fittings needed in a complete heating, ventilating, and air-conditioning  system.  The  Steelworker  must  be very   versatile   because   the   most   difficult   part   of sheet-metal work is the installation of a product that has been built in a shop and is installed on a site at a later  time. All  of  the  variables  and  problems  that  occur during the installation process cannot be covered here; however, this section will cover some of the different hanging  and  connecting  systems  used  by  the sheet-metal  worker.  The  type  of  connecting  system used depends upon where the duct system is installed, its size, how many obstructions there are, and also, what type of structure the system is hanging from or connected to. SHOP PROCEDURES The small sheet metal shops in the NCF or in a Public Works Department are normally tasked with single  fabrication  jobs  for  an  NCF  project  or  small repair projects. These shops usually employ a small number  of  Steelworkers  as  part  of  a  multi-shop environment. The senior Steelworker assigned to a shop  is  tasked  with  the  plan  development  and estimating of materials. The layout Steelworker makes up most of the fittings in the shop and is responsible for  stockpiling  patterns  and  tracings  on  standard fittings  used  for  sheet-metal  duct  systems. NOTE: You should fabricate an entire job at the shop,  rather  than  deliver  an  incomplete  system  to  the jobsite. SHOP DRAWINGS A shop drawing is a plan view or an elevation view of a fitting, duct, or other object that is drawn either by the freehand sketch method or by using drafting instruments. It maybe useful to get assistance from an Engineering  Aid  for  complex  duct  systems  or  fittings. One of the better methods is to draw a complete set of standard  fittings  and  then  add  the  required  dimensions to fit the job. The  dimensions  shown  on  the  views  of  a  shop drawing are finished dimensions. Once the finished dimensions  have  been  determined,  one-half  inch  must be added to each end to obtain the raw size of the pattern.   This   dimension   produces   a   cut   size dimension. The type of material, gauge number, and type  of  seam  may  be  added  to  the  shop  drawing  if desired. Usually these are specified on the drawings and  on  the  pattern  sheets. DUCT MATERIAL Metal sheets, wire, band iron, and angle iron are the  most  widely  used  materials  in  sheet-metal fabrication. The types of metal sheets are plain, flat sheets and ribbed sheets or corrugated sheets. The sheets  are  made  of  such  materials  as  black  iron, galvanized iron, tin plate, copper, aluminum, stainless steel,  or  Monel.  Galvanized  and  black  iron  sheets  are the  most  commonly  used  material  in  sheet-metal work. The  thickness  of  a  sheet  is  designated  by  a  series of numbers called gauges. Iron and steel sheets are designated by the U.S. standard gauge which is the accepted standard in the United States. REINFORCEMENT  AND  SUPPORT The  recommended  gauge  thicknesses  of  sheet metal  used  in  a  standard  ventilating  and air-conditioning  system  with  normal  pressure  and velocities  are  shown  in  table  2-2.  Where  special rigidity  or  stiffness  is  required,  ducts  should  be constructed of metal two gauges heavier than those given in the table. All insulated ducts 18 inches or greater on any flat side should be cross broken, as shown in figure 2-92. Cross breaking maybe omitted if the duct is insulated with approved rigid type of insulation and sheet metal two gauges heavier is used. The maximum length of any section of ductwork will not exceed 7 feet 10 inches; this measurement allows  individual  sections  to  be  fabricated  from  an 8-foot  sheet  of  metal  with  a  2-inch  allowance  for connection  tabs.  If  lengths  of  7  feet  10  inches  are considered   too   long   for   a   specific   job,   it   is recommended  that  the  duct  system  be  constructed with sections of 3-foot 9-inch multiples. Many   duct   systems   run   into   unplanned obstructions,  particularly  in  renovation  work,  such  as electrical connections and wiring, structural members, Figure 2-92.—Cross-broken flat surfaces 2-32