JIG BORING
The jig
boring machine is the most accurate
of all machine tools. This was first developed in the year 1910 in Switzerland
and used as a locating machine. The real
jig borer was first built in the year 1917 by Pratt and Whitney
1.
Spindle head 2. Quill 3. Column 4. Spindle housing 5.7. Spindle 6. Table 8. Bed
Jig
boring machines are now used for production of jigs, fixtures, tools and other
precision parts which require high degree of accuracy. They are
characterized- by provisions of highest accuracy through rigidity, low thermal
expansion and precise means of measuring distance for accurately locating and
spacing holes. The machining accuracy is very high, within a range of 0.0025
mm. A jig boring machine resembles in, appearance to a vertical milling
machine, but so far its operation and accuracy are concerned there cannot be
any comparison between the two. The -spindle and other parts of the machine are
extremely rigid to, resist deflection -and the vibration is minimum. The spindle runs in preloaded antifriction
bearings,. The spindle housings are made in invar having a very low coefficient
of linear expansion. The jig boring machine requires to be operated in
temperature controlled rooms where temperature can be maintained constant. This
is essential to prevent inaccuracy in the machine and in the work being
manufactured due to thermal expansion of the metal.
Parts of a Jig borer
A jig
borer has the essential element of a vertical spindle milling machine. For the
sake of attaining high accuracy, it is
generally built lower to the floor and
is much more rigid and of accurate
construction.
On the base of it, a saddle is supported which moves in and out from the operator to the column. To supplement
the saddle movement, a table is
provided on it, which can move to
right or left. It has a massive column
which supports the spindle housing
and is capable of moving up and down the column ways. Thus the third position adjustment is achieved.
Block
diagram of jig borer
The
spindle moves inside a quilt and is supported by the housing or spindle head.
In order to add to the rigidity of the machine tool, the quill is made capable
of moving up and down inside the housing thus giving a telescope mechanism. In
order to eliminate the lost motion for purpose of high accuracy and precision,
the spindle quill and housing are manufactured under extremely careful and
exacting conditions and to minimise the errors due to expansion, the housing is
made of invar cast iron The spindle is hardened, ground, stabilised and tapped
and pre-loaded ball bearing are used., The drying mechanism-provided is capable
of giving speeds ranging from about 30 to 1500 RPM. So that best cutting speed
is available for each size of hole.
For aligning the job and centering it, a dial
indicator fitted on the spindle is used. Usually the operator located the
points etc. from either a finished edge or from a suitable hole while setting
up for boring operation. The dial indicator is used to pick up the surface and
then with reference to it, the spindle can be set.
Types of jig boring machine
There are mainly two types of Jig boring machine :
1. Vertical milling machine type
2. Planer type
Vertical
milling machine type : It resembles in construction to a vertical milling
machine. The spindle rotates on a
vertical column and the horizontal
table rests on the bed in front of the column. The positioning of the work mounted on the table may be obtained
by compound movements of the table, perpendicular and parallel to the column
face.
Planer type : It consists of two
vertical columns at the two sides of the table and is mounted on the base. The
table has reciprocating movement for adjustment of the work. The spindle is mounted on the crossrail
bridging the two vertical columns. In a planer type Jig borer, two co-ordinate
movements for hole location are provided by the longitudinal movement of the
table and the cross movement of the spindle
along the crossrail.
THE JIG GRINDER
The need for accurate hole locations in hardened* material led to the
development of the jig grinder in
1940. While it was originally developed to position and grind accurately straight or tapered holes,
may other uses have been found for
the jig grinder over the years. The most
important of these has been the grinding of contour forms which may include a
combination of radii, tangents, angles, and
fiats.
The advantages of jig grinding are :
1. Holes distorted during the hardening process
can be accurately brought to correct size and position.
2. Holes and contours requiring taper or draft
may be ground. Mating parts, such as
punches and-dies can be finished to size, eliminating the tedious Job of hand
fitting.
3. Because more accurate fits and better surface
finishes are possible, the service life of the part is greatly prolonged.
4. Many 'parts requiring contours -can be made
in a solid form, rather than in sections
as was formerly necessary.
Jig Grinder Parts
The jig
grinder is similar to a Jig borer, both having precision-ground lead screws
capable of positioning .the table within 0.002 mm accuracy over its entire
length. Both are vertical spindle machines and employ the same basic cutting
principle encountered in single-point boring.
The main difference between these two machines is in the spindles.
The Jig grinder is equipped with a high-speed pneumatic turbine grinding spindle for
holding and driving the grinding
wheel.
The spindle construction permits outfeed grinding
wheel. The spindle construction permits outfeed grinding and also the grinding
of tapered holes.
Grinding
Head Outfeed
A horizontal dovetail slide connects the grinding head to the main
spindle of the jig grinder. The grinding head may be offset from the center of
the main spindle to grind various size holes. The amount of eccentricity of the
grinding head can be accurately controlled by the internally threaded outfeed
dial which is mounted on the non rotating yoke at the top of the jig-grinding
spindle. The dial is graduated in steps of 0.002 mm permitting accurate control
of the hole size during grinding.
Coarse
adjustment of the grinding wheel position is obtained by a fine pitch adjusting
screw within the dovetail slide. This coarse-adjusting screw is accessible
only when the machine spindle is
stopped.
Grinding Methods
The
removal of material from a hole with a conventional grinding wheel is carried
out by two methods : outfeed and plunge grinding. Each method has its
advantages,, and at times both can be used effectively to grind the
same hole. Small holes, less than
6.35 mm in diameter, can be effectively ground by using diamond-charged
mandrels. Holes larger in diameter than the normal machine range can be ground
effectively if an extension plate is used between the grinding spindle and the
main spindle. With the use of an extension plate, holes up to 228.6 mm En diameter may be ground.
Outfeed Grinding
Outfeed grinding is similar to internal grinding where the wheel is fed
radiaily into the work with passes as fine as 0.002 mm at a time. The cutting
action takes place with the periphery of the grinding wheel. Outfeed grinding
is generally used to remove small amounts of stock when high finish and
accurate hole size are required.
Plunge Grinding
Plunge
grinding with a grinding wheel can be compared to the cutting action of a
boring tool. The grinding wheel is fed radiaily to the desired diameter and
then into the work. Cutting is done with the bottom comer of the wheel only. It
is a rapid method of removing excess stock, and if the wheel is properly
dressed, it produces satisfactory finishes for some jobs. The sharp cutting
action which results from the small contact area of the wheel keeps the work
cooler than outfeed grinding.