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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 In the case of deep drilling, the drill should be withdrawn at intervals to relieve chip packing and to ensure the lubricant reaches the point.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 

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: 

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. 

2. Standard or straight reamer. 

3. Piloted reamer, with the end reduced to provide accurate alignment. 

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. 

## Drill Stops 

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. 

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## Drill Bushings and Guides 

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] 

Drill bushing types: 

1. Tube—hand-held in an existing hole 

2. Commercial—twist lock 

3. Commercial—threaded 

## Drill Bushing Holder Types 

There are four types of drill bushing holder: 

1. Standard—fine for drilling flat stock or tubing/rod; uses insert-type bushings. 

2. Egg cup—improvement on standard tripod base; allows drilling on both flat and curved material; interchangeable bushings allows flexibility. [Figure 4-51] 

3. Plate—used primarily for interchangeable production components; uses commercial bushings and self- feeding drills. 

4. Arm—used when drilling critical structure; can be locked in position; uses interchangeable commercial bushings. 

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