.105 ) thickness for wheels of all diameters. Illustration 56 Cutting common brick with a metal centered segmental diamond abrasive wheel on a masonry saw It is recognized that advancements in material science may allow machine guards to be developed with strength characteristics equal to, if not greater than, the 12 gauge (.105 ) thick steel required Concrete saws On machines specifically designed and used for concrete sawing (see section 1.3.4, page 12) the maximum angular exposure of the cuttingoff wheel periphery for safety guards shall not exceed 180. (See figures 28, 29 and illustration 57.) Machines specifically designed for concrete sawing, using only steel centered diamond cutting-off wheels not exceeding 16,000 SFPM, shall use steel guards of at least to 12 gauge (.105 ) thickness for wheels of all diameters. E Concrete saws Illustration 57 Cutting a concrete deck with a gasoline-powered concrete saw It is recognized that advancements in material science may allow machine guards to be developed with strength characteristics equal to, if not greater than, the 12 gauge (.105 ) thick steel required. 64

16 Wall saws On machines specifically designed and used for wall sawing (see section , page 16), the maximum angular exposure of the cuttingoff wheel periphery for safety guards shall not exceed 180 (see figures 28 and 29). Machines specifically designed for wall sawing, using only steel centered diamond cutting-off wheels not exceeding 16,000 SFPM, shall use aluminum guards of at least.090 thickness for wheels of all diameters Rotary tool (burr) grinding On machines designed and used for grinding steel or carbide rotary tools (see section , page 14) the maximum angular exposure for safety guards shall not exceed 180. This exposure shall begin at a point not more than 65 above the horizontal plane of the wheel spindle. (See figure 30.) E Wall saws Illustration 58 Wall saw securely mounted to work surface Figure Cutting-off wheel grinding On machines specifically designed and used for cutting off material, the maximum angular exposure of the cutting-off wheel periphery for safety guards shall not exceed 180 (see figure 31) and the guard shall be designed according to the specifications listed in table 8. It is recognized that advancements in material science may allow machine guards to be developed with strength characteristics equal to, if not greater than, the.090 thick aluminum required. Figure 31 65

17 Table 8 Minimum basic thicknesses for peripheral and side members for safety guards used with cutting-off wheels * Material Used in Construction of Guard Maximum Thickness of Cutting-Off Wheel Speed not to Exceed 6 through 11 inches Cutting-Off Wheel Diameters inches Over 11 through 20 inches Over 20 through 30 inches Over 30 through 48 inches Over 48 through 72 inches SFPM A B A B A B A B A B Structural Steel (Min. Tensile Strength 60,000 psi) 1/2 inch or less 14,200 1/16 1/16 3/32 3/32 1/8 1/8 3/16 3/16 1/4 1/4 1/2 inch or less 16,000 1/8 3/32 1/8 1/8 3/16 1/8 1/4 3/16 5/16 1/4 The recommendations listed in the above table are guides for the conditions stated. Other material, designs or dimensions affording equal or superior protection are also acceptable. *See section , page 24, for cutting-off wheel definition Band type guards for stationary grinding machines Band type guards shall conform to the following general specifications: a. The bands shall be of steel plate or other material of equal or greater strength. They shall be continuous, the ends being either riveted, bolted or welded together in such a manner as to leave the inside free from projections. b. The inside diameter of the band shall not be more than 1 larger than the largest rotating diameter of the wheel or its holder. c. The band shall be of sufficient width and its position kept adjusted so that at no time will the wheel protrude beyond the edge of the band a distance greater than indicated in table 9, page 68, or the wall thickness (W), whichever of these dimensions is smaller. Exception: Band type guards used in conjunction with segments individually chucked in suitable holding mechanisms to form a grinding unit may have a door for access through the band to facilitate segment change and adjustment. As a guide for the construction of band type guards, figure 32, and table 10, page 68, have been prepared. Other designs affording equal or better protection are also acceptable. E Band type guards for stationary grinding machines Illustration 59 An example of a well constructed band type guard 66

18 Figure 32 Illustration 60 A properly constructed band type guard. Notice that the exposure of the abrasive wheels is in conformance with the requirements listed in table Construction guide for steel guards The recommendations listed in tables 4 through 12 are guides for the conditions stated. Other material, designs, or dimensions affording equal or superior protection are also acceptable. Exception A: Machines specifically designed for concrete sawing, using only steel centered diamond cutting-off wheels not exceeding 16,000 SFPM, shall use steel guards of at least 12 gauge (.105 ) thickness for wheels of all diameters. Exception B: Machines specifically designed for wall sawing, using only steel centered diamond cutting-off wheels not exceeding 16,000 SFPM, shall use aluminum guards of at least.090 thickness for wheels of all diameters. E 4.4 Construction guide for steel guards When steel centered diamond cutting-off wheels are operated properly; i.e. at the correct speed and in an undamaged condition; there is no risk of the failure of the steel center. The only risk is from small particles of the material being cut or possibly the small diamond segments coming off the steel center. As the peripheral speed of all diameters of steel centered diamond cutting-off wheels is about the same, the potential energy level of these projectiles is the same regardless of wheel diameter. The materials listed in exceptions A and B have proven their capability of preventing through penetration of the above relatively low mass items. Thus, it is not necessary to manufacture guards of the thickness of material as required for abrasive-type cutting-off wheels as shown in table 8. 67

19 Table 9 Exposure versus wheel thickness Overall Thickness of Wheel (T) Inches Maximum Exposure of Wheel (C) Inches 1/2 1/4 1 1/2 2 3/ /2 5 and over 2 Table 10 Guide for construction of band type guards; maximum wheel speed 7000 SFPM Minimum Material Specifications Sheet Steel min. tensile strength 60,000 psi Diameter of Wheel Minimum Thickness of Band A Minimum Diameter of Rivets Maximum Distance Between Centers of Rivets inches inches inches inches Under 8 1/16 3/16 3/4 8 to 24 1/8 1/4 1 Over 24 to 30 1/4 3/8 1 1/4 Table 11 Guide for construction of drawn steel guards for wheels 2 thick and less, 10 and less in diameter. Used on bench and pedestal grinders. Material Used in Construction of Guard Sheet Steel Minimum Tensile Strength 60,000 psi Maximum Thickness of Wheel 2 through 5 Wheel Diameter inches Over 5 through 8 Over 8 through 10 For Speeds Up Through (SFPM) inches A B A B A B 5, /16 1/16 3/32 1/16 3/32 1/16 9, /16 1/16 3/32 1/16 The recommendations listed in the above table are guides for the conditions stated. Other material, designs or dimensions affording equal or superior protection are also applicable. Illustration 61 Shows the guard wall symbols A and B used in tables 8, 11 and 12 68

20 Table 12 Minimum basic thickness of peripheral and side members for abrasive wheel safety guards Material Satisfactory for Speeds up to 8000 SFPM 9000 SFPM SFPM SFPM Material used in Construction of Guard Gray Iron (Minimum Tensile Strength 20,000 psi) Malleable Iron (Minimum Tensile Strength 50,000 psi) Steel Castings (Minimum Tensile Strength 60,000 psi) Structural Steel (Minimum Tensile Strength 60,000 psi) Maximum Thickness of Abrasive Wheel 3 through 6 inches Over 6 through 12 inches ABRASIVE WHEEL DIAMETERS Over 12 through 16 inches Over 16 through 20 inches Over 20 through 24 inches Over 24 through 30 inches Over 30 through 48 inches A B A B A B A B A B A B A B inches inches inches inches inches inches inches inches 2 1/4 1/4 3/8 5/16 1/2 3/8 5/8 1/2 7/8 5/8 1 3/4 1 1/ /16 5/16 1/6 5/16 1/2 3/8 3/4 5/8 1 5/8 1 1/8 3/4 1 3/ /8 5/16 1/2 7/16 5/8 1/2 1 5/8 1 1/8 3/4 1 1/4 7/8 1 1/2 1 1/8 8 5/8 9/16 7/8 3/4 1 3/4 1 1/8 3/4 1 1/4 7/8 1 1/2 1 1/8 10 3/4 11/16 7/8 3/4 1 3/4 1 1/8 3/4 1 1/4 7/8 1 1/2 1 1/ / / / /16 1 1/16 1 3/4 1 3/ /8 1 1/8 1 3/8 1 1/8 1 1/2 1 3/ /8 2 1/4 5/16 3/8 5/16 1/2 3/8 5/8 1/2 3/4 5/8 7/8 3/4 1 7/8 4 5/16 5/16 3/8 5/16 1/2 3/8 5/8 1/2 3/4 5/8 7/8 3/4 1 1/8 7/8 6 3/8 5/16 1/2 7/16 5/8 1/2 3/4 5/8 7/8 5/8 1 3/4 1 1/4 7/8 8 1/2 7/16 5/8 1/2 3/4 5/8 7/8 5/8 1 3/4 1 1/4 7/8 10 1/2 7/16 5/8 1/2 3/4 5/8 7/8 5/8 1 3/4 1 1/4 7/ /16 11/16 13/16 11/16 1 3/4 1 1/8 7/8 1 3/ /8 3/4 1 3/4 1 1/8 7/8 1 1/2 1 1/8 2 1/4 1/4 5/16 5/16 3/8 3/8 1/2 7/16 5/8 1/2 3/4 5/8 7/8 3/4 4 1/4 1/4 1/2 1/2 1/2 1/2 9/16 1/2 5/8 1/2 3/4 5/8 1 3/4 6 3/8 1/4 3/4 5/8 3/4 5/8 3/4 5/8 13/16 11/16 13/16 11/16 1 1/8 3/4 8 7/8 3/4 7/8 3/4 7/8 3/4 7/8 3/4 15/16 13/16 1 3/ /8 1 7/ /8 15/16 1 1/ /16 1 1/ /4 1 1/8 1 1/4 1 1/8 1 1/4 1 1/8 1 1/4 1 1/8 1 13/16 1 7/ /8 1 1/4 1 3/8 1 1/4 1 7/16 1 5/16 2 1/ /16 2 1/8 1/16 5/16 1/4 5/16 1/4 5/16 1/4 5/16 1/4 3/8 5/16 1/2 3/8 4 1/8 1/16 3/8 5/16 3/8 5/16 3/8 5/16 3/8 5/16 3/8 5/16 1/2 3/8 6 3/16 1/16 1/2 3/8 7/16 3/8 7/16 3/8 7/16 3/8 7/16 3/8 3/4 1/2 8 1/2 3/8 9/16 7/16 9/16 7/16 9/16 7/16 5/8 1/2 3/4 1/2 10 9/16 7/16 5/8 1/2 5/8 1/2 5/8 1/2 5/8 1/2 7/8 5/8 16 5/8 9/16 3/4 5/8 3/4 5/8 13/16 11/16 1 1/16 13/ /16 11/16 13/16 11/16 7/8 3/4 1 3/16 15/16 The recommendations listed in the above table are guides for the conditions stated. Other material, designs or dimensions affording equal or superior protection are also acceptable. 69

21 4.4.1 Material requirements and minimum dimensions for portable grinding machines See tables 4, 5, 6 and 7 for minimum thickness of peripheral and side members for various types of safety guards and classes of service for portable grinders, as referred to in section 3.1. The recommendations given in tables 4, 5, 6 and 7 and figures 11, 12, 13 and 14 are guides for the conditions stated. Other materials, designs or dimensions affording equal or superior protection are also acceptable Material requirements and minimum dimensions for stationary grinding machines See tables 8, 9, 10, 11 and 12 for minimum basic thickness of peripheral and side members for various types of safety guards and classes of service for stationary grinders, as referred to in section 3.1. The recommendations given in tables 8, 9, 10, 11 and 12 and figures 16 to 32 are guides for the conditions stated. Other materials, designs or dimensions affording equal or superior protection are also acceptable Optional materials If materials other than those given in tables 4 to 12 and figures 11 to 14 and 16 to 32 are used, the thickness of the peripheral and side members shall be such that the resultant safety guard will provide equal or better protection than a similar guard constructed according to the tables and figures stated above Cutting-off wheels over 16 diameter operating at higher than standard speeds Cutting-off wheels larger than 16, operating in excess of the SFPM specified in this standard, shall have wheel guards constructed of materials and using methods verified by the manufacturer to be suitable. 70

22 4.4.5 Wheel guard with removable side covers Radial strength of the guard assembly shall prevent distortion of the peripheral member in the event of wheel breakage. Guard side covers must be replaced before grinding machine is turned on and used again Material specifications The minimum thickness specifications given in tables 4 to 12 are based on the following: American National Standard Specification for Gray Iron Casting, G (R1972) (ASTM A48-64 (1971)). American National Specification for Mild-to- Medium Strength Carbon-Steel Castings for General Application, G (ASTM 27-72). ASTM A47-74, Specification for Malleable Iron Castings. ASTM A283-74, Specification for Low and Intermediate Tensile Strength Carbon Steel Plates of Structural Quality. ASTM A663-72, Specification for Merchant Quality Hot-Rolled Carbon Steel Bars Subject to Mechanical Property Requirements. ASTM A675-72, Specifications for Special Quality Hot-Rolled Carbon Steel Bars Subject to Mechanical Property Requirements. Other material affording equal or superior protection and which lend themselves equally well to the desired type of construction may also be used. 71