This article first appeared in Practical Engineering 1940 Vol1 No26. The article is accurate as of 1940. This article provides information on techniques employed in Industry at the time.
In addition to scratch-brushes, there are steel wire wheels for power-brushing. They are of heavier construction than scratch-brushes and can be used for a vast number of jobs. Among these we may mention cleaning castings, removing the burr or frase from gears after machining the teeth, cleaning up welded and brazed joints, closing the pores in aluminium castings, removing paint and enamel from metal, decarbonising internal combustion-engine parts, and so on. It will be seen from these that this type of wheel has a much more severe action on the job than has a scratch-brush.
A noteworthy feature of the steel wire wheel is the built-up construction which permits the replacement of worn sections. This is a sound idea from the point of view of economy because on most jobs one part of the wheel wears more than another, and as this method of assembly permits the interchange of the sections, the wear may be made more even than would be the case with a one-piece construction.
It is not generally appreciated that there is usually a marked distinction between a surface which has been polished preparatory to plating and a surface which has been polished as a final operation. A very high finish is required as a preliminary to plating because the high reflectivity of nickel and chromium (particularly the latter) causes a mark on the surface of the job to appear out of all proportion to its depth. For this reason it is difficult to obtain a really high-class finish on cast iron unless it is of a particularly good quality, possesses a close, fine grain, and is free from blowholes.
In commercial cast iron it is almost .always possible to discern the grain of the base metal when the article is nickel or chromium plated, and very often dull patches are apparent where the grain of the metal varies slightly, due perhaps to chilling in the mould or some similar condition. Large castings such as stove tops or oven doors are usually treated with about four grades of emery, 60, 90, 120 and 180 grit being probably the most popular.
If the castings are smooth and clean, the first stage may be omitted. Sand-blasting makes it possible in some cases to omit the first two stages if the metal is of good quality. After polishing with the 180-grit emery the article may be glazed with a bob dressed with 180-grit and treated with bobbing grease. Good results are obtained if the grease is rubbed into the stationary wheel with the hand as a preliminary step.
The final operation is carried out with a fibre wheel and a brushing emery compound. Medium-quality work which is to be polished only may be treated with 60, 90 and 120 grit, the last operation being glazing with a 120-grit wheel and bobbing grease.
It will be evident from the foregoing remarks on the polishing of cast iron that the processes involved are dependent on three things : 1—The condition of the surface at the commencement of operations ; 2—the quality of the metal; 3—the class of finish required.
For the polishing of steel articles the processes are essentially the same as for cast iron, but as the original surface is invariably in a better condition for polishing, some of the first operations may be omitted. Thus, for plain polished steelwork which has been drawn through dies (tubing or bar) or machined (bolts, etc.) a single polishing with a 120-grit glazing bob may be sufficient.
It is largely a matter of opinion as to what constitutes a satisfactory finish for a particular case, and the treatment of a polishing job at one place may vary considerably from the treatment of a similar job elsewhere. Operations may be added or left out to suit the individual requirements dictated by the standard of quality to be maintained. It must be emphasised that a higher finish is required on goods which are to be plated, as distinct from goods which are to be just polished.
As brass is a much softer metal than iron or steel it is a simpler matter to obtain a satisfactory finish on a brass article, although blow holes in brass castings cannot, of course, be polished out unless they are only just on the surface.
For good-quality castings and machined parts in brass it is often sufficient to use 120-grit emery on a felt bob, and to follow this with a 180-grit glazing wheel. The final operation is mopping with a tripoli compound and a fairly hard cutting mop. Cases do arise, however, where the original surface is in a much less satisfactory condition. In this event more stages of cutting out with emery are introduced, starting with a coarser grade.
It is a generally-accepted theory that the polishing mop, in addition to its cutting action on metal also causes it to flow, and this is considered to be an important factor in the production of a high finish. Probably the best illustration of this flowing action under the mop occurs when polishing brass castings containing small blowholes. After polishing, these holes will be found to have dragged and produced lines in the metal surface, commencing at the blowholes and travelling in the direction of the mop.
An unusual finish for brass articles is obtained by lightly sand-blasting and polishing certain parts only, leaving the remainder with the sand-blasted surface. This is particularly effective when the article is nickel or chromium plated. Heating-installation globe valves may be treated in this manner, the flats of the hexagon base, and the valve bib and stem, being suitable parts for polishing for the relief effect. For a first-class finish on brass which is to be lacquered, the polishing with tripoli may be followed by "colouring" with rouge or lime-finishing compound. A soft cotton or sheepskin mop is required for this job.
Frequent reference has been made to 60-grit emery, 120-grit emery, and so on. In order to remove any doubts which may exist as to the exact meaning of these names it may be stated that the "grit" of the abrasive used is a number which indicates the grain size. It is actually the linear measure or the mesh of the sieve through which the abrasive will pass. Thus 60-grit emery will pass through a sieve having 60 holes to the linear in., or 3,600 holes to the sq. in.
Barrelling or tumbling is a process which plays an extremely important part in the manufacture of a multitude of small articles. It will be appreciated that the cost of polishing by hand vast numbers of small items like collar studs, screws, hooks, cable clips, pram-wheel hub caps, cycle spokes, washers, meat skewers, pencil clips and so on would be exorbitant. Barrelling consists essentially of rotating a quantity of the work pieces together with a suitable polishing, scouring, or burnishing agent, in a container at such a speed that the mass tumbles over and over in a similar manner to concrete in a drum concrete mixer.
The actual process is not quite so simple as this, however. The shape and weight of the article dictates the design of the barrel. A particular design eminently suitable to one job might give an unsatisfactory finish to another which is only slightly different. One rule which always applies is that the parts to be treated must not possess sharp edges or points.
Some articles are not adapted on their own to barrel finishing, but if a quantity of scrap metal of suitable size and shape is added to the charge, satisfactory results can often be obtained. Rumbling stars are manufactured for the express purpose of inclusion with articles possessing recesses which would not otherwise receive treatment.
The abrasives used include emery, sharp sand, granite chippings, pumice powder, tripoli powder, coke dust, and so on. Mixed with oil, emery is very effective for removing scale from articles, and for scouring the surface of rough parts. The other abrasives may be mixed with water for scouring purposes. Burnishing is achieved by the use of hard steel balls possessing a highly-polished surface. These, too, are generally used with water.
Polishing mops and wheels must be driven at a suitable speed to obtain the best results and effect the maximum economy of polishing materials. The machine used for this purpose is termed a polishing lathe. It is specially designed to suit the job in hand, the essentials being:
1.—The spindle must be located at a convenient height dependent upon whether underhand or overhand polishing is to be carried out.
2.—The spindle must be rigid and free from vibration at the high speeds often required.
3.—It must possess sufficient overhang to avoid fouling the headstock with large work pieces.
4.—A convenient method of cutting off the motive power supply to the spindle in case of accidents, or when changing the polishing tools, must be provided.
5.—In cases where the polishing tools need frequent changing the method of mounting should facilitate this.
6.—The spindle speed must be suitable to the job in hand.
The first consideration is dictated by the type of work. For large castings where wheels up to 36in. in diameter are used, overhand polishing is adopted.
The top of the wheel rotates away from the operator and the work is applied to the top side. In this case the spindle is fairly low down near the floor. Underhand polishing is carried out with smaller wheels mounted so that the underside rotates away from the operator and the work is applied on this face. The spindle in this case is, of course, higher than for overhand polishing. Rigidity of the spindle is particularly necessary when using abrasive-covered felt bobs which are fairly heavy. If the spindle is subject to vibration, not only do the bobs tend to work loose in use, but the polishing operation is made difficult by the movement of the wheel.
For extra large work (e.g., towel rails), polishing lathes with long snouts are available. The ends of the snouts carry additional bearings which provide support for the extra long spindles. Occasionally a heavy wheel will show signs of working loose on the spindle nose, or on a particularly awkward job some part may become involved with the wheel or spindle. In cases like these it is necessary to stop the spindle in as short a time as possible. If a wheel does come off a taper nose when travelling at 3,000 r.p.m. it can do consider-able damage. For this reason care must be exercised when mounting hard felt bobs, scratch brushes, and so on.
The quickest method of mounting a wheel on the spindle is by means of the taper nose just mentioned. In cases where considerable wear is likely, this nose piece is made short and detachable to facilitate replacement. The taper end carries a thread which will screw into the hole in the centre of a polishing wheel and make its own thread inside the hole. (The hole in the wheel is plain when new.) For a double-ended spindle it is necessary to have a pair of detachable noses, i.e., one with left-hand threads and one with right-hand threads.
It is very dangerous to mount a bonded grinding wheel with a lead centre bush on to a taper thread. The bursting action due to the taper may cause the wheel to break when in use. In addition to this the wheel will not run true. All abrasive wheels of this type should be mounted between metal side plates and suitable friction washers.
In general, the polishing lathe with a self-contained motor is the most satisfactory. There is no belt to foul long jobs which are being polished, and the maintenance costs are reduced to a minimum. The motors used reach peak r.p.m. almost instantaneously and are easily brought to rest. In addition to these, the failure of a motor does not affect the remainder of the plant. If, however, the conditions of the motive power supply make a belt drive preferable to an independent one, there are several methods to choose from including a method of driving by means of an under-floor pulley.
In order to obtain the most satisfactory results in the minimum of time and with the minimum expenditure of polishing materials, it is absolutely essential that the spindle be run at the correct speed. The main factors controlling this are the size and type of the wheel which is used. As the important point is to secure the correct surface speed for the wheel, the number of revolutions which it must make in one minute are dependent upon its diameter. In consequence, as a large wheel wears down it should be transferred to a faster-running spindle.
Good average peripheral speeds are 5,000 to 6,000 ft. per min. for abrasive dressed wheels and 6,000 to 9,000 ft. per min. for mops. For general work, using wheels of about 10 in. diameter, two spindle speeds of 3,000 and 4,000 r.p.m. will prove satisfactory for most jobs.
An unpleasant feature of metal grinding and polishing is the large amount of dust, fluff, compound and so on which is thrown out from the wheel. It is compulsory to provide an efficient exhaust system to deal with this matter and thus safeguard the health of the operators.
The usual method is to fix a hood behind the wheel, of such shape that it does not interfere with the polishing process. (It is possible to arrange the top of the hood on hinges so that it may be swung out of the way when dealing with extra large jobs.) The hood is connected to a main trunking by branch pipes, the exhausting being effected by a centrifugal fan at the extremity of the trunking. The fan discharges into a dust extractor where the dust is separated from the air and may be collected for disposal. This method is applicable to large installations, but where only one or two lathes are required, it is possible to equip these with a small dust-exhausting plant situated near the machines.
In small establishments a single polishing lathe is expected to perform a number of jobs including grinding, using a bonded wheel. For this purpose spindles may be fitted with clamping plates. The spindle is threaded for a short distance past the taper thread so that a large nut may be screwed on, followed by the clamping plates and friction washers, preferably rubber (for me with bonded wheels), and finally another large nut for clamping purposes. When wheels which are fitted with metal centre bushes are used, the clamping plates are used without the rubber washers. Metal centre bushes do not screw on to a taper thread. It is important that the rubber washers be used on bonded wheels, as there is a danger of the wheel breaking under the clamping pressure if these are omitted.
Many repetition jobs are capable of being polished by means of automatic machines. In addition, strip material, rod, and tubing, may also be treated in this manner. Individual items are generally held in suitable chucks which are rotated by small independent motors so that all faces of the work are presented to the mop. In some instances, three or four chucks are mounted on a head which is also capable of being rotated. In this way additional mops are brought into play in order to polish those parts of the job which are inaccessible to the first mop. The primary requisite of an article which is to be polished by means of automatic equipment is symmetry about its axis of rotation when fixed in the chuck on the machine. Suitable jobs are hubcaps, reflectors, cycle hubs and rims, lamp-glass rims and meter bezels.
Rod and tubing may be satisfactorily polished by means of abrasive bands, the work being rotated and fed through the machine automatically. The now well-known principle of centreless grinding may be applied with advantage to automatic polishing machines for cylindrical work. The essentials of this principle are a large wheel rotating at high speed and a smaller wheel rotating at a relatively low speed. The larger wheel performs the cutting operation, the smaller one serving to rotate the work which will not "follow" the fast-running wheel. A narrow blade supports the job and no further cutting takes place when the material is reduced to the required size.