The sawmill industry mainly uses carbide- or stellite-tipped circular saw blades.
Circular blades are used for splitting and cutting.
© Metsä Group/Studio Tomi Aho
Materials and properties of circular blades
The sawmill industry mainly uses
- carbide- or
- stellite-tipped circular saw blades.
The blade is made of a special hot- or coldrolled steel that is doped with alloying elements that improve toughness, strength and hardness. A hot-rolled steel frame is hardened and tempered strained in a thermal treatment that will ensure the permanence of these properties.
The thickness of the blade material depends on the thickness of the manufactured circular blades. The blade preform is cut from a plate of steel using laser or water jet cutting technology.
Teräaineen paksuus riippuu valmistettavien pyöröterien paksuudesta. Teräaihio leikataan teräslevystä käyttäen laserleikkausta tai vesileikkuutta.
Elements that affect the operation of the circular saw blade:
- Material of the body
- Correct and even hardness throughout the blade
- Permanence of tension
- Shape and number of teeth
- Sufficiency of the gullet
- Rotation speed, cutting height and feeding speed
- Carbide tips/stellite:
-
Hardness/toughness
- Size of the tip
- Positioning of the tip
-
Properties of circular blades
Main properties are:
- The circular saws used at sawmills are 250–1,200mm in diameter. Most commonly 400 – 600 mm.
- At its thinnest, the blade can be 1.5mm but, in general, they are 2.5–4.0mm thick.
- Cutting speeds are around 75–90m/s.
- The blade’s cutting speed is the same regardless of blade diameter. Blades are tensioned according to the cutting speed.
- The feeding speed is dependent on the blade’s rotation speed and the number of teeth on the blade, 30 – 220 m/min.
There are three important factors to identify concerning the blade tooth shape:
- The number of its teeth which depends on the blade diameter as well as on teeth pitch and feed length per tooth. The cutting edge of the tooth can be straight or grinded into a particular shape.
- The most important factors in the gullet structure are the gullet rounding radius and the angle of the back which prevent sawdust from compacting. This can also be prevented by means of differently shaped steps on the tooth.
- The purpose of the gullet is to collect sawdust cut by the blade edge and remove it from the kerf. The important thing about the gullet shape is that it must be empty of sawdust when it leaves the kerf. The sharpness of the gullet edges prevents the sawdust from getting in between the blade and the wood.
Shape of the teeth
© Antti Saikkonen
There are three important factors to identify concerning the blade tooth shape:
- clearance angle (symbol α)
- cutting angle or wedge angle (symbol β)
- rake angle (symbol γ)
When cutting and edging is done at a high speed, a rake angle of 20–35 degrees is used. The cutting angle of a circular blade depends on the dimensions of the timber.
One important factor in a blade is the number of its teeth which depends on the blade diameter as well as on teeth pitch and
In rip sawing, the feed length should be 0.8–1.3mm/tooth. The number of teeth in ripping blades is usually 20–70. In crosscut blades with a bigger diameter, the number of teeth can be over a hundred.
The cutting edge of the tooth can be straight or grinded into a particular shape. Straight teeth are suitable for multi-blade sawing machines and edgers.
Circular blades also have wiper slots that are on the surface of the blade body and help the blade stay clean and remove sawdust from the kerf. The blade body may also include sawing holes.
There are also various replacement teeth blades and a patented spring set teeth version.
The shape and number of teeth determine efficiency and sawing quality. The shape of the teeth also determines the shape and size of the gullet in front of the teeth. Circular saw parts: A: Tooth back with step, B: curved back, C: E-cooling hole / closed wiper slot, D: copper rivet, E: small gullets, F: adjustable scribing blade, G: grouped teeth, K: cooling hole, R: wiper slot, open and closed, S: guard teeth, T: special slot, U: open wiper slot.
Choosing circular blades
Circular saw blades are made to suit a particular sawing machine and sawing process © Heinolan Sahakoneet Oy
Choosing a circular blade depends on:
- The machinery supplier and the product being made
- Durability and the need for maintenance
- Challenges in sawing
- the speed of the sawing line
- resin
- blade overheating
- winter conditions (frozen raw material)
- breakage of the blades, which means that blades have to be replaced in the middle of a shift.
The most important technical characteristics are the quality of the blade body, carbide or stellite soldering joints, flange diameter, number of teeth, rake angle, side and back clearance and sawing clearance. Moreover, details related to fastening and positioning, such as the condition of the flanges, are important. Incorrectly sized blades will soon become damaged and dull.
If a blade has too high of a teeth pitch
- a feeding rate per tooth that is too low
- the blade does not cut but instead scrapes the wood causing the cutting edge of the blade to wear dramatically.
- Sawdust will be too fine and it will be difficult to get it out of the kerf.
- Fine sawdust will try to get between the blade and the wood causing friction and heating of the blade.
When the feeding rate per tooth is too high:
- the gullet will get clogged
- the blade will overheat
- the sawing result will be poor
Maintenance of circular blades
Circular saws require constant maintenance. Blades must be serviced regularly but they must also be serviced or replaced if they get damaged unexpectedly.
© Koskisen Oy
Maintenance activities:
- Blades to be serviced are washed to remove any resin, sawdust and other impurities.
- The blade body condition and tension are checked and adjusted if necessary.
- Sharpening and side grinding of circular blades is done using automatic CNC controlled sharpening machines.
- If carbide or stellite tips have come off, the blade can be fitted with new tips by soldering them onto the body.
- The tip area must be grinded before soldering the new tip on to ensure that the tip will be secure.
- The blade body can also be repaired unless the damage is too severe.
Some sawmills maintain their blades in a separate blade room where the blades are sharpened and some tips can also be replaced.
Often sawmills outsource their blade maintenance to an outside service provider so that the sawmills send the blades to the service provider or the service provider picks up the blades for regular maintenance.
