2.2. Cutting and Finishing Steel
2.2.1. What methods are available for cutting steel, and what is the corresponding range of utility for each method?
The following methods are commonly used to cut steel:
1. Friction sawing, which is performed with a high-speed rotary blade, is commonly used by steel producers and is limited only by machine size. This cutting method, however, is no longer commonly used in fabrication shops.
2. Cold sawing, whether by rotary saw, hack saw or band saw, is limited only by machine size.
3. Oxygen-acetylene (and related fuel) flame cutting, which can be mechanically or hand-guided, is commonly used for general cutting and edge preparation operations, such as coping, beveling, notching, etc.; its utility is virtually unlimited.
4. Plasma cutting, which is mechanically guided, is generally useful for cutting plate up to 1 in. thick.
5. Laser cutting, which is mechanically guided, is generally useful for cutting plate; thickness limitations vary.
6. Shearing, which is performed with mechanical presses, is generally useful for cutting plates and angles and is limited only by machine size and capacity.
Additional minor material removal and finishing may also be accomplished by one of the methods listed in 2.2.2
Some of the cutting methods in 2.2.1 result in surfaces that are finished without further treatment; see 2.2.3 and 2.2.4. When this is not the case, the following methods are commonly used to provide finished surfaces:
1. Milling, which is commonly used to bring members to their required length and end finish.
2. Face machining, which can be used to finish large areas to exact dimensions.
4. Grinding, which is commonly used for edge preparation, including treatment of flame-cut edges, removal of burrs, etc., when required
2.2.3. Can the end of a column, as received from the rolling mill, be considered to be a finished surface?
Yes, provided the mill cut is square to the column axes and meets the surface roughness requirements in ASME B46.1 (see 2.2.6)
No. As stated in the AISC Specification Section M2.6, "Compression joints that depend on contact bearing ... shall have the bearing surfaces of individually fabricated pieces prepared by milling, sawing or other suitable means." The AISC Code of Standard Practice Section 6.2.2 Commentary states that "most cutting processes, including friction sawing and cold sawing, and milling processes meet a surface roughness limitation of 500 in. per AISI/ASME B46.1." Cold-sawing equipment produces cuts that are more than satisfactory.
Structural steel preferably should be thermally cut by mechanically guided means. However, mechanically guided cutting may not be feasible in some cases, such as the cutting of copes, blocks, holes other than bolt holes (see 2.4.1 and 2.4.2) and similar cuts. Accordingly, hand-guided thermal cutting should be allowed as an alternative. Regardless, thermally cut surfaces must meet the appropriate roughness limitations as summarized in 2.2.6.
Inadvertent notches or gouges of varying magnitude may occur in thermally cut edges, depending upon the cleanliness of the material surface, the adjustment and manipulation of the cutting head and various other factors. When thermally cut edges are prepared for the deposition of weld metal, the AISC Specification Section M2.2 provides acceptance criteria that consider the effect of discontinuities that are generally parallel to the applied stress on the soundness of welded joints. Additionally, correction methods for defects of various magnitudes are stipulated therein. When thermally cut edges are to remain unwelded, the following surface condition guidelines are recommended:
1. If subjected to a calculated tensile stress parallel to the edge, edges should, in general, have a surface roughness value not greater than 1,000 in. as defined in ASME B46.1.
2. Mechanically guided thermally cut edges not subjected to a calculated tensile stress should have a surface roughness value not greater than 2,000 in. as defined in ASME B46.1.
3. Hand-guided thermally cut edges not subjected to a calculated tensile stress should have a roughness not greater than 1⁄16 in.
4. All thermally cut edges should be free of notches (defined as a V-shaped indentation or hollow) and reasonably free of gouges (defined as a groove or cavity having a curved shape). Occasional gouges not more than 3⁄16 in. deep are permitted. Gouges greater than 3⁄16 in. deep and all notches should be repaired as indicated in 2.2.7.
2.2.7. When surface roughness for thermally cut edges does not meet the limitations in 2.2.6, how is the surface repaired?
Roughness exceeding the criteria in 2.2.6 and notches not more than 3⁄16 in. deep should be removed by machining or grinding and fairing-in at a slope not to exceed 1:10. The repair of notches or gouges greater than 3⁄16 in. deep by welding should be permitted. The following criteria are recommended:
1. The discontinuity should be suitably prepared for good welding.
2. Low-hydrogen electrodes not exceeding 5⁄32 in. in diameter should be used.
3. Other applicable welding requirements of AWS D1.1 should be observed.
4. The repair should be made flush with the adjacent surface with good workmanship.
5. The repair should be inspected to assure soundness.
Reentrant corners should provide a smooth transition between adjacent surfaces, but generally need not be cut exactly to a circular profile. The recommendation in Part 9 of the AISC Manual is that an approximate minimum radius of ½ in. is acceptable. However, the primary emphasis should be that square-cut corners and corners with significantly smaller radii do not provide the smooth transition that is required. It is acceptable to provide radius transitions by drilling (or hole sawing) with common-diameter drill sizes (not less than ¾ in.).
When the corner of a cope has been square-cut, a common solution is to flame-cut additional material at the corner to provide a smooth transition. Note that the sides of the cope need not meet the radius transition tangentially. Any notches that occur at reentrant corners should be repaired as indicated in 2.2.7.