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Shear Knives Maintenance Guide
In many stamping plants, a shear can be the heart of the entire
operation. If the shear is down, all following operations can
cease. Should sheared parts be wrong or have excessive burr,
troubles may start in the stamping operations that follow.
This article will initially discuss shears, then knives and operating practices and maintenance. Basically. there are two types of shears used in stamping plants. They are commonly known as squaring shears and are either mechanically- or hydraulically operated. Shears are also used in cut-to length lines. Squaring shears can be built with underdrive or overdrive designs. Normally. Overdriven shears are furnished with a gap or throat depth, whereas underdriven shears have solid frames and are gapless. There are cast frames, welded frames, bolted frames, and combinations of each type. The principle of shearing is similar to the action of an ordinary set of scissors. The angle at which the blades come together is the blade rake, and the distance between the blades is clearance. Mechanical shears are usually built with a fixed rake and hydraulic shears with an adjustable rake. There are pros and cons for each type. Shears in cut- lines are always a mechanical type and usually use a bow-tie-type knife with a straight knife. Shear Knives
Most shear builders purchase their knives from a knife manufacturer, although some builders do manufacture their own knives. A reputable knife manufacturer can produce a shear knife for the operator's specific needs, given complete information. Basically, there are five factors that govern the choice, analysis, and hardness of tool steel to "engineer a knife for the job." These factors are:
Most knife manufacturers have a wealth of information and specifications on many shears produced. Should specifications be needed, the information in Figure 1 is required.
Most knife manufacturers produce their products to dimensional tolerances as published by The American Society of Mechanical Engineers ANSI B94.53. Clearances
Increasing clearances will reduce tonnage required to make a cut up to a point. Excessive clearance will result in increasing these tonnage requirements. In addition to shearing, some bending will take place. Many shears are equipped with knife clearance adjustments to encourage the operator to make the various changes. Shear builders differ somewhat as to the suggested clearances between knives for shearing different thicknesses and types of material. In the absence of information in this regard, as a general rule of thumb., the clearance should be set at 7 percent of the material thickness being sheared. When making cuts at or near the capacity of the shear, the center of the upper knife should be bowed .002 inch nearer the lower knife than at the ends. This adjustment is necessary to overcome deflection of the upper ram. There are several suggested clearances (see Figure 2). Common sense and experimentation will help establish knife clearances for other metals.
Maintenance of Shear Knives
High-quality tool steels and workmanship have gone into the production of shear knives. The utmost care has been given to the proper analysis of the tool steel to be used, as well as the hardening, tempering, and grinding of the finished product. They have been carefully oiled and packed in heavy wooden boxes for shipment to ensure arrival at the customer's plant in perfect condition. If the knives are to give long satisfactory service, they must be given the care due any precision instrument. Storage. When not in use, the knives should be stored, preferable in wooden boxes, in a dry, protected area. Do not store knives where heavy objects can drop or be thrown on them. Following regrinding and prior to storage, the knives should be coated with a rust-inhibiting oil. This will prevent pits resulting from corrosion on the knife surface that can become focal points for cracks. Handling. Shear knives can resist considerable shock when well -supported throughout their full length in the rigid knife seats of the shear. However, they can be severely shipped or broken if dropped while being moved between the storage area and the shear or the grinder. Leveling. As with any machine tool, a shear should be leveled precisely. This should be checked at frequent intervals and adjustments made, if needed. Shearing accuracy and knife life can be impaired by a shear being out-of-level. Installation
Great care should be taken when installing knives in a shear to be certain that the knife seats are clean, all burrs are removed, and the knives have been wiped clean. This step is important to assure firm support of the knives in the shear to prevent breakage and assure proper clearances Damage to the cutting edges can be prevented by supporting the upper knife on wooden blocks while it is being changed. For the exact procedure to follow in changing knives, refer to the operator's manual furnished by the shear builder. Shearing
Never shear material thicker or of greater tensile strength than that for which the shear was designed. The indicated capacity of the shear is the maximum thickness to be sheared, regardless of the width or length being sheared. Do not attempt to shear hardened material. After turning or changing knives, be certain to remove all tools from the shear table, or they may be pushed between the knives accidentally. If knives have been reground, shims of proper thickness should be used under the lower knife to set the cutting edge level with the table top. Grinding
It is false economy to delay sharpening of dull knives. If the knives are used beyond the point when they should have been reground, an excessive amount of knife material will have to be removed to bring the knives to a proper cutting condition. Shearing with dull knives may result in the following:
As a general rule, four-edge shear knives are reground, with the major stock removal on the edges and faces touched up slightly to clean up and maintain proper parallelism. A word of caution - some shears from international manufacturers do not have vertical adjustment to allow shimming of the lower knife and are face-ground only. A new set of knives being ground for the first time would be rendered useless if they are edged and faced. Be sure to inform the grinder if this the case. The accuracy and quality of a cut made by a knife are partially dependent on how the knife was ground. The thickness and width of the knife must be held parallel within very close limits throughout the knife length. The great majority of shear knife grinding is done on machines equipped with magnetic chucks. It is important that the shear knives be fully demagnetized after sharpening. Failure to do this will allow metallic slivers or dust to adhere to the knife edges, causing them to dull prematurely. Many stamping plants require die blades in their operation. These die blades are similar to shear knives, except they are manufactured to closer tolerances on the width and thickness (see Figure 3).
Die blades can be purchased in any required length and, in many cases, on short delivery. Many die blades were manufactured from carbon steels. However, the new analysis and hardness of the high carbon, high chrome used in die blades has established records of three to five times the service of the carbon steels. The design of the blade allows four edges to be used before blades require resharpening. For more information about anything you see on this web site
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