Definitions

requisitely

Reamer

[ree-mer]
A reamer or ream is a tool used in machining to make existing holes dimensionally more accurate and to improve surface finish. Reamers are used mostly in the metalworking areas of machining. They may be used as hand tools or in the spindles of machine tools such as milling machines or drill presses.

Overview of design and function

Reamer body

A typical reamer consists of a set of parallel straight or helical cutting edges along the length of a cylindrical body. Each cutting edge is ground at a slight angle and with a slight undercut below the cutting edge. Reamers must combine both hardness in the cutting edges, for long life, and toughness, so that the tool does not fail under the normal forces of use. They should only be used to remove small amounts of material. This ensures a long life for the reamer and a superior finish to the hole.

The spiral may be clockwise or counter-clockwise depending on usage. For example, a tapered hand reamer with a clockwise spiral will tend to self feed as it is used, possibly leading to a wedging action and consequent breakage. A counter-clockwise spiral is therefore preferred even though the reamer is still turned in the clockwise direction.

Shank types (drive types)

For production machine tools, the shank type is usually one of the following: a standard taper (such as Morse or Brown & Sharpe), a straight round shank to be held by a collet, or a straight round shank with a flat for a set screw, to be held by a solid toolholder. For hand tools, the shank end is usually a square drive, intended for use with the same type of wrench used to turn a tap for the cutting of screw threads.

Reaming versus drilling to size

The geometry of a hole drilled in metal by a twist drill may not be accurate enough (close enough to a theoretically true cylinder of a certain precise diameter) and may not have the requisitely 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 < 0.2mm (.008 in) for soft materials and < 0.13mm (.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.)

Types of reamers

Adjustable hand 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 respectively 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.

Straight reamer (precision)

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 centering 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/+0.02mm(0.0008") 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 0.02mm or less than 0.001".

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.

Hand reamer

A hand reamer has a longer taper or lead in at the front than a machine reamer. This is to compensate for the difficulty of starting a hole by hand power alone. It also allows the reamer to start straight and reduce the risk of breakage. The flutes may be straight or spiral.

Machine reamer

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 are essential on a machine reamer to clear the swarf automatically.

Rose reamer

A rose reamer has no relief on the periphery and is offset by a back taper to prevent binding. They are primarily used as roughing reamers.

Shell reamer

Shell reamers are designed for reaming bearing and other similar items. They are fluted almost their whole length.

Tapered reamer (precision)

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 slow 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 joins 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.

Morse taper reamer

A morse taper reamer is used manually to finish morse taper sleeves. These sleeves are a tool used to hold machine cutting tools or holders in the spindles of machines such as a drill or milling machine. The reamer shown is a finishing reamer. A roughing reamer would have serrations along the flutes to break up the thicker chips produced by the heavier cutting action used for it.

Combination Reamer

A combination reamer has two or more cutting surfaces. The combination reamer is precision ground into a pattern that resembles the part’s multiple internal diameters. The advantage of using a combination reamer is to reduce the number of turret operations, while more precisely holding depths, internal diameters and concentricity. Combination reamers are mostly used in screw machines or second-operation lathes and not with Computer Numerical Control ( CNC ) machines because G-Code can be easily generated to profile internal diameters.

Combination reamers can be made out of cobalt, carbide, or high-speed steel tooling. When using combination reamers to ream large internal diameters made out of material with lower SFM , carbide tips can be brazed onto a configured drill blank to build the reamer. Carbide requires additional care because it is very brittle and will chip if chatter occurs. It is common to use a drill or combination drill to remove the bulk of material to reduce wear, or the risk of the part pulling off on the combination reamer.

Tapered reamer (non-precision)

A tapered reamer may be used cleaning burrs from a drilled hole, or to enlarge a hole. The body of the tool tapers to a point. This type of reamer consists of a body which, typically, is up to 1/2 inch in diameter, with a rod cross piece at the large end acting to form a handle. It is especially useful for working softer metals such as aluminum, copper, and mild steel. A similar tool can be seen on select Swiss Army knives, such as the Electrician model, to be used on conduit.

Materials

Reamers are normally made of high-speed steel, cobalt steel, solid carbide, or are carbide tipped with an alloy steel body. The cutting edge usually has a hardness between Rc 63 and Rc 67 for steel and from to Rc 76 to Rc 82 for carbide.

Additional uses and types

Often, hobby grade radio control cars need reamers to have the holes drilled for the mounting of the bodies. Although similar to metal reamers, radio control body reamers are small, and the tip is usually conic, with flue areas on each side to help create the holes.

Additional Information

These websites contain more specific information about the reaming process.

See also

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