by Charles G. Monnet, Jr.
Part Four 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.
Setting Knives, Part Two
3. Setting Straight Knives On Square Heads By Use of Setting Block. When setting straight knives on square heads on the machine a setting block as illustrated in figure 64 below can be used to good advantage. The knives are placed on the head and tightened very lightly. The setting block is put against the knife edge and tapped lightly until the edge of the block makes full contact along the edge of the head. The block can be made of brass or very hard wood. Unless it is made very accurately it has no value. Many people are of the opinion that there is no need for setting more than two straight knives on a square head due to the fact that in practice they are not usually jointed. When making long runs, there is something to be said for using four knives. There is as much likelihood of getting 3 out of 4 of the knives cutting as there is in getting 2 out of 2 cutting. Even though more than one knife is cutting this does not mean that more than one is producing a "finish" cut. See the article under the section on Moulder Operating Practice titled "Two Knives Cut Even Though Only One Produces Finish".
4. Set Knives To Proper Projection. Generally, straight knives on square heads are set with a projection of approximately 1/16". A less projection may be used for hard, close-grained woods where a fine finish is desired. When running soft woods at heavy cuts and fast feeds more projection is required. More projection is required for all heavy cuts to give more hip clearance, save power, and reduce the strain on the heads, knives and bolts. The amount of projection should be governed by the kind and condition of the wood being planed, the rate of feed, amount of cut and the finish required. The projection of knives on square heads should be kept down to a minimum, with regard to the factors just mentioned, because excess projection of the knives puts a terrific strain on the knives and bolts.
5. Making "Combination Set-Up". When running a wide pattern on square heads more than one knife (or pair of knives) should be used to form the pattern. The use of several knives distributed around the head to make the pattern is called a "combination set-up". (see figure 93 on page 97) In making a combination set-up it is often desirable to use a pair of more of narrow knives called spikes. These are knives which are under 1½" wide on the cutting edge. They may be as narrow as ¼" edge length.
a. Use of "Caps" and "Spikes". Spikes are held on square heads by means of caps. (see figure 65 above.) The hole in the cap must be large enough that a standard 5/8" bolt will go through it easily. The hole is usually about 21/32" diameter. A spike is put under each side of the cap. Extra wide spikes may protrude a little from the side of the cap. This is all right if it is not overdone. The thickness of the spike cap should be sufficient to withstand cutting thrust without springing abnormally and they should be long enough to give proper support to the spikes. In cases where only one spike is needed for the cutting operation a shorter spike (usually known as a "slug") should be used under the cap on the other side. This short spike should always be set so that it does not project over the lip of the cutter head. This is illustrated by figure 66 below. S is the "slug" spike and C is the cap. In actual practice there would be another spike (SS) with greater projection (the cutting spike) on the other side of bolt B. The cap C would never be projected beyond arc R which shows the minimum projection, M, that a knife would ever have. This projection would be the same as for round heads which is 1/16" to 1/8". On farther projecting spikes such as SS the cap could be set as CC due to the greater clearance arc RR. However, this should not be done because on most patterns some part of the moulding would be ruined by any projection of the cap beyond R.
Spikes have an advantage over slotted knives since they can be ground on both ends. One end may be ground to make a round nose and the other end to make a bevel. Slug spikes used on opposite sides of the head should not only be of the same weight, but of the same dimensions to give proper balance. Spikes which have been ground down beyond use for cutting can be used as "slug" spikes. When using slotted knives that are projected unusually far out on the head caps can be used to good advantage. The cap strengthens the knife against the excessive cutting thrust which is always present on knives set-out beyond the normal projection. Care should be used in setting the caps in all cases because if they are not on the head straight one corner may hit part of the pattern.
6. Tightening Knives. When square heads are used, judgment must be used in tightening the bolts which held the knives. They should not be drawn so tight that the bolt is strained and weakened. The average man with a regular wrench exerts a force of 1600 times the diameter of the bolt. For example the average mechanic exerts 10,000 pounds on a 5/8" diameter bolt. Square heads should never be used for flat surfaces at high speeds.
7. Use Good Bolts. Alloy steel makes the safest and best bolts for square heads. They should be of the same size and weight so they will be balanced. They should not touch the bottom. Many accidents have resulted from this cause. The workman thinks he has the knife held firmly to its place, but the pressure is at the end of the bolt and not at the head. If there is a minute space between the knife and head, shavings will drive under and break the strongest bolt that can be made.
The two things which cause most bolts to break are over-tightening of the bolts and bending of the knives. The continuous over-tightening of a bolt beyond its elastic limit causes a progressive fracture, which causes the bolt to break when in use. When a bolt breaks in this manner, there is usually serious damage done to the machine and frequently human injury. When the nut on a bolt is brought down snug, any further tightening stretches it. No harm is done if the stretching does not go beyond the elastic limit. If the limit is exceeded, then some of the fibres are ruptured and a fracture begins. At each tightening (when elastic limit is exceeded) more fibres are ruptured and the rupture becomes worse. When the bolt breaks, only a part of it shows to be a fresh break. Often, mistakenly, the old fracture is thought to be a defect in the bolt, when actually, it is just the old fracture which has been built up for some time. To avoid fracturing bolts, stop when the bolt is tight. Do not use a wrench longer than furnished by the manufacturer and never put a piece of pipe on the wrench handle. (See figure 62 on page 65) Bending of knives is often the cause of bolt breakage. Knives projected a long ways out on the head may start to bend in the cut, then chips drive under the knife causing it to bend further and dig into the wood or catch against the chipbreaker. If the bolt did not break, the head would tear out and break. When the knife is bent backwards it shows the weakness to be in the knife. When it is necessary to project knives way out on the head be sure to use brace caps.
8. Template-Knife Numbering System. When using a plywood template (or card) for making combination set-ups as explained above it is a good practice to have a system for numbering the knives that make up the set-up. The number of each knife can be written on the template. The proper number for the knife can be etched on the knife. Suitable cabinets should be built with pegs and shelves so that all the knives of one type cn be hung together. For example, nosing cutters of all sizes will be in one group, cove cutters in another, etc. There are many good systems, some simple, some elaborate, for numbering the knives and keeping track of them for use in various set-ups. Your system can be tailored to fit the needs of your own plant.
C. Milled-To-Pattern Heads.
1. Use Gauge For Setting Knives. The majority of milled-to-pattern knives have corrugated backs. Knives of this type should be set out the same number of corrugations. They should be set "width-wise" by use of a setting gauge such as on all Dependable Cutter Head Grinders or by use of a knife setting stand such as the Dependable S-1 pictured on page 64. Milled-to-pattern knives not having corrugated backs should be set for projection and width-wise by use of one of the two methods mentioned. They can be set by use of a scale, but this is a slow, time consuming job. Milled-to-pattern knives should be set "projection-wise" to the cutting circle for which the head was made.
2. Tightening Knives. Milled-to-pattern knives should be tightened in the same way as round heads. Do not tighten one clamp completely but tighten each one slightly and go around the head several times until proper tension is on each knife. Six knife heads in particular can be sprung by incorrect tightening of the knives. Do not use wrenches larger than those supplied with the head for tightening the knives.
D. Setting Knives To Proper Cutting Circle. All planers, moulders, etc. have a standard cutting circle to which the knives should be set. The standard cutting circle for the Dependable Electro-Unit Drive Moulders is 6½". The cutting circle of a round head is the diameter of an imaginary line made by the edge of the knives as the head revolves, and/or the diameter from the tip of one knife to the tip of the opposite knife. See figure 67. The cutting circle of a milled-to-pattern head is the diameter of an imaginary line made by the deepest ground point of the knives and not (like round heads) from the tips of the knives. See figure 67. The diameter of a head is the diameter of the body of the head. Figure 67 shows the diameter of both round and milled-to-pattern heads.
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.