by Charles G. Monnet, Jr.
Part Five of a six part series providing in-depth information on proper sharpening and balancing procedures for jointer knives. Copies of the 167 page Knife Grinding and Woodworking Manual are available from Charles G.G. Schmidt & Co., Inc.
Cutting Angles and Bevels
Required for Knives and Cutters
A. Properties of Wood Making Various Angles Necessary. In order to obtain the maximum efficiency out of moulders and similar wood cutting machines it is necessary to take into consideration the physical and structural properties of the wood being machined. Some of the main structural features as related to cutting are: 1. Grain formation, which may be straight, wavy, spiral, crossed, interlocked, or irregular. 2. Cell or cell wall content, which may contain crystals, resin, wax, silica, and moisture. 3. Texture which may be fine, medium or coarse. The physical properties are: soft, medium, and hard. Elasticity is another property. In addition to the physical and structural properties that must be considered, decorative properties must be considered because a good part of the wood machined is worked to this objective. The properties to be considered under this category are the color variations of the growth rings and the medullary ray formation.
B. Types of Angles. The nomenclature of knives has varied from manufacturer to manufacturer and each organization or writer has had his own terms. The terms used in this manual as shown in figure 68 are the ones that have been most commonly used. Recently, the Forest Products Research Society and the Machine Knife Association have come up with the terms shown in figures 69, 70, and 71. They are in hopes these will be adopted by the woodworking industry.
1. Cutting Angle. The cutting angles of a knife is the angle of the knife in relationship to the cut, or the angle formed by the face of the knife and a radial line from the center of the head. A radial line is a line drawn from the cutting edge of the knife through the center of the head. (See figure 68 on page.) The cutting angle of a knife exerts great influence on the finish produced, the feed resistance and the power consumed in cutting.
Cutting angles may run all the way from the greatest forward hook of a knife to an actual scraping cut. Theoretically, the cutting angle could even go to a negative rake. Only in extreme cases should knives ever be set at the radial or pure scraping cut and never at a negative rake. The correct angle for a particular wood depends on its hardness, type of grain, degree of dryness (moisture content) and the desired finish on the stock.
2. Back Bevel Angle. A back bevel on a knife is made by grinding a bevel on the regular grinding bevel of the knife and forms what is called a back bevel angle. This is an angle from the grinding bevel of the knife to a line drawn from the (back) bevel ground. (See figure 68 on page 70.) When a knife leaves the factory it is made with one bevel which is called the grinding bevel (a line drawn from the face of the knife and the bevel is called the grinding angle.) When the knife has been used and is resharpened the grinder operator may blunt off the knife instead of following the factory made angle. This gives the knife two angles instead of the one originally on the knife. Therefore, the new angle formed by the old bevel and the new bevel is called the back bevel angle. The sharpness angle is increased by the amount of the back bevel angle.
3. Front Bevel Angle. A front bevel on a knife is made by grinding the face of the knife which forms what is called a cutting bevel (or front bevel angle). This is an angle from the face of the knife to a line drawn from the bevel ground. In other words, a front bevel is a bevel ground on the face side of the knife, the side which approaches the cut. A front bevel does not have to be a long bevel because only a small amount of the knife edge engages on this side. (See figure 68 on page 70.) Front bevels are generally ground to provide the correct cutting angle for a particular kind of wood. However, front bevels can be used to act as chipbreakers. When using a front bevel it is usually advantageous to use a reduced grinding angle, otherwise, the sharpness angle may be too blunt. Many operators do not realize that there is not a better or safer chipbreaker made than a knife properly ground on the face side. When a knife is ground with a front bevel there is no danger of shavings being forced under the knife and breaking it off. On surfacers and jointers it is worth while to grind a face bevel on the knives and set them out a little to save the wear on the lip under the knife. If it wasn’t for the use of front bevels, to change the cutting angle of the knives, it would be necessary to have several heads each with the knives set in at a different angle, for working the various kinds of wood to best advantage. The greater the angle of bevel ground on the face of the knife, the more the cutting angle of the knife is reduced. Figure 72 shows how the cutting angle of a knife on a square head can be reduced by use of a face bevel.
4. Grinding Angle. The grinding angle of a knife is the angle formed by grinding a bevel on the back of a knife. The grinding angle is the angle between the face of the knife and the bevel ground. (see figure 68 on page 70.) Another name for the grinding angle is knife bevel. A small grinding angle will make it possible to joint the knives more times before the heels become too wide. However, another factor that should be considered, especially when cutting hardwoods or knotty softwoods, is that the grinding angle must be large enough to give the knife edge strength to resist deflection, wear and nicking. When using thin high speed steel knives in round heads the width of the knife bevel, and therefore the acuteness of the grinding angle, is limited because the entire knife bevel must be above the body of the head and as you know, the knives must not project too far out on the head. There is danger of chips packing behind the knife if the bevel extends below the periphery of the head.
5. Sharpness Angle. The sharpness angle of a knife is the angle between the cutting face and the bevel forming the cutting edge of the knife. The sharpness angles is the same as the grinding angle unless front of back bevels have been ground on the knife. (See figures 68, 70, and 71 on page 70.) When these are ground then the sharpness angle is increased by an amount equal to the front bevel angle and/or the back bevel angle. When the cutting angle is reduced the sharpness angle is increased and vice versa.
6. Clearance Angle. The clearance angle is the angle between the grinding bevel (or back bevel if there is one) and the tangent to the cutting circle. (See figure 68 and 70 on page 70.) The value of this angle is relative to the sharpness angle. Actually very little clearance angle is necessary so this is determined by the required cutting and sharpness angles. Of course without the clearance angle, the heel of the knife would pound the stock being cut. When working hardwoods knives must have adequate clearance. The same knives (not having proper clearance) might get by in softwood. The larger the clearance angle the greater the number of times jointing can be done without developing too wide a heel. However if the angle is too large the cutting edge of the knife will be too weak and knife breakage may occur. Jointing may reduce the clearance angle to zero, or even a negative value. When this happens the heel of the cutter will pound into the wood to varying degrees depending on the width of the heel and the species of the wood. The lack of a clearance angle increases the cutting friction and the impact on the cutter edge.
C. Surface Finish Should Determine Cutting Angle. There are many factors which determine the proper cutting angle for moulder or planer knives. The most important is probably the surface finish which is required. The proper surface finish depends on the end use of the planed stock. As stock is fed through a moulder having accurately jointed knives, one knife enters the stock to cut and lifts a shaving followed by the next knife in the head which cuts in a similar manner. The amount of space (limited by peaks at the height of the cut) between consecutive knife marks on the stock determines the degree of smoothness of the planed surface. Consequently, the surface finish is thought of in number of "knife marks per inch." Only a general rule can be given for correct number of knife marks per inch for proper surface finish as the number should vary depending on the kind and condition of the wood and the end use of the stock. Generally, for rough planing, 6 to 8 knife marks per inch is satisfactory and 16 to 20 knife marks per inch is necessary for finish planing. However, when machining some hardwoods as many as 40 to 45 knife marks per inch may be necessary to obtain a quality surface. See Chart Giving Recommended Knife Marks on page 128.
D. The Affects Of Various Cutting Angles. The machining (cutting) process consists of the separation of the wood substance at the cutting edge and the deflection of the severed material. As the knife edge penetrates deeper into the wood the severed material is forced up the knife face. This deflection is brought about by a vertical force acting normal to the knife face and another force parallel to the knife face required to overcome the frictional resistance to movement of the severed material against the knife. The greatness of these forces depends partly upon the amount of deflection of the severed material. The forces increase as the cutting angle is reduced. As the parallel force is reduced there is a greater force exerted in the direction of feeding but a decreased force at right angles to the face of the board being planed. This vertical force causes any tearing out that may occur in the wood ahead of the knife. The machined surface is ruined if, due to the direction of grain, tearing out occurs below the plane of cutting.
When the cutting edge of the knife has passed the center of the wave mark it begins to cut with a lifting action. Unless the correct cutting angle is used the cells in front of the cutting edge may be torn apart. The tendency of the knife edge to lift the cells at this part of the cut increases or decreases in relation to the cutting angle. When using a large angle the lifting action is much greater. (See figure 72.) By grinding a face bevel on the knife as shown in figure 73 the approach angle of the knife is such that it does not give any lifting action to the cutter. Tearing is likely to be particularly bad when machining wavy or interlocked wood. This is due to the direction of the cells being so that part of them are cut against the grain. This enables the knife edge to exert a greater leverage against the cells thus tearing them apart. When machining wood which is figured and likely to "tear out" by grinding a front bevel on the knives, thereby decreasing the cutting angle to between (5° and 20°) a faster feed rate can be used and still a good finish can be obtained. Since feed speed, depth of cut and cutter projection also influence the lifting action of the cells, when it is not practical to change the cutting angle, by using a slow feed rate a fairly good finish can be obtained. Better results will be obtained in this case if very light cuts are made. The slower feed rate reduces the leverage on the cells because each knife is taking a smaller cut.
A reduction in cutting angle increases cutting and feeding resistance, and causes the knives to dull sooner. The greater power consumption, caused by decreasing the cutting angle, varies with the different types of wood, degree of dryness, spindle speed, feed speed and other factors. However, the average increase in power consumption, under normal conditions, is 25% increased when the cutting angle is reduced from 30° to 20° and 100% increase power consumption when the cutting angle is reduced from 30° to 10°. Proper cutting speeds are important, but they do not affect the finish nearly as much as the proper cutting angles.
The Forest Products Laboratory at Madison, Wisconsin conducted a series of tests on the machining of Southern Hardwoods. The results of their tests on cutting angles are shown in Figure 74.
|Kind of wood||Defect-free pieces at cutting angle of--|
For each cutting angle shown the test pieces were planned under two different conditions: (1) At 36 feet per minute feed and 3,600 r.p.m. and (2) on the opposite side at 54 feet feed and 5,400r.p.m. The moisture content was 6 percent for this entire series of tests.
Figure 74.* Effect of cutting angle on quality of work.
E. Various Bevels Required For Different Types of Wood. Wood machines best when cut with the grain as opposed to cutting against or into the grain. (See figure 75 on page 74.) This is because the wood cells are stronger and tougher longitudinally than radially or tangentially. This is shown by the way the cells or cell walls tear apart and leave a rough surface when cut across, unless special angles, bevels, etc. are used when machining. The cells tear apart because the cutter edge, as it digs under or into the cells, exerts a force greater than the middle lamella which binds them together. The cells tend to tear away in their entire length rather than sever with a clear clean cut and this causes the rough, ragged edges which produce a rough surface. The cells of the various species vary greatly and for this reason various cutting angles, bevels, etc. are required for different woods.
When machining the various woods it is necessary to change the sharpness and strength of the cutting edge of the cutter to suit the particular species of wood. This is done by grinding a front bevel or back bevel on the knife. Any change of the cutting angle or grinding angle caused by the above changes the sharpness angle. The sharpness angle being the angle between the cutting face and bevel forming the cutting edge of the knife. The wear of the cutting edge of a knife depends on the physical and structural properties of the wood being machined. The wear of the knife edge is due to the friction from cutting and is thought by some to be an electrochemical process. Cutter wear is relative to the cutting angle, direction of the grain, thickness and toughness of the cell walls and the content of the cells. Therefore, very hard and/or abrasive woods require a much larger sharpness angle in order to give the cutter edge strength to withstand the extra friction and impact in cutting. Soft, clean woods can be worked with a small sharpness angle which gives a very sharp knife edge. Grinding a front or back bevel on the cutter will increase the strength of the cutting edge. So will increasing the grinding angle. When grinding a front bevel, to reduce the cutting angle for machining interlocked grain, the same sharpness angle can be maintained by reducing the grinding angle.
Some woods have a high elasticity. This varies in relation to the moisture content. The cells of wood having this characteristic bend away when subjected to the forces exerted by the cutter edge. The cells not having been cut cleanly "bounce" back after machining and give the surface a wooly finish. Drying the wood to a low moisture content gives more rigidity to the cells and a somewhat better cutting action. However, for best results when machining wood of this type use a sharp cutting edge and a large cutting angle. Also, the use of a high speed feed will give better results as it also has the effect of giving increased rigidity to the cells. The use of a small cutter projection or crystals, having an abrasive action on the cutters wear the edges rapidly. When machining wood of this type if a larger sharpness angle is used longer cutter life between grindings will be obtained. When possible, the use of the maximum practical feed speed will reduce rubbing to a minimum and will give a better cutting action.
Due partly to the different kinds of woods being worked and partly to the different angles at which knives are held in different makes and types of heads, every shop usually has one or more standard knife bevels which are kept the same by use of gauges. It is impossible to work the various kinds of wood to best advantage with any one cutting angle or grinding bevel. In fact softwood, such as white pine, bass wood, popular, and wood of this class, is worked to best advantage without any front bevel at all, using a cutting angle of 30 degrees in round heads from a radial line, for green or wet stock and 25 degrees for kiln dried material. A radial line is a line drawn from the cutting edge of the knife through the center of the arbor.
Editor's note: copies of the 167 page Knife Grinding and Woodworking Manual are available from Charles G.G. Schmidt & Co., Inc. You can click on their link to reach their web site, and send an e-mail request for more information, or call them at 201-391-5300.
The entire series will be available at woodweb's archive section.