A reamer is a rotary cutting tool designed to finish a hole with accurate size and smooth surface finish. Reamers are used in metalworking to machine a previously created hole. Typically a hole is drilled to under size keeping a small allowance of material and then using a reamer, the hole is enlarged to the precise dimension. This metal cutting operation is called reaming.
Why Use a Reamer?Reamers are used primarily for two reasons.
- To achieve precise hole diameter
- To accomplish high quality surface finish
It is the quickest operation to get Precision hole and a reamed hole relatively smooth surface.
Drilling vs reaming
Drilling is a primary metal cutting operation, meaning it can create a hole on a solid work piece. Reaming is a sizing and finishing operation by which a previously formed hole is enlarged to the precise size.
The modern machines can drill holes with good accuracy for the general purpose tolerance (±0.050mm) especially when solid carbide drills are used. However a precision reamer can size the hole to exact dimensions within the range of 0.0025mm.
The surface finish achieved by drilling generally ranges from N9 to N8 while a typical reaming operation can produce N6 or even N5 grade holes (Ra value of 0.4 to 0.8 µm or 16 to 32 µinches).
Other metalworking operations that produce similar results are horning, boring and internal cylindrical grinding. However reaming is simplest and less time consuming when compared to other hole finishing operations.
Type of Reamers
Wide range of reamer is available according to the application, purpose and the machine on which it is used. Here are some of the most popular ones.
They are fitted with cutting blades that are adjustable to cover a range of size from the previous sized reamer to the next larger sized one. To change the diameter, loosen captive nut on one end and tighten the nut on other end.
Adjustable reamers are generally not suitable for mass production and precision metalworking since the dimension cannot be controlled very accurately. Also, the movable blade design with no spiral in the flutes result in chattering marks. These are mostly used in repair work and deburring the holes.
Fixed Straight Reamers
A Straight reamer has fixed size and is used to finish a hole by slight material removal. The high precision reamers are normally fixed reamers. They are made out of a single piece of HSS (high speed steel) or carbide. In case of larger sized reamers with carbide blades, they are permanently brazed to the body. This enables us to attain exact sized holes consistently.
They are used for manual operation. The cutting end has a lead taper which allows the user to drive the tool easily into the hole. At the shank end a square is provided to hold the tap using a tap wrench or similar holding devices.
Hand reamers are suitable for finishing holes on work pieces that cannot be loaded on a machine. However they are not recommended for reaming blind holes since the lead taper at the tip will result in smaller diameter towards the bottom of the blind hole. In such cases it is better to use machine reamer.
Machine Reamers or Chucking Reamer
They are used on machines like drill press, milling machines, lathe etc. Machine reamers are available with a variety of shanks including straight shank, Morse taper and other taper shanks that fits into the machine. In case of smaller reamers a taper sleeve such as Morse taper may be used. They have lead taper to a very small length at the tip which works fine since usually reaming is done immediately after drilling and hence the drilled hole is accurately aligned to the reamer. Machine reamers are commonly used in precision machining and tool making industry where tight tolerance of hole position and diameter is important.
These are used to produce taper holes. Depending on the angle of the taper a single drilling or multiple step-drilling will be performed prior to reaming. Tapered holes are used assembly of machine parts, aircraft wing assembly etc. where a tapered pin will be fitted inside the hole which can only be removed by knocking from the smaller end of the whole. This ensures the pin stays in position during the operation of the machine.
Shell reamers are designed with no shank and cutting portion to the full length. An arbor is fitted the hole in the center with the lugs on the arbor going inside the slots to drive the cutting tool. These type of reamers are mainly used for finishing the bearing and reaming holes that are very deep.