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cutting technology

Circular saw blade geometry

introductory remarks

The machining of wood and wood-based materials, especially the shaping by machining, is a central topic in woodworking and wood processing. The moving cutting wedge in the form of the saw tooth, the planing knife or the cutter blade separates the material by cutting it by chip formation. One speaks of cutting with geometrically determined cutting edge by rotating tools. The optimum cutting process depends on a bundle of factors, not least on the cutting geometry, i.e. on the coordination of the various angle sizes on one cutting edge.
 

Cutting edge geometry

Without going into the usual designation in the tool reference system, the orthogonal plane of the carbide tooth, the angles at the cutting wedge can be described as follows:

α = Clearance angle, the angle between the cutting plane and the back of the cutting edge (free surface).

β = wedge angle, the angle between the back of the cutting edge and the face of the cutting edge

γ = rake angle, the angle between the cutting face and the vertical on the cutting plane at the point of contact of the cutting tip.

δ = cutting angle, the angle between the cutting face and the cutting plane (cutting angle = wedge angle + clearance angle)

In addition, there are additional angle designations for circular saw blades and milling cutters.

The sum of the angles of α, β + γ is 90°. If one of these three angles is changed, another changes. In order to find out the optimum cutting edge geometry for the respective material (e.g. softwood, hardwood, coated chipboard, aluminium alloy, etc.), empirical investigations often have to be carried out, whereby the feed path, i.e. the total length of the machined workpieces, the required surface quality of the machined material and cutting edge wear play an important role. As far as cutting angles are concerned, it can generally be said that a defined cutting operation is not possible with a clearance angle of 0°, because if the clearance angle is too small, the back of the cutting edge rubs against the workpiece in the mesh arc and the cutting forces increase. If the rake angle is too large, the cutting wedge is weakened, the risk of cutting chipping increases, but the cutting forces decrease. If the rake angle is too small, the cutting work has a scraping tendency.

Tooth shapes

In the practice of sawing, certain tooth shapes have developed for the respective machining cases depending on the type of material and the cutting directions, e.g. solid wood lengthwise or crosswise. Therefore, numerous carbide circular saw blades with standardized tooth geometries are offered, covering a wide range of applications. Different tooth shapes were developed for the different cutting directions and materials. Whether the cross-cutting of solid wood results in more or less large tears depends, among other things, on the tooth shape and its angles. Further target parameters are work safety and the slight feed force during saw-cutting. For example, the Holz Berufsgenossenschaft has determined on its recoil test bench that the cutting width and, above all, the number of teeth are of essential importance for the risk of recoil with saw blades. Carbide (HW) circular saw blades for the machining of wood and wood materials, aluminium alloys and plastics have largely substituted the saw blades of alloyed steels on the market, i.e. with the exception of special applications with thin cutting widths and special workpiece materials. The brazed cutting part of the HW saw blades can be designed and configured differently for different machining applications.

Flat tooth (F)

The flat tooth is the simplest tooth form. It can also be described as the basic shape of the tooth. It can be easily sharpened by machine without complex adjustment. Each tooth, soldered onto the circular saw blade in the panel seat, cuts at the cutting edges: Therefore, the cut surface quality on both sides mainly depends on the axial run-out accuracy of these cutting edges. The guidance of the saw blade is only slightly supported in the cut-off.

Flat tooth with corner bevel (F FA)

To protect the cutting edge, the flat tooth is equipped with a small so-called protective bevel of 0.3 x 45° to 0.5 x 45°. This tooth form is called flat chamfer. In contrast to the trapezoidal tooth, the chamfer is kept very small.
Alternate tooth (W)

If the flat tooth is alternately equipped with an axis angle (angle of inclination), an alternate tooth forms on the circumferential cutting edge. The resulting corner angle can be reinforced by alternating back grinding (free surface grinding).
 

Hollow tooth (H)

The hollow tooth gets its name from the concave breast grinding (chip surface grinding), which gives it the characteristics of a flat tooth, which is provided on both sides of the breast with an alternating axis angle. Thus each tooth flank produces a pulling cut on both cut surfaces of the separating cut. By grinding a concave rake surface, a slight concave shape is also created at the cutting edge.

Hollow tooth with chamfer (KTH)

Also the hollow tooth, actually a flat tooth with hollow-ground breast, frequently receives a protective bevel at its cutting edge corner, which is particularly endangered by hollow grinding. This protective bevel is also carried out in the range of 0.3 to 0.5 mm x 45°. This tooth form is particularly used for plastic-coated panel materials and plastics. The designation KTH comes from plastic trapezoidal hollow tooth. The term trapezoid is not quite appropriate here either, since this tooth design is also a flat tooth with a low chamfer. The trapezoidal tooth, however, usually has a chamfer, which is about ¼ to 1/3 of the cutting width. Geometrically strictly speaking, the flat tooth with chamfer is of course a trapezoidal tooth. The cutting edges, i.e. all teeth, are cut advantageously on both sides as with the flat tooth. This results in a high surface quality. The chamfering process moves the quality-forming cutting edge corner in the direction of the flank, i.e. it is gently set back when the cutting edge is engaged. A longer service life is the desired result.

Trapezoidal tooth (T)

For the trapezoidal tooth, the same applies as for the flat tooth with chamfering. In addition, the guide function of the chamfering surface is of primary importance for the trapezoidal tooth: the saw blade works with less vibration. In most cases, the trapezoidal tooth is used in the group toothing as a pre-cutting tooth. If the chamfer of the flat tooth is enlarged to the middle, the roof tooth is obtained.

Roof tooth (D)

This is usually carried out with a corner angle of 30°. The roof tooth scores the coating of the plate in the middle of the tooth and then performs a pulling tooth engagement acting over the cutting edge up to the cutting corner. This tooth form is therefore frequently used in group toothing for plastic-coated plates.

Roof tooth with protective chamfer (D FA)

Provided with a protective bevel of 0.3 x 45°, the cutting edge corner of the roof tooth receives more stability: tooth shape DFA. This tooth form is used for DP circular saw blades and group teeth.
When sawing coated panels in synchronization with pre-scoring, the flat tooth is often used, as the cutting edge entry from above and below produces clean edges. Depending on the material to be cut, different tooth shapes are used in the main saw blade for the machining of coated chipboards in conjunction with scribing saws, i.e. in the combination process of scribing saws and main saws. These HW tooth forms were developed in order to achieve better cutting results and, above all, longer service lives when countercutting the main saw blade.

One-sided corner angle, one-sided pointed (ES)

One-sided pointed ES If the machining case requires a defined cutting quality only on one cutting surface, the tooth form ES is used. This can be formed by corner angle grinding or, more frequently, by one-sided axis angle generation in the case of flat teeth, as is the case, for example, with chipper saw blades. The latter is used for chip flight away from the chain conveyor. The cutting quality increases advantageously and the number of teeth can be reduced for a higher economic efficiency. Of course, left and right versions are available. When looking at the tooth face, for example, the term ESR is used for the chamfering of the tooth on the right, i.e. the tip of the tooth formed in this way is on the left. Compared to flat teeth, alternate teeth have the advantages of subdivision of the cut, the lead of the tooth tips and, in the case of axial angles, also of the "pulling cut". A minor disadvantage compared to the flat tooth is the greater sharpening effort, especially with regard to a symmetrical formation of the alternating tooth geometry. This symmetry is a prerequisite for a perfect cut of the alternate tooth circular saw blade.

Group toothing

The so-called trapezoidal flat tooth TF, actually the trapezoidal flat tooth, the hollow roof tooth flat tooth (DH) and the alternate tooth flat tooth (WWF) are examples of group teeth. At T + F the group tooth is composed alternately of slightly higher trapezoidal teeth and slightly lower flat teeth. A favourable cut subdivision is achieved by this very frequently used group toothing: The slightly protruding trapezoidal teeth take over the pre-machining, the flat teeth bring about the cutting edge quality. The English-speaking countries speak of triple spacing.

Roof hollow tooth (DH)

The group toothing of the hollow tooth design with the concave chip surface grinding and the combination of roof tip and flat form of the cutting edge, i.e. the free surface, is particularly successful in the European area. If a scoring tool has to be dispensed with, the cutting edge geometry of the DH saw blade ensures low tear-out cutting edges on the exit side of the coated panel. Regrinding is a time-consuming process, as long tool life after sharpening can only be achieved again with perfect grinding of the circumference (back) and hollow tooth face. The roof tooth acts as a pre-cutter and supports the guidance of the tool in the cutting gap. With additional effort the cutting corners are sometimes provided with a protective chamfer (DHFA).

WWF and WAWAF

The group teeth alternate tooth left, alternate tooth right, partly a repeat of Wl and Wr, then a flat tooth, which stands back approx. 0.2 mm, as well as the design of the alternate tooth group with alternate axis angles are mainly used in the Asian region. These tooth form combinations are also used in our room for the separation of picture frame strips, which have to be ground at great expense. To have the alternating tooth tip pre-cut to produce a low tear edge and to have the flat tooth clear the kerf is a machining process frequently used in woodworking. If, however, the hardness of the material to be cut is such that the tips of the alternate teeth are at high risk of breaking out, the alternate tooth is also provided with a protective chamfer.

Alternate tooth with protective chamfer (WFA)

This chamfering process is similar to that of the flat tooth. GUHDO-Werk also uses the designation SCH2 for this. The alternating tooth with chamfer is used relatively frequently for cutting high-strength, thin-walled aluminium alloys with success. Compared to the trapezoidal flat tooth, it enters the sensitive aluminium profile with less cutting force.

Tooth form grinding with facet BTF

Other tooth forms and group teeth exist, but they are mainly found in special saw blades. Complex tooth geometries are realized in metal cutting. Protective chamfers on the chip face and counter chamfers on the cutting edge corner optimize the cutting and chip splitting. Here the so-called Braunschweig tooth is only one variant of the tooth geometries. Chip breaking grooves on the free surface and chip breaking grooves on the rake surface complete these geometry variants.
GUHDO-Werk uses for the Steel-CUT circular saw blades the tooth form flat bevel with an alternating bevel formation cutting width B/3 with 5° and for the BTF circular saw blades a bevel at free surface and rake surface for the stabilization of the cutting wedge of the TF tooth geometry at the steel sheet cutting .