Of all metal-working operations the production of true flat surfaces is perhaps one of the most difficult if reliance has to be placed on hand tools and hand methods, for it depends just about completely on the personal skill of the workman. But a point offset from the spindle centre of a vertical milling machine must when rotated describe a flat plane in space if there is no axial movement. Therefore, provided the spindle is truly square to the table, an offset cutting tool must generate a flat surface on a work-piece attached to the table. Model engineering, just the same as full size engineering, demands the production of a great many flat surfaces, so the ability of the machine to perform this task in a simple way, without expensive tooling, is extremely important to the home worker.
The cheapest tool for the purpose is the flycutter, usualfy consisting of a small toolbit set in some kind of holder. There are commercially made holders available, but it is quite easy to make satisfactory holders at home, and they serve just as well Three home-made flycutters are shown in Fig. 14. Each is just a Morse taper arbor with an enlarged head having a slanting hole drilled in it to take a cutter bit (•J- in. in these samples) with a screw to lock it in place. The head diameters are 1 j in., in. and 2} in. so the faces that can be machined at one pass are roughly ^ in. to y in. wider in each case. They were made by boring through short pieces of steel of these sizes to suit the parallel parts of Morse taper arbors. It is not perhaps widely enough known that tool merchants can, if they will, supply Morse taper arbors of this kind, which are a stock product of the large drill makers. This method of fabricating flycutters by using a ready made arbor with a head Loctited on saves a good deal of time and some heavy steel. The effectiveness of tools made in this way is beyond question. Fig. 15 shows a bracket clamped against a large angleplate and being milled with one.
Fig. 16 shows one working on a steel connecting rod which has to be reduced from a circular section at each end. The rod is about 9 in. long, so it is held in two vices at the same time, and each end is taken down to finished size before it is turned over. Packings are used, different at each end to ensure the finished surface is above the vice jaws, to avoid cutting
Fig. 14 Set of three flycutters into them, and these packings ensure the Owning two vices alike may at first rod is at the right attitude for keeping the thought seem something of a luxury, but milled surfaces parallel to the axis. as soon as long articles have to be dealt
Fig. 15 Flycutting a bracket
Fig. 15 Flycutting a bracket
with the benefits are at once apparent. reduced to a tapered section to cut up into
Another flvcutting operation is shown wedge blocks for connecting rods of the in Fig. 17 where a steel bar is being type in the previous picture. These wedge
Fig. 17 Flycutting tapered bar material
Fig. 18 F/ycutting cylinder soleplate blocks are needed for adjusting the bearings in the rod ends. The rectangular section bar is held in a vice on a tilting angle-plate which has been set at 6 degrees to the table of the miller with a Starrett combination protractor. The tapered form will be seen on the end of the completed piece lying on the angleplate. This is an easy way of getting a special section which cannot be bought, and which would, to say the least, be tedious to make by filing.
These flycutter holders do not allow much adjustment of the radius of the cutter bit, but with some makes of boring head there is quite a lot of adjustment. For example the Dore boring head permits of using a cutter in a f in. diam. bar at any radius up to 2$ in., and by setting the saddle in or out on the slide body the radius can be adjusted by fine amounts to suit any job within the range. Fig. 18 shows an old type, pre-war boring head being used to face a cylinder sole-plate for a slide valve engine model of 2 j in. stroke.
Of course, multi-cutting-edge face mills permit machining a surface quicker than a single point tool can do, and with less snatch and jerking, but commercially made they are very expensive, and in the home workshop the greater productivity is not usually of much consequence. Nevertheless, for anybody willing to spend the time needed they can be made in the home workshop, with several cutter bits mounted in one mild steel body. Fig. 19 shows a face mill of this kind, which was made originally to screw on the spindle of a Myford lathe to do some repetitive milling of a fairly heavy nature, now no longer required, but it is still a good general purpose tool. It has 12 tool bits £ in. diam. set into flat bottomed holes, all
Fig. 19 FacemiH
Fig. 19 FacemiH
ground off to the same projection, and sharpened to a diameter of approx. 2y in.
In Fig. 20 it is shown milling the face of a half-flywheel iron casting for a model stationary engine. The casting is supported by a special angle plate type of fixture, the pattern for which was made in an hour. Without this fixture the operation
Fig. 20 Milting flywheel joint face
Fig. 20 Milting flywheel joint face
would be somewhat difficult. If the casting was held in a vice on the table the point of cutting would be a long way from the holding point, and movement of the casting under the pressure of cutting would be not easy to prevent. Vibration and chatter would be more likely. It very often happens that the only way to get a satisfactory job is to make some equipment specially for it. This is not usually wasteful, especially if a duplicate component is ever required, but the equipment is usually found adaptable for some other job later. Doing metal cutting by knife-and-fork methods can soon lead to disaster. The other half of the wheel casting, with the cast-in teeth for the barring 'rack', can be seen in the bottom half of the picture. The wheel is 9J in. diameter and has 96 teeth.
Broad flat surfaces can be. and sometimes have to be, produced by successive parallel passes with an endmill much narrower than the face required. Apart from taking more time than a tool with a wide sweep, minute ridges tend to be left where the passes overlap, and these may have to be removed later by filing or scraping. So while this method is feasible the flycutter or boring head is better where there is room to use it, and the cutter bits are cheaper than endmills and easily sharpened like any lathe tool.
However, an example of work where a small cutter and successive passes must be used is shown in Fig. 21 where a flat bedplate slide for the crosshead on a model stationary engine is being milled. The surface being cut is in a recess £ in. deep and the corners cannot be dealt with by a tool cutting the full width, as the radius left would be too great Note the stop bar bolted to the table. Accurately squared with the table it provides not only correct location for the casting (which was followed by others) but also insurance against slipping.
In the full sized engines these slides were always planed, and every engine-building shop had planers for this kind of work. In the one where I worked there were several of different sizes, and the largest, built by Joshua Buckton of Leeds, could plane any casting up to 20 ft. long, 12 ft. wide and 12 ft. high. It was said at that time to be the largest in Yorkshire, and certainly it often did castings for other firms. Cutting could be done in both direc tions of the table travel at equal speeds, or in one direction with a quick return the other way. Each of the four toolheads had power operation independent of table movement, so that cross-planing could be done through bearing recesses, etc. One of the pictures shows this operation on a model being done by milling. Each head could also be swivelled so that angular faces could be planed also.
After the planing of crosshead slides they were tackled by the fitters and scraped to a portable surface plate This was coated sparingly with a mixture of lamp black and oil. slid to and fro on the slide, lifted off, and then all the black marks scraped away. The surface plate was then put on again and a fresh lot of marks made, which in turn were scraped away. This work went on for many hours, indeed on a big slide two men could spend two or three days. For such work the surface plate would be so large that two men could not lift it without the use of the shop crane. Eventually after a long time the finish obtained would be regarded as acceptable. It then consisted of a very large number of extremely shallow depressions between the marks, and each of these proved to be an oil
Fig. 22 Milling bearing jaws in bedplate
Fig. 22 Milling bearing jaws in bedplate pocket. When the engine was eventually put to work, with the cross-head having had similar treatment, the result was that the cross-head ran to and fro on a film of lubricant which reduced wear to a very small amount. Engines in textile mills would run 60 years and at the end you would find the scraper marks still there. The oil was continuously renewed by brass combs attached to the crosshead which picked up oil from a well at each end of the slide. An engine running night and day, as many of them did, with a speed of about 80 r.p.m. would make approx. 3600 million cross-head strokes in that time! Not a bad performance?
When flat surfaces have to be produced at right angles to the table it is necessary to use the side of an endmill. This may be quite unavoidable on some components, such as the model engine bedplate shown in Fig. 22. There is not much choice about milling out the jaws for the crankshaft bearings. This is an operation which the big planer used to do with the power drive on the heads of the cross-rail.
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