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.
[Illustration: Fig. 25. Example of Precision Boring on Milling 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.
[Illustration: Fig. 26. Obtaining Vertical Adjustment by Means of Depth
Gage and turned Plug in Chuck]
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.
[Illustration: Fig. 27. Adjusting for Center-to-center Distance by use
of Plugs and Micrometer]
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.
[Illustration: Fig. 28. Accurate Method of Aligning Spindle with Button
on Jig-Plate]
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.
[Illustration: Fig. 29. Locating Work from Edges of Angle-Plate by
means of Depth Gages and Size Blocks]
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.
[Illustration: 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.
[Illustration: Fig. 31. Making a Vertical Adjustment by Measuring to
Ground Plug in Spindle]
[Illustration: 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.
[Illustration: 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.
[Illustration: Fig. 34. Center Locating Punch]
[Illustration: 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_.
[Illustration: Fig. 36. Section of Center Locating Punch]
[Illustration: 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.
[Illustration: 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.
[Illustration: 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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