TM 1-1500-204-23-9 (3) Round-nose  chisel.    Round-nose  chisels make  circular  grooves  and  chip  inside  corners  with  a fillet. (4) Diamond-point chisel.  The diamond-point chisel is used to cut V-grooves and sharp corners. b. Repair.      Nicked   or   battered   chisels   may   be repaired by grinding.  If necessary, temper can also be restored. (1) Grinding.  Chisels are usually ground to a 70-degree angle, but that may be as high as 90-degrees for harder substances.  Proceed as follows: (a) Set the rest on the grinding wheel to adjust for the desired bevel. WARNING Eye     protection     will     be     worn     when grinding chisels.  Failure to comply may result in serious injury to personnel. (b) Move  the  chisel  from  side  to  side while  grinding  so  that  the  cutting  edge  will  be  slightly curved. (c) Ensure   that   the   bevels   are   kept centered or the cutting edge will not be centered. (d) Preserve temper by dipping frequently in water. (2) Retempering.      If   temper   has   been   lost during grinding, retemper as follows: (a) Heat  the  entire  chisel  to  cherry  red in a gas furnace or a charcoal fire. (b) Grasp  the  chisel  in  the  center  with tongs  and  dip  the  cutting  end  in  clean,  cold  water  to  a depth of 1 1/4 inch. (c) Turn  the  chisel  and  dip  the  head  in the water to a depth of about 1 inch. (d) Quickly   polish   the   hardened   ends with  a  file  or  abrasive  cloth,  Federal  Specification  P-C- 451,  and  observe  the  color  returning  from  the  heated center of the chisel to the ends. (e) Dip   the   cutting   end   each   time   it becomes  purple,  and  the  head  each  time  it  becomes blue. (f) When   the   red   disappears,   dip   the entire chisel. c. Care.    Observe  the  following  practices  for  the care and upkeep of chisels: (1) Protect   the   cutting   edges   by   installing protective covers. (2) Store  in  racks  or  where  they  may  not  be chipped or broken. (3) Lubricate  with  a  light  coat  of  oil  before storing. (4) Regrind  broken  or  chipped  edges  before using. 4-10.  Punches.    Punches  usually  are  made  of  carbon steel   tempered   on   both   ends.      They   generally   are classified as solid punches and hollow punches and are designed    according    to    their    intended    use.        Hollow punches  vary  in  size.    Solid  punches  vary  both  in  size and in point design. a. Solid    Punches.        Solid    punches    are    named according  to  their  shape  and  are  designed  for  various purposes. (1) Types and uses. The following paragraphs describe the common types of punches used in aircraft maintenance. (a) Prick punch.  A prick punch, shown in figure 4-58, is used to place reference marks in metal. It is also often used to transfer dimensions from a paper pattern  directly  onto  the  metal.    It  is  relatively  slender and  is  tapered  to  a  point  of  about  30  degrees.    The following   precautions   should   be   taken   when   using   a prick punch • Never strike a prick punch a heavy blow with the hammer   because   it   could   bend   the   punch   or cause   excessive   damage   to   the   item   being worked. • Do   not   use   a   prick   punch   to   remove   objects from  holes  because  the  point  of  the  punch  will spread   the   object   and   cause   it   to   bind   even more. 4-21