Let Us Spray Those Worn Parts
Page 30
If you've noticed an error in this article please click here to report it so we can fix it.
With Spares at a Premium, Reclamation by Metal Spraying Comes into its Own. W. E. Ballard, in an Informative I .A.E. Paper, Details Applications and Limitations of the Process
DURING the early days of our apprenticeship, our L./credulity was such that we were ready to believe—but only once—in the existence of the putting-on tool. Conjured up in the kindly imagination of some chargehand or foreman for the consolation of the innocent and inexpert turner, who had taken off a " thou " or so too much, this mythical appliance was, in those days, no more than the basis of one of those jokes—mostly of a rather rougher nature—that served to while away the irksome hours of confinement to a machine-shop from six in the morning to six at night.
To-day, however, not only do putting-on tools really exist, but also they have become of vital importance: so much so indeed, that their construction and use form the subject Of an I.A.E. paper this month. Mr. W. E. Ballard, A.I.C., director of Metallisation, Ltd., is the author, and his paper is headed "Reclamation of Worn Parts by Metal Spraying."
He names as two distinct advantages of the process that it is nearly cold, and therefore, free from the difficulties iuherent to welding, and that the apparatus is comparatively portable, needing no chemicals in vats, as with electro-deposition. He stresses, however, that it has definite 'limitations. •
Whilst three V types of metal spraying are described in the paper, the author states that only one is suitable for reclamation by using metals such as steel, including stainless steel, and nickel, .and he deals exclusively with this. In this system the metal used is in the form of a wire, 1 mm. to 4 in, in diameter, which is fed into the tool or
spraying pistol. Also fed to the pistol are compressed air, at 40-60 lb. per sq. in., supplied at the rate of 15-30 cubic ft. per min., combustible gas (coal gas, 'acetylene, hydrogen or propane) at 25-35 lb. per sq. in. and 30-60 cubic ft. per hr., and oxygen at the same pressure as the combustible gas, but in rather smaller quantity.
Combustion and Wire Advance Synchronized Functions ' The wire is advanced in the tool by a mechanism driven by an air turbine, whilst the combustible gas and the oxygen mix in a chamber within the main nozzle of the pistol, and flow down small grooves surrounding an inner nozzle, through which the wire passes. On ignition of the gas, the flame melts the wire which must be fed into the tool at the same rate as it becomes molten,
About 95 per cent. of the compressed-air supply is discharged 'around the gas feed, and converges on the hot zone of the flame, catching up the molten metal in 0.01-mm. particles, and imparting to them the speed of a rifle bullet. When these particles meet a suitably roughened surface, they adhere and form a coating which, Mr. Ballard states, remains cool at• a distance of 9-5 ins, from the nozzle. Considering the structure of the coating, the author likens each particle to a small pressure die casting, and describes the whole as an "infinite " number of such particles, flattened by the impact and locked together hy a form of microscopic dove-tailing.
The tensile strength of sprayed steel he gives approximately as only 2 tons per sq. in„ but the compression strength is high, the structure resembling cast-iron.
Turning to preparation of parts for spraying, he writes that the usual method is shot-blasting, which affords a clean and sufficiently rough surface, adding that there is no adherence by alloying, but only by interlocking.
Shot-blasting for preparation is effected at 30-60 lb. per sq. in. with angular steel grit capable of passing a 15 or 30 mesh-per-inch sieve. Degreasing may be advisable first. Alternatively, to steel grit, carborundum or Blastyte may be used, especially for case-hardened parts. If no shotblasting apparatus is available, shafts, etc., may be prepared by turning on them a rough thread 1/32-in. deep and 20-30 to the inch. The author comments that most parts for reclamation are prepared in this latter manner.
Mr. Ballard outlines two tests of deposit strength, one of these comprised building up with 0.8 per cent, carbon steel, a collar 3/16-in, thick, and 1-in, wide, on a 1-in. shaft. The last-named was passed through a hole and pulled until the collar burst, which happened when a force of 22.45 tons was exerted.
In respect of lubrication, the theory is that because a sprayed surface consists of minute pores which retain oil tenaciously, it should be ideal from this aspect, but there is evidence, states the author, that that is the case.
Under the heading Technique of Reclamation, he gives valuable advice on repairing worn parts. In the case of a shaft, this is mounted in the lathe, machined down, if necessary, to allow for deposit thickness and roughthreaded. Then the pistol is mounted on the tool post, the deposit applied, and the shaft finally ground (or turned, but the former is preferable) to size. During spraying, the peripheral speed should not exceed 30 ft. per mm. Subsequent hardening is unnecessary. Very hard coatings can be obtained with high-carbon-steel wire.
Spraying bores is not so simple, because the coating, in shrinking, tends to loosen. A skilled. operative, however, can accomplish' such work successfully.
Technique of Repairing
Cracks by Metal Spraying Cracked water jackets can be repaired by spraying, provided that no material stress tending to open the crack is created, when, say, the head-nuts are tightened. The best method is to grind a rectangular-section channel along the crack, shot-blast and fill it by spraying; using a stencil to avoid building up metal at the sides of the grooves.
The job must not be allowed to get hot. Other methods are also explained. Cylinder heads, according to Mr.
Ballard, are not generally ideally suited for repair by spraying, but nevertheless, many have been successfully treated by the process.
He refers to a satisfactory repair to ax aluminium-alloy head with Screwed-in valve seats, one of the recesses for which had a completely stripped thread. Cracks in cylinder-block top faces, and around valve seats, have been successfully repaired by spraying. Bores cannot usually be treated, but, if opened up for liners and bored oversize (possibly through the employment of semi-skilled labour), the liners can be sprayed and ground to the oversize. Crankcases, worn or cracked, are good subjects, provided that in the case of a crack all oil can be removed.
Gudgeon pins can be built up with complete success, whilst crankshafts have actually represented the largest field for spraying. A tip given for dealing with these is to plug the oil holes until after final grinding, not forgetting to take out the plugs before assembly! The author recommends 0.7 per cent, carbon steel.
Splined shafts are bad subjects, but no fewer than nine specific shafts of other type are named as perfectly suitable, whilst under the same general heading kingpins are included. Balland roller-bearing housings can be sprayed, but not shafts on which rollers run direct. Steel is usually employed, but often a good job can be made with zinc. In conclusion, Mr. Ballard just touches on the protective effect of zinc spraying, naming wheels which rust where the paint gets chipped off with the tyfe lever, and springs, as saitable subjects. He finally comments that, although in normal times replacement may be more economical than reclamation, just now the position is very different. He adds, furthermore, that, in any case, saving of time may be a valuable factor, and suggests that the metal-sprayed part may actually possess advantages over the standard part—as, for example, improved anti-friction or oil-retaining properties,