Guillotine Shears or Machine
These are foot operated or power driven. These are used where large sheets are required to be cut with straight edges into strips and rectangular blanks. Sheet width of 600mm x 2mm thick and up to 1200mm x 1.6mm thick can be accommodated on treadle operated guillotine. Guillotines have a moving top blade, which is operated by a foot treadle and a spring, which returns the blade to the top of its stroke. The table is provided with guide to maintain the squareness of work piece. An adjustable stop is provided to cut number of pieces of same size. When the treadle is operated, a clamp descends to hold the work in position while cutting takes place and this also act as a guard to prevent injury.
These are hand operated shears fitted on the bench which can cut more quickly than hand snips. These are used for cutting large sheets of thick gauges. Mild steel plates up to 3/16″ thickness can be cut in it. These shears vary in shape and size. The blades are ground at 87º. The movable blade is fixed with a handle in order to get more leverage while cutting. If the blades are worn out, these should be ground to correct angle. These are classified by the length of the cutting blades.
Various types of hand operated shears are as follows:
(a) Hand Lever Bench Shears. These are used for straight line cutting of sheet metals up to 3.0mm thick.
(b) Hand Operated Throatless Bench Shears. These may be used for either convex or concave cutting by suitable manipulation of the sheet up to thickness of 4.7mm.
(c) Hand Lever Corrugated Bench Shears. These are used for straight shearing of corrugated material up to the thickness of 1.6 mm.
All shearing machine in general should have the following:
(a) The rigidity or robustness of the cutting member mountings is essential so that defection of the cutting members is kept to a minimum during the cutting action.
(b) A satisfactory means of adjustment should be provided between the cutting members. This enables the cutting clearance to be adjusted and the position of the cutting members to be maintained with regard to the feed of the material.
Nibblers or Nibbling Machines
Nibbling means constant cutting. Nibblers are portable power shearing machines. These are either electrically or pneumatically operated. These may be used for cutting straight line or irregular curves. Pneumatic machines are much safer for working on sites as the power source being an air compressor. One main advantage of nibbling over shearing is that there is less distortion of the work.
Two types of portable nibblers are:
(a) Shear Type Nibbler
(b) Punch Type Nibbler
Shear Type Nibbler
It is used for accurate straight line or curved cutting of material up to 4.5mm thickness. It is a short- stroke power shear fitted with a rapidly reciprocating cutting blade. Each stroke makes a cut of approximately 3mm in length. The speed of the cutting blade is between 1200 and 1400 strokes per minute. The machine is fitted with a pair of very narrow flat blades. The top blade is fixed to the moving member or ram, and the bottom blade on a spiral extension on U- frame. Top bladeusually moves to and fro the bottom blade (fixed) at fairly high speed. The lighter machines have a minimum cutting radius of 16 mm and the heavier ones about 50 mm. There is a provision for vertical adjustment to allow for re- sharpening of the blade by grinding, and an adjustment behind the bottom blade to allow for setting the cutting clearance.
Punch Type Nibbler
In this machine a punch and die is employed instead of shearing blades. The advantage of this machine is that it can perform certain operations which cannot be accomplished on other shearing machines. For example it can be used to cut out apertures, which could otherwise only be produced by means of specially designed punches and dies set up in a powerful press required.
This type of nibbler is generally used for rapid and accurate straight line or curved cutting of material from 1.62 mm to 3.2 mm thickness approximately. Heavy- duty machines are also available for cutting steel up to 6.35 mm thickness.
Punch type nibblers are available in various sizes. The top cutting tool (a punch) reciprocates at fast short strokes at the rate of 350 to 1400 strokes per minutes over a diameter. It nibbles out the material by the simple principle of overlap punching. To produce a smooth clean edge only a slight finishing is necessary. Standard punches of 4.8, 6.35, 9.5 mm diameter are employed with different sizes of machines, and the maximum linear cutting speed is approximately 1.8 mtr / minute.
Portable nibbling machine are now available with rectangular punches which produce a cleaner cut compared to circular punches.
Circular Cutting Machine
This machine operates on the rotary cutter principle, i.e. cutters rotate and produce a continuous cutting action with very little distortion of the material. These machines are used for cutting circles from sheet metal and circular hole in sheet metal. These are both hand operated and power driven. These are available in various sizes. Small machines cut circles from 2½″ to14″ diameter up to 18SWG sheet metal whereas large machines cut from 4″ to 40″ diameter up to 16 SWG.
A good practice method of adjusting the cutters is to cut a true circle in paper. If a machine does it in a satisfactory manner, then it will shear sheet metal without leaving burrs on the edge.
Power Cutting Machine
In workshops, power saw are used for larger amount of cutting work. The machinery may be of a light duty with a capacity of about 150mm x 150mm or a heavy duty cutting material up to 300mm x 300mm. There are two main types of power saws:
(a) The Reciprocating Power Saw (Power Saw)
(b) Do All Cut Machine (Band Saw)
The Reciprocating Saw has a blade and action similar to that of a hacksaw. The work, usually heavy bar is clamped in a vice with the feed being controlled by dash pot which reduces vibrations of the saw.
Do All Cut Machine
The light duty band saw has a continuous flexible blade driven vertically past the work which is moved by hand on a horizontal table. This saw is suitable for cutting wood, plastics and thin sheets, usually of soft materials.
Safety Precautions for Do All Cut Machine
The following safety precautions are to be observed:
(a) Blade should be suitable for material to be cut.
(b) The blade should be properly tightened and the teeth are pointing downwards.
(c) Use the guide for straight cutting.
(d) Do not change the gear when machine is on.
(e) Adjust the airflow towards the cutting points.
(f) Do not overfeed otherwise the blade will break.
(g) Feed only when the machine is in full operation.
(h) Switch off in case the blade is gripped in the cut.
(i) Do not twist the job when machine is in operation.
(j) Only authorized operator to operate ‘Do All Cut Machine’.
Difference between Cutting and Shearing
Both cutting and shearing operation are used to separate out sheet metal into different pieces. Yet there are a few differences between cutting and shearing.
Precautions for Power Shearing
Following are the precautions to be observed while working on Power shearing machine:
(a) The machine should be faultless and have safety devices.
(b) There should be clearance between cutting members.
(c) The cutting tool must be sharp.
(d) Machine should be rigid and robust.
(e) There should be good arrangement for holding and setting the heavy sheets and plates.
(f) Machine should be well lubricated.
(g) The stroke should be set according to the requirement.
Safety Precautions in Sheet Metal Work
Accidents in sheet metal shop are due to negligence when handling sheets the cuts are observed on fingers. Following safety precautions are suggested to avoid injury:
(a) Use hand gloves before handling a sheet metal.
(b) Do not try to lift the cut portion of the sheet at the time of cutting or shearing.
(c) Do not try to test the quality of cut edge with naked hand.
(d) Place the sheets in such manner that there is no danger of their fall.
(e) Do not try to bend or perform other operations on the sheet without tools.