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UNLESS special setting-up provision has been made, machining a cylinder for a small internal combustion engine can present something of a problem, at least on the first occasion, or until experience has been gained. The difficulties encountered-more so when a thin sleeve has to be machined-are due to the relatively thin wall thickness of the part, and the need for a firm hold during the operation, with no distortion at the finish.Even a heavy chucking grip over the base flange of a cylinder will not necessarily be a complete safeguard against movement during the boring, and may well cause distortion for the bore to be out-of –round when the cylinder id unchucked. A heavy grip for initial rough boring, followed by an easing of chuck jaw pressure-to minimise distortion for finishing-may on occasion prove successful, with some risk of movement when the pressure is eased. Holding on the outside of cooling fines, whose diameter despite the thinness furnishes additional resistance against circular distortion, is another possibility at times, providing the fins are ‘sufficiently thick and true at their outer edges, and providing also a grip can be obtained over several-which necessarily requires deep chuck jaws.

 

Machining Small Cylinders

 

 

Another way-which, however, demands some extra machining-is to make provision on the pattern for a base flange larger and longer than functionally necessary, to serve for chucking, then to machine this surplus away after boring the cylinder-mounting it on a mandrel for the purpose. But when no such provision has been made, or when the component is a cylinder sleeve, other methods must be adopted. For a cylinder with base flange and fins, or for a plain thin sleeve, the first set-up can be as at A, on a centred and threaded mandrel. This passes right through the bore cored in the cylinder or sleeve, and subject to clearance therein should be as substantial as possible for a powerful hold to be obtained through the nuts on the washers Y and Z. Rough-nesses at the ends where the washers abut should be smoothed off first by filing; and the total length of the mandrel should be such that a carrier can be applied on end to take the drive between centres. At this set-up, with discreet machining, the flange and spigot end of a cylinder can be trued; or a thin sleeve may be turned over its whole length. In the case of a cylinder, the subsequent set-up for boring can be as at B. A locating ring is made from any suitably-sized piece of steel, brass or aluminum alloy, which is parallel, or can be faced parallel, and the bored to fit on the spigot, reasonably but not exceptionally tightly. This provides a backing for the cylinder on the faceplate, where it can be held by the- flange with clamps-four preferably for a square flange, locating one at each comer.

 

Following boring-as accurately and smoothly as possible-the cylinder can be mounted by the bore for finishing the ends-the face of the flange, the spigot to length and diameter. The end of the bore can also be chamfered at this set-up to give a lead-in for rings.The mandrel can be a piece of material held in the chuck, centred, then turned and smoothed-with a fine file—for the cylinder to be an easy push fit most of the way, tight at the finish. Alternatively, expansion hole from the end, taping for a coned screw, and slitting lengthwise or the mandrel can be threaded, tapered, and a nut and split sleeve employed. In the case of a thin sleeve or cylinder liner, a boring set-up can be made as at D, after machining the outside. Two sufficiently-long blocks are bolted to the angle plate on the faceplate, the lower one separately so as not to move after boring, when the upper is loosened to insert the liner. 

  

32. Some Facts about Tapers
May 23, 2016

Apart from many other uses a taper is often the means of securing a flywheel, a sprocket, a gear or pulley to a shaft either with or without a key. For such purposes, a taper is usually mechanically more satisfactory than a plain interference fit between shaft and bore, a sliding fit with a drive-in key, or a sliding fit with a locating shoulder and, a nut for holding.

The angle of a taper has a considerable influence on the grip exerted when components are pulled together; and the smaller the angle, or the less the change in diameter for unit length, the more powerful the hold. A taper may be dimensioned or designated in two common ways, A. The angle, P, may be given irrespective of the size of the shaft, which may be convenient when the angle is a whole degree, such as 5 deg., or whole degree and a simple fraction, such as 6 deg. Alternatively, the smaller and larger diameters, Q and R, may be given and the distance, S, between them. In such an event, the taper may also be given as so much per foot or per inch as the case may be.

 

On a lathe, a fairly quick taper is machined by setting the topslide at an angle and using this for machining; while a slow or gradual taper longer than can be machined from the topslide is produced by setting over the tailstock, or using a set-over centre when the work is mounted between centres. Again, on lathes so equipped, a taper turning attachment may be used to control movement of the cross-slide, leaving the work in longitudinal alignment with the lathe bed. Some experiments are virtually always necessary in setting up for machining a taper, since no graduation, particularly of the topslide, is sufficiently accurate. Moreover, it is important for the tool to be at centre height, otherwise, variations in shape and angle occur, B. When the tool is at centre height it lies on the horizontal centre line of the shaft, where the slide angle is the same as the taper. If the tool is dropped, however, to plane, T, the effect on the larger diameter, U, is much less than on the smaller diameter, V. In obtaining well-fitting tapers, size, angle and finish, both externally and internally, are extremely important. If one has the choice when fitting two parts on a “one-off” job the shaft is better finished last, as it is the more easily adjusted for size to secure longitudinal location, and the simpler to provide with a good finish, or on which to correct small inaccuracies, from careful use of a fine (Swiss) file and/or abrasive cloth. Variations in longitudinal location with size are shown at C, where the component moves from W to X as the bore is increased.

 

Some Facts about Tapers

 

 

If there is no choice in procedure, as when fitting a new sprocket to an existing shaft, the taper is picked up on the topslide with the shaft in the lathe and a mandrel turned to the same taper, but smaller. This is used for preliminary testing, though the final testing should be with the shaft itself. Illustrations, D, show some ways in which an internal taper can be faulty. The bore may be large on the outside or inside, when on entering the mandrel and “feeling”, slackness can be noticed. Pushing in the mandrel and twisting also shows where the taper is touching. When the taper is faulty, the topslide requires adjustment. Finish is important, since if the mandrel rides on ridges the fitting will be poor though the angle may be correct. Slight correction and improvement follows from grinding the parts together, but too much reliance should not be placed on this. For small bores, silver-steel tools, E, can be made by bending as Y or turning to leave a diameter, Z, both of which can be filed to shape, then hardened and tempered. Of course, with a taper reamer available, or made from silver steel sizing and finishing are simplified.

 

 

Employing the lathe on a jig-boring principle, with the tool rotating and the work mounted on the vertical slide with longitudinal feed by saddle movement inevitably raises problems of setting and feeding the tool in use. For holes, it is a boring tool, of course, though parallel bosses and spigots can be tackled by the same method.Usually the tool is mounted in the four-jaw independent chuck for its radial setting to be conveniently obtained by adjusting the chuck jaws. This is done by starting at small diameters and working outwards for holes, and at large diameters working inwards for bosses and spigots. A considerable amount of work may be done in this way; though if dimensional accuracy is important, some difficulties will almost certainly be encountered on approaching finished sizes in the absence of fine, controlled radial feed of tools. It is not disputed, of course, that setting up can be done in the four-jaw chuck to virtually “spot on” accuracy given a dial indicator, time and patience in ordinary use. But it is always noticeable on getting down to fine dimensions, that the mere pressure of the chuck jaws can have a marked effect. Slackening will set a marked wobble into the work; and the tightening of one without slackening the other can be quite sufficient to correct a wobble or start one. In turning it may not be of great importance, because once a setting is obtained it is finished. But working with the tool rotating and needing to be set for each cut, it can present a problem towards the finish when there is no certainty as to depth of cut applied.  

 

Setting rotating boring tools

 

Fine radial feed or setting of tools can be obtained in either of two ways on the principle of the inclined plane, or on that of the eccentric. The inclined plane gives a small radial movement for a larger endwise one, and the eccentric provides a similar redial movement for considerable rotational displacement. First, however, there may be the problem of locating the work accurately by scribed lines to a truly spinning needle, which, to avoid the chuck jaw push-over effect, can be mounted in a holder as at A. The holder is held in the chuck and the needle-soldered in a hole in the central rod-set true at the tip by adjustments to four small screws. The inclined plane method of tool feed can be arranged as at B, using an angle plate on a faceplate. The fixed block is bolted to provide the guide and reference base for the tool-holder, which is kept pressed up to it and unclamped, and move endwise for setting cut. Inclination can be according to needs; but about 1-1/2 deg. will give a feed of 1/40, increasing diameter by about 0.001 in. for 0.020 in. endwise movement. Eccentric feed for a tool can be provided as at C. The body is circular to hold in the chuck and has a few eccentricity in the bore, while the taper-ended toolholder is fitted in a ring which can be turned in the body with the draw bolt slackened. Setting up adjustment can be made locating the tool holder in the ring, and final fine cuts obtained turning the ring in the body. With a four-jaw chuck, roughing cuts can be applied by adjusting the jaws.The alternative is as at D, with a circular plate located on an eccentric spigot in the spindle bore, and clamped back to the faceplate. The tooholder is held by studs and nuts to the plate, with major adjustment made through slots. The spigot can be turned eccentrically by mounting the plug in the spindle with a strip of shim stock one side. To set tools initially, an indicator can be mounted as a E, on the base to push across the lathe bed, picking up the reading from a mandrel turned to required diameter X, or using a bridge gauge as at F, a slip gauge Y, of radius dimension, will give the reading for the tool tip.

 

 

30. Feeds in Lathe Milling
Apr 24, 2016

In setting up work for milling in the lathe, an important influence is the manner in which the feed will be applied. The lathe spindle runs in the normal direction when driving a milling cutter – that is, the top of the cutter rotates towards the operator or looking from the headstock towards the tailstock, the cutter runs in a clockwise direction. Drills and end-mills rotate in this manner; and although a saw or slotting cutter could be reversed it would mean running the lathe spindle the opposite way, with the possibility of the chuck or driving plate unscrewing. With the work mounted on the vertical slide, feed can be applied either from the cross-slide or from the vertical slide with the longitudinal feed of the saddle on the bed employed for putting on cut or locating the work in relation to the cutter, though this arrangement may be modified according to circumstances. Having regard to cutter rotation, and the way in which feed may be applied, the next point is the manner in which the cutter will contact the work. In ordinary milling, the feed forces the work on to the cutter, as at A, and the thrust of feed and cutter is always in opposition. This means that is the cross-slide feed is used the work should be mounted above the cutter and a start made near the operator for the work to be traversed away from him. To maintain the same conditions with cross-slide feed the work could be mounted to run under the cutter and brought from the far side towards the operator. But in many instances the cross-slide feed is too limited for this. On occasion, from some convenience there may be in setting up or because the hazards are overlooked, the conditions of operation may be reversed and the down-cut milling principle introduced, as at B, with the possibility of breaking the cutter or spoiling the work from chatter or digging-in. 

 


Feeds in Lathe Milling

 

This is because, as the cutter rotates, the tooth which is nearing the end of its cut has a tendency to carry the work with it, taking up the slack away from the feedscrew. When this happens the next tooth to strike the work has a greatly augmented cut to contend with, and in a bad case the tooth may be chipped or the cutter broken in halves; while there is always the danger of the work being moved or the finish spoilt. Even with the slide gibs tightened so that the feed is stiff the risk remains. Consequently, whenever possible the down-cut principle, as at B, should be avoided in milling in the lathe. Using vertical-slide feed in a downwards direction, conditions are satisfactory when the work is on the far side of the lathe away from the operator, as at C. With the work mounted near to the operator the feed should be upwards, as at D, as when slotting a piece of material to make a fork. A slight tendency to dig in may be noticed on commencing a cut if this is to be very deep. The remedy is to take a series of light cuts. The principles apply in end milling when the cut is on the side of a bar, as at E (top), and in machining a slot or channel (bottom), the cut is balanced. Where a slot is to be widened the method at F should be adopted. Cutting on the top of the slot, cross-slide feed should be away from the operator; cutting on the bottom, it should be towards operator. The same applies when milling a dovetail, as at G, where feed conditions should be as F. A keyway for a Woodruff or “half-round” key is machined as at H (top) with a direct feed on to the cutter; usually downwards from the vertical slide. A long keyway bottom can be cut with cross-slide feed under conditions similar to A. 

 

 

Though toolmaker’s buttons are the usual means of locating systems of hole centres accurately, there are various adaptations and extensions of the principle which can be conveniently employed. This applies if more than one part has to be dealt with, or if two halves or matching portions (of a casing, say) must be in accurate alignment. A mounting plate or jig plate with holes in required positions may then be used for setting up though in making the plate the holes would be located with buttons. Again, a button plate in which buttons (which may be of a special type) can be located either side may be required for matched components. The ordinary button is attached to the work or component, as at A1, with a screw and washer, set true using an indicator, then removed for the hole to be bored. If a button is made with a recess concentric with the outside diameter, a small sleeve can subsequently be used to locate it in a plate, as at 2, on either side as required. But if this arrangement is questioned from the point of view of accuracy, the type of button at 3 can be used, this having an integral spigot to locate it in its hole. The initial layout of the plate is effected, of course, with ordinary buttons for the holes to be bored for the special type.

 

Buttons and Mounting Plates

 

 

Examples of matching components on which accuracy is essential are flywheels which also form the webs of a crankshaft as for an inside flywheel i.c. engine. Differences in the throw of the crankpin in the two flywheels would inevitably lead to wobble on the mainshafts, as would occur if the crankpin holes were bored out of square. Accuracy can be ensured, however, by either using a button direct or employing a mounting plate. To use a button direct, as at B (left), each fly wheel is bored for the main shaft, and a plug inserted for measuring over to locate the button, which can then be brought to spin truly and the flywheel clamped to the faceplate. To employ a mounting plate (right), this must first be provided with two accurately spaced holes (using buttons); then with the mounting plate trued and clamped or bolted to the faceplate, a plug will locate the flywheel which can be separately clamped.

 

Such a mounting plate can be trued on the faceplate with a plug in the hole to be engaged by the indicator, or by using the indicator with a swinging arm direct in the hole. Parts with more than two holes can be set up; and the method is of equal value in the case of a radius outside a component, as at C. Here the mounting plate has been prepared with hole X at the radius point; so when this hole is spinning truly, and the casting is located by a stepped plug in the other, then clamped, an accurate set-up is ensured. Use of a button plate taking spigoted buttons either side enables halved or matching parts to be set up separately, so that bores which may not come to the outside can be accurately located. A common example is the timing case of a single cylinder i.c. engine where the cam spindles or bushes are enclosed. In a small size, the button plate, as at D, can be prepared, then located to the crankcase half by a plug in the main shaft bore, for screw or stud holes to be drilled, by which the plate can be held. A button having been fitted, the whole crankcase half can be adjusted on the faceplate to bring the button plate can be taken off and the work proceed. Other holes are dealt with in the same manner. For the timing case or cover, a mounting plate is prepared from the button plate, and the centre cut out, so with the case attached at the back, on a set-up as at E, the button plate (with button reversed) can be set up for truing, then afterwards removed for machining.

It is a merit of a screw-cut threat that, whatever faults it may possess, it is always true with the end face or any shoulder on the work-assuming, of course, that all operations have been performed at a single setting, or that for the screw cutting operation the work has been properly set up. The thread may he fine or coarse, badly-shaped, undersize or taper, but at least it is never out-of-square. This inherent squareness can be an important or even vital feature in the practical construction and assembly of components. It does not follow automatically when, for speed or convenience, threads are produced and in its absence, assembly or functional difficulties may occur. Two faces, whether shoulder-type or taper, pulled together by accurate screw-threads, about extremely firmly, and the reaction is a purely axial one completely free from tilt. Parts such as caps and plugs, can be pressure-tight without effort or precaution and squareness of alignment follows when, for example, such a construction is employed for attaching a web on a crankshaft. In the absence of squareness and accuracy, a wedge-shaped gap is left between the faces of screwed components, as at A, ans this can result in pressure leakage, or difficulty in making a joint with a washer or strain, malalignment and insecurity in the case of a constructional feature. Where the fault is on the thread of studs, or in the directional alignment of tapped holds, there may be direct assembly difficulty from inability to fit a cover. For correction, a component with an internal thread can sometimes be mounted on an accurate threaded mandrel in the lathe, and the face trued by a light cut. A faulty stud can be renewed; but if a tapped hole is out of true, about the only thing to do is fit a stud tightly, screw on a nut, and with hammer blows bring the stud up square.

 

Accuracy in Thread Cutting

 

Better that correction at any times to ensure that threads are produced squarely, or at least as closely as possible approaching that condition. In many instances, particularly if components are already on the lathe, it may be convenient to screw cut threads t about there-quarter depth, then clean and size them with a die-which will be relieved of a considerable amount of work. The same is true for internal threads large enough to be screw cut before finishing with a tap-a procedure essential for threads too large to tap straight out, but needing to be quickly and uniformly sized. On work of a size normally tapped, accuracy can be ensured, as at B, by supporting the tap from the tailstock centre with the tap wrench resting on the top slide, pulling the chuck round by hand, and feeding up the tailstock barrel. If there is no centre at the end of the tap, a small hollow centre must be used in the tailstock. For work requiring an external thread, as at C, the principle can be employed with an ordinary die-holder, supporting this from the topalide, and backing up the die from a flat or pad centre in the tailstock-when the die should be fitted the reverse way to normal for its throat, as usual, to run first on the work. Accuracy of threads in medium and small tapped holes follows from using a guide either tapped squarely itself or drilled square to the nominal diameter of the tap. Location to the hole to be tapped can be obtained from a short length of rod of core diameter when it is plain, as at D. Then guide and work can be clamped. A bench or pillar drill, its chuck locating, not gripping, the shank of a tap wrench, may be used for accurate tapping as at E, the work held or clamped on the table, and the tap holder turned by fingers. Starting a die squarely, as at C, it will often continue truly on work transferred to the vice, or a guided die-holder can be used, of which a simple example is as at F.

 

 

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