Knife Grinding and Woodworking Manual 6 -- Excerpts Part 6
by Charles G. Monnet, Jr.
Part Six 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
The reason that some operators have to sharpen their knives more often than others working the same kind of wood on the same kind of machine is because they use the wrong cutting angle and sharpness angle. These greatly affect the durability of the initial sharpness of the knife edges. Small cutting angles of 10 degrees or less give best results when machining wavy or interlocked grain. Large cutting angles, up to 35 degrees, may be used for straight grained wood.
When machining woods such as ash, birch, hickory and maple a slight front bevel should be ground on the knives to prevent torn and chipped grain as extremely sharp knife edges do not produce the best results. When machining cypress, pine, poplar, spruce and other soft hardwoods the knife edges should be as sharp as possible to prevent raised or burnt grain.
Another factor to consider in regard to cutting angles is the impact a knife has when it strikes the wood to cut the chip. As the cutting angle is reduced, the force of the impact is increased. When the knives have too heavy a heel from over-jointing, the impact force can become great enough to break the knife or cutter. Even if the impact force is not great enough to break the knife good work will not be obtained because the heel of the knife will cause compressed grain. The effect of knife impact depends to a great extent on the kind and condition of the material being machined. Some woods have greater natural elasticity or rebound characteristics than others. The amount of elasticity of the wood determines the amount of heel that can be permitted on a knife without getting compressed grain. Wood which is softer, or contains a relatively high moisture content requires more clearance angle than dry, hard woods. For smooth machining each knife should produce uniform impact. This cannot be obtained unless accurate grinding and jointing is done.
F. Condition Of Material Helps Determine Bevel. The correct angle for a particular wood depends on its hardness, type of grain, degree of dryness, and finish desired. Generally speaking, as the wood increases in dryness the cutting angle is decreased. When running curly birch, hard maple, etc. it is necessary to front bevel the knives a great deal to prevent tearing of the grain. The more the knives are front beveled the more they approach a scrape rather than a cut. The more nearly a knife approaches a scraping action, the less liable it is to tear the stock. Wood which is kiln-dried should be machined with more of a scraping cut than green wood. When machining wood of 7% or less moisture content the knives should more nearly approach a scraping cut. Front beveling can be used to prevent tearing of flat stock. (See Torn Grain on page 121 and Chipped Grain on 120.) White pine, cypress, etc. tear the least and require little if any front beveling of the knives.
Generally, smaller cutting angles are required for kiln-dried than for air dried material. However, this depends largely on the type of wood. On kiln-dried hard wood such as oak, birch, and maple it is necessary to front bevel 10 or 15 degrees, from a radial line. Wood becomes much more brittle with dryness, chip deflection is improved, and any tearing that occurs is less likely to continue below the surface. Some interlocked wood will machine well (when kiln-dried to less than 8 degrees) at a 15 degree to 20 degree cutting angle, while the same wood air dried may require 10 degree cutting angle to prevent tearing. With wavy grain wood, such as some types of sycamore, the dryer the wood, the smaller the cutting angle has to be to prevent tearing. Soft and wooly natured woods require fine, sharp edges on the knives for the best finish on the stock. Therefore, large cutting angles and sharpness angles should be used. Even on the very hardest of wood, which is sun green or partially dried, it is impossible to carry any great amount of front bevel, because scraping cut will cause the grain to raise and make a fuzzy surface. A large cutting angle has been found best for machining relatively soft and open grained wood. This has been found to be especially true if the wood is wet or green.
The degree of dryness of any wood has a lot to do with the liability of tearing. Moderately dry material (classified as dry) will tear more easily than wet stock. Exceedingly dry material (classified as bone dry) will tear worse than moderately dry material. Therefore, the cutting angle must be changed by front beveling in order to machine this wood at high speed without tearing. Wood may vary in moisture content all the way from almost zero to over 150% of its maximum dry weight. There is little variation in the physical properties of wood until the moisture content is reduced below the fibre saturation point. This is about 25%. As the moisture content of the wood is reduced below the fibre saturation point, the cell walls begin to shrink. By this shrinkage, the wood becomes more compact, harder and less resilient. This change in the condition of the wood requires a change in cutting angles, bevels, etc. for best machining results.
G. Long Bevel Weakens Knife. Use no more knife bevel than is necessary because a long bevel on a thin knife weakens the cutting edge and is more difficult to grind without injury to the knife. This is particularly true when machining hardwoods. It may be better to use a shorter bevel and to grind more often, but under no circumstances, should the bevel be allowed to rub on the work because it will draw the temper in a few seconds. The shorter bevel often pays by saving stock, even though it does take more power and must be sharpened oftener. Veneer knives should always be ground with the same angle of bevel. Extreme cutting angles cannot be used on tungsten carbide knives because of its weak tensile strength. Due to this the carbide must be backed up by steel having greater tensile strength.
H. Use of Medium Bevel. One can readily see that it would be impossible to strike a medium bevel, that would properly work both hard and soft wood. Quite often in smaller factories, it is sometimes necessary for the operator to determine the angle which will work the best on the majority of the kinds of wood that he is working because one machine is called on to work all classes of wood. Under such circumstances it is found that a cutting angle of 20 degrees, from a radial line, works to a very good advantage. If the larger percentage of the work is hardwood, a 10 to 15 degree cutting angle would perhaps work to a better advantage, while if the larger percentage runs to softwood, 25 to 30 degrees would be best. The cutting angle should not be smaller than necessary to prevent tearing because a smaller cutting angle requires more power, creates greater feed resistance, causes quicker dulling of the knives and may cause the machined surface to become fuzzy. Generally, dogwood requires a smaller cutting angle because it has a greater tendency to tear below the surface of the cut.
I. Cannot Get Proper Bevels With Portable Grinder. Some plants grind the knives on their planer cutter heads by use of a portable grinder which is attached to the machine. This has a disadvantage in that the concave grinding done by the small diameter wheel greatly weakens the knife edge. Another disadvantage is that it is impossible to grind a back bevel on the knives by the use of the portable grinder. Still another disadvantage is that the knives have to be ground dry and there is danger of ruining them due to excessive heat. After a few light grindings by this method, they cannot be ground any longer due to the danger of the grinding wheel damaging the cylinder, so they have to be taken out and ground on a regular knife grinder.
J. Cutting Angles For Different Type Heads.
Knives are never reversed in round heads as they sometimes are in square heads because a negative cutting angle is produced. The wood is compressed instead of being cut cleanly because due to lack of proper clearance chips and shavings cannot get away easily. Unless a moderate amount of front beveling will produce the required cutting angle, it is more practical to buy heads with the proper cutting angle.
2. Milled-To-Pattern Heads. Changing bevels on milled-to-pattern bits to get larger or smaller cutting angles does not work as it changes the pattern. See figure 31 on page 44. This shows how a change in the cutting angle changes the pattern cut by the cutter. Milled-to-pattern bits have to made with the proper cutting angle for the kind and condition of the material to be planed.
3. Square Heads. On square heads the cutting angle is controlled by the length of the sides of the head and the projection of the knives. Each knife must project far enough beyond the lip of the head to provide clearance for the head bolts, and for this reason the cutting angle cannot exceed 45 degrees. Dependable square heads are made with a standard 42 degree cutting angle. This can be modified to some extent by changing the amount of projection of the knife beyond the lip of the head. See figure 35 on page 45. This shows that a knife with standard projection for dressing flat stock has a cutting angle of 42 degrees, but if the knife is projected out approximately 1" on the head the knife will have a cutting angle of 30 degrees. Projection of the knife being limited, if a still smaller cutting angle of 30 degrees. Projection of the knife being limited, if a still smaller cutting angle is needed, the knives must be face beveled (angle ground on face of knife) or placed in the head bevel down instead of up for very small cutting angles. See figure 72, page 72.
4. Carbide Heads. The present practice is to use the same cutting angles and grinding bevels on carbide knives used in round heads and bits used in milled-to-pattern heads that are used for regular high speed steel. This can only be given as a general rule because in some cases the angle may be so sharp that proper backing cannot be given to the carbide tip and the knife will not hold up. However, grinding bevels (also called knife or bit bevel) have been used up to 40 degrees in some cases with apparently good results. When angles of this extreme nature are used the problem of backing the knife properly with steel having high tensile strength becomes more and more difficult due to the long bevel the backing-steel cannot come down behind the carbide insert as far as it could on the shorter bevels. Use the chart of cutting angles on page 77 and page 78.
K. Use of Chart. Due to the wide variety of machining characteristics found in different lots of the same species, and often in a single tree, it is practically impossible to set up one correct cutting angle for a particular wood. The variation of machining characteristics is great in some types of wood, while in other types the wood is very uniform. Therefore, the use of a chart or charts is at best a rough guide to the selection of the proper cutting angle and knife bevels. Tests have proven that for Eastern Hardwoods best results are obtained with a cutting angle of 10 degrees to 20 degrees if the moisture content is below 20%. In the Douglas-Fir Regions it has been found that cutting angles of 20 degrees to 25 degrees give the best results. Figure 76 shows a chart of cutting angles and grinding bevels recommended and used with good success over a period of years. Figure 77 on page 78 shows a chart published by the Forest Products Laboratory as a result of a series of tests conducted on hardwoods. Our suggestion would be to use these charts as a starting point in conduction trial and error tests to determine the cutting angle best suited for the wood you are machining. Of course, if the angles and bevels in the chart give the desired result then there is no need for further experimentation.
L. Honing Bevel On Cutters. There may be times when it is advantageous to hone face bevels on knives rather than grind them. Tearing may take place when machining wavy grained wood with ground-to-pattern or milled-to-pattern cutters. By use of a carefully honed face bevel on the cutters, thus reducing the cutting angle, good work may be obtained without reducing the feed speed as is often necessary in these cases. When machining this type of wood with straight cutters on square heads it may be quicker to hone a bevel on the face of the knives than to remove, grind and re-set the knives. The bevel honed may not have to be very large for it to be effective. Often a 1/32" wide bevel will be sufficient to give good results. Hard India oilstones that do not wear away very rapidly are recommended for this work. When honing bevels be careful not to use too great an angle thereby removing all of the positive cutting angle.
FIGURE 76. CA is cutting angle. B is grinding bevel.
Cutting Angles Recommended by Forest Products Laboratory for Hardwoods
*Courtesy of Forest Products Laboratory.
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.
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