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 flats.
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 out feed grinding wheel. The spindle construction
permits out feed grinding and also the grinding of tapered holes.
Grinding Head Out feed
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 1he main spindle. With the use of an
extension plate, holes up to 228.6 mm in diameter may be ground.
Outfeed Grinding
Outfeed grinding is similar to internal grindin.9 where the wheel is fed
radially 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 radially to the desired
diameter and then into the work. Cutting is done with the bottom corner 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.
