Carbide Tipped Reamers
Carbide-tipped reamers are basically the same in design as high-speed-steel reamers, with the exception that the cutting blades have carbide tips. They may be used on a wide range of materials, including all the common industrial metals and the machinable plastics. The extreme hardness of cemented carbides enables them to withstand wear and abrasion; however, these materials are relatively brittle, and the reamer body, the flutes or blades, and the carbide tips themselves must definitely be shaped and dimensioned to resist deflection. Nearly all carbide-tipped reamers are intended for machine reaming.
The terminology and definitions for reamers adopted and published by the ANSI are generally applicable to carbide-tipped reamers.
Certain angles and clearances are embodied in the cutting elements of carbide-tipped reamers to obtain cutting action in the proper places, prevent friction, and clear the chips.
Practically all the cutting is done on the chamfer, which is in the form of a truncated cone on the entering end of the reamer. This chamfer is shaped to start the reamer properly in the hole.
A clearance on the chamfer of 6 to 8 degrees on the carbide tip and a secondary clearance of 10 to 15 degrees on the steel body have proved to be satisfactory for most workpiece materials. Under good work conditions and proper alignment of the reamer and the work, a back taper of 0.003 in/in of flute length is adequate. Back taper should be increased to prevent binding of only slightly dull tools when reaming very abrasive materials. In general, circular land widths may range from 0.007 inches on ¼ inch-diameter to 0.020 inches on 1 ½ inch-diameter reamers. Primary relief angles on the carbide tip range from 15 degrees on a ¼ inch-diameter to 7 degrees on a 1 ½ inch-diameter reamers (see Fig 3.55).
Solid carbide reamers are available in diameters from 1/16 to 3/8 inches, and larger on special order. Since sintered carbide is a rigid material, a solid carbide reamer will finish a hole within close tolerances. If a hole is drilled slightly out of round or out of parallel, a solid carbide reamer will usually not follow the irregularities but will finish a true hole around its own axis. Rough and finish-reaming operations often can be combined into one through the use of solid carbide reamers.
Number Of Flutes: Carbide-tipped roughing reamers may have either an even or an odd number of flutes, depending upon the reamer size and the application (Table 3-25). Standard and finishing types usually have an even number of flutes. The spacing of the flutes around the reamer body is sometimes uneven to minimize the possibility of chatter.
Kinds Of Carbide-Tipped Reamers: Carbide-tipped reamers may be made for either right- or left-hand flutes, or with straight flutes, and may incorporate one, two, or more sections of different diameters for cutting two or more diameters in one operation (Fig. 3-56).
Carbide-tipped reamers in straight flute design, with strips of tungsten carbide the full length of the flutes, are ideal for precision reaming to close tolerances. The wear on the diameter from bushings is eliminated and back taper can be reduced to a minimum, as danger of pickup on the diameter is reduced.
Solid carbide reamers are more practical in small sizes because of their rigidity, heat resistance, and strength. Tipped reamers in small sizes below ¼ inch are very fragile because of the removal of steel necessary to install the tip. Also, the rapid absorption of heat causes brazing failures in dry operation.
For best efficiency, carbide-tipped reamers house be used only on machines that are in good condition. Loose or misaligned spindles can cause serious damage to a reamer, and the resulting cut will not conform to the desired size or finish.
The magnitude of lead and rake angles, width of land, amount of clearance, and whether straight or spiral flutes should be employed are all related to the kind and characteristics of the material in the workpiece. (Table 3-26).
Carbide-tipped reamers are generally covered with a plastic coating over the tips for the protection of the cutting edges. This coating may be removed in such a matter that it can be replaced when the tool is not in use. The coolant or lubricant, when used, should be in abundant supply and should wash over the tool at all times to avoid cracking of the carbide from overheating and quenching.
Grinding Suggestions: Reamers should be sharpened as soon as they become dull enough to prevent proper cutting. It usually is necessary to sharpen only the cutting edge or the chamfer, but if it is necessary to regrind the diameter, the correct circular-land width must be maintained, as well as the back taper.
Carbide-tipped reamers may be reground on a standard cutter grinder with a 220-grit resinoid-bonded diamond cup wheel. The grinding wheel should be rotated in a direction to cut from the face of the tip toward its back (Fig 3-57). The use of a lubricant during grinding lengthens grinding wheel life and lessens the danger of a surface cracking or “crazing” of the carbide from overheating. Do not quench carbide tools in water.