JIG GRINDER

JIG GRINDER

Introduction:-

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.