The three fundamentals steps for making a hole by drilling. The steps are: 1. Locating 2. Cutting 3. Checking.
Step # 1. Locating:
In making any hole by drilling, boring or any other operation, its exact location at the desired position in the work is utmost.
It can be accomplished in the four important ways discussed below:
(a) Lay Out:
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In this method the lines are scribed on the desired profile and the punch marks are pricked. The tool is then adjusted so as to move on these marks.
(b) Buttoning:
This method makes use of tool maker’s buttons, and is used when it is desired to locate and make holes with greater accuracy than is possible by ordinary layout. The tool maker’s buttons are available in sizes of 7.5, 10 and 12.5 mm and 25 mm in diameter and about 12.5 mm high. Flat washers and machine screws are supplied with the set.
To locate holes by this method, the centres are first located roughly by scriber, scale and dividers to an accuracy of about 0.25 mm and then prick punched. Holes are then drilled and tapped to receive the screws. Next the buttons are fastened in place with screw and washer.
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It is to be noted that the hole in the button is sufficient for relocation and the screw is drawn up lightly so that the buttons can be tapped lightly with a hammer into final position. The buttons are next altered in position by measuring with an accurate instrument so that the axes of the buttons will be the final axes of the bored holes.
After checking the distances finally the screws are tightened very hard. The workpiece is then mounted on the face plate and the axis of one button at a time is made concentric with the axis of spindle by changing the position of face plate and observing the movement of button against a fixed dial indicator.
The button and screw are then removed, and the boring operation carried out. It may be noted that the original tapped hole does not influence the accurate location of the final hole. Also the diameter of the button has no relation to the final size of the hole: Accuracy of this method is of the order of ± 10 µ m.
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(c) Transfer:
In this method a master plate is used having desired holes located in it. This method is useful when the relative positions of the hole locations are known and not their dimensions. The master plate is used just like jig and is clamped with the workpiece.
(d) Co-Ordinate Location:
In this method, the holes are located by working to known dimension with measuring devices extraneous to the workpiece.
Step # 2. Cutting:
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The various methods used for removal of metal are single point boring, drilling, reaming, counter boring, reaming through use of jigs and hardened steel bushing, grinding, lapping and boring by using diamond charged mandrels for very small holes of diameter as small as 0.075 mm.
Step # 3. Checking:
After the completion of job, it must be inspected thoroughly for its dimensional accuracy to ensure that the finished product is within the specified tolerances.
It is very important for the proper use of jig borer that it is kept in the temperature controlled room as its overall accuracy is of the order of 1 micron for various operations.