How to Setup Metal Working Business, Part 2 Finishing

loading...

Cutting and Machining

Cutting can be done with a hammer and cold chisel, which is hard work but often preferable to saws that wear too quickly.

Another common method is flame cutting if welding equipment is available.

Tinsnips are used for light work on thin material (up to about 1 mm steel or 1.5 mm on softer metals). They are cheap and versatile, but slow, and tend to buckle the edges of the cut metal.

Various patterns of snips are available, some intended for cutting deeply into large sheets and others made more maneuverable within the cut. Some have straight blades, and others, intended particularly for cutting holes and curved edges, have curved blades.

Hand lever shears bolt to a bench or a stand, and are of greater capacity than tin snips (e.g. 3 mm steel). Their use is generally, restricted to straight cuts, although it is possible to trim the edges of a sheet to a convex curve by successively cutting thin strips from the corners. They can be used to cut to a point on a sheet, rather than being restricted to cutting right across, and so can be used, for example, to cut out squares from the corners of a sheet in preparation for folding into a box. Having the shearing action of a pair of scissors, they tend to distort the cut edges. However, this difficulty is overcome in one type of machine, in which the sheet lies flat on the table, and a narrow strip is sheared out along the line of the cut.

a] Guillotines are arranged to cut across the entire width of a sheet, and can be treadle operated (typically with a capacity of 1 m width and 1.5 mm thickness, or power operated. These machines are quick and accurate in use, and leave clean-cut edges practically free of distortion. They are used universally for the preparation of straight-sided shapes, except where cuts into, but not right across, the sheet are needed.

b] Holepunch

c] Drilling

d] Lathes and Milling Machines

Machines such as lathes and milling machines are an expensive investment for a small
metalworking enterprise and so are not discussed in this technical brief. They can, however, vastly improve the quality and versatility items that can be made providing the ability to make screws and gears.

Jointing – The jointing processes used on sheet metal include:

  • Folding and seaming
  • Riveting
  • Soldering
  • Brazing
  • Welding

Folding and Seaming. The edges are folded over each other, is used particularly by tinsmiths, and in the volume production of thin sheet metal parts. A jenny or folding machine is used to make the folds, and then with either a seaming machine or a hammer the folds are closed together to make the seam. The seam can be sealed if necessary with soft solder, or by hot dip galvanizing the finished product.

Rivets are usually set by hand using a hammer and appropriate sets. The technique is low in capital but takes time and skill. However, pop-riveting, in which a hand held tool is used to set a hollow rivet, much quicker, as it requires access from only one side of the job. The cost of the pop-rivets, which are made to suit the type of setting tool (“gun”) are expensive. Except for the jointing of aluminum and pre-coated steel, riveting has mainly given way to welding.

Soldering is used largely in tinplate and copper and brass, although it can be used on galvanized and bare steel. It is not usually used for aluminum, as powerful fluxes are necessary. The process consists of running molten solder into the joint, using either a flame or a heated iron (made of copper) to heat the joint while the solder is applied. Cleaning and the use of flux are necessary to enable the solder to take. Fluxes can be proprietary, or zinc chloride can be used. This is made by dissolving zinc in hydrochloric acid (spirits of salts) until the action ceases. A little extra acid added after bottling improves the cleaning power of the flux.

Brazing is a process similar to soldering, in principle, (and is often called hard soldering). Instead of using soft solder (melting point 200°C), brass is used (melting point 900°C). Flame heating is almost always used, although the use of a forge is possible. A gas-air torch or a powerful paraffin blowlamp is convenient sources of heat, as is oxyacetylene equipment if it is available. As flux, proprietary preparations and borax are equally effective. A brazed joint is often as strong as the surrounding metal, and in copper work especially it is an excellent and neat joint.

Welding is the most generally used jointing process. It consists of melting together the joint edges and allowing the result to solidify, with or without additional (filler) metal.

Oxyacetylene equipment is the most versatile, as it can be used for welding, brazing, cutting and heating. Steel down to about 0.5 mm can be welded and gaps filled easily, but distortion tends to be a difficulty because of the somewhat diffuse heat. The main expenses are in the rental of the cylinder and the supply of the gases. The need for a nearby supply network limits the usefulness of the oxyacetylene process to industrialized areas.

Metallic arc welding (“stick welding”) is economical and versatile. A transformer type welder of 180 amps, will weld steel of thickness between 1.5 and 10 mm relatively easily, and with care it is possible to weld steel outside this range of thickness. With special electrodes other ferrous metals can be welded with this equipment. A welding generator with D.C. output can weld all metals in common use, but the equipment cost is much higher. Consumable electrodes are the main material expense.

With a carbon arc torch, an arc welder can be used on jobs normally tackled with oxyacetylene equipment, but the stability of the arc flame tends to be poor, making it difficult to weld thin material. However, for brazing, it is quite adequate.

Finishing

Any product will look much more attractive if a suitable finish has been added and it will be easier to sell. There are a number of finishes that can be applied to metal products. In most cases, the finishes will help protect the product from corrosion. The simplest is to paint them. Other finishes are:

  • Galvanising of steel is done by emersion into a bath of molten zinc which will chemically bond to the surface. This protects the steel from corrosion.
  • Electroplating produces a deposited layer of metal onto a product by the application of an electric current so a suitable solution.
  • Enamelling. Products can be given a hardwearing but attractive finish by enamelling them. Once the product has been coasted then it needs to be heated in a kiln.

Quality Control

When working in a small workshop there are certain principles that will help in producing a good product.

  • Plan the sequence of manufacturing so that all the accurate surfaces are machined at one setting. The finished surfaces are not held in a vice or chuck.
  • Take care over marking out and measuring parts that accurately made add quality to a product. Measuring at every stage of manufacture ensures that errors are seen early on when they can be put right. Time spent marking out is not a waste of time.
  • Remove all Burs. Burs are the rough edges created when metal is cut. These should be removed from edges and holes.
  • Don’t use emery cloth to hide inaccurate work. If the surface finish of a product is bad it means the tool is in bad condition, it is blunt, the wrong shape or being used at the wrong speed.
  • Lean to weld properly and make strong welds. A good weld will be strong and will not need dressing. Dressing a poorly welded joint can hide the weakness of the join and can cause problems later on.
  • Finishing is very important and a good finish will help sell the product but it should
    not be used to hide bad work. No amount of paint can make a bad product look good.

Product Development and Design

The products need to be attractive to potential customers. This can be achieved by making items of high quality, that are attractive, that are affordable and address customers needs.

source: www.practicalaction.org

Loading...

Leave a Reply

Your email address will not be published. Required fields are marked *