To digress a little, with the right technique it is incredible the accuracy to which the home machinist can work with some effort. I have an interest in astronomy and lapping is a fundamental process in the art of telescope mirror making. Here, a chunk of low-expansion plate glass is first ground (by hand) using a second plate of glass moved across the top with abrasive between the two. This produces a spherical depression. This rough (relatively speaking) shape is then lapped with jeweler's rouge and a pitch lap (again all by hand) to a precise paraboloid surface. The degree of accuracy is described in terms of fractions of the wavelength of light (we are talking 0.000005" variation over, say, an 8" diameter mirror). This is an extreme example and there is no other practical use for such fine limits in normal model engineering (thankfully!)
Nevertheless, the basic method used in telescope mirror making is the same as that used in the workshop, viz., a lap made of a material which is softer than the workpiece is charged with an abrasive and the process of working the two surfaces together produces a fine finish. The reason why the lap is usually softer than the workpiece is to avoid embedding abrasive into the latter's surface. Instead, the abrasive is embedded into the lap which then acts much like very fine emery paper with a firm shape and surface. Indeed, one of the finest laps is made by rolling a round bar of copper in diamond (or sapphire) dust between two steel plates, this embeds the abrasive in the surface of the copper and excess dust is brushed off. This produces a long-lasting lap capable of the finest work. It is important to note that lapping should not be relied upon to remove significant amounts of metal, less than a thou should be aimed for and the smaller the amount the better. This means of course that the initial workpiece prior to lapping should be machined to the finest tolerances possible so as not to rely on lapping to 'get it to size'. Having said that, lapping a bore will successfully remove small constrictions and produce a bore which is truly parallel. The difficulty though is that the process of lapping a bore can often result in slight bell-mouthing and wherever possible the workpiece should be made slightly longer and reduced to length after lapping.
Laps may be of a simple construction of a fixed dimension, effectively solid rods or plates of soft metal like copper, aluminium, brass etc., the exact material being dependent upon the work in hand. For lapping bores the tool may usefully be made expandable such that progressive removal of metal may be achieved. In either case it is required to allow passage of the lapping medium. In the case of the solid lap (as opposed to an expanding lap) the surface is machined with multiple shallow grooves (say, 0.5" apart) the purpose of which is to hold and distribute evenly the abrasive compound which is doing the work. A simple flat plate (scraped true with reference to a surface plate) used for lapping flat work can be criss-crossed with a pattern of V-grooves. A simple type for lapping bores can be made from copper tube with holes drilled through, the tube itself then acts as a reservoir for the abrasive and the holes spread it evenly around the lap. Hollow or female laps are used to finish the outside of workpieces such as pistons. Again, the lap can either be made as a precision bored length of copper or brass rod with cross-holes to hold the abrasive, or arrangements can be made to enable the bore of the lap to be constricted to provide an extended working range.
Expanding laps can be made to several designs. There are the 'barrel' type which, as the name suggests, are not parallel but are expanded by compressing a flexible piece of tubing, and there are the parallel-expanding types which are usually split tubes expanded by wedges or a taper shaft (mated with a taper bore) under control of a bolt or nut. Both these types are illustrated below:
The abrasives are commercial items and are made of the same materials as grind stones (silicon carbide, aluminium oxide, carborundum etc), the more exotic materials like diamond and sapphire dust are of course very expensive but with care a small amount will last a lifetime of lapping. Final polishing is achieved with fine 'flour' abrasives, or jeweler's rouge (ferric oxide powder), or metal polish. The important thing to remember is that the common materials are available in a wide range of grit sizes. It's a critical part of the method that lapping be progressive, starting with the coarser grades and finishing with the finer grades. It's also extremely important that all traces of the coarser grade abrasive be removed from both the work and lap before proceeding to a finer grade. Normally the abrasive will be mixed with very light oil or paraffin to form a slurry which is able to work itself between the work surfaces. With reference to the description of how a lap works you will understand that if coarse particles of abrasive have been embedded into a soft metal lap it cannot then be used for final polishing, to do so will risk scratching the fine polished surface. Clearly, as an engineer you will have to judge the degree of finish necessary on the finished item. Lapping a cast-iron model steam locomotive cylinder is a less critical operation than lapping the cylinder and piston of diesel engine where high compression is required.
Lets take a look at a typical lapping job - that of producing a fine finished bore and piston for an IC engine. In fact, piston and bore are both lapped in separate operations (NOT both together). All of these operations will be carried out in the lathe (and I need hardly mention the importance of keeping lapping compounds off the machine, particularly the chuck and slideways). For the bore an expanding lap is ideal, and this should be some 3-4 times the total length of the bore. The first grade of abrasive would be mixed with light machine oil (10W or lighter) and liberally coated on the inside of the workpiece. Similarly, the slurry would be added to the outside (and inside assuming it is of the ventilated type) of the lap. The lathe would be started at about 300rpm (for a nominal 1" bore) and the lap passed rapidly through the bore, keeping it moving back and forth without it coming out the bore. How to hold the lap? well, perhaps the best way is with a 'floating' tailstock holder, and failing this holding with the hand is a method as good as any. Be careful when holding the lap by hand as it's possible it may jam, hold it lightly and expect the unexpected. Remember also that unless the lap is maintained dead parallel with the bore (an almost impossible task) it will tend to bell-mouth the bore a little - hence the reason for making the work a little longer than finished size and trimming to length later. When the inside of the bore has achieved an all-over grey appearance, with the fine scratches appearing even and criss-crossing both ways, and with no evidence of any deeper scratches (as might be left by the reamer) it's time to move onto the next finer grade. The work will have to be removed from the chuck to clean it properly, and this should be done with clean paraffin oil followed by hot soapy water. The same procedure applies to the lap and all traces of the abrasive must be removed. The process continues until you reach the 'flour' grade of abrasive by which time the finish on the workpiece should be very fine indeed. A final polished finish, should this be deemed necessary, can be achieved using metal polish (diluted Autosol, or some liquid chrome cleaner). The lap should be a separate 'finishing' lap so there is no chance of contamination with the coarser grades of abrasive which might be embedded in the main lap. The piston is treated in a similar way except of course the lap is female. Work will continue with the coarse abrasive until (using the un-trimmed bore as a gauge) the piston will not *quite* enter the bore. At this stage finer grade abrasives are used and work continues until the piston will just enter the bore tightly. At this stage, it is usual to finish mating the two parts by using metal polish and briefly using the piston to lap the bore directly. Great care needs be taken but this method ensures that the fit is good for the entire length of the bore.
To lap a flat plate, such as the port face of a cast-iron slide valve cylinder, requires a lap which is somewhat larger than the workpiece. It is easier to slide the cylinder around on the flat lap than vice-versa. A good flat lap can be made from a slab of 3/8" or 1/2" steel plate with a piece of 16g brass sheet Araldited to one face, this then being machined flat in the lathe or milling machine and then scraped true with reference to a surface plate before machining a pattern of grooves to hold the abrasive. In use, the workpiece should be moved around on the lap in a figure of 8 motion to even out cutting effect of the abrasive.
Today, it is possible to buy large diamond-impregnated steel plates for sharpening of cutting tools. The larger ones (8" x 3") in the finest grade also make excellent laps as the baseplate is quite flat (at least in the better and more expensive examples). These plates certainly can replace coarse grade abrasive lapping leaving only the final polishing stage.
Reference was given earlier to the difficulties of lapping taper bores. The simple reason for this is that longitudinal movement of the lap cannot occur because of the taper. Without such movement uneven stock removal takes place which leaves a surface which is not even (there are bumps and circular hollows formed by accumulation of abrasive at random points). There is no real cure for this and in industry the problem is simply resolved by resorting to internal and/or external grinding techniques. The sort of work that is being considered here is Morse taper shanks or sockets and like shapes. The home worker will probably never have to lap such surfaces anyway, and careful turning and reaming will produce acceptable if not ideal results.
(c) Chris Heapy 1996.
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