Metal Spraying on Machine Tools

Metal Spraying on Machine Tools

Metal Spraying on Machine Tools - Metallizing or metal spraying is the process of applying molten metal to a base metal surface in the form of a fine spray that bonds to the base metal and forms a metal coating. There are two principal methods:

Reclamation of a Splined Shaft by Electro-Deposition

(i) Wire Metallizing (or flame spraying) which deposits metallic materials by the use of a combustion flame.

(ii) Plasma spraying, this uses plasma arc as heat source rather than, a flame as in flame spraying.

Fig. shows flame spraying (wire Metallizing) process. The material to be sprayed is taken in wire form is fed into the oxy-acetylene flame, there it melts by the heat of the (neutral) flame, gets atomized and blown by a stream of compressed air onto the base metal surface where it spreads around and interlocks with projections, imbed in pits and freezes quickly upon contact with the base metal surface.

Separate particles overlap and intertwine with one another to form a coherent structure. The sprayed material is generally applied in layers less than 0.25 mm thick. The wire metallizing is a fast and, low cost method of applying a wide variety of metal coatings such as carbon steel, zinc, lead, stainless steel, brass and bronze, over a grit-blasted or nickel-aluminide undercoated substrate.

Such coatings are often used for shaft build up and general restoration of worn or mismatched parts. Wire metallizing is very much used in corrosion protection.Metal spraying is ideally suited for the reclamation of worn parts. 

Meters Dry Compressed Air

Typical deposition rates are given as zinc 62, aluminium 15 and steel 10, lb. per hour.For the deposition of metal on shafts a motorised design of the gun is available which can be fixed to the toolpost of the lathe on which the shaft is being supported and rotated. 

To be suitable for the reception of spraying, a clean rough surface is necessary, and this may be obtained in general by shotblasting with an angular steel grit or, in the case of a shaft, good adhesion is given by turning a rough, ragged screw thread having a thread of about 20-30 threads per inch and about 1/32 in. deep. All traces of grease must be removed, by degreasing if necessary.

 The structure of the deposited metal consists of a conglomerate of tiny flattened particles of metal shaped like saucers and piled indiscriminately one on top of the other. The adhesion of this deposit to the base depends on a form of mechanical interlocking and will depend on the shape and condition of the base surface.

The structure of the deposit is such that for reclaimed surfaces the finished layer should be at least 1/64 in. thick so that a slight additional reduction may be necessary to worn parts in order that the sprayed metal may not be less than tool after the final sizing. A characteristic of sprayed metal is its porosity, and this seems to be in its favour for shafts running in bearings, as a certain amount of oil is absorbed and retained as a lubricant.

The final sizing of sprayed metal is best, conducted by grinding as its structure does not lend itself to very good results under a cutting tool.Steel of any carbon content may be sprayed and the higher carbon contents give increasing hardness of coating.

If the job is kept cold when spraying with wire round about 1% C, the chilling of the deposited metal results in the characteristics of the usual quenching phenomena taking place and the deposit is very hard.Most other metals may be sprayed and the process may be used with bronze wire to deposit metal on to the surfaces of worn bearing bushes.