Sawing is a machining process primarily used to separate material into required lengths or shapes. Most sawing processes use a blade containing a series of cutting teeth, although abrasive cut-off machines use a bonded abrasive wheel rather than a toothed blade.
Metal sawing is widely used for preparing bar, tube, structural sections, plate and other stock before subsequent manufacturing operations. The choice of sawing process depends on the material, section geometry, required accuracy, production quantity and the quality of the cut required.
Metal sawing machines take several forms. Four important types are:
| Process | Cutting principle | Typical use |
|---|---|---|
| Power hacksaw | A straight toothed blade moves with a reciprocating cutting action. | General stock cutting using a simple and robust machine. |
| Cold / circular saw | A rotating toothed circular blade removes chips from the workpiece. | Accurate cutting of bar, tube, sections and other stock. |
| Bandsaw | A continuous flexible toothed band travels around two or more wheels. | Stock cut-off and, on suitable vertical machines, contour cutting. |
| Abrasive cut-off saw | A high-speed abrasive wheel removes material by abrasive cutting. | Cutting materials and sections suited to abrasive rather than toothed sawing. |
A power hacksaw uses a straight saw blade mounted in a rigid bow or frame. The blade moves backwards and forwards across the workpiece with a reciprocating motion.
Material is normally removed during one direction of the stroke. During the return stroke the blade load is reduced or the blade is lifted slightly from the workpiece to reduce unnecessary rubbing and tooth wear.
Cutting force may be produced by the weight of the saw frame or by a controlled mechanical or hydraulic feed system, depending on the machine. The workpiece is normally held in a vice while the saw frame moves through the section.
Power hacksaws are mechanically simple and robust, but the reciprocating action includes a non-cutting return stroke. Continuous-band sawing and other processes have therefore replaced power hacksaws for many production applications.
A cold saw uses a rotating toothed circular blade to remove chips from the workpiece. The term distinguishes the process from abrasive cut-off sawing: a cold saw uses defined cutting teeth rather than an abrasive disc.
Cold saws are used for cutting bar, tube, structural sections and other metal stock. Machines range from manually operated workshop machines to automatic production systems incorporating powered material feed, clamping and length control.
Circular metal-saw blades may use high-speed-steel cutting edges or carbide-tipped teeth depending on the machine and application. Blade diameter, tooth form, tooth pitch and cutting conditions are selected for the material and section being cut.
A bandsaw uses a continuous flexible blade which travels around wheels within the machine. Because the blade moves continuously through the workpiece, there is no non-cutting return stroke.
Metal-cutting bandsaws are commonly divided into horizontal cut-off machines and vertical machines.
Horizontal bandsaws are widely used for cutting bar, tube, structural sections and bundles of stock to length. The workpiece is normally clamped while the saw head or blade assembly feeds through the material.
Machines may be manually controlled, semi-automatic or fully automatic. Production machines can incorporate stock feeding, automatic clamping, length measurement and repeated cutting cycles.
Vertical bandsaws pass the blade vertically through a work table. Depending on the machine and blade, they can be used for straight, angular and contoured cuts.
Contour cutting requires the blade width and other blade characteristics to be compatible with the required curvature. A blade which is too wide for a tight radius cannot follow the required path without excessive twisting or interference.
Tooth pitch is the spacing of the saw teeth and is commonly specified either directly as a distance or as the number of teeth per unit length. The appropriate pitch depends strongly on the geometry and thickness of the section being cut.
A coarse pitch provides larger spaces between teeth for chip formation and removal. A finer pitch provides more teeth in contact with thin sections and can reduce the tendency for individual teeth to snag or overload.
Some bandsaw blades use a repeating sequence of different tooth spacings rather than a single constant pitch. Variable-pitch blades can help distribute cutting forces and reduce repetitive excitation associated with a uniform tooth spacing.
The appropriate blade should be selected using current blade-manufacturer guidance for the workpiece material, section and machine.
Saw performance is influenced not only by tooth pitch but also by the geometry and arrangement of the teeth.
The tooth form determines the geometry presented to the workpiece and influences chip formation and cutting forces.
On many saw blades the teeth are set alternately or in groups so that the cut produced is wider than the body of the blade. This provides clearance for the blade to pass through the workpiece.
The kerf is the width of material removed by the saw during cutting. It is normally greater than the thickness of the main blade body where tooth set or tooth geometry projects beyond the blade.
Kerf is important when planning material usage and cut position because the material within the kerf becomes waste.
Bandsaw cutting speed is the linear speed at which the blade teeth move through the workpiece. Appropriate band speed depends on factors including:
Harder or more difficult-to-machine materials will often require different cutting conditions from free-machining materials, but simple rules such as "harder material always means a particular lower speed" are not sufficient for selecting production cutting data.
The blade must be fed into the workpiece with sufficient force to allow the teeth to form and remove chips without overloading the blade or machine.
Insufficient feed can allow the teeth to rub rather than cut effectively, while excessive feed can overload teeth, cause inaccurate cutting and reduce blade life.
Feed requirements also change as the effective section presented to the blade changes. This is particularly relevant when cutting hollow, structural or irregular sections.
Hollow sections require particular attention to blade selection because the number of teeth engaged changes as the blade passes through the section.
A thin-wall tube may initially present only a small thickness of material to the cutting teeth. A blade pitch that is satisfactory for a solid bar of the same outside diameter may therefore be unsuitable for the tube.
The orientation of angle, channel, beam and other structural sections can also influence tooth engagement, cutting forces and cutting time. Workpiece orientation should therefore be considered as part of the sawing process rather than simply choosing whichever orientation is easiest to clamp.
The workpiece must be securely supported and clamped during sawing. Movement during the cut can damage the blade, produce an inaccurate cut and create a safety hazard.
Long bar, tube and structural sections should be supported appropriately on both the machine and external supports where required. The cut-off section should also be considered so that it cannot move unexpectedly, trap the blade or fall when the cut is completed.
Abrasive cut-off machines use a rotating bonded abrasive wheel rather than a toothed saw blade. Material is removed by abrasive cutting as the wheel passes through the workpiece.
Abrasive cutting can be useful for materials and sections which are suitable for rapid abrasive separation. Machines range from manually fed cut-off saws to automated production equipment.
The process produces different cutting conditions from a cold saw or bandsaw. Heat generation, sparks, abrasive-wheel wear, dust and the condition of the cut surface must all be considered.
| Process | Characteristics | Typical considerations |
|---|---|---|
| Power hacksaw | Reciprocating straight blade with simple machine construction. | Robust traditional process, but includes a non-cutting return stroke and is now uncommon in higher-productivity production. |
| Cold saw | Rotating toothed circular blade. | Can provide controlled, accurate cut-off of suitable metal stock with good repeatability. |
| Bandsaw | Continuous flexible toothed blade with relatively narrow kerf. | Versatile for bar, tube and structural stock; machine and blade selection can cover a wide range of sections. |
| Abrasive cut-off | High-speed bonded abrasive wheel. | Useful for suitable hard materials and rapid cut-off, but heat, sparks, wheel wear and surface condition require consideration. |
Depending on the sawing process, cutting fluid may provide cooling, lubrication and assistance with chip removal. The required fluid and delivery method depend on the blade, workpiece material, machine and cutting operation.
Some saws operate dry while others are designed around flood coolant, minimum-quantity lubrication or another controlled fluid system.
See also: Cutting Fluids .
A satisfactory saw cut depends on the machine, blade, workholding and cutting conditions. Problems observed during sawing can include:
Possible causes include incorrect blade selection, unsuitable tooth pitch, incorrect feed or blade speed, inadequate blade tension or guidance, poor workholding, worn machine components and ineffective cutting-fluid application.
Saw-cut surfaces are often not the final functional surfaces of a component. Where a cut face will subsequently be turned, milled, ground or otherwise finished, sufficient machining allowance should remain to remove saw-cut irregularity and establish the required datum and finished dimension.
The required allowance depends on the sawing process, cut quality, component size and the requirements of the subsequent manufacturing operation rather than on a single universal value.
Powered saws contain exposed or potentially accessible cutting elements, moving mechanisms and workpieces which may produce sharp chips, swarf, sparks or cut-off sections. Suitable guarding, workholding and operating procedures are therefore essential.
Machines should be operated in accordance with the manufacturer's instructions and applicable workplace safety requirements. Guards and other protective devices must not be defeated to improve access or production rate.
Before clearing a jam, changing a blade or performing maintenance, the machine should be made safe and isolated as required by the machine instructions and workplace procedure.
Selection of a sawing process should consider:
Production sawing machines can incorporate powered stock feeding, automatic clamping, programmable cut length and repeated cutting cycles. These features allow sawing to form part of an automated material preparation system rather than being treated as an isolated manual operation.
For high-volume work, material handling and the time required to load, position, clamp and unload stock can be as important as the actual time taken for the blade to pass through the material.
The following resources provide further information relating to sawing machines and safe machine use.