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SCREW THREADS IN nuts are, in most instances, produced by taps, though large threads and those which do not come within a recognised rate are produced on machines such as lathes, that is they are screwcut. Over a wide range of sizes, the production of threaded holes in metal is not in itself a difficult operation, as it demands only suitable drills, taps, wrenches, and some knowledge of threads to be produced-plus the skill which comes with practice. Standard taps are shown in Fig. 1. The taper tap has a long taper at the end for starting easily in a hole. The second tap is less tapered to follow behind the first one, and the plug, or third tap, is parallel down to the end. 

 

Threads and How to make them

 

 

Proportions of a thread

The main proportions of a thread are the pitch, angle, and depth. The pitch is the distance. advanced by the thread in one complete turn. In many instances a pitch is chosen which results in complete numbers of turns in 1 in. Thus, pitch is expressed as the number of turns per inch, or t.p.i. The angle of thread is chosen according to design requirements, and is standard for different rates; common angles are 60 deg., 55 deg. and 47-1/2 deg. The depth is governed by the angle, and by the radius at the top and at the bottom, though threads can be sharp or flattened at top and bottom. The depth, however, is always the same proportion of the pitch for a particular rate of thread.

 

 

 

Rates of threads

The different rates of threads, are given in text books, but the newcomer to practical work need be concerned with only three. These are the British Standard Whitworth (B.S.W. or Whitworth), the British Standard Fine (B.S.F.), and the British Association (B.A.). In the two first rates, the diameter of the thread is given as a fraction of an inch, and the pitch or t.p.i. is related to the outside diameter. B.S.W. is a coarser rate than B.S.F., and has fewer t.p.i. for a given diameter; at present it is used in general and agricultural engineering, while B.S.F. is common on cars and motor cycles, though for special purposes, one or other rate may be used. The angle of both threads is 55 deg. The B.A. rate is a small millimeter thread, used in electrical, instrument, and’ model work. Outside diameters are given in millimetres and fractions of a millimetre, and the pitch similarly. Thus, the pitch does not produce exact t.p.i. These threads are given numbers from 0 to 25, and to deal with them it is only necessary to know the outside diameter, and the tapping and clearing drill sizes-obtainable from tables The angle of this thread is 47-1/2 deg. The tapping drill size produces the core diameter of the hole,. and this must be suitable for the particular rate of thread, so that a full thread is produced, and no more. If the hole is small, too much metal will be left, and it will be difficult or impossible to produce a thread, or in small sizes, the tap may be broken.

 

 

 

Truncated threads

Alternatively, if the hole is too large. the thread produced will be truncated or flat at the bottom instead of rounded, and such a thread is weak depending on the extent of truncation. If the metal to be tapped is tough, however, a certain amount of truncation is not undesirable. Tables show the correct size tapping drill for standard rates of thread as mentioned,, and these should be followed; it is then only necessary to centre punch the position of the hole in the metal, and drill the hole squarely. If the hole goes right through the metal, a taper tap can be used, followed by a second or plug. If the hole is “ blind ” however, the taper tap will not bite, and a second tap is required for a start, with the plug tap to finish the thread to the bottom of the hole.

 

 

 

How to tap

When tapping, work should be mounted firmly, the tap presented squarely and rotated by means of the wrench. After a turn or two, resistance will increase and it is necessary to ease the tap back carefully, then advance again as far as possible, the process being repeated until the hole is tapped; complete withdrawal and cleaning of the tap is also necessary on occasion. For lubrication, thin oil may be used for steel, and paraffin for duralumin and aluminium. Worn taps, like worn dies, produce threads tight in the core or on the outside diameter, due to loss of radius.

 

 

A RIGHT-ANGLE microscope made from a star diagonal can be used in the chuck of a lathe to give optical precision for two important settings. The first is to true the axis of the tailstock to that of the spindle after the tailstock has been set over for taper turning. The second is to centre cross lines on work on the vertical slide, a necessary preliminary to machining with revolving tools. These extra settings represent a new departure for a microscope on the amateur’s lathe. They bring the instrument into the class which includes jig borer microscopes and optical centre finders for centre lathes and drilling machines.

The principle of operation has been used in industry for several years. It was introduced by Watts for optical plumbing in theodolites and was patented by them. They then used it for their Watts Microptic Centre Locator, an instrument which was smaller than a jig borer microscope, and suitable for use on drilling machines, milling machines and centre lathes.

It is obvious that the microscope is a useful if not indispensable tool for a modem workshop. By using it, a turner learns the principles and tricks of the latest practice. Construction is not difficult for anyone with experience of general turning.

 

Setting up with a Star Diagonal

 

 

 

To mount the microscope in the chuck, a parallel spindle is fitted to the star diagonal in the same way as the taper shank for fitting to the tailstock. The same clamp can be used. The parallel spindle is in a bearing which is gripped in the three-jaw or four-jaw chuck. The end of the spindle has a thread for two nuts which are locked when they have been adjusted to a firm friction grip. The fit of the spindle in the bearing and the adjustment of the nuts must give a firm mounting without any trace of shake. Diagram A shows the set-up.

The microscope is focused on two lines on a flat (pad) centre in the tailstock. These lines are scribed with a pointed tool so that they precisely straddle the axis of the pad. The tailstock is clamped to the bed, and the barrel is advanced until the lines are clearly seen. Then the barrel is clamped and the microscope adjusted.To avoid inaccuracy, the lines are scribed on the pad by the method illustrated at diagram B. The pad is turned from mild steel with a taper for the tailstock barrel. Then it is mounted in a bush in the fourjaw chuck. This bush should be bored in the chuck to take the pad. The scribing tool is set just above (or below) centre height, and a stop is used to two opposite jaws of the chuck. By this method, the lines must lie equally each side of the axis. The principle of operation has been used in industry for several years. It was introduced by Watts for optical plumbing in theodolites and was patented by them. They then used it for their Watts Microptic Centre Locator, an instrument which was smaller than a jig borer microscope, and suitable for use on drilling machines, milling machines and centre lathes.

 

To adjust the microscope, we first turn it to position CI. The optical axis, as seen from V, must then coincide with the lathe axis. This gives a line of sight from W to X. To counteract wobble in the chuck mounting, we turn the chuck while making observation in the microscope. The field as seen by the scribed lines moves between extreme positions Y and Z, which are 180 deg. apart. From one or the other, the chuck is turned 90 deg. and the position V is marked with chalk. It is obvious that the microscope is a useful if not indispensable tool for a modem workshop. By using it, a turner learns the principles and tricks of the latest practice. Construction is not difficult for anyone with experience of general turning. To mount the microscope in the chuck, a parallel spindle is fitted to the star diagonal in the same way as the taper shank for fitting to the tailstock. The same clamp can be used. The parallel spindle is in a bearing which is gripped in the three-jaw or four-jaw chuck. The end of the spindle has a thread for two nuts which are locked when

they have been adjusted to a firm friction grip. The fit of the spindle in the bearing and the adjustment of the nuts must give a firm mounting without any trace of shake. Diagram A shows the set-up. The line in the microscope ocular should lie between the two scribed lines on the pad, as at D. This setting can be obtained from the optical micrometer. Then the microscope is turned over to position Cz; the setting should be the same. If it is not, we adjust the tailstock to half the error, and also adjust the optical micrometer. Then a recheck is made from the first position CI. With a few adjustments, spot-on accuracy is obtained. For work on the vertical slide, you use the microscope in this way for the vertical line of the crossed pair, but bring the line precisely to the one in the ocular. For the horizontal line, the microscope is set vertically, and the chuck is turned 90 deg. so that V is at the side. Then you bring the horizontal line precisely to the one in the ocular. The spindle for mounting the microscope can be 1/2 in. dia. and the bearing I in. dia. X 1 1/4 in. long. A thread not coarser than 26 t.p.i. should be used for the spindle locknuts.

 

 

WHILE the slide movements provided on a metal turning lathe for sliding, surfacing and angular work on a horizontal plane are normally sufficient, the scope of the lathe can be extended to include many kinds of work beyond ordinary turning by the fitting of attachments which provide a vertical movement. Many of these devices, varying in complexity and utility, have been described in ME, and have been put on the market by makers of lathes and their accessory equipment. The most popular attachment is the vertical slide, which was first used by instrument makers and horologists and was later adapted to larger lathes up to 3 1/2 in. or more in centre height. Many model engineers consider the vertical slide indispensable. One form has a perpendicular slideway, integral with the base or rigidly fixed to it, so that the sliding table can move only in a vertical plane, and the other has a slideway which can be swivelled about a horizontal axis to provide for oblique sliding. Attachments to carry a milling spindle or a dividing head are often mounted on the vertical slide table, or are made with a self-contained elevating movement. It occurred to me that the three functions of the vertical work table, milling spindle and dividing head could quite easily be combined in one compact unit; this idea is not new, but it has seldom been carried out as neatly or thoroughly as it might be.One of the most important requirements of any fixture used for milling in the lathe is that it should be as rigid as possible to avoid deflection, which is liable to destroy accuracy, and cause digging-in or chattering of the cutting tool. At the very best, there are limits to the usefulness of a light lathe for milling operations, as its slides are not designed for this work, and it is often necessary to mount the work (or the cutting tool) at some distance from its point of support. Most vertical slides nowadays are of very sturdy design, but this is of little use if the entire fixture is liable to deflection. The number of separate parts or articulated joints must necessarily affect rigidity, and the non-swivelling vertical slide is generally to be preferred.

 

 

Milling Attachment with Three Functions

 

 

But the weakest feature, indeed the Achilles heel, of the orthodox vertical slide is its attachment by a single bolt to the cross-slide. No matter how broad the base of the slide, or the strength of the fixing bolt, there is a limit to the security which can be obtained by its anchorage to a T-slot in the cross-slide or boring table of the lathe. A practical improvement, though one not easy to carry out without an effect on the adaptability of the fixture, would be to provide the base with a broad foot to take two or more well-separated fixing bolts. The overhang of the sliding work table from the base of its vertical support is another factor which affects rigidity. Many fixtures, especially those which swivel, do not provide the support to withstand the levering action caused by rotating cutters-generally much greater than the continuous action of a comparable single-point tool. In the appliance shown, the form of vertical support is unusual. It is intended to provide rigidity with the minimum overhang of the work table or milling spindle. Its principle is logical, and by no means without precedent in machine tool design, as the prismatic form of slideway or lathe bed. Has been used in some of the best machine tools. But the major

reason for the use of a square pillar is ease of production in the home workshop. The pillar itself can be made from stock mild steel bar, which is obtainable in. sufficiently accurate shape to need no more than a little filng and scraping, with the normal checking. Exactness of angle on the four sides is not highly important so long as the sliding parts are fitted to

the bar; this also can be done by filing and scraping.

 

 

 

 

The pillar is mounted on a part-circular baseplate with flats on the sides to bring it within the width of a normal cross-slide-there is no advantage in any overlap-but of an adequate diameter that will provide adequately stable mounting. Slots in the base give a latitude which will accommodate variations in the spacing of the T-slots in the crossslide, and also the angular position of the base. A 1/2 in. hole is drilled through the centre of the pillar and counterbored at both ends to provide! a register recess. It is secured to the base by a long tension stud, which can be tightened much more securely than is possible with the central anchorage bolt usually fitted on a vertical slide. Alternatively, the pillar could be permanently attached to the base by brazing or welding, but this might restrict the rotation of the pillar for setting the table at angles other than 90 degrees to the lathe axis, unless the base-plate is modified. On the side of the pillar opposite to the vertical table, a housing is provided for either a milling spindle or a simple form of dividing head. The bearing for both fittings is of the self-contained quill type; you can remove or replace it without upsetting adjustments. In the general arrangement drawings, the dividing spindle is shown in position; it is designed to take the same nose and socket fittings as the lathe itself, so that work can be transferred from one to the other. The tail end of the spindle takes any of the lathe change wheels, which can be indexed by a spring plunger mounted on a banjo clamped to the quill. This gives a range of divisions which will cope with many-perhaps most workshop requirements. The range can easily be extended by the addition of a worm gear or other multiple indexing device. If milling or indexing is never likely to be required, the spindle housing may be replaced by a V-grooved saddle, similarly fitted to slide on the pillar with the vertical table. But the housing takes up so little room, and is so unlikely to get in the way or affect operation of the slide, that it is worth fitting even if its use is not at first envisaged. Only vertical movement of the slide is provided, in the interests of simple construction and rigidity. In my experience, angular movement of the slide is rarely needed; you can generally obtain the essential object by mounting the work obliquely on the table. The spindle housing, in the form shown, is also restricted to the horizontal position. For some kinds of work, provision for swivelling the housing about the horizontal axis, and also for fitting an overarm support bar would be an advantage, but these refinements have already been anticipated in the design and will be incorporated in a modified version. The elevating movement of the slide is provided by a screw, the bearing of which is in an overhanging plate attached to the top of the pillar. It engages a tapped hole in the housing or saddle casting, and is operated by a disc handwheel graduated to serve as an index. This is, in my opinion, much better than the popular ball handle with a separate index disc; it gives a more open spacing of divisions owing to its larger diameter and is therefore easy to read. But this is, of course, an optional fitting, and may be modified to suit your preference. The screw is intended to be 3/8 in. BSF 120 t.p.i., so that if 50 divisions are engraved on the disc, each will represent a vertical movement of 1/1,000 in. Although feedscrews for machine slides generally have square or Acme threads, and are coarser than most standard V forms, the Vs together with their corresponding internal threads, are quite serviceable and provide finer adjustment. They are likely to be at a disadvantage only when frequent and rapid traverse is required.

 

In making this appliance, you should begin with the square pillar. Having obtained a suitable piece of square steel bar, or machined it from the solid, you check it for general truth, and mark it out carefully for central drilling. It is then set up in the four-jaw chuck, and its true running is checked over the four flats-not over the corners, which may vary in sharpness. As you cannot steady the projecting end of the bar (except by making a special fitting) the overhang may cause some lack of rigidity, tending to affect the accuracy of drilling. It will therefore be worth while for you to allow an extra 1/2 in. or so of length, which, after the centre-drilling and supporting on the back centre, you can turn circular and run in the three-point steady; this extension will of course have to be parted off afterwards, but it can be retained until you have machined the register recess.The pillar need not be made to the specified length (though this will give a convenient range of vertical movement for most purposes on lathes of about 3 1/2 in. centres). Neither is it essential that the hole should be exactly central. But it must be at least parallel to the axis, so that when the ends are faced at the same setting as the drilling operation the pillar will stand exactly vertical on a level surface. Take all possible pains to be accurate. Though errors in machining can be corrected in fitting, it is much better if they do not occur in the first place. Your most suitable tool for forming the recess at each end of the pillar is a counterbore. You can make one by fitting a double-ended high-speed or silver steel bit in a mild steel bar 1/2 in. diameter, to suit the drilled hole. The exact size of the recesses is not critical, as the spigots of the baseplate and cap can be fitted to them.The baseplate can be made from an iron casting or a piece of steel plate. In the machining, the important points are and the fit of the spigot. The position of the slots for bolting it to the cross-slide may be modified to suit the spacing of the T-slots in the lathe on which it is to be used. For the tension stud, a suitable length of 1/2 in. mild steel bar is screwed at each end to fit the tapped hole in the baseplate, and to take a standard nut at the top. Screwed rod or studding may be used, but it is not so strong as a properly made double-ended stud. I recommend simple iron castings for both the sliding table and the quill housing. You can cast in the T-slots in the table by providing a corebox and putting core prints on the pattern, but unless they are cast smooth and accurate they may be more trouble than they are worth. It is quite possible to machine the slots from the solid, or from rudimentary grooves cast directly from the pattern, when the table is fitted to the pillar and mounted on the cross-slide.

 

The table casting is first machined on the front surface. After it has been mounted in the four-jaw chuck, and reversed for similar treatment on the joint face, you can machine the four edges square by mounting the casting face down on an angle plate. To make sure that these edges are in reasonably true angular relation to the V groove, clamp a round bar in the groove and use it as a location gauge. If you do not have a shaping or milling machine you can file and scrape the V surfaces to fit the pillar. Use an accurate flat bar, or a slip of plate glass, to test the individual side surfaces, with the aid of marking colour or “mechanics’ blue.” You can again use the round bar as a gauge to check both the squareness of the groove with the edges of the table, and its parallel truth with the face. Lay the table on a surface plate with the bar in the groove, and measure its underside distance from the plate at each end with inside calipers, or over the bar with a dial test indicator. After the quill housing has been machined on the joint face, you can deal with it in the way that you fitted the V groove. When offered up to the pillar, the joint faces may not make contact, and there is a choice between increasing the depth of the grooves or of fitting shims between the faces. To machine the bore of the quill clamp, the casting may be mounted on an angle plate and squared off carefully from the V groove. The bore should be dead parallel and both ends faced to provide a true seating for the quill when fitted either way round. Before splitting the clamp, the holes for the clamp screws should be drilled, tapped and spot faced.

 

 

 

IN the normal way, collet chucks are used on lathes with hollow spindles, which admit of long material being fed through from the rear ends. By this means small parts can be machined, parted off, and the material moved along in the collets without need for cutting into lengths with a hacksaw. The same is true, of course, when a three-jaw or four jaw chuck is used. However, a great many jobs perhaps most in a small workshop require much less than the whole length through the lathe spindle; and one of the solid type will generally provide sufficient distance from the front of the chuck jaws back into the taper where the centre is fitted. There is naturally more overhang with this than with collets, which can have the minimum projection from the spindle,. and there is more size and weight m the chuck, which may seem out of proportion for small parts. But given the limit on length of material, it is only necessary to provide mountings for collets, or make up special types, to take advantage of the facilities these can offer.For standard collets which are used in a nose-piece, a mounting is as at A. On a hollow-spindle lathe, not adapted for collets, the nosepiece is mounted on a small back plate, and the collets are operated with a draw-tube or rod through the spindle. Here a mounting plate is bolted with packing to the faceplate, the nosepiece is bolted or held -by screws to the mounting plate, and there is a special nut for the collets, with radial holes for operating by a tommy bar, between the mounting plate and the faceplate.The mounting plate can be flat rectangular mild steel, wide enough to take the flange of the nose-piece, while the packing can be two parallel pieces of square mild steel. Drilled, cleaned up by filing, and bolted up to the faceplate, these pieces can be centre punched for refitting in original positions.  

 

Collets for Chuck and Faceplates

 

 

After centring, the plate is drilled, and the recess machined for the flange of the nose-piece. If this part is being made, it can be bored and tapered after attaching by screws to the mounting plate. The nut for the collets-in round mild steel-can be turned with a step to locate in the bore in the mounting plate, and fitted either with the faceplate removed from the spindle, or by detaching the mounting plate. Abutting to the end of the spindle, the nut pushes the collet out for freeing the work. An example of a collet for use in a jaw chuck is as at B. For making this, a piece of mild steel rod is faced, centred, and drilled with a clearance hole in what will be the back. Then it is reversed in the chuck, and the process gone through again for the length where the work will be held finishing by reaming if possible. Turned down to take a clamp, the diameter is undercut at the shoulder and slit lengthwise-to be springy and contract easily. A centre punch dot to No 1 jaw admits of true resetting after unchucking.

 

An adaptation of the collet is as at Cl. The reversed piece of mild steel is turned down, but left solid, the end filed half through, and a loose piece fitted with the clamp. With the collet rechucked, this end is drilled and reamed for the work-and for grip, the loose piece is eased at the joint line by filing. Collets to contract by a nut can be of two types, as at C2 and C3. In the first type, the nut has a coned end which bears on the split end of the collet jaws; while in the second type, the nut has a taper thread (from not putting the tap right through) running on a taper thread obtained from regulating the opening of the die which is used for cutting it. A plain bush, of course, can often be used as a collet, split wrth a cut along the side. For slitting the ends of collets. blocks as at- D can be bored for holding, and a step filed or machined to accept and guide the saw blade.

 

 

 

64. Useful Tool Grinding Jig
Nov 26, 2017

THE object of this article is to enable the model engineer to produce accurate flat faces on the tool-faces which can be struck again instantly on a regrind. Perhaps its first recommendation is that it can be produced by anyone possessing a 3& in. lathe. Next, in order of merit, is that it can be produced from scrap.As a matter of expediency the prototype has been built up from aluminium alloy angles and plate, and for this reason, perhaps, the scantlings are on the heavy side. In steel, all thicknesses, with the possible exception of the top plate, could be reduced, but this is a matter of individual taste. Any grinding jig can, of course,‘be offered up to the wheel at any unusual angle, but usually an alteration in the angular inclination entails a consequent shift towards, or away from, the wheel face if safe and efficient support is to be given to the tip of the tool. The next most desirable feature of the appliance is that it can be secured permanently to the bench, with the edge of the top table no more than just clear of the wheel. The top table rolls round an axis coincident with its front edge-protractor markings on one side angle enabling it to be set at any desired angle within its scope. The addition of another protractor on the top plate enables the operator to carry out double angle grinding. Not all tools may be ground with equal facility, of course, but with the aid of a few simple accessories, any normal tool can be precision ground as to front and side clearance and side rake. This category would include right- and left-hand knife tools, roughing tools with flat faces, screw cutting and parting tools. Round-nose tools with a common clearance at front and sides may also be ground by using the top table alone, set to the appropriate angle.

 

Useful Tool Grinding Jig

 

 

The jig illustrated is used at the side of the wheel and it is possible that modification to the guard may be necessary to accommodate it. It is essentially intended for tool finishing and would, therefore, be set up to the fine wheel of a double-ended grinder. When forming a new tool from a stock bit, the roughing-out is done by hand on the coarse wheel. The angle is tested from time to time by touching it to the fine wheel with the jig set to the appropriate angles-the final grind is done on the fine wheel, of course. Once established, the tool angles can be struck again at any time. The setting of the jig and the grinding of a normally blunted tool is a matter of seconds; honing, too, is reduced to the minimum

 

The table top

For this a slab of plate h in. x 104 in. x 4 in. is required. It should be flattened to a straightedge to reduce filing or machining. It is then cut to shape using hacksaw and file. The centre recess on the front edge is made to accommodate the nut and washer securing the wheel. It is not really necessary to machine the top face but it should be smooth and flat to promote easy manipulation of the protractor guide. A groove, & in. wide x { in. deep is machined in the top face to accommodate the guide bar. This can be done by setting up the plate on angle plates secured to the cross slide with the centre line of the groove at lathe centre height-a & in. end mill in the S.C. chuck is employed.

 

It is unlikely that the full length of the groove can be machined at one setting as few slides on small lathes have the necessary travel, but a careful resetting will enable this operation to be completed. That part of the inner edge of the plate remaining after it is gapped for the wheel nut must be beveled to an included angle of 60 deg. or slightly less. Later it will be necessary to cut two small notches in the outer edge & in. wide x & in. deep to give passage to the trunnion bearing flange when the angularity of the top table approaches the maximum.

 

The sole plate

This is a simple rectangular piece of plate which can be made by hacksaw and file. It should be reasonably flat otherwise it may distort when screwed down to the bench and tend to jam the trunnions. If the wheel is the full 8 in. dia., a recess in the upper surface will need to be cut. Holes for the holding down screws are left to individual requirements.

 

 

 

Top trunnions

It will be seen that the bearing edges of these are machined to a radius struck from a centre outside their surfaces. They are formed from 24 in. x 24 in. x fin. angle bar, each 2# in. long. The outside faces of the flanges should be checked for 91, deg. angularity and corrected if necessary.The cross-sectional edges should be squared off to assist setting-up for machining. For the purpose of roughing out the radii, a tin or card template may be used with advantage for marking out ; the surplus is then readily removed with a hacksaw.

Before setting up for machining, guide lines (Fig. 1) are lightly marked on the facedate-these fix the positio& o$ the edges of the angle bars. With the scribing block on the lathe bed. scribe the first mark + in. yor the finished thickness bf the top plate)

above the lathe centre, carrying the line right across the faceplate. Now turn the mandrel through 90 deg. so that this first line is vertical to the shears.

 

Scribing the location lines

Scribe two further lines (Nos. 2 and 3), 7132 in. above and below the lathe centre. These are the locating lines for the inner edges of the right- and left-hand trunnion angles. The sketch shows how these lines appear when completed. Return the mandrel to its original position and bolt on the angle plate with its heel edge set to the line, in the lower back quadrant. Secure the right-hand trunnions to the angle plate, taking care that its outer flange overhangs the edge of the angle plate by about $ in. (Fig. 1). The vertical flange of the tnmnions is set parallel to the faceplate with its toe in line with No. 2 line, i.e., 7/32 in. off-centre. In slow back-gear turn this edge until the tool is just touching the toe of the 24 in. edge. The heel of the trunnion should now measure 2; in. (approximately) and the radius about 2% in. The left hand trnnnion should be machined in a similar fashion, but with the angle plate mounted on the faceplate in the lower, front quadrant. If two angle plates are available, both trunnions can be machined in one operation.

 

Cutting the zero line

With a sharp V-pointed tool, the centre line fol the radial slots should be marked + in. from the turned edge; likewise the line which borders the protractor marking 5/32 in. from the edge. With the same tool mounted on its side at lathe centre height a zero line is cut on the face of the trunnion at an angle of 45 deg. Some form of dividing is desirable for cutting in the protractor markings but failing this, a draughtsman’s protractor can be set up with the exercise of a little ingenuity and used as a dividing head with sufficient accuracy for most requirements. Short lines at every degree, with slightly longer ones at the 5 deg. and 10 deg. stations are engraved by feeding in the cross slide for the requisite amount. Figures (& in.) stamped later at every 5 deg. station assist in setting when the jig is in use.

 

 

 

Protractor markings 

It will be noted that protractor markings need be engraved on one trunnion only. The opposite trunnion requires only the centre line for the slot to be marked in at this stage. The slots could be milled in before dismounting by the use of a slide rest mounted milling spindle. Failing this, the slots are drilled out and finished bv filing. Do not omit to bevel the inside edges of the-top flinges to an angle of 60 deg., as shown. In the initial stages the trunnion  bearings should be dealt with. As far as roughing-out is concerned this is done in a similar fashion to their mating angles C, again bearing in mind that there is a left- and a right-hand unit. The angle sections in this instance are 39 in. long.

 

 

 

Setting up trunnion bearings

Again they are set up on the angle plate secured to the faceplate in the settings appropriate for a boring operation. It is possible that the off-set required may tax the faceplate beyond its capacity, in which case the work piece could be mounted with its inner surface to the angle plate (Fig. 2), due allowance being made for flange thickness if the angle plate has to be reversed. For this operation, the lines inscribed on the faceplate are 1 and 2, 3% in. above and below centre height and 3 25132 in. to the left of centre. Machining is carried on with a boring tool until its point is just scraping the toe of the flange as before. It is a wise plan to check the radius produced, as the operation approaches completion by using the mating trunnion as a gauge. A smear of blue on the bearing edge of the latter will indicate progress and machining should be stopped when the two surfaces are shown to be mating. A touch with a scraper at the bench should then ensure smooth action between the two parts.

 

Clamp assemblies

The shapes of these 8 in. plates are identical, but, on assembly, they need to be handed to right and left, therefore the securing studs must be screwed through the plate to suit. No. 2 B.A. clear holes are drilled and countersunk. The clamp plates, secured to the trunnion angles in the correct setting, are used as jigs for marking off the tapped holes in the bearing angles. It is wise to lock the clamp plates in position with the studs at the extreme ends of the slots-this precaution enables the table to be returned to zero without checking when in use. With the trunnions and bearings and clamp plates assembled after drilling 2 B.A. clear holes in the top and base flanges, they are secured to the table top and sole plate in turn for drilling the tapped holes in the latter paas. The setting should be carried out with maximum care for, as will be realised, the set-up is now the equivalent of a shaft rolling in a bearing.

 

 

 

Ensuring smooth working

If the trunnions are not axially true in their bearings the smooth working of the apparatus will be destroyed for the same reason that a bent shaft will not turn in a bearing. To prevent this, the trunnion and bearing angles should be secured temporarily to the top and sole plates by means of clamps. When the desired freedom of working is ensured the holes are marked off, or drilled by using the angles as jigs.

With the fastening screws in place, the main unit is substantially complete. It is almost certain that packing will be needed to bring the surface of the top plate reasonably into line with the centre of the stone. A slab of seasoned hard wood should be chosen and bedded to the bench top to avoid rockobviously the top surface of the packing should make an exact right angle with the side of the stone.

 

 

 

Nuts for holding down

For the purpose of holding down, & in. square nuts may be sunk into the under surface of the packing before the latter is screwed down to the bench-the holding down screws then screw in from the top. The holes for these through the packing should be drilled a little oversize to give some latitude to the setting of the jig.

 

Top plate protractor guide

The drawing is self explanatory and the only difhculty is the marking of the scale. Here again, in the absence of dividing apparatus, a draughtsman’s protractor can be made-to serve. A special mark should be made at 278 deg. on either side of the zero line, thus being half-the-angle of Whitw-orth screwcutting tools. For the sake of stiffness the guide strip which engages in the groove in the top plate should be of steel. It should bear on the bottom of the groove and the top surface should stand a thou. or two proud of the surface of the top plate A. This ensures that the clamp will hold it securely without distortion. Needless to say, the strip should be. a sliding fit in the groove. The remaining accessories, the extension guide (G), the square (H) and the “ boat ” (J) are simple and easy to make. The G-clamp can be bought for a shilling or so at any tool dealers.

 

 

 

 

 

 

Guide is left floating

The protractor is used for all normal, simple tools such as right- and left-hand knife tools, roughing and parting tools and, in fact, any tool which has flat facets which do not deviate too far from the zero angle. In grinding the tools mentioned, the protractor guide is left floating and is worked to and fro with one hand along the groove, while the tool is nresented to the stone with the other hand, so distributing the wear on the latter as far as possible. It will be noted that if the angle of the tool is too acute, the influence of the guiding edge of the protractor is lost, particularly on short tools. Tool-holder and boring-bar bits, for example, are not easy to align with the protractor swung round to a large angle. In this circumstance, the extension guide (G) is brought into use. The protractor is set to the desired angle and fixed at a suitable station on the top plate by means of the G-clamp. The extension guide is laid along the edge of the protractor with one end near to the stone, thus supplying the necessary guiding edge to the short tool

 

 

 

Making it a short interlude

The square will be found necessary when grinding acute angles, which operation is illustrated in the sketch. It is also used when grinding the top faces of tools with the table set to give the necessary side rake and the protractor set for zero, positive or negative back rake, the latter being necessary when grinding certain tool-holder bits, particularly those for screw cutting in a Jones and Shipman tool holder, for example. The boat (J) is used for underlining tools such as planer and shaper tools. A little preliminary practice with the apparatus will transform the chore of tool grinding into nothing more than a very short interlude in the more serious business of getting on with the job, as well as removing the question of tool angles from the realms of guesswork.

 

 

 

63. Tips For Tapping
Nov 14, 2017

PERHAPS the greatest cause of tap breakage is excessive pressure applied at an angle to the axis of the hole. It is also the cause of the hole being out of alignment.Some time ago I had to tap a number of 2 BA holes about 1/2 in. apart and fix lengths of screwed rod. The plate was about 3/16 in. thick and when finished the spaces between the rods were anything but parallel. This incident, together with two broken taps, was the origin of the following device, designed to eliminate tap breakage when turning back to clear the tap. It also acts as a guide to ensure the tapping pressure is evenly applied, and controls the feed.Construction is simplicity itself. Only ordinary straight turning is needed if a solid bar is available. Alternatively two pieces of pipe welded together and turned to fit can be used but this seems somewhat clumsy.

 

Tips For Tapping

 

 

A 5 in. length of 1 3/4 in. dia. Shafting or b.m.s. round is -chucked in the three-jaw and turned down to 1 in. dia for 3 3/4 in. in length. With fixed steady supporting the 1 in. dia. end, the bar is bored through with a 1/4in. or 3/16 in. dia. long reach drill using high speed and plenty of lubricant. Don’t forget, especially when drilling holes of abnormal depth, to ease back the drill many times to clear the flutes-unless, of course, the swarf is ejecting continuously.The hole is opened up in stages to 11/16 in. dia. Check the diameter of the tap holder and if necessary open out to a slide fit. The tap holder shown is the Eclipse No 143. Reverse the bar in the chuck and turn a recess to suitable depth and diameter to fit the drill chuck, i.e. dimensions A and B. This should be a firm non-sloppy fit over the nose of the chuck.

 

Next, either mill a 1 l/32 in. wide slot in each side of the barrel or make one by drilling through, cutting up with the hacksaw and finishing with a file, to allow the cross-bar to pass through. This slot will enable the tap holder to slide downwards as the threading proceeds. For the smaller size Eclipse No 43 holder for 11/16 in. barrel dia., bore or drill out to 1/2 in., and for the length of slot 2 1/2 in. (see diagram), substitute 1 1/2 in. The rest of the dimensions can be scaled down to suit. The compression spring should be fairly stiff for this size holder but weaker for the No 43 size, i.e. BA and 3/16 in. Whit. and below. The length of the spring should be such that in its normal length it should allow the cross-bar to be about 1/2 in. from the open end of the slot. The diameter of the spring should be a free fit inside the bore of the barrel.

 

Although primarily designed for use with a drill press, if the work is too large to be ‘accommodated it is easy to rig up a suitable jig. A plain turned spigot could replace the chuck. This could be clamped to the work or bench. However, with the drill press, the work already drilled to correct tapping size is located and bolted or clamped to the table and either packed up, or the drill head lowered until the tap engages the hole and the spring in the barrel becomes slightly compressed. The cross-bar is then turned and the gadget takes care of the rest. The drill remains stationary, except, of course that the chuck revolves with the rotation of the crossbar and tap holder

 

 

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