Chapter III: Locating Work for Boring on Milling Machine

It is often desirable to perform boring operations on the milling machine, particularly in connection with jig work. Large jigs, which because of their size or shape could not be conveniently handled in the lathe, and also a variety of smaller work, can often be bored to advantage on the milling machine. When such a machine is in good condition, the necessary adjustments of the work in both vertical and horizontal planes, can be made with considerable accuracy by the direct use of the graduated feed-screw dials. It is good practice, however, when making adjustments in this way, to check the accuracy of the setting by measuring the center distances between the holes directly. For the purpose of obtaining fine adjustments when boring on the milling machine, the Brown & Sharpe Mfg. Co. makes special scales and verniers that are attached to milling machines, so that the table may be set by direct measurement. By attaching a scale and vernier to the table and saddle, respectively, and a second scale to the column with a vernier on the knee, both longitudinal and vertical measurements can be made quickly and accurately, and the chance of error resulting from inaccuracy of the screw, or from lost motion between the screw and nut, is eliminated.

Checking Location of Holes by Micrometer-and-plug Method

One method of checking the accuracy of the location of holes bored in the milling machine, is to insert closely fitting ground plugs into the bored holes and then determine the center-to-center distance by taking a direct measurement across the plugs with a micrometer or vernier caliper. For example, if holes were to be bored in a jig-plate, as shown in Fig. 1, assuming that hole _A_ were finished first, the platen would then be moved two inches, as shown by the feed dial; hole _B_ would then be bored slightly under size. Plugs should then be accurately fitted to these holes, having projecting ends, preferably of the same size. By measuring from one of these plugs to the other with a vernier or micrometer caliper, the center distance between them can be accurately determined, allowance being made, of course, for the radii of each plug. If this distance is incorrect, the work can be adjusted before finishing _B_ to size, by using the feed-screw dial. After hole _B_ is finished, the knee could be dropped 1.5 inch, as shown by the vertical feed dial, and hole _C_ bored slightly under size; then by the use of plugs, as before, the location of this hole could be tested by measuring center distances between _C-B_ and _C-A_.

An example of work requiring the micrometer-and-plug test, is shown set up in the milling machine in Fig. 25. The large circular plate shown has a central hole and it was necessary to bore the outer holes in correct relation with the center hole within a limit of 0.0005 inch. The center hole was first bored and reamed to size; then an accurately fitting plug was inserted and the distances to all the other holes were checked by measuring from this plug. This method of testing with the plugs is intended to prevent errors which might occur because of wear in the feed-screws or nuts, that would cause the graduated dials to give an incorrect reading. On some jig work, sufficient accuracy could be obtained by using the feed-screw dials alone, that is, without testing with the plugs, in which case the accuracy would naturally depend largely on the condition of the machine.

A method that is a modification of the one in which plugs are used to test the center distance is as follows: All the holes are first drilled with suitable allowance for boring, the location being obtained directly by the feed-screw dials. A special boring-tool, the end of which is ground true with the shank, is then inserted in the spindle and the first hole, as at _A_ in Fig. 1 is finished, after which the platen is adjusted for hole _B_ by using the dial as before. A close-fitting plug is then inserted in hole _A_ and the accuracy of the setting is obtained by measuring the distance between this plug and the end of the boring-tool, which is a combination tool and test plug. In a similar manner, the tool is moved from one position to another, and, as all the holes have been previously drilled, all are bored without removing the tool from the spindle.

Another modification of the micrometer-and-plug method is illustrated in Figs. 26 and 27. It is assumed that the plate to be bored is finished on the edges, and that it is fastened to an angle-plate, which is secured to the table of the milling machine and set square with the spindle. A piece of cold-rolled steel or brass is first fastened in the chuck (which is mounted on the spindle) and turned off to any diameter. This diameter should preferably be an even number of thousandths, to make the calculations which are to follow easier. The turning can be done either by holding the tool in the milling machine vise, or by securing it to the table with clamps. In either case, the tool should be located near the end of the table, so as to be out of the way when not in use.

After the piece in the chuck is trued, the table and knee are adjusted until the center of the spindle is in alignment with the center of the first hole to be machined. This setting of the jig-plate is effected by measuring with a micrometer depth gage from the top and sides of the work, to the turned plug, as illustrated in Fig. 26. When taking these measurements, the radius of the plug in the chuck is, of course, deducted. When the plate is set the plug is removed from the chuck and the first hole drilled and bored or reamed to its proper size. We shall assume that the holes are to be located as shown by the detail view, Fig. 26, and that hole _A_ is the first one bored. The plug is then again inserted in the chuck and trued with the tool, after which it is set opposite the place where the second hole _B_ is to be bored; this is done by inserting an accurately fitting plug in hole _A_ and measuring from this plug to the turned piece in the chuck, with an outside micrometer as indicated in Fig. 27. Allowance is, of course, again made for the radii of the two plugs. The horizontal measurement can be taken from the side of the work with a depth gage as before. The plug is then removed and the hole drilled and bored to the proper size. The plug is again inserted in the chuck and turned true; the table is then moved vertically to a position midway between _A_ and _B_, and then horizontally to the proper position for hole _C_, as indicated by the depth gage from the side of the work. The location can be verified by measuring the center distances _x_ with the micrometer. In a similar manner holes _D_, _E_, _F_ and _G_ are accurately located.

If the proper allowances are made for the variation in the size of the plug, which, of course, is made smaller each time it is trued, and if no mistakes are made in the calculations, this method is very accurate. Care should be taken to have the gibs on all sides fairly tight at the beginning, and these should not be tightened after each consecutive alignment, as this generally throws the work out a few thousandths. If the reductions in the size of the plug, each time it is turned, are confusing, new plugs can be used each time a test is made, or the end of the original plug can be cut off so that it can be turned to the same diameter for every test. If the center distances _x_ are not given, it is, of course, far more convenient to make all the geometric calculations before starting to work.

The Button-and-plug Method

The use of the button method as applied to the milling machine, is illustrated in Fig. 28, where a plain jig-plate is shown set up for boring. The jig, with buttons _B_ accurately located in positions corresponding to the holes to be bored, is clamped to the angle-plate _A_ that is set at right angles to the spindle. Inserted in the spindle there is a plug _P_, the end of which is ground to the exact size of the indicating buttons. A sliding sleeve _S_ is accurately fitted to this plug and when the work is to be set for boring a hole, the table and knee of the machine are adjusted until the sleeve _S_ will pass over the button representing the location of the hole, which brings the button and spindle into alignment. When setting the button in alignment, all lost motion or backlash should be taken up in the feed-screws. For instance, if the button on the jig should be a little higher than the plug in the spindle, do not lower the knee until the bushing slips over the button, but lower the knee more than is required and then raise it until the bushing will pass over the button. This same rule should be followed for longitudinal adjustments.

After the button is set by this method, it is removed and the plug in the spindle is replaced by a drill and then by a boring-tool or reamer for finishing the hole to size. In a similar manner the work is set for the remaining holes. The plug _P_ for the spindle must be accurately made so that the outer end is concentric with the shank, and the latter should always be inserted in the spindle in the same relative position. With a reasonable degree of care, work can be set with considerable precision by this method, providing, of course, the buttons are properly set.

Some toolmakers use, instead of the plug and sleeve referred to, a test indicator for setting the buttons concentric with the machine spindle. This indicator is attached to and revolves with the spindle, while the point is brought into contact with the button to be set. The difficulty of seeing the pointer as it turns is a disadvantage, but with care accurate results can be obtained.

Size Block and Gage Method

Another method which can at times be employed for accurately locating a jig-plate in different positions on an angle-plate, is shown in Fig. 29. The angle-plate is, of course, set at right angles to the spindle and depth gages and size blocks are used for measuring directly the amount of adjustment. Both the angle-plate and work should have finished surfaces on two sides at right angles to each other, from which measurements can be taken. After the first hole has been bored, the plate is adjusted the required distance both horizontally and vertically, by using micrometer depth gages, which should preferably be clamped to the angle-plate. If the capacity of the gages is exceeded, measurements may be taken by using standard size blocks in conjunction with the depth gages.

It is frequently necessary to bore holes in cast jig-plates or machine parts, which either have irregularly shaped or unfinished edges. A good method of locating such work is illustrated in Fig. 30. The part to be bored is attached to an auxiliary plate _A_ which should have parallel sides and at least two edges which are straight and at right angles to each other. This auxiliary plate with the work, is clamped against an accurate angle-plate _B_, which should be set square with the axis of the machine spindle. A parallel strip is bolted to the angle-plate and the inner edge is set square with the machine table. After the first hole is bored, the work is located for boring the other to the edge of the auxiliary plate, and horizontal measurements _y_ between the parallel and the plate. These measurements, if quite large, might be taken with micrometer gages, whereas, for comparatively small adjustments, size blocks might be more convenient.

Vernier Height Gage and Plug Method

When a vernier height gage is available, it can often be used to advantage for setting work preparatory to boring in a milling machine. One advantage of this method is that it requires little in the way of special equipment. The work is mounted on an angle-plate or directly on the platen, depending on its form, and at one end an angle-plate is set up with its face parallel to the spindle. An accurately finished plug is inserted in the spindle and this plug is set vertically from the platen and horizontally from the end angle-plate, by measuring with the vernier height gage. After the plug is set for each hole, it is, of course, removed and the hole drilled and bored or reamed.

The way the plug and height gage is used is clearly illustrated in Figs. 31 and 32. The work, in this particular case, is a small jig. This is clamped directly to the machine table and at one end an angle-plate is also bolted to the table. This angle-plate is first set parallel with the traverse of the saddle or in line with the machine spindle. To secure this alignment, an arbor is inserted in the spindle and a test indicator is clamped to it by gripping the indicator between bushings placed on the arbor. The table is then moved longitudinally until the contact point of the indicator is against the surface plate; then by traversing the saddle crosswise, any lack of parallelism between the surface of the angle-plate and the line of saddle traverse will be shown by the indicator.

When the work is to be adjusted horizontally, the vernier height gage is used as shown in Fig. 32, the base of the gage resting on the angle-plate and the measurement being taken to an accurately ground and lapped plug in the spindle. For vertical adjustments, the measurements are taken between this ground plug and the machine platen as in Fig. 31.

Locating Holes to be Bored from Center-punch Marks

The problem of accurately locating holes to be bored on the milling machine has received much attention, and the method generally used when accuracy has been required is the button method, which was previously described. So much time is required for doing the work by this method, however, that numerous efforts have been made to obtain equally good results in other ways.

The increasing demand for rapidity combined with accuracy and a minimum liability of error, led to the development of the system described in the following: A center-punch mark takes the place of the button, from which to indicate the work into the proper position for boring. The fundamental principle involved is to lay out, accurately, two lines at right angles to each other, and correctly center-punch the point where they intersect. With proper care, lines may be drawn with a vernier height gage at right angles, with extreme accuracy, the chief difficulty being to accurately center the lines where they cross. For semi-accurate work this may be done with a common center-punch but where extreme accuracy is required this method is not applicable, as the average man is incapable of marking the point of intersection accurately.

The diagram, Fig. 33, illustrates, in a simple way, the procedure adopted in laying out work by this system. The base _E_ is in contact with a surface plate while the line _BB_ is drawn with a height gage; then with side _F_ on the plate the line _AA_ is drawn. It will be seen that these lines will be at right angles to each other, if the bases _E_ and _F_ are square. Work done by this method must have two working surfaces or base lines, and these must be at right angles to each other. There is no difficulty in drawing the locating lines _AA_ and _BB_ correctly, either with a vernier height gage or with a special micrometer gage reading to 0.0001 inch, the only difficult element being to accurately center-punch the lines where they intersect as at _D_. It is assumed that two holes are to be bored, so that the intersection at _C_ would also be center-punched.

The scriber point of the height gage should be ground so that it will make a V-shaped line, as shown by the enlarged sketch _G_, rather than one which would resemble a saw-tooth, as at _H_, if a cross-section of it were examined with a microscope. This is important because when the lines are V-shaped, an accurate point of intersection is obtained.

As it is quite or almost impossible to accurately center-punch the intersection of even two correctly drawn lines, by ordinary means, the punch shown in Figs. 34 and 36 was designed and an extended experience with it on a very high grade of work has demonstrated its value for the purpose. It consists essentially of a small center-punch _O_ (Fig. 36) held in vertical position by a holder _P_ which is knurled to facilitate handling. Great care should be exercised in making this tool to have the body of the punch straight, and to have it stand at right angles to the surface to be operated upon, for the slightest inclination will cause the finished hole to be incorrect, no matter how carefully the lines are drawn. The 60-degree point must be ground true with the axis. The holder for the punch stands on three legs, located as indicated, and ground accurately to a taper fit in the holder, where they are secured by watch screws bearing on their tops. The lower ends are hardened, and terminate in an angular point of 55 degrees (the point of the vernier scriber being 60 degrees). The edges are sharp, and slightly rounded at the ends, so that the legs will slide along a line smoothly. The points _V_ and _U_ (Fig. 36) have edges that are in line with each other, while the point _T_ has an edge at right angles to the other two. The center of the punch is located at equal distances from all the legs, and is held off the work normally by a leather friction acted upon by a set-screw in the side of holder _P_.

If this tool is placed upon lines of the form shown at _G_, Fig. 33, the legs _V_ and _U_ may be slid along horizontal line _B-B_, Fig. 33, until the sharp edge of leg _T_ drops into line _A-A_. When this occurs the punch _O_ is lightly tapped with a hammer, and the resulting mark will be accurately located in the center of the intersection of the lines. It is good practice to make the work very smooth before drawing the lines, and after laying them out, to stone them so as to remove the slight burr raised in drawing them. A drop of oil is then rubbed into the lines, and the surplus wiped off. This procedure permits points _V_ and _U_ to run very smoothly along the line, and the burr having been removed, the edge of leg _T_ drops into the line very readily with a slight click. As it is not advisable to strike punch _O_ more than a very light blow, it marks the work but slightly, and a more distinct indentation is made with the follower punch shown in Figs. 35 and 37. This punch is made like the previous one, so that it will stand at right angles to the work. The sectional view (Fig. 37) shows the punch _A_ supported by the holder _E_ which has four legs cut away on the sides so that the point of the punch may be seen. When this punch is in position, it is struck a sufficiently heavy blow to make a distinctly visible mark. The work is now ready to be placed upon the work table of the milling machine, and indicated for boring the holes, an indicator being used in the milling machine spindle.

An indicator which has been found especially valuable for this purpose is shown in Figs. 38 and 39. It is of the concentric centering type, and with it the work is brought concentric with the axis of the spindle. The arbor _I_ is provided with a threaded nose on which disk _D_ is screwed. This disk has four holes in its rim, equally-spaced from each other. Hardened, ground, and lapped bushings _b_ are put into these holes to receive plug _A_ which is made a gage-fit both in these holes and in hole _B_ in the outer end of sector _C_. This sector is held by a split sleeve to the barrel _L_ which carries the 60-degree centering-rod _K_ that comes into contact with the work to be bored. The spherical base of barrel _L_ fits into a corresponding concave seat in the nose of the arbor at _H_, and is held in place by a spring _E_ which connects at one end to the cylindrical stud in the base of the barrel, and at the other to the axial rod _M_ by which it and the other connecting parts may be drawn into place, and held by the headless set-screw _J_, bearing on a flat spot on the tang end of the rod.

Now, if plug _A_ is removed from bushing _b_ the point of the centering-rod _K_ may be made to describe a circle. At some point within this circle is located the center-punch mark on the work to be bored. The holes in the rim of the faceplate all being exactly the same distance from an axial line through both the arbor _I_ and centering-rod _K_, it follows that the center mark on the work must be so located by horizontal and vertical movements of the work table that pin _A_ may be freely entered in all the four holes in the rim of disk _D_. When that occurs, the center coincides with the axis of the spindle.

The point of the center-punch _A_ (Fig. 37) should have an angle slightly greater than the angle on the centering-rod _K_, as it is impossible to locate the work in the preliminary trials so that the center of the work will be coincident with the axis of the spindle, and unless the precaution mentioned is taken, the true center on the work is liable to be drawn from its proper location when trying to bring the work into such a position that the plug will enter all the holes in the disk. As the work being operated on is brought nearer to the proper location by the movements of the milling machine table, spring _G_ will be compressed, the center rod sliding back into barrel _L_. This spring is made so that it will hold the center against the work firmly, but without interfering with the free rotation of the sector _C_ around disk _D_. When the work is located so that the plug enters the holes, the gibs of the machine should be tightened up and the plug tried once more, to make sure that the knee of the machine has not moved sufficiently to cause the work on the table to be out of line. The work table is now clamped to prevent accidental horizontal shifting, and the work is drilled and bored.

In using this indicator the milling machine spindle is not rotated together with arbor _I_, only the sector being turned around the disk. The tool is set, however, in the beginning, so that the axes of two of the bushings _b_ are at right angles to the horizontal plane of the machine table, while the axes of the other holes in the disk are parallel with the top of the work table. The centering-rods are made interchangeable and of various lengths, to reach more or less accessible centers. Fig. 38 shows the indicator with one of the long center-rods in the foreground.

The only part of the milling machine on which dependence must be placed for accuracy is the hole in the spindle, and this is less liable to get out of truth, from wear such as would impair the accuracy, than are the knee, table, or micrometer screws. The only moving part is the sector, and this, being light, is very sensitive.

A series of 24 holes was laid out and bored in one and one-half day by the method described in the foregoing. Measurements across accurately lapped plugs in the holes, showed the greatest deviation from truth to be 0.0002 inch, and running from that to accuracy so great that no error was measurable. This same work with buttons would have required considerably more time.

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