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The Subject of Drills - the Different Forms and Sizes & Hints about Grinding - Shop Talks with the Young Mechanics by W. H. Vandervoort, 1898

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The twist drill which has come into such universal use, has superseded the old, flat, forged drill which, for so many years, held without rival the first position as a tool for producing circular holes in metal.  For the needs of its day, it served its purpose well.

The advancements along mechanical lines demanded a better tool for this work, however, and the twist drill resulted, brought out in practically the same form as now used, the principal recent improvements being mostly in slight changes in forms, and its more accurate production due to improved methods of manufacture.

The flat drill, as used for metal work, is generally of the form shown in Fig. 28.  It is made from round stock, is forged thin at the lips, and ground as shown in figure, with three cutting edges - A, B and C. 

This is a very accommodating sort of a tool, being capable of producing a number of holes of different diameters, yet, approximately, the width of the drill.  The disadvantage of this adjustability, however, lies in the fact that the size of hole wanted cannot ordinarily be produced.

The flat drill has no lands, as that part of the twist drill between flutes is called, to steady and guide it in the work. Consequently, the hole drilled will usually not be round, and should be the point of the drill strike the side of a small blow hole or a soft spot in the metal being drilled-as frequently happens in working cast metal-the point will drift towards this spot, thus making a hole that is neither round not straight. This is shown in Fig. 29.

In order to drill holes approximately to size with the flat drill, it is necessary that the cutting lips be most carefully ground. The angle of the lips with the axis of the drill must be equal, otherwise one cutting edge will perform all the work, and will dull quickly, due to this double duty.

The pressure on the cutting lip will crowd the point, causing it to revolve in a small circle about the center of revolution. This will cause the other flute to cut slightly at its outer end, thus producing a hold of larger diameter than the width of flat. This is shown is Fig. 30.

The cutting lips should be of equal length with their intersection in the axis of rotation of the drill. If one lip is longer than the other, the diameter of the hole drilled will depend on the length of this long lip, as it will rotate about C, its central axis, as shown in Fig. 31.

In case the intersection of the lips does not fall on the axis of the drill, the one li is thereby made longer than the other, and the hole drilled will again be large, as the tool will spring an amount sufficient to allow it to revolve about C instead of its true axis, and the length of the long flute, again determines the diameter of the hole drilled, as shown in Fig. 32.

The first cost of the flat drill is small, and the results obtained by its use usually poor.  Its only advantage lies in the fact that it can readily be forged and tempered to do work on extremely hard metals.

The flat drill, ground thin and tempered hard, is a valuable tool for drilled hard steel or chilled iron, as it will in that form take hold of metal that the twist drill will not touch.  It also makes a convenient extension drill, as it can be readily formed on the end of a long bar of steel.

The flat drill is not adapted to the drilling of deep holes, as it does not free itself of chips. It is largely used for roughing out cored holes, preparatory to boring, which work is very destructive, due to scale and sand, to the land clearances of twist drills. When so used in a lathe, the drill is held against the dead center and fed forward by the rail screw, the work revolving.

About 1860, twist drills, having milled flutes, were first placed on the market. Previous to this date, however, drills with flutes produced by filing and twisting of the flat stock had been used to a limited extent.

In Fig. 33 is shown a taper shank twist drill. A A are the flutes, B B are the lands, C-the metal between the flutes-the web, D D the lips, E the shank and F the tang. The center or grinding line is the fine line running along the bottom of each flute, and serves as a guide to the lips in grinding so their intersection will fall in the center of the drill.


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