Efficient grinding depends primarily upon the proper setup of the machine being used. If the machine is not securely mounted, vibration will result, causing the grinder to produce an irregular surface. Improper alignment affects grinding accuracy, and it is good practice to check the security and plumb of the machine every few months. It is advisable to place a strip of cushioning material under the mounting flanges, along with any necessary aligning shims, to help absorb vibration.
When a grinding wheel is functioning properly, the abrasive grains cut very small chips from the workpiece and at the same time a portion of the bond of the wheel is worn away. As long as the bond is being worn away as fast as the abrasive grains of the wheel become dull, the wheel will continue to work well. If the bond is worn away too rapidly, the wheel is too soft and will not last as long as it should. If the cutting grains wear down faster than the bond, the face of the wheel becomes glazed and the wheel will not cut freely.
Precision and semiprecision grinding may be divided into the following classes:
Cylindrical Grinding. Cylindrical grinding denotes the grinding of a cylindrical surface. Usually, “Cylindrical grinding” refers to external cylindrical grinding and the term “internal grinding” is used for internal cylindrical grinding. Another form of cylindrical grinding is conical grinding or grinding tapered workpieces.
Surface Grinding. Surface grinding is the grinding of simple plain surfaces.
Tool and Cutter Grinding. Tool and cutter grinding is the generally complex operation of forming and resharpening the cutting edges of tool and cutter bits, gages, milling cutters, reamers, and so forth.
The grinding wheel for any grinding operation should be carefully chosen and the workpiece set up properly in the grinding machine. Grinding speeds and feeds should be selected for the particular job. Whenever practical, a coolant should be applied to the point of contact of the wheel and the workpiece to keep the wheel and workpiece cool, to wash away the loose abrasive, and to produce a better finish.
In grinding, the speed of the grinding wheel in SFPM and the feed of the grinding wheel are as important as, and sometimes more important than, proper wheel selection. Occasionally, the grinder spindle should be checked with a tachometer to make sure it is running at its specified RPM. Too slow a speed will result in waste of abrasive, whereas an excessive speed will cause a hard grinding action and glaze the wheel, making the grinding inefficient. The feed of the grinding wheel will determine to a cetain extent the finish produced on the work and will vary for different types and shapes of grinding wheels.
Factors Governing Speed. The various factors governing the speed in SFPM of a grinding wheel are as described below.
Safety. The grinding wheel should never be run at speeds in excess of manufacturer’s recommendations, Usually, each grinding wheel has a tag attached to it which states the maximum safe operating speed.
If a wheel is permitted to exceed the maximum safe speed, it may disintegrate and cause injury to the operator and damage to the grinding machine.
Condition of the Machine. Modern grinding machines and machines that are in good condition can safely turn a grinding wheel at speeds greater than machines that are older or in poor condition. Most grinding machines are equipped with spindle bearings designed for certain speeds which should not be exceeded. Poor quality will result from vibrations caused by inadequate rigidity or worn bearings that are not in the best condition. High speeds will intensify these defects.
Material Being Ground. The material being ground will generally determine the grain, grade, structure, and bond of wheel to be selected. For example, if the wheel is too soft for the material being cut, an increase in speed will make the wheel act harder. Conversely, if the wheel is too hard, as lower speed will make the wheel act softer.
Type of Grinding Wheel. The type of grinding wheel employed for a particular operation is one of the major considerations in the proper selection of cutting speed. In general practice, the wheel will be selected for the material to be cut. The recommended cutting speed can then be determined by the wheel type, bond, and grade of hardness (Table 5-1 in Appendix A).
Calculating Wheel Size or Speeds. Both cutting speeds in SFPM and rotational speed in RPM must be known to determine the size wheel to be used on a fixed-speed grinding machine. To determine the grinding wheel size, use the following formula:
|D =||12 x SDFPM|
- SFPM = Cutting speed of wheel (In surface feet per minute).
- RPM = Revolutions per minute of wheel.
- D = The calculated wheel diameter (in inches).
To obtain the cutting speed in SFPM when the wheel diameter and RPM are given, use the same formula in a modified form:
|SDFPM =||D x RPM|
To obtain the rotational speed in RPM when the wheel diameter and desired cutting speed are known use the formula in another modified form:
|RPM =||12 x SDFPM|
As a grinding wheel wears down and as it is continually trued and dressed, the wheel diameter decreases, resulting in loss of cutting speed. As this occurs, it necessary to increase the rotational speed of the wheel replace the wheel to maintain efficiency in grinding.
In cylindrical grinding, it is difficult to recommend any work speeds since these are dependent upon whether the material is rigid enough to hold its shape, whether the diameter of the workpiece is large or small, and so forth. Listed below are areas to consider when performing cylindrical grinding:
Surface grinding machines usually have fixed work speeds of approximately 50 SFPM or have variable work speed ranges between 0 and 80 SFPM. As with cylindrical grinding, the higher work speeds mean that more material is being cut per surface foot of wheel rotation and therefore more wear is liable to occur on the wheel.
The feed of the grinding wheel is the distance the wheel moves laterally across the workpiece for each revolution of the piece in cylindrical grinding or in each pass of the piece in surface grinding. The following methods are recommended for determine feeds:
Methods for determining depth of cuts are recommended for determining feeds.
Most grinding machines are equipped with coolant systems. The coolant is directed over the point of contact between the grinding wheel and the work. This prevents distortion of the workpiece due to uneven temperatures caused by the cutting action. In addition, coolant keeps the chips washed away from the grinding wheel and point of contact, thus permitting free cutting.
Clear water may be used as a coolant, but various compounds containing alkali are usually added to improve its lubricating quality and prevent rusting of the machine and workpiece.
An inexpensive coolant often used for all metals, except aluminum, consists of a solution of approximately 1/4 pound of sodium carbonate (sal soda) dissolved in 1 gallon of water.
Another good coolant is made by dissolving soluble cutting oil in water. For grinding aluminum and its alloys, a clear water coolant will produce fairly good results.
Offhand grinding is the process of positioning and feeding the workpiece against a grinding wheel by hand. Offhand grinding is used for reducing weld marks and imperfections on workpieces, and general lathe tool, planer tool, shaper tool, and drill grinding. Deciding depth of cut and feed is based on the operator’s knowledge of grinding.
Offhand grinding is performed on utility grinding machines which generally have fixed spindle speeds and fixed wheel size requirements, so that the cutting speed of the wheel is constant and cannot be changed for different materials. Therefore, the operator must use care in feeding and not overload the wheel by taking too heavy a cut, which would cause excess wear to the grinding wheel. Similarly, he must be careful not to glaze the wheel by applying excessive pressure against the wheel.
The one variable factor in most offhand grinding is the selection of grinding abrasive wheels, although limited to one diameter. For example, a softer or harder wheel can be substituted for the standard medium grade wheel when conditions and materials warrant such a change. Lathe tool grinding is described in Chapter 7. Drill sharpening and drill grinding attachments and fixtures are described in Chapter 4.
Milling cutters must be sharpened occasionally to keep them in good operating condition. When grinding milling cutters, care must be exercised to maintain the proper angles and clearances of the cutter. Improper grinding can result in poor cutting edges, lack of concentricity, and loss of form in the case of formed tooth cutters. Milling cutters cannot be sharpened by offhand grinding. A tool and cutter grinding machine must be used.
The bench-type tool and cutter grinding machine described here is typical of most tool and cutter grinding machines. It is designed for precision sharpening of milling cutters, spot facers and counterbores, reamers, and saw blades. The grinding machine contains a l/4-HP electric motor mounted to a swivel-type support bracket which can be adjusted vertically and radically on the grinder column. The column is fixed to the grinder base which contains T-slots for attaching grinder fixtures used to support the tools that are to be ground.
The motor shaft or wheel spindle accepts grinding wheels on each end. One end of the spindle contains a wheel guard and tool rest for offhand grinding of lathe tools and so forth. Cup, straight, and 15º bevel taper abrasive grinding wheels are used with this machine. Fixtures used for grinding tools and cutters include a center fixture for mounting reamers, taps, and so forth between centers; an outside diameter fixture for chucking arbor-type milling cutters and shanked peripheral cutting edge tools; and an end mill fixture for supporting end cutting tools to the grinder base.
Use the following methods and procedures when grinding formed milling cutters.
A positive rake angle is a rake angle that increases the keenness of the cutting edge. A negative rake angle is one that decreases or makes the cutting edge more blunt.
Figure 5-17. Correct and incorrect grinding of formed milling cutter teeth.
The grinding wheel should be set up so that the wheel traverse is aligned with the face of one tooth (Figure 5-18). The alignment should be checked by moving the grinding wheel away from the cutter, rotating the cutter, and rechecking the traverse on another tooth. After this alignment is accomplished, the depth of cut, is regulated by rotating the cutter slightly, thus maintaining the same rake angle on the sharpened cutter. The depth of cut should never be obtained by moving the cutter or grinding wheel in a direction parallel to the wheel spindle. Doing this would change the rake angle of the cutter.
Figure 5-18. Aligning formed milling cutter and grinding teeth.
Plain milling cutters with saw-tooth type teeth are sharpened by grinding the lands on the periphery of the teeth. The lands may be ground using a straight grinding wheel or a cup-shaped grinding wheel.
The important consideration when grinding this type of cutter is the primary clearance angle or relief angle of the land (Figure 5-19). If the primary clearance angle is too large, the cutting edge will be too sharp and the cutter will dull quickly. If the primary clearance angle is too small, the cutter will rub rather than cut and excessive heat will be generated.
Figure 5-19. Milling, cutter nomenclature.
The primary clearance angle (Figure 5-19) should be between 3° and 5° for hard materials and about 10° for soft materials like aluminum. For cutters under 3 inches in diameter, a larger clearance angle should be used: 7° for hard materials and 12° for soft materials.
The clearance angle for end and side teeth should be about 2° and the face of these cutters should be ground 0.001- or 0.002-inch concave toward the center to avoid any drag.
To grind the lands of milling cutter teeth to primary clearance angle, the teeth are positioned against the grinding wheel below the wheel’s axis (Figure 5-20).
Figure 5-20. Grinding primary clearance angle.
To obtain the primary clearance angle when grinding with a straight wheel, lower the indexing finger or raise the grinding wheel a distance equivalent to 0.0088 times the clearance angle times the diameter of the grinding wheel. For example, to find the distance below center of the indexing finger (Figure 5-20) for a cutter with a 5° clearance angle, being ground by a straight wheel 6 inches in diameter, the calculation is as follows: 0.0088 x 5 x6 = 0.264 inch. The indexing finger would then be set 0.264 inch below the wheel axis. The milling cutter axis should also be 0.264 inch below the wheel axis.
To obtain the primary clearance angle when grinding with a cup wheel, the formula for a straight wheel is used except that instead of wheel diameter being used in the formula, the cutter diameter is used. In this case, the index finger is set to the calculated distance below the axis of the milling cutter (Figure 5-20) instead of below the axis of the wheel.
The land of each tooth (Figure 5-19) should be from 1/32 to 1/16-inch wide, depending upon the type and size of the milling cutter. As a result of repeated grinding of the primary clearance angle, the land may become so wide as to cause the heel of the tooth to drag on the workpiece. To control the land width, a secondary clearance angle (Figure 5-19) is ground. This angle is usually ground to 30°, although the exact angle is not critical. Generally, an angle between 20° and 30° is sufficient to define the land of the tooth.
The peripheral teeth of end milling cutters are ground in the same manner as the teeth of a plain milling cutter. When grinding the end teeth of coarse-tooth end milling cutters, the cutter is supported vertically in a taper sleeve of the end mill fixture and then tilted to obtain the required clearance angle. The end mill fixture is offset slightly to grind the teeth 0.001 to 0.002 inch lower in the center to prevent dragging. A dish-shaped grinding wheel revolving about a vertical spindle is used to grind end milling cutters.
After the milling cutter is ground, the cutting edges should be honed with a fine oilstone to remove any burrs caused by grinding. This practice will add to the keenness of the cutting edges and keep the cutting edges sharper for a longer period of time.
Cylindrical grinding is the practice of grinding cylindrical or conical workplaces by revolving the workpiece in contact with the grinding wheel. Cylindrical grinding is divided into three general operations: plain cylindrical, conical grinding (taper grinding), and internal grinding. The workpiece and wheel are set to rotate in opposite directions at the point of contact (Figure 5-21).
Figure 5-21. Direction of rotation for cylindrical grinding.
TT = (WW x FF x WRPM) + 12
- TT = Table travel in feet per minute
- WW = Width of wheel
- FF = Fraction of finish
- WRPM = Revolutions per minute of workpiece
- 12 = Constant (inches per foot)
The fraction of finish for annealed steels is 1/2 for rough grinding and 1/6 for finishing; for hardened steels, the rate is 1/4 for rough grinding and 1/8 for finishing.
l-inch-wide wheel is used to rough grind a
hardened steel cylinder with a work RPM of 300.
For example, After the calculations have been completed, set the machine for the proper traverse rate, turn on the table traverse power feed, and grind the workpiece.
Check the workpiece size often during cutting with micrometer calipers. Check the tailstock center often and readjust if expansion in the workpiece has caused excessive pressure against the drilled center in the workpiece.
The finishing cut should be slight, never greater than 0.001 inch, and taken with a fine feed and a fine grain wheel.
If two or more grinding wheels of different grain size are used during the grinding procedure, each wheel should be dressed and trued as soon as it is mounted in the grinding machine.
Most conical grinding is performed in the same manner as plain cylindrical grinding. Once the grinding machine is set up, the table is swiveled until the correct taper per inch is obtained. Steep conical tapers are normally ground by swiveling the headstock to the angle of taper. Whichever method is used, the axis of the grinding wheel must be exactly at center height with the axis of the work.
The internal grinding attachment is bolted to the wheel head on the universal tool and cutter grinder. The RPM is increased by placing a large pulley on the motor and a small pulley on the attachment.
The workpiece should be set to rotate in the direction opposite that of the grinding wheel. The following step-by-step procedure for grinding the bore of a bushing is outlined below as an example.
If two or more grinding wheels are used to complete internal grinding, true each wheel after mounting it to the spindle of the internal grinding attachment.
Surface grinding or grinding flat surfaces, is characterized by a large contact area of the wheel with the workpiece, as opposed to cylindrical grinding where a relatively small area of contact is present. As a result, the force of each abrasive grain against the workpiece is smaller than that applied to each grain in cylindrical grinding. In surface grinding the grinding wheel should be generally softer in grade and wider in structure than for cylindrical grinding.
The following sequence is provided as a step-by-step example of a typical surface grinding operation.