drilling temperatures. For steel, sulfurized mineral cutting oil is  superior. Sulfur has an affinity for steel, which aids in holding  the cutting oil in place. In the case of deep drilling, the drill  should be withdrawn at intervals to relieve chip packing and  to ensure the lubricant reaches the point. As a general rule, if  the drill is large or the material hard, use a lubricant. Reamers, used for enlarging holes and finishing them  smooth to a required size, are made in many styles. They  can be straight or tapered, solid or expansive, and come  with straight or helical flutes.  Figure 4-48  illustrates three  types of reamers: The cylindrical parts of most straight reamers are not cutting  edges, but merely grooves cut for the full length of the reamer  body. These grooves provide a way for chips to escape and  a channel for lubricant to reach the cutting edge. Actual  cutting is done on the end of the reamer. The cutting edges  are normally ground to a bevel of 45° ± 5°. Reamer flutes are not designed to remove chips like a drill.  Do not attempt to withdraw a reamer by turning it in the  reverse direction because chips can be forced into the surface,  scarring the hole. A spring drill stop is a wise investment.  [Figure 4-49] Properly adjusted, it can prevent excessive drill penetration  that might damage underlying structure or injure personnel  and prevent the drill chuck from marring the surface. Drill  stops can be made from tubing, fiber rod, or hard rubber. There are four types of drill bushing holder: There are several types of tools available that aid in holding  the drill perpendicular to the part. They consist of a hardened  bushing anchored in a holder.  [Figure 4-50] Reamers Drill Bushings and Guides Drill Bushing Holder Types Drill Stops 4. Arm—used when drilling critical structure;  can be locked in position; uses interchangeable  commercial bushings. 3.  Plate—used primarily for interchangeable production  components; uses commercial bushings and self- feeding drills. 2.  Egg cup—improvement on standard tripod base;  allows drilling on both flat and curved material;  interchangeable bushings allows flexibility.  [Figure 4-51] 1.  Standard—fine for drilling flat stock or tubing/rod;  uses insert-type bushings. Drill bushing types: 3. Commercial—threaded 2.  Commercial—twist lock 1.  Tube—hand-held in an existing hole 3.  Piloted reamer, with the end reduced to provide  accurate alignment. 2.  Standard or straight reamer. 1.  Three or four fluted production bullet reamers are  customarily used where a finer finish and/or size is  needed than can be achieved with a standard drill bit. Figure 4-50.  Drill bushings. Figure 4-48.  Reamers.  Arm-type bushing holder Bushing holder 1 2 3 HSS Figure 4-49.  Drill stop.  4-19