Hand Threading and Reaming - Reamers
The geometry of a hole drilled in metal by a twist drill may not be accurate enough (close enough to a true cylinder of a certain precise diameter) and may not have the required smooth surface finish for certain engineering applications. Although modern twist drills can perform excellently in many cases—usually producing sufficiently accurate holes for most applications—sometimes the stringency of the requirements for the hole's geometry and finish necessitate two operations: a drilling to slightly undersize, followed by reaming with a reamer. The planned difference between the drill diameter and the reamer diameter is called an allowance. (It allows for the removal of a certain small amount of material.) The allowance should be < 0.2 mm (.008 in) for soft materials and < 0.13 mm (.005 in) for hard materials. Larger allowances can damage the reamer. The drilled hole should not be enlarged by more than 5% of the drilled diameter. Drilling followed by reaming generally produces hole geometry and finish that is as close to theoretical perfection as possible. (The other methods of hole creation that approach nearest to perfection under certain conditions are boring [especially single-point boring] and internal cylindrical grinding.)

Reamers is either hand or machine reamers.
Type of reamers
Straight fluted reamer- A straight reamer is used to make only a minor enlargement to a hole. The entry end of the reamer will have a slight taper, the length of which will depend on its type. This produces a self centring action as it enters the raw hole. The larger proportion of the length will be of a constant diameter.
Reamed holes are used to create holes of precise circularity and size, for example with tolerances of -0/+0.02 mm.  This will allow the force fitting of locating dowel pins, which need not be otherwise retained in the body holding them. Other holes, reamed slightly larger in other parts, will fit these pins accurately, but not so tightly as to make disassembly difficult. This type of alignment is common in the joining of split crankcase halves such as are used in motorcycle motors and boxer type engines. After joining the halves, the assembled case may then be line bored (using what is in effect a large diameter reamer), and then disassembled for placement of bearings and other parts. The use of reamed dowel holes is typical in any machine design, where any two locating parts have to be located and mated accurately to one another - typically as indicated above, to within 0.02 mm.

Another use of reamed holes is to receive a specialized bolt that has an unthreaded shoulder - also called a shoulder bolt. This type of bolt is commonly used to replace hot peened rivets during the seismic retrofit of structures.

Straight fluted reamer
 
Spiral fluted reamer - The reamer is also straight but the flute is in a spiral.  This spiral fluid reamer ensure a smooth finish without any chatter marks

Spiral fluted reamer
 
Taper reamer - A precision tapered reamer is used to make a tapered hole to later receive a tapered pin. A taper pin is a self tightening device due to the shallow angle of the taper. They may be driven into the tapered hole such that removal can only be done with a hammer and punch. They are sized by a number sequence (for example, a No.4 reamer would use No.4 taper pins). Such precision joints are used in aircraft assembly and are frequently used to join the two or more wing sections used in a sailplane. These may be re-reamed one or more times during the aircraft's useful life, with an appropriately oversized pin replacing the previous pin.
 

Straight fluted taper reamer

Spiral fluted taper reamer
 
 
Adjustable reamer - An adjustable hand reamer can cover a small range of sizes. They are generally referenced by a letter which equates to a size range. The disposable blades slide along a tapered groove. The act of tightening and loosening the restraining nuts at each end varies the size that may be cut. The absence of any spiral in the flutes restricts them to light usage (minimal material removal per setting) as they have a tendency to chatter. They are also restricted to usage in unbroken holes. If a hole has an axial split along it, such as a split bush or a clamping hole, each straight tooth will in turn drop into the gap causing the other teeth to retract from their cutting position. This also gives rise to chatter marks and defeats the purpose of using the reamer to size a hole.
 

Adjustable reamer
 
Machine reamers - A machine reamer only has a very slight lead in. Because the reamer and work piece are pre-aligned by the machine there is no risk of it wandering off course. In addition the constant cutting force that can be applied by the machine ensures that it starts cutting immediately. Spiral flutes have the advantage of clearing the swarf automatically but are also available with straight flutes as the amount of swarf generated during a reaming operation should be very small.

 
Faults with taps, dies and reamers

Breaking of taps - Small taps (5 mm and smaller) has a tendency to break, especially in threading hard materials. 

To prevent breaking of taps ensure the following:

  • Only small movement forward and large movements back. 

  • Remove the tap regularly to clean cuttings. 

  • Use sufficient  and correct cutting fluid.

  • Do not use blunt taps.

  • Use correct size tap handle as to big tap handle will result on excessive torque on tap and it will then break.

  • Be careful with blind holes so that to stop when tap reach bottom of hole.

Follow this useful link for faults and how to fix them: https://www.guhring.com.au/troubleshooting/troubleshooting.html
 
The Function of cutting fluids and their application
When reaming and tapping it is essential to use cutting fluid to improve the process. A cutting fluid has the following functions when reaming and tapping:
  • To wash away cuttings formed by the tap, die or reamer
  • To prevent building up of material at cutting edge
  • To improve surface finish
  • Remove heat and cool down tap, die and reamer
  • Allow for greater cutting speeds for improve production rates
  • More accurate holes and threads
  • To increase the live of the tap, die or reamer
Recommended Cutting Fluid for Various Materials
MATERIAL DRILLING REAMING TAPPING TURNING THREADING MILLING
Aluminium Soluble Oil Kerosene Kerosene & Lard Oil Soluble Oil Kerosene Mineral Oil Soluble Oil Mineral Oil Soluble Oil Soluble Oil Kerosene & Lard Oil Soluble Oil Lard Oil Lard Or Mineral Oil
Brass Dry Soluble Oil Kerosene & Lard Oil Soluble Oil Dry Soluble Oil Lard Oil Dry Soluble Oil Soluble Oil Lard Oil Soluble Oil Dry
Bronze Dry Soluble Oil Lard Oil Mineral Oil Soluble Oil Lard Oil Dry Soluble Oil Lard Oil Dry Soluble Oil Soluble Oil Lard Oil Soluble Oil Lard Oil Dry
Cast Iron Dry Soluble Oil Air Jet Soluble Oil Mineral Lard Oil Mineral Lard Oil Soluble Oil Mineral Lard Oil Dry Dry Sulfurized Oil Dry Soluble Oil
Copper Dry Soluble Or Lard Oil Kerosene Mineral Lard Oil Soluble Oil Lard Oil Dry Soluble Oil Mineral Lard Oil Soluble Oil Soluble Oil Lard Oil Soluble Oil Dry
Malleable Iron Dry Soda Water Dry Soda water Soluble Oil Soluble Oil Lard Oil Soda Water Dry Soda Water
Monel Metal Soluble Oil Lard Oil Soluble Oil Lard Oil Mineral Lard OilSulfurized Oil Soluble Oil Lard Oil Soluble Oil
Steel Alloys Soluble Oil Sulfurized Oil Mineral Lard Oil Soluble Oil Mineral Lard Oil Sulfurized Oil Mineral Oil Soluble Oil Lard Oil Sulfurized Oil Soluble Oil Mineral Lard Oil
Steel Forgings Low Carbon Soluble Oil Sulfurized Lard Oil Lard Oil Mineral Lard Oil Soluble Oil Mineral Lard Oil Soluble Oil Lard Oil Soluble Oil Soluble Oil Mineral Lard Oil Soluble Oil Mineral Lard Oil
Tool Steel Soluble Oil Sulfurized Oil Mineral Lard Oil Soluble Oil Sulfurized Oil Lard Oil Mineral Lard OilSulfurized Oil Soluble Oil Lard Oil Sulfurized Oil Soluble Oil, Lard Oil