A variety of tools and methods can be used to cut straight lines or gentle curves in sheet metal. This decision may be influenced by the amount of distortion that can be tolerated, as well as the location of the cut within the sheet.
Snips and Shears
Snaps and shears are both shears and act like scissors, cutting like scissors. Shears cut plastic cards, and they are called that because of the way they cut. Shearing forces do not involve any bending of the sheet and act parallel to its cross-section.
A shear or snip applies the same forces in opposite directions across a cross-section so that one side is pushed down and the other is pushed up simultaneously. As shear forces act parallel to the cross-section, they produce shear stresses, which when they exceed the shear strength of the material fracture (shear) across the cross-section.
Despite its cutting action, snips don’t remove metal from the sheet at the cut. During the cutting process, the sheet is compressed along the edges, leaving stresses on the edges and causing the edges to stretch.
Cutting around curves can result in an edge that looks chewed because of the complex forces involved in shearing and bending, which is not desirable on some projects. Although this can be dressed later, it will require additional work, and the final position of the edge may not match the planned one.
Straight blades and robust side cheeks are available with scissors and they can be used to cut both straight lines and gentle curves. Cutting with scissors is similar to using curved blades, but assists in cutting tighter curves.
Additionally, as the sheet parts around the jaws of these snips, the sides of the sheet will curl due to the scissor action that tears the sheet. Snips with substantial side cheeks are especially prone to this.
As the snips work their way further into the cheeks, the cut edges are widely separated. In short, the cheeks are shaped so that the cut edges are guided over the sheet, causing significant bends and distortions.
Maintain the jaw flat against the sheet during cutting and in contact with it to minimize the distortion. An edge-cutting operation from one side of a sheet will produce an offcut that is spiral coiled and is distorted from multiple angles. It is difficult to straighten the narrow strip after it has curved and twisted.
Compound Action Snips
Aviation snips, also known as compound action snips, create higher cutting forces with a system of links between the blade and the handle. These are easier to use than standard snips.
Standard or curved blades are available, as well as left- or right-handed snips. Small serrations in the jaws of aviation snips make it easier to shear the material, making lines more precise. On some work, the serrations left on the edges are not desirable because they match the pattern on the cut edges.
The Monodex Cutter
A monodex cutter reduces distortion by using a different action on thin sheets. The tool’s upper jaw supports the sheet, and squeezing the handles forces a blade through the sheet; this shears the sides of a thin strip away from the sheet.
Cut length is short, and the cutter nibbles along the length of the cut. By cutting a thin strip of metal, typically 2.7mm wide, the cutter reduces distortion by supporting the metal with its upper jaw. An alternative version of this tool is designed specifically for cutting corrugated sheet, in which the flat soleplate has been replaced by a curved sole designed to fit the corrugations. The tool is perfect for cutting roofing materials.
Powered Shears
Powerful shears can be used to cut narrow strips (usually approximately 5mm or 3*16in wide) from sheets. A moving blade shears two edges simultaneously with the upper soleplate of the shears, as with the Monodex cutter.
To advance the cut, simply push the cutter forward. The length of the cut is unlimited, which makes this tool ideal for cutting long materials. In the fast and smooth cutting motion, the blade moves upward and downward around 4,500 strokes per minute.
As the tool moves forward, the cut strip curls to one side of the tool thanks to the internal guide strip. Often, this isn’t the best solution because the curl twists downwards in a tight coil, marking the sheet as it twists.
A better solution would be to guide the strip further upwards before it coils, keeping it above the sheet’s surface. It may be necessary to pause the cuts at intervals during a long cut so that the coil can be snipped off.
As a result, the soleplate can simply rest on the sheet when starting the tool. Start by pushing the tool forward, then tilting it very slightly up or down until a ‘sweet spot’ appears and the tool glides smoothly and quickly forward.
This tool is particularly effective at following straight lines. The cutting of the blade is straight, so gentle curves are relatively easy to follow. Tight curves, however, are harder because the blade cuts straight.
By running the soleplate’s edge along a thin guide strip clamped to the work, accuracy and speed can be achieved. Due to the fact that the sole-plate is not closed at the front, when this tool cuts a strip from a sheet it does not terminate the cut at the end of the strip.
As a consequence, the tool cannot produce a finished end to the slot. The sheared strip will fall off the metal as the blade enters the hole by drilling a hole of 5mm diameter at the intended end of the cut.
As shears cannot begin a cut at an edge, a pilot hole large enough to accommodate the moving blade must be drilled before cutting within the boundaries of the sheet: for the shears shown, this is approximately 45mm.
If the shears are firmly guided and moved forward slowly at first, they will begin cutting on a curve, even if the cut should begin at a right angle to a straight edge.
Hacksaws
The right blade can be used to make straight cuts with a hacksaw. If you are cutting sheets 0.8mm and thicker, a blade with 32 teeth per inch (tpi) will be necessary, with approximately three teeth in contact with the work at all times.
32tpi is useful for sheets of roughly 0.5mm thickness, because the blade is typically used at a 45 degree angle to the thickness of the sheet. So the hacksaw blade will cut on a forward stroke, the teeth must be pointing forward, away from the operator.
Conventional hacksaws have a throat depth that ranges from 90 to 150 millimeters, depending on the angle between the blade and the work, which limits the amount of sheet metal they can cut.
Cutting sheets should be done using hacksaws, and the edges should be kept in place to prevent downward pressure from the cutting action. Place the sheet on a bench with a flat edge.
Alternatively, you can fold a workbench or a roller stand for support of the other edge. In order to minimize the risk of the sheet being dragged upwards as the hacksaw blade is pulled back, it may be helpful to sandwich the metal sheet between two other sheets of hardboard, MDF or plywood, clamped close to the line of the cut.
The Sheet Saw
Sheet saws, panel hacksaws, and Shetack saws are hybrid machines that combine conventional saws and hacksaws. Behind the hacksaw blade, a deep, rigid sheet acts as support, rather than a tubular frame, allowing truly unlimited cuts. Though they are rarely for sale, this type of saw is no longer produced.
Pad Saws
Pad saws contain a handle with a slot designed to hold a hacksaw blade (or a shortened blade, which works best), and they are capable of making straight cuts if they are handled carefully and firmly.
The blade will be under tension when cutting with the teeth facing the handle, reducing the likelihood of it breaking. The sheet is therefore lifted upwards by the ‘pull’ stroke, making clamps essential.
While cutting, you should keep the blade upright; tilting to the side will snap the blade. When using a powered model, make sure you keep the blade upright. Modern electric industrial saws are capable of cutting metal with fine-toothed blades, so they are better suited to this purpose than hacksaws or panel saws.
However, they require firm guidance in terms of throat depth and cut length. Small DIY saws with coarse blades are not recommended for cutting sheet metal because of their coarseness.
Bench Mounted Shears and Cutters
Bench-mounted shears use a scissor action, like hand-held shears, but the long lever and the sturdy blade make cutting relatively easy. This kind of shear suffers from the disadvantage that cut material is likely to distort as it slides along the sides of the shears behind the blade.
As with hand-held shears, the action of cutting will set up stresses on either side of
the cut, and the resulting edges will tend to distort. The Gabro bench-mounted cutters act in the same way as the Monodex, removing a thin channel of material by shearing on either side of
the blade.
Unlike other cutting machines, these machines come with their own tables and stands, so they can be used independently. The thickness of the blades varies with the capacity of the machines, removing widths of 3.3mm or 4.5mm from steel sheets with thicknesses up to 1.6mm or 3.2mm respectively. Current models have cut lengths of 70mm or 108mm per stroke.
With this design of cutter, a thin strip of material is removed from the sheet, avoiding distortion and allowing long cuts with repeated strokes of the blade, as the sheet can pass on either side of the blade. With this tool, the slot end is finished, unlike powered shears.
Sheet Metal Guillotine
An electric motor, a pneumatic pump, or a foot pedal can be used to operate the sheet metal guillotine, which has a long blade that shears an entire edge using the edge of the table as the backing. Cutting across a sheet keeps the cut edge straight and undistorted, despite the fact that this is an action of shearing with what is essentially a large pair of scissor blades.
Due to the length limitation of the blade, but the fact that guillotines are available in large sizes means that industrial users only have to choose the appropriate size of machine and the appropriate capacity to suit their needs.
On the table, the cutting edge of the table is aligned with the side guide, which is at right angles to the blade. Upon operation, the machine clamps down on the sheet in order to hold it in place, and the blade shears the sheet from side to side.
For repetition work, the machine can be fitted with a rear stop, which eliminates the need to measure and mark sheets individually, but simply aligns them with the side guides, feeds them through the machine until they reach the rear stop, and cuts the paper.
It is available in miniature versions which can be fitted to fly presses or short hydraulic presses. The cutting edge in a fly press is much shorter (for example, 100mm), but the action remains the same. As the press is operated, a blade is forced flush against the end of the table or short support and the material is sheared.
The Corner Notcher
In preparation for folding a sheet into a box side or other shape, one frequently requires cutting a square or rectangular notch from the corner. An angle notcher makes two simultaneous cuts, forming a corner cut-out in one operation, with two blades at right angles.
In a production situation, this tool is a valuable asset as it accomplishes precisely what is required in one operation.
Vertical Bandsaw
When cutting sheets with a vertical bandsaw, it is possible to do so accurately and conveniently. The blade of this type of saw is in the form of a continuous loop carrying teeth. It is somewhat similar to the blade on a jigsaw, but much deeper in depth from the front to the back.
The blade slides through a slot on a horizontal table, cutting downward. Cut edges pass either side of the sheet as it is fed horizontally into the blade. This means that sheets of any length can be cut without binding or obstruction.
By using the guide arm, a straight cut can be created a defined distance from the edge of a sheet.
When cutting large radius curves, with care, even though the blades are generally relatively deep. The depth of the throat of the saw is a limiting factor (the distance from the side of the blade to the frame of the saw at the first point of contact).
Metal-cutting bandsaws move the blade much slower than wood-cutting bandsaws, which are commonly used for cutting wood.
Steel is typically cut at speeds of 100 meters per minute (m/min) (300 feet per minute) for low-carbon steel sheet, 80 meters per minute (250 feet per minute) for brass, and 130 meters per minute (400 feet per minute) for aluminum.
Harder materials require slower speeds, and a typical multi speed bandsaw can cut metal at speeds of up to 250m/min and wood at speeds of up to 400m/min. Because wood and metal have different tooth designs, a metal-cutting blade is needed.
Metal is cut very quickly by wood-cutting blades. Metal-cutting vertical band saws should be selected based on the following factors:
- There is a high level of quality in the blade guides above and below the table. Blade wobble results in wavy-edged cuts. Good guides tend to be costly, but they’re well worth it.
- It refers to how big the gap is between the blade and the table where it passes through. Metal sheet, especially thin metal sheet, requires support as close as possible to the edge of the blade.
- Many inserts are available to fit the table’s hole, and can be cut so that the slot clearance at the sides of the blade is narrow, which makes it easier for the sheet to be supported at the point of cutting, and results in a crisper cut.
- To help prevent thin sheets from becoming wedged under the guide, a cutting guide arm clamps firmly to the table.
Bandsaws can cause serious injuries when misused. There is a high risk of blade contact due to the considerable length of the exposed blade and constant feeding of materials into it.
The bandsaw is a common tool in abattoirs and industrial butchers, and is particularly efficient at cutting bones. Always adjust the top blade guides to the highest point on the sheet, giving your face as little exposure to the blade as possible – and always maintain a healthy sense of respect for the machine.
Circular Saw Blades
Sheet metal can be cut using specially designed circular saw blades, whether in a saw table or a hand-held circular saw. TCT blades have a specially designed tooth shape (the “ATP” profile) but are usually tungsten carbide tipped (TCT).
Aluminum 3mm can be cut easily, but there are other metals that can also be cut. You should remember that cutting a thin sheet with a table saw can be dangerous, since the sheet is likely to slip under the side guide arm of the saw and become trapped. It is not recommended to cut metal using this method because any reaction from the saw will throw the sheet back at you.
Nibblers
A powered nibbler uses a shearing action, but instead of removing a thin strip, it uses a punch to remove a small circle. By punching rapidly (typically 1,300 to 2,200 strokes per minute) a channel is nibbled from the sheet.
Using a smaller punch (typically 2mm) on thinner material (up to 1.6mm) will allow curves up to 50mm wide to be cut, whereas a larger punch (typically 4mm) will allow curves up to 3.2mm wide, but must be limited to a minimum radius of approximately 128mm.
The nibbler leaves a jagged edge, but the high speed of punching means that the serrations are very small. Use caution when handling them. As the tool is pushed forward rapidly, the serrations grow larger, at the same time the serrations become smaller as it moves forward slowly, which makes the edge look increasingly smooth, but it is still sharp.
There is waste produced by nibblers in the form of many tiny crescent-shaped pieces of paper. These are sharp as well, and should be swept up and not handled.
In contrast to the powered shears, the nibbler is much more capable of following a curve. Its ability to maintain a straight edge depends on the skill and steady hand of the operator, and it is much easier to run the edge of the cutting column along a guide to produce a straight edge.
Because the cutting action involves a punch passing through the material, the powered nibbler produces a finished end to any slot it cuts, although that end will be a semi-circular shape because the punch used is circular.
A nibbler is held in a purpose-made stand to allow accurate and speedy cutting of circles. The base which supports the sheet being cut is set level with the upper face of the anvil in the nibbler (located in the lower part of the column).
Although this particular adaptation is used for cutting short curves, the length of the cut is unlimited, and the addition of a simple straight guide bar at the side would allow straight cuts to be made with ease. A larger base would provide support for larger sheets.
When cutting a closed shape like a circle within a sheet, powered nibblers are unable to start a cut far enough away from the edge of the sheet; therefore, a pilot hole must be cut first, and the nibbler lowered into the hole before it can begin cutting. It is recommended to make the pilot hole at least twice as large as the minimum radius required by the nibbler.
Electric Jigsaw
An electric jigsaw can cut metal sheets varying in thickness from approximately 1 mm to over 6mm. In addition to cutting tighter radii, smaller circles, and more complex shapes, jigsaw blades are narrower (from teeth to back of blade) than hacksaw blades.
Because the blade is suspended unobstructed below the flat foot of the jigsaw, they can also make long uninterrupted cuts straight or curved. As a result of the cutting pressure, the foot prevents the sheet from rising up as the blades cut.
To prevent teeth from catching and lifting the jigsaw off the sheet, it is important to press firmly downwards while cutting. A glue foot should be used for cutting brass and other metals with a finish on their top faces, to prevent scraping of the metal.
The edge of a jigsaw foot can be slid against a straight-edge guide clamped to a piece of metal in order to achieve straight cuts, despite jigsaw blades having a tendency to wander.
The same guidelines apply to jigsaws and hacksaws regarding tooth pitch. For most sheets, that means a fine blade, with three teeth in contact with the thickness of the sheet. The hardness of wood-cutting blades is not sufficient for metal-cutting ones, so metal-cutting grades should be used instead.
For soft metals, such as aluminum and brass, good quality wood-cutting blades can be used, but the blade will not last as long. Aluminum also benefits from a coarser blade since fine teeth tend to get clogged.
While the coarseness of the blade is important, it has to be balanced against ease of cutting, the tendency for a coarse blade to snag and force the jigsaw upward, and the quality of the finished edges. It is imperative to use sharp blades whenever cutting brass.
Handled carefully, the jeweler’s piercing saw can make pretty much any shape. 6/0 blades are the finest, and No. 8 blades are the coarsest, and should be fitted to the frame with the teeth facing down, towards the handle. You should apply considerable pressure to the blades. The teeth are cut by pulling the saw gently downward with just enough forward pressure.
As you move the saw up and down to turn a corner, pause on the spot and gently turn the blade, letting it turn on its own. The best blades have rounded backs, which make them easier to turn. When the blade loses its sharpness, or breaks, be prepared to replace it.
The tool works best when it is used with practiced touch. Jewelers who often cut sheet are required to have it supported close to where the cut will take place. A bench peg, also known as a bench pin, is attached to the edge of the bench and allows the sheet and saw to be moved quickly to follow a curve while providing support.
The Fretsaw
In many ways, the fretsaw is similar to a piercing saw, though it has a more robust blade. On some saws, you can change the stroke rate as well as power up and down.
When it comes to metal, the stroke rate has to be slower than when it comes to wood or some plastics. The fretsaw is capable of cutting intricate shapes because it is fitted with metal-cutting blades. Both the saw and the sheet are turned while cutting with a hand-held piercing saw. Fretsaws are used to cut with a stationary saw and blade, while the sheet is turned to follow the cut line.
In metal cutting, fretsaw blades are available in grades 0000 (70tpi) through 9 (25tpi). The blade tends to cut to one side when forced to cut at a rapid pace; therefore, cutting should be done at a pace dictated by the blade itself, as with hand saws.
Cutting speeds (i.e., the stroke rate) should be slow, blades should be fitted with the teeth pointing downward, and stroke length should be shortened, especially for thin materials, to lower the rate at which blades break. You can tape sheets to plywood or MDF backing boards or you can sandwich them between two sheets of plywood or MDF.
Despite the downward direction of the blade on the fretsaw, the blade tends to lift the sheet off the table on the back stroke. The only way to resist this is to press down on the saw blade with the fingers.
Strong downward pressure will tend to make it difficult to maintain forward movement and fine control of the direction of cut, so there is a compromise here, best worked out with practice. Take care when your fingers are near the blade, and keep the plasters within reach.
DEBURRING EDGES
The edges of sheets tend to be sharp, and cut edges may be sharp, with needle-like slivers in places, so it is important to deburr edges to prevent injury. This also allows the edges to mate properly with adjacent parts, or to sit flat on other flat sheet parts. Keeping the cutting edges on your tools sharp is the first step.
Basic deburring tools include files, carbide wheel tools, belt sanders, and -purpose-designed sheet deburring machines.
Files
The files are perfect for deburring short sheet lengths. Make sure you file across the edge at an angle of 45 degrees. Afterward, make several strokes along the sharp edge between the flat side of the sheet and the narrow edge, slanting the file 45 degrees to the flat side as well as 45 degrees to the vertical.
Generally, a finer file should be used on thinner sheets. The best hand files for filing a flat edge are engineering hand files, which have a long, flat, rectangular blade.
A half-round file (flat on one side, semicircular on the other) works well for curved edges. Engineer’s files range in coarseness from coarse to medium to bastard, second cut, smooth, and dead smooth.
It may be necessary to use a finer Swiss pattern file if you are working with brass sheet. This is a hand file that has a range of finer cuts ranging from 00 (coarsest) through 0, 1, 2, 4, 6, and 8 (smoother) then 0/1, 0/2, 0/4, 0/6 and 0/8 (extremely fine).
It should be noted that a 0 cut will leave a fairly coarse finish, while an 8/0 will leave a very fine finish indeed. There is no doubt that coarser files are more suitable for quick deburring, but following this up with smaller and smaller files leaves a fine finish.
To deburr curved cuts or holes of varying sizes, half-round files and files with circular cross-sections are appropriate. Edges being filed should either be placed face down on a flat bench top or clamped between two bars placed close to the edge to prevent distortion and noise.
A vice can be used to grip small pieces of sheet close to the cut edge, thus providing support.
Carbide Wheel Tools
Two hardened wheels attached to a handle with a hand guard prevent injury when using specially designed deburring tools. It is simply a matter of dragging the tool along the cutting edge of the sheet as it passes between the wheels, which lie at right angles to the sheet.
Using a swivelling deburring tool with a hardened blade, deburr holes or curved cuts one side at a time. A thin sliver of metal is cut when the blade is drawn along the sharp edge, effectively bevelling the edge. Deburr both sides of the cut with this procedure, so both sides of the cut are deburred.
Belt Sander
A power file performs the same task as a file, but more quickly. If the belts remain stationary too long, they may remove too much metal. Be sure to keep the belts moving.
To deburr edges and corners quickly, an abrasive belt tool can be used. A belt that uses aluminum oxide as an abrasive is suitable for metal-working tools.
Deburring does not mean that the sheet is being ground to shape. File should be held in such a way that the belt does not go towards the edge of the sheet. By doing so, you reduce the chances of being snatched, caught, or dug in.