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.
Fig. 30. Method of Holding and Locating Casting
of Irregular Shape, for Boring Holes
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.
Fig. 31. Making a Vertical Adjustment by Measuring to Ground Plug in Spindle
Fig. 32. Making a Horizontal Adjustment by measuring from Angle-Plate to Ground Plug
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.
Fig. 33. Diagram Illustrating Rapid but Accurate Method of
Locating Holes to be bored on Milling Machine
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.
Fig. 34. Center Locating Punch
Fig. 35. Center Enlarging Punch
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.
Fig. 36. Section of
Center Locating Punch
Fig. 37. Section of
Center Enlarging Punch
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.
Fig. 38. Indicator used for Aligning Punch Marks with 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.
Fig. 39. Sectional View of Indicator shown in Fig. 38
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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Transcriber’s Notes:
The illustrations have been moved so that they do not break up paragraphs and so that they are next to the text they illustrate.
Typographical and punctuation errors have been silently corrected.