Sunday, November 17, 2013

Jig and Fixture Design - Principles


PRINCIPLES OF JIG DESIGN

Jigs and fixtures may be defined as devices used in the manufacture of duplicate parts of machines and intended to make possible interchangeable work at a reduced cost, as compared
with the cost of producing each machine detail individually.
Jigs and fixtures serve the purpose of holding and properly locat- ing a piece of work while machined, and are provided with neces- sary, appliances for guiding, supporting, setting, and gaging the tools in such a manner that all the work produced in the same jig or fixture will be alike in all respects, even with the employ- ment of unskilled labor. When using the expression "alike," it implies, of course, simply that the pieces will be near enough alike for the purposes for which the work being machined is intended. Thus, for certain classes of work, wider limits of variation will be permissible without affecting the proper use
of the piece machined, while in other cases the limits of varia- tion will be so small as to make the expression "perfectly alike" literally true.

Objects of Jigs and Fixtures. The main object of using jigs and fixtures is the reduction of the cost of machines or machine details made in great numbers. This reduction of cost is ob- tained in consequence of the increased rapidity with which the
machines may be built and the employment of cheaper labor,
which is possible when using tools for interchangeable manu-
facturing. Another object, not less important, is the accuracy
with which the work can be produced, making it possible to
assemble the pieces produced in jigs without any great amount of
fitting in the assembling department, thus also effecting a great
saving in this respect. The use of jigs and fixtures practically
does away with the fitting, as this expression was understood in
the old-time shop; it eliminates cut-and-try methods, and does
away with so-called "patch- work" in the production of machin-
ery. It makes it possible to have all the machines built in the
shop according to the drawings, a thing which is rather difficult
to do if each individual machine in a large lot is built without
reference to the other machines in the same lot.

The interchangeability obtained by the use of jigs and fixtures
makes it also an easy matter to quickly replace broken or worn-
out parts without great additional cost and trouble. When
machines are built on the individual plan, it is necessary to fit
the part replacing the broken or worn-out piece, in place, involv-
ing considerable extra expense, not to mention the delay and the
difficulties occasioned thereby.

As mentioned, jigs and fixtures permit the employment of
practically unskilled labor. There are many operations in the
building of a machine, which, if each machine were built indi-
vidually, without the use of special tools, would require the work
of expert machinists and toolmakers. Special tools, in the form
of jigs and fixtures, permit equally good, or, in some cases, even
better results to be obtained by a much cheaper class of labor,
provided the jigs and fixtures are properly designed and cor-
rectly made. Another possibility for saving, particularly in the
case of drill and boring jigs provided with guide bushings in the
same plane, is met with in the fact that such jigs are adapted to
be used in multiple-spindle drills, thereby still more increasing
the rapidity with which the work may be produced. In shops
where a great many duplicate parts are made, containing a
number of drilled holes, multiple-spindle drills of complicated
design, which may be rather expensive as regards first cost, are
really cheaper, by far, than ordinary simple drill presses.

Another advantage which has been gained by the use of jigs and fixtures, and which should not be lost sight of in the enu- meration of the points in favor of building machinery by the use
of special tools, is that the details of a machine that has been
provided with a complete equipment of accurate and durable
jigs and fixtures can all be finished simultaneously in different
departments of a large factory, without inconvenience, thus mak-
ing it possible to assemble the machine at once after receiving
the parts from the different departments; and there is no need
of waiting for the completion of one part into which another is
required to fit, before making this latter part. This gain in
time means a great deal in manufacturing, and was entirely
impossible under the old-time system of machine building, when
each part had to be made in the order in which it went to
the finished machine, and each consecutive part had to be lined
up with each one of the previously made and assembled details.
Brackets, bearings, etc., had to be drilled in place, often with
ratchet drills, which is a slow and always inconvenient operation.

Difference between Jigs and Fixtures. To exactly define the word "jig, " as considered apart from the word "fixture,"
is difficult, as the difference between a jig and a fixture is often-
times not very easy to decide. The word jig is frequently, al-
though incorrectly, applied to any kind of a work-holding appli-
ance used in the building of machinery, the same as, in some
shops, the word fixture is applied to all kinds of special tools.
As a general rule, however, a jig is a special tool, which, while it
holds the work, or is held onto the work, also contains guides
for the respective tools to be used; whereas a fixture is only
holding the work while the cutting tools are performing the oper-
ation on the piece, without containing any special arrangements
for guiding these tools. The fixture, therefore, must, itself, be
securely held or fixed to the machine on which the operation is
performed; hence the name. A fixture, however, may sometimes
be provided with a number of gages and stops, although it does
not contain any special devices for the guiding of the tools.

The definition given, in a general way, would therefore clas- sify jigs as special tools used particularly in drilling and boring
operations, while fixtures, in particular, would be those special
tools used on milling machines, and, in some cases, on planers,
shapers, and slotting machines. Special tools used on the lathe
may be either of the nature of jigs or fixtures, and sometimes the
special tool is actually a combination of both, in which case the
term drilling fixture, boring fixture, etc., is suitable.

Fundamental Principles of Jig Design. Before entering upon a discussion of the minor details of the design of jigs and fixtures, the fundamental principles of jig and fixture design will be briefly outlined. Whenever a jig is made for a compo- nent part of a machine, it is almost always required that a corre- sponding jig be made up for the place on the machine, or other part, where the first-mentioned detail is to be attached. It is,
of course, absolutely necessary that these two jigs be perfectly alike as to the location of guides and gage points. In order 'to
have the holes and guides in the two jigs in alignment, it is advis-
able, and almost always cheaper and quicker, to transfer the
holes or the gage points from the first jig made to the other. In
many instances, it is possible to use the same jig for both parts.
Cases where the one or the other of these principles is applicable
will be shown in the following chapters in the detailed descrip-
tions of drill and boring jigs.

There are some cases where it is not advisable to make two jigs, one for each of the two parts which are to fit together. It
may be impossible to properly locate the jig on one of the parts
to be drilled, or, if the jig were made, it may be so complicated
that it would not be economical. Under such conditions the
component part itself may be used as a jig, and the respective
holes in this part used as guides for the tools when machining
the machine details into which it fits. Guide bushings for the
drills and boring bars may then be placed in the holes in the
component part itself. In many cases, drilling and boring opera-
tions are also done, to great advantage, by using the brackets
and bearings already assembled and fastened to the machine
body as guides.

One of the most important questions to be decided before mak- ing a jig is the amount of money which can be expended on a
special tool for the operation required. In many cases, it is
possible to get a highly efficient tool by making it more compli-
cated and more expensive, whereas a less efficient tool may be
produced at very small expense. To decide which of these two
types of jigs and fixtures should be designed in each individual
case depends entirely upon the circumstances. There should be
a careful comparison of the present cost of carrying out a certain
operation, the expected cost of carrying out the same operation
with an efficient tool, and the cost of building that tool itself.
Unless this is done, it is likely that the shop is burdened with a
great number of special tools and fixtures which, while they
may be very useful for the production of the parts for which they
are intended, actually involve a loss. It is readily seen how
uneconomical it would be to make an expensive jig and fixture
for a machine or a part of a machine that would only have to
be duplicated a few times. In some cases, of course, there may
be a gain in using special devices in order to get extremely good
and accurate results.

Locating Points. The most important requirements in the design of jigs are that good facilities be provided for locating the
work, and that the piece to be machined may be easily inserted
and quickly taken out of the jig, so that no time is wasted in
placing the work in position on the machine performing the work.
In some cases, a longer time is required for locating and clamp-
ing the piece to be worked upon than is required for the actual
machine operation itself. In all such cases the machine per-
forming the work is actually idle the greater part of the time, and,
added to the loss of the operator's time, is the increased expense
for machine cost incurred by such a condition. For this reason,
the locating and clamping of the work in place quickly and
accurately should be carefully studied by the designer before
any attempt is made to design the tool. In choosing the locat-
ing surface or points of the piece or part, consideration must be
given to the facilities for locating the corresponding part of the
machine in a similar manner. It is highly important that this
be done, as otherwise, although the jigs may be alike, as far as
their guiding appliances are concerned, there may be no facility
for locating the corresponding part in the same manner as the
one already drilled, and while the holes drilled may coincide,
other surfaces, also required to coincide, may be considerably
out of line. One of the main principles of location, therefore,
is that two component parts of the machine should be located
from corresponding points and surfaces.

If possible, special arrangements should be made in the design of the jig so that it is impossible to insert the piece in any but the correct way. Mistakes are often made on this account
in shops where a great deal of cheap help is used, pieces being
placed in jigs upside down, or in some way other than the cor-
rect one, and work that has been previously machined at the
expenditure of a great deal of time is entirely spoiled. There-
fore, whenever possible, a jig should be made " fool-proof ."

When the work to be machined varies in shape and size, as,
for instance, in the case of rough castings, it is necessary to have
at least some of the locating points adjustable and placed so
that they can be easily reached for adjustment, but, at the same
time, so fastened that they are, to a certain extent, positive. In
the following chapters different kinds of adjustable locating
points will be described in detail.

Clamping Devices. The strapping or clamping arrangements should be as simple as possible, without sacrificing effectiveness, and the strength of the clamps should be such as to not only hold
the piece firmly in place, but also to take the strain of the cutting
tools without springing or " giving." When designing the jig,
the direction in which the strain of the tool or cutters acts upon
the work should always be considered, and the clamps so placed
that they will have the highest degree of strength to resist the
pressure of the cut.

The main principles in the application of clamps to a jig or fixture are tha they should be convenient for the operator, quickly operated, and, when detached from the work, still con-
nected with the jig or fixture itself, so as to prevent the oper-
ator from losing them. Many a time, looking for lost straps,
clamps, screws, etc., causes more delay in shops than the extra
cost incurred in designing a jig or fixture somewhat more com-
plicated, in order to make the binding arrangement an integral
part of the fixture itself. Great complication in the clamping
arrangements, however, is not advisable. Usually clamping
arrangements of this kind work well when the fixture is new, but,
as the various parts become worn, complicated arrangements
are more likely to get out of order, and the extra cost incurred in
repairing often outweighs the temporary gain in quickness of
operation.


The judgment of the designer is, in every case, the most im-
portant point in the design of jigs and fixtures. Definite rules
for all cases cannot be given. General principles can be studied,
but the efficiency of the individual tool will depend entirely upon
the judgment of the tool designer in applying the general prin-
ciples of tool design to the case in hand.

When designing the jig or fixture, the locating and bearing points for the work and the location of the clamps must also be
so selected that there is as little liability as possible of springing
the piece or jig, or both, out of shape, when applying the clamps.
The springing of either the one or the other part will cause in-
correct results, as the work surfaces will be out of alignment with
the holes drilled or the faces milled. The clamps or straps
should therefore, as far as possible, be so placed that they are
exactly opposite some bearing point or surface on the work.

Weight of Jigs. The designer must use his judgment in re- gard to the amount of metal put into the jig or fixture. It is
desirable to make these tools as light as possible, in order that
they may be easily handled, be of smaller size, and cost less in
regard to the amount of material used for their making, but, at
the same time, it is poor economy to sacrifice any of the rigidity
and stiffness of the tool, as this is one of the main considerations
in obtaining efficient results. On large-sized jigs and fixtures,
it is possible to core out the metal in a number of places, without
decreasing, in the least, the strength of the jig itself. The
corners of jigs and fixtures should always be well rounded, and
all burrs and sharp edges filed off, so as to make them convenient
and pleasant for handling. Smaller jigs should also be made
with handles in proper places, so that they may be held in posi-
tion while working, as in the case of drilling jigs, and also for
convenience in moving the jig about.

Jigs Provided with Feet. Ordinary drill jigs should always be provided with feet or legs on all sides which are opposite the holes for the bushings, so that the jig can be placed level on the
table of the machine. These feet also greatly facilitate the
making of the jig, making it easier to lay out and plane the differ-
ent finished surfaces. On the sides of the jig where no feet are
required, if the body is made from a casting, it is of advantage
to have small projecting lugs for bearing surfaces when laying
out and planing. While jigs are most commonly provided with
four feet on each side, in some cases it is sufficient to provide the
tool with only three feet, but care should be taken in either case
that all bushings and places where pressure will be applied to the
tool are placed inside of the geometrical figure obtained by con-
necting, by lines, the points of location for the feet.

While it may seem that three feet are preferable to use, because the jig will then always obtain a bearing on all the three feet,
which it would not with four feet, if the table of the machine
were not absolutely plane, it is not quite safe to use the smaller
number of supports, because a chip or some other object is liable
to come under one foot and throw the jig and the piece out of
line, without this being noticed by the operator. If the same
thing happens to a jig with four feet, it will rock and invariably
cause the operator to notice the defect. If the table is out of
true, this defect, too, will be noticed for the same reason.

Jig feet are generally cast solid with the jig frame. When the jig frame is made from machine steel, and sometimes in the case of cast-iron jigs, detachable feet are used.

Materials for Jigs. Opinions differ as to the relative merits of cast iron and steel as materials from which to construct the jig and fixture bodies. The decision on this point should depend
to a great extent upon the usage to which the fixture is to be put
and the character of the work which it is to handle. For small
and medium sized work, such as typewriter, sewing machine,
gun, adding machine, cash register, phonograph, and similar
parts, the steel jig offers decided advantages, but for larger work,
such as that encountered in automobile, engine, and machine tool
fixtures, the cast-iron jig is undoubtedly the cheaper and more
advisable to use. The steel jig should be left soft in order that
at any future time additional holes may be added, or the existing
bushings changed as required. With a cast-iron jig this adding
of bushings is a difficult matter, as the frame is usually bossed
and "spot finished" at the point where the bushings are located,
and it is very difficult to build up on the jig frame in order to
locate or change the bushings. When designing the jig, these
points should be remembered and provision made for them,
where possible.

General Remarks on Jig Design. One mistake, quite frequently made, is that of giving too little clearance between the piece to be machined and the walls or sides of the jig used for it.
Plenty of clearance should always be allowed, particularly when
rough castings are being drilled or machined in the jigs; besides,
those surfaces in the jig which do not actually bear upon the
work do not always come exactly to the dimensions indicated on
the drawing, particularly in a cast-iron jig, and allowance ought
to be made for such differences.

In regard to the locating points, it ought to be remarked that, in all instances, these should be visible to the operator when placing the work in position, so that he may be enabled to see
that the work really is in its right place. At times the construc-
tion of the piece to be worked upon may prevent a full view of
the locating points. In such a case a cored or drilled hole in the
jig, near the locating seat, will enable a view of same, so that the
operator may either see that the work rests upon the locating
point, or so that he can place a feeler or thickness gage between
the work and the locating surface, to make sure that he has the
work in its correct position. Another point that should not be
overlooked is that jigs and fixtures should be designed with a view
of making them easily cleaned from the chips, and provision
should also be made so that the chips, as far as possible, may fall
out of the jig and not accumulate on or about the locating points,
where they are liable to throw the work out of its correct position
and consequently spoil the piece.

The principles so far referred to have all been in relation to the holding of the work in the jig, and the general design of the jig for producing accurate work. Provisions, however, should
also be made for clamping the jig or fixture to the table of the
machine, in cases where it is necessary to have the tool fixed
while in operation. Small drilling jigs are not clamped to the
table, but boring jigs and milling and planing fixtures invariably
must be firmly secured to the machine on which they are used.
Plain lugs, projecting out in the same plane as the bottom of
the jig, or lugs with a slot in them to fit the body of T-bolts, are
the common means for clamping fixtures to the table. For
boring jigs, it is unnecessary to provide more than three such
clamping points, as a greater number is likely to cause some
springing action in the fixture. A slight springing effect is almost
unavoidable, no matter how strong and heavy the jig is, but, by
properly applying the clamps, it is possible to confine this spring-
ing within commercial limits.

Jigs should always be tested before they are used, so as to make sure that the guiding provisions are placed in the right relation to the locating points and in proper relation to each other.

Summary of Principles of Jig Design. Summarizing the principles referred to, the following rules may be given as the main
points to be considered in the designing of jigs and fixtures:

1. Before planning the design of a tool, compare the cost of production of the work with present tools with the expected cost of production, using the tool to be made, and see that the cost of building is not in excess of expected gain.

2. Before laying out the jig or fixture, decide upon the locating points and outline a clamping arrangement.

3. Make all clamping and binding devices as quick-acting as possible.

4. In selecting locating points, see that two component parts of a machine can be located from corresponding points and surfaces.

5. Make the jig " fool-proof "; that is, arrange it so that the work cannot be inserted except in the correct way.

6. For rough castings, make some of the locating points adjustable.

7. Locate clamps so that they will be in the best position to resist the pressure of the cutting tool when at work.

8. Make, if possible, all clamps integral parts of the jig or fixture.

9. Avoid complicated clamping arrangements, which are liable to wear or get out of order.

10. Place all clamps as nearly as possible opposite some bearing point of the work, to avoid springing.

11. Core out all unnecessary metal, making the tools as light as possible, consistent with rigidity and stiffness.

12. Round all corners.

13. Provide handles wherever these will make the handling of the jig more convenient.

14. Provide feet, preferably four, opposite all surfaces containing guide bushings in drilling and boring jigs.

15. Place all bushings inside of the geometrical figure formed by connecting the points of location of the feet.

1 6. Provide abundant clearance, particularly for rough castings.

17. Make, if possible, all locating points visible to the operator when placing the work in position.

18. Provide holes or escapes for the chips.

19. Provide clamping lugs, located so as to prevent springing of the fixture, on all tools which must be held to the table of the machine while in use, and tongues for the slots in the tables in all milling and planing fixtures.

20. Before using in the shop, for commercial purposes, test all jigs as soon as made.

Types of Jigs. The two principal classes of jigs are drill jigs and boring jigs. Fixtures may be grouped as milling, planing, and splining fixtures, although there are a number of special fixtures which could not be classified under any special head.

Drill jigs are intended exclusively for drilling, reaming, tap-
ping, and facing. Whenever these four operations are required
on a piece of work, it is, as a rule, possible to provide the neces-
sary arrangements for performing all these operations in one
and the same jig. Sometimes separate jigs are made for each
one of these operations, but it is doubtless more convenient
and cheaper to have one jig do for all, as the design of the jig
will not be much more complicated. Although it may be pos-
sible to make a distinction between a number of different types
of drill jigs, it is almost impossible to define and to get proper
names for the various classes, owing to the great variety of
shapes of the work to be drilled. There are, however, two general
types that are most commonly used, the difference between
them being very marked. These types may be classified as
open jigs and closed jigs, or box jigs. Sometimes the open jigs
are called clamping jigs. The open jigs usually have all the drill
bushings in the same plane, parallel with one another, and are
not provided with loose or removable walls or leaves, thereby
making it possible to insert the piece to be drilled without any
manipulation of the parts of the jig. These jigs are often of
such a construction that they are applied to the work to be
drilled, the jig being placed on the work, rather than the work
being placed in the jig. The jig may be held to the work by
straps, bolts, or clamps, but in many cases the jig fits into or
over some finished part of the work and in this way the jig is
located and held in position.

The closed drill jigs, or box jigs, frequently resemble some
form of a box and are intended for pieces where the holes are
to be drilled at various angles to one another. As a rule, the
piece to be drilled can be inserted in the jig only after one or
more leaves or covers have been swung out of the way. Some-
times it is necessary to remove a loose wall, which is held by
bolts and dowel pins, in order to locate the piece in the jig.
The work in the closed drill jig may be held in place by set-
screws, screw bushings, straps, or hook-bolts.

The combination drilling and boring jig is another type of
closed jig designed to serve both for drilling and boring opera-
tions. Before designing a combination drill and boring jig,
the relation between, and number of, the drilled and bored
holes must be taken into consideration, and also the size of the
piece to be machined. In case there is a great number of holes,
it may be of advantage to have two or even more jigs for the
same piece, because it makes it easier to design and make the
jig, and very likely will give a better result. The holes drilled
or bored in the first jig may be used as a means for locating the
piece in the jigs used later on. Combination drill and boring
jigs are not very well adapted for pieces of large size.

Open Jigs. Open jigs of the simpler forms are simply
plates provided with bushed holes which are located to cor-
respond with the required locations for the drilled holes. While
holes are sometimes drilled by first laying out the holes directly
upon the work, it is quite evident that this method of drilling
would not be efficient if a large number of duplicate parts had
to be drilled accurately, as there is likely to be more or less
variation in the location of the holes, and considerable loss of
time. In the first place, a certain amount of time is required
for laying out these holes preparatory to drilling. The operator,
when starting the drill, must also be careful to make it cut
concentric with the scribed circle, which requires extra time,
and there will necessarily be more or less variation. To over-
come these objections, jigs are almost universally used for hold-
ing the work and guiding the drill, when drilling duplicate parts,
especially when quite a large number of duplicate pieces must
be drilled.

The ring-shaped jig shown at A in Fig. i is used for drilling
the stud bolt holes in a cylinder flange and also for drilling the
cylinder head, which is bolted to the cylinder. The position of
the jig when the cylinder flange is being drilled is shown at
B. An annular projection on the jig fits closely in the cylinder
counterbore, as the illustration shows, to locate the jig concentric
with the bore. As the holes in the cylinder are to be tapped or
threaded for studs, a "tap drill," which is smaller in diameter
than the bolt body, is used and the drill is guided by a remov-
able bushing b of the proper size. Jigs of this type are often
held in position by inserting an accurately fitting plug through
the jig and into the first hole drilled, which prevents the jig
from turning with relation to the cylinder, when drilling the
other holes. When the jig is used for drilling the head, the
opposite side is placed
next to the work, as
shown at C. This side
has a circular recess or
counterbore, which fits
the projection on the
head to properly locate
the jig. As the holes in
the head must be slightly
larger in diameter than
the studs, another sized
drill and a guide bushing
of corresponding size are
used. The cylinder is, of
course, bored and the
head turned before the drilling is done.

Jigs of the open class, as well as those of other types, are
made in a great variety of shapes, and, when in use, they are
either applied to the work or the latter is placed in the jig.
When the work is quite large, the jig is frequently placed on it,
whereas small parts are more often held in the jig, which is so
designed that the work can be clamped in the proper position.
The form of any jig depends, to a great extent, on the shape of
the work for which it is intended and also on the location of
the holes to be drilled. As the number of differently shaped
pieces which go to make up even a single machine is often very
great, and as most parts require more or less drilling, jigs are
made in an almost endless variety of sizes and forms. When all
the holes to be drilled in a certain part are parallel, and es-
pecially if they are all in the same plane, a very simple form of
jig can ordinarily be used.

Box Jigs. A great many machine parts must be drilled on
different sides and frequently castings or forgings are very
irregular in shape, so that a jig which is made somewhat in
the form of a box, and encloses the work, is very essential, as
it enables the guide bushings to be placed on all sides and also
makes it comparatively easy to locate and securely clamp the
part in the proper position for drilling. This type of jig, which,
because of its form, is known as a closed or "box jig," is used
very extensively.

A box jig of simple design is shown in Fig. 2. This particu-
lar jig is used for drilling four small holes in a part (not shown)
which is located with reference to the guide bushings B by a
central pin A attached to the jig body. This pin enters a hole
in the work, which is finished in another machine in connection
with a previous operation. After the work is inserted in the
jig, it is clamped by closing the cover C, which is hinged at one
end and has a cam-shaped clamping latch D at the other, that
engages a pin E in the jig body. The four holes are drilled by
passing the drill through the guide bushings B in the cover.

Another jig of the same kind, but designed for drilling a
hole having two diameters through the center of a steel ball,
is shown in Fig. 3. The work, which is shown enlarged at A,
is inserted while the cover is thrown back as indicated by the
dotted lines. The cover is then closed and tightened by the
cam-latch Z), and the large part of the hole is drilled with
the jig in the position shown. The jig is then turned over and
a smaller drill of the correct size is fed through guide bushing
B on the opposite side. The depth of the large hole could be
gaged for each ball drilled, by feeding the drill spindle down to
a certain position as shown by graduation or other marks, but
if the spindle has an adjustable stop, this should be used. The
work is located in line with the two guide bushings by spherical
seats formed in the jig body and in the upper bushing, as shown.
As the work can be inserted and removed quickly, a large num-
ber of balls, which, practically speaking, are duplicates, can
be drilled in a comparatively short time by using a jig of this
type.

A box jig that differs somewhat in construction from the
design just referred to is illustrated at A in Fig. 4, which shows
a side and top view. The work, in this case, is a small casting
the form of which is indicated by the heavy dot-and-dash lines.
This casting is drilled at a, b, and c, and the two larger holes a
and b are finished by reaming. The hinged cover of this jig
is opened for inserting the work by unscrewing the T-shaped
clamping screw s one-quarter of a turn, which brings the head
in line with a slot in the cover. The casting is clamped by tighten-
ing this screw, which forces an adjustable screw bushing g down
against the work. By having this bushing adjustable, it can
be set to give the right pressure, and, if the height of the cast-
ings should vary, the position of the clamping bushing could
easily be changed.

The work is properly located by the inner ends of the three
guide bushings ai, bi, and ci, and also by the locating screws I
against which the casting is held by knurled thumb-screws m
and n. When the holes a and b are being drilled, the jig is
placed with the cover side down, as shown at A in Fig. 5, and
the drill is guided by removable bushings, one of which is shown
at r. When the drilling is completed, the drill bushings are
replaced by reamer bushings and each hole is finished by ream-
ing. The small hole c, Fig. 4, is drilled in the end of the cast-
ing by simply placing the jig on end as shown at B, Fig. 5.
Box jigs which have to be placed in more than one position
for drilling the different holes are usually provided with feet
or extensions, as shown, which are accurately finished to align
the guide bushings properly with the drill. These feet extend
beyond any clamping screws, bolts, or bushings which may
protrude from the sides of the jigs, and provide a solid support.
When inserting work in a jig, care should be taken to remove
all chips which might have fallen upon those surfaces against
which the work is clamped and which determine its location.

Still another jig of the box type, which is quite similar to
the one shown at A, Fig. 4, but is arranged differently, owing
to the shape of the work and location of the holes, is shown
at B in the same illustration. The work has three holes in
the base h, and a hole at i which is at an angle of 5 degrees
with the base. The three holes are drilled with the jig stand-
ing on the opposite end y, and the angular hole is drilled while
the jig rests on the four feet k, the ends of which are at such an
angle with the jig body that the guide bushing for hole i is prop-
erly aligned with the drill. The casting is located in this jig
by the inner ends of the two guide bushings w and the bushing
o and also by two locating screws p and a side locating screw q.
Adjustable screws t and t\ in the cover hold the casting down,
and it is held laterally by the two knurled thumb-screws u
and v. If an attempt were made to drill this particular part
without a jig (as would be done if only a few castings were
needed) it would have to be set with considerable care, provided
the angle between hole i and those in the base had to be at
all accurate, and it would be rather difficult to drill a number
of these castings and have them all duplicates. By the use of
a jig, however, designed for drilling this particular casting,
the relative positions of the holes in any number of parts are
practically the same and the work can be done much more
quickly than would be possible if it were held to the drill-press
table by ordinary clamping appliances. Various designs of jigs
will be described in Chapter VII.

Details of Jig Design. The general principles of the design
and use of jigs have been explained. The details of jig design
will now be considered. Generally speaking, the most im-
portant parts of a jig are the guide bushings for the drills and
other tools, the clamping devices, and the locating points,
against which the work is placed to insure an accurate posi-
tion in the jig. The guides for the cutting tools in a drill jig
take the form of concentric steel bushings, which are placed in
the jig body in proper positions.

The drill bushings are generally made of tool steel, hardened
and lapped, and, where convenient, should be ground inside
and out. They should also be long enough to support the
drill on each side regardless of the fluting, and they should be
so located that the lower end of the bushings will stop about
the same distance above the work as the diameter of the drill,
so that chips will clear the bushings readily. Where holes are
drilled on the side of a convex or a concave surface, the end of
the bushing must be cut on a bevel and come closer to the part
being drilled, to insure the drill having adequate support while
starting into the work. The bushings should have heads of
sufficient diameter. Long bushings should be relieved by in-
creasing the hole diameter at the upper end. The lower end
of the bushing should have its edges rounded, in order to permit
some of the chips being shed from the drill easily, instead of
all of them being forced up through the bushing. It is also
good practice to cut a groove under the head for clearance for
the wheel when grinding the bushing on the outside. A com-
plete treatise covering dimensions and design is given in the
chapter on "Jig Bushings."

In order to hold the work rigidly in the jig, so that it may
be held against the locating points while the cutting tools
operate upon the work, jigs and fixtures are provided with
clamping devices. Sometimes a clamping device serves the
purpose of holding the jig to the work, in a case where the
work is a very large piece and the jig is attached to the work
in some suitable way. The purpose of the clamping device,
however, remains the same, namely, that of preventing any
shifting of the guiding bushings while the operation on the
work is performed. The clamping device should always be an
integral part of the jig body in order to prevent its getting lost.
Different types of clamping devices are shown and described
in the chapter on "Jig Clamping Devices. "

The locating points may consist of screws, pins, finished
pads, bosses, ends of bushings, seats, or lugs cast solid with
the jig body, etc. The various types used are described in
detail in the chapter on "Locating Points and Adjustable
Stops."

Source:  https://archive.org/details/jigfixturedesign00joneuoft

More resources
http://nptel.iitk.ac.in/courses/Webcourse-contents/IIT%20Kharagpur/Manuf%20Proc%20II/pdf/LM-33.pdf

http://www.brighthubengineering.com/machine-design/47195-the-3-2-1-principle-of-jig-fixture-design/

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