Parallels are rectangular metal blocks used in milling machines to hold the work-piece to a level that matches with the cutting tool. Parallels ensure that the work-piece is always parallel to the surface of the machine table. Their main purpose is to ensure that the work-piece is well exposed to allow smooth working of the machine. They are usually made of stainless steel, cast iron, or tool steel to enhance their grip, as other metals cannot weaken these alloys easily. Several varieties and sizes of parallels exist depending on the type of work to perform. Parallel strips are ground steel bits, which are sold as matching pairs. Parallels are optional, as their uses are determined by the job at hand.
Milling machines are essential tools in machining operations. They carry out the milling operations through making required relative motions between the work-pieces and tools. Milling machines utilize parallels to hold firm the work-piece. Parallels are ground flat plates that are usually manufactured using hard steel (Smid 67). Parallels come in pairs, which are placed at the same dimensions with their matching faces. They have series of holes that are drilled on their front side to enable them position the work-piece or fit in clamps to remove any sharp edge on the work-piece. Some parallels have excellent surface tolerance that enables them to be lightly bonded through revolving two parallels together.
Special adjustable parallels have regulating feature that permits them to be used in different solid type parallels, as they can be easily inserted in numerous openings and enlarged to desired sizes. Adjustable parallels have two pieces that are dovetailed. In general, parallels have four faces, which include the front, the back, and the two sides, but some parallels have irregular shapes with a smooth end surface. The surface of each parallel determines how it is manufactured, as a grained surface depicts grinding while a smooth surface indicates lapping.
Parallels are made in different sizes and thicknesses to allow them to support work-piece that lacks a flat outline. Parallels are sold in pairs depending on their thicknesses. The thin parallel set comes in 10 pairs and measures 1/8 inch thick and 6 inches long, but with different heights that include ½ inch, 5/8 inch, ¾ inch, 1 inch, 1 1/8 inch, 1 ¼ inches, 1 1/8 inches, 1 ½ inches, and 1 5/8 inches. The 4-pair parallel sets measure six inches long with different heights. The smallest parallels, or ultra thin parallels, have a thickness of 1/32 inch and a length of 6 inches.
The wavy parallel sets come in 9 pairs where the width varies from ½ inch to 1 inch, but the length is fixed at 1/8 inch. The height also varies based on use. Adjustable parallel sets come 6 pairs and each pair measures 3/8 inch to ½ inch; ½ inch to 11/16 inch; 11/16 inch to 15/16 inch; 15/16 inch to 1 5/16 inches; 1 5/16 inches to 1 ¾ inches; and 1 ¾ inches to 2 ¼ inches. The magnetic chuck parallels are 1 inch long, 2 inches wide, and 4 inches high.
The materials to be used in parallels should be strong enough to withstand tension and stress during milling and grinding. Parallels are made from high-grade alloy steel that is hardened and grinded. These alloys include stainless steel, tool steel, and cast iron. Cast iron is preferable in making parallels because it minimizes tress in the metal and the possibility of metal twisting is relatively low. Stainless steel can withstand corrosion, as well as heat damage, and its strength allows manufacturers to produce materials of different thicknesses.
Parallels are usually utilized in machine operations for milling, turning, or grinding. They are quite useful for layout work when operating milling machines. They are commonly applied in vices to offer support and setting jobs accurately. In addition, lifting the work-piece above the vice enhances drilling of holes. Parallel strips are utilized to offer clearance so that milling can be carried out without destroying the machine table. Figure 1 below illustrates parallels in a vise.
Figure 1: Parallels holding the work-piece in a vise
In this case, the work-piece is pushed downward to the parallels, enabling the milling cutter to move over the work-piece. Parallels of similar size are utilized to create the desired height on the work-piece. Parallels should be high enough to enable the milling cutter to cut at the right level, as excessive height minimizes the holding capacity of the jaws. After setting the work-piece, additional tightening is unnecessary as it may raise the work-piece off the parallels.
Angle parallels are suitable for supporting work-piece at a specific angle in a vice. Adjustable parallels can be used in different machine tools as they have different gages and can be expended to fit different sizes. However, parallels may not be appropriate for ultra precision work, because they are not attached to any piece of the milling machine (Smid 67). Parallels can also be utilized in the case where the vise develops a problem on its face, to reduce the possibility of holding the work-piece hazardously.
“Parallels.” Victor Machinery Exchange, Inc., n.d. Web. 26 May 2015.
Smid, Peter. Cnc Control Setup for Milling and Turning: Mastering Cnc Control Systems. New York: Industrial Press, 2010. Print.