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road and workshop
Page 55
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by Handyman
Benchwise: lathe sense (20)
I WAS DEALING last week mainly with the application of the end-milling tool in the lathe and some of the work it can undertake, remembering that the work is stationary and is best held in a universal vertical slide. Of course, it must be appreciated that the parallel end-mill has its limitations in that its approach into the casing of a water pump, for instance, is at a direct right angle to the face it is to shape, therefore it cannot be expected to machine an area other than that permitted by the width of the casing aperture. While this can be adequate for a gland-seating face no wider than the opening, in order to face the whole end area for the sweep of, say, a water-pump impellor, which is greater than the opening, another version of the milling tool, a cutter, must be used. And here we have a very wide range indeed, with almost unlimited scope for repair work.
Quite expensive However, before plunging into the field of parts reclamation, remember that milling cutters are quite expensive items although they may not be much to look at, being rather like a mixture of cycle chain sprockets, ratchet wheels and miniature circular saws, the heavier or broader duty cutter looking little more than a small gear wheel with swept back teeth. Sensible care and storage is required; never stack them together or just drop them into a generaltool box as they can damage each other, so store them separately in a wooden box, and arrange pegs or dowels at spindle size to locate them clear of each other. Clean and oil them after use, and store clear of damp; also, to avoid the chance of condensation in a closed box let the cutter cool down after use and before storing, because of the danger of rust forming at the cutting edges.
As they are expensive, there are no thanks to be gained by rushing out and ordering a big set of cutters at one go! Where I find that a man is skilled and has an interest in this work, I encourage him to learn the full use of this equipment and to build up his kit progressively as he finds that particular tasks are likely to be repeated; by this means each tool is an asset and will pay for itself.
Bear in mind also that milling cutters require some power to drive them, particularly as you go on to the broader cutting faces, and this applies also where the operation is deep slotting with a cutter that has cutting edges on the side too; therefore you will be calling for a good deal more power than any normal turning operation. Further to this and in line with my comments on the end-mill; all slack must be removed from slides and feed screws that will be used during an operation, otherwise serious chatter can set up, and this chatter will be seen 'on the work; it will also worsen as the job proceeds rather like the mechanic overloading and misusing the adjustable hand reamer in a brass bush.
Therefore set up your machine to be on the stiff side and do not operate with even barely perceptible slack, as the load is heavy and even a "thou" or so of slide or screw freedom will cause the work to move away from the tool pressure.
Another important point, in view of the possibility of chatter, is that where the work is mounted on a vertical slide and clamped there, even with the strongest clamps there can be some little "springing" in evidence, even just that "thou" or so. Where the operation calls for a direct up-and-down load between cutter and work, it pays to fit a steel step in the bottom slot of the vertical slide, and build up firm packings from tthis step to the underside of the work. This foundation should be applied wherever possible as it adds the final touch to a troublefree mounting, and further eliminates the chance of chatter.
The cutters are nothing more in general shape than a plain gear or sprocket, with a central drive hole, and therefore must be firmly mounted on a spindle, unlike the end-mill which is formed on its own shaft. Again there are two common types: one is the tapered arbor—seated in the lathe mandrel taper—which has a parallel section at the outward end, reduced in diameter from a good shoulder at the large end of the taper.
This parallel section has sufficient length to take the broader cutter; outboard from the parallel section is the thread and nut. Obviously, various thickness cutters will be used; therefore a series of plain spacer washers can be needed to fit on the plain section and take up the gap to the cutter. These spacers can be of varied thickness and can be in sleeve form to cut down the numbers required, but all must be well finished at their end faces; any old washer will not do as any unevenness will or can cause the cutter to wobble, and you will end up with a wide slot or other discrepancy that you cannot afford.
The cutter should be perceptibly proud of the parallel section, in order that the smaller bore plain washer on the thread section can transmit full load from the nut, just as described on the companion flange to shaft assembly dealt with recently. Milling cutters are also mounted on plain arbors to work between centres, either driven by the chuck or a carrier.
These arbors are equipped with a shoulder stop and you position your cutter and build-up from there with sleeves and machine-faced plain washers. However, remember that the final sleeve or washer at the nut end, must be sited so that full nut load can be applied to the cutter.