Page images
PDF
EPUB

SPEED OF DRILLS

Learn to run drills at their proper speed to secure the most work with fewest grindings and breakages. The best practise is to use a speed that will give 30 feet a minute cutting speed for steel, 35 feet for cast iron and 60 feet for brass. This means that the cutting edge must run fast enough to make these speeds. For drilling steel with a-inch drill this means 1834 revolutions a minute, while for brass it would be 3668 revolutions. The table gives the speeds without any figuring for all drills up to 3 inches. These speeds require plenty of lubricant. This is for carbon steel drills.

These speeds may have to be reduced with very hard material, but should not be exceeded in ordinary cases. High-speed steel drills will stand about double these speeds. It is easy to memorize the speeds of the drills most in use.

[blocks in formation]

FEED OF DRILLS

The feed of drills is usually given in parts of an inch per revolution, 0.004 to 0.007 inch for drills of inch and smaller and 0.007 to 0.015 inch for larger drills being recommended. This has been worked out into the table for the standard speeds to show inches of feed per minute for the three speeds given, which is more convenient. This is not an iron-clad rule but should be used with judgment. For high-speed steel these figures can be just about doubled.

DRILL TROUBLES

Twist-drills will stand more strain in proportion to their size and weight than almost any other tool, and when a good drill gives trouble it is pretty safe to say some of the conditions are wrong.

If it chips on the edge, the lip clearance is too great and fails to support the cutting edge or the feed is too heavy. Ease off on the feed first and then watch the grinding.

If it splits in the web it is either ground wrong, i.e., does not have the center lip at the angle of 45 degrees or the feed is altogether too heavy.

If the outer corner wears, it shows that the speed is too great. This is particularly noticeable on cast iron.

DRILL POINTERS

In most cases it is better to use high speeds almost to the point where the drill corners commence to wear with a light feed than to use slower speed and heavy feed.

This is specially true of drilling in automatic machines where the holes are not more than twice as deep as the diameter where drills are flooded with lard oil. With deeper holes the chips are harder to get rid of and it is better to use slower speeds and heavier feeds as the drilled hole gets deeper.

Watch the drill chip and try to grind so that it will come out in a small compact roll. It is better to have this continuous clear to bottom of hole if possible.

In drilling brass use a heavier feed especially on automatic machines, as it helps to work out the chips. If you lubricate at all, flood the work. Twist-drills ground as for steel often catch and "hog in" on brass, especially at the bottom of the hole, where it breaks through. To avoid this, grind the lead or rake from the cutting edge.

In drilling hard material use turpentine as a lubricant.

Drills feed easier by thinning the extreme point if this is carefully done. This is important in hand feeding.

SPECIAL DRILLS AND THEIR USES

Ratchet-drills have a square taper shank, are used in hand-ratchet braces and in air-driven drills. Used in bridge building, structural and repair work.

The shell drill, Fig. 10, is used after a two-groove drill in chucking out cored holes or for enlarging holes that have been made with a two-groove drill. It has a taper hole and a number of sizes can be

used on the same arbor.

Wire drills and jobbers or machinists drills both have round shanks and only differ in size. Wire drills are made to a twist-drill gage and the others to a jobbers or fractional gage.

Blacksmith drills all have a 1-inch shank 2 inches long, so as to all fit the same holder. There is a flat on the shank for set-screw. The straightway or Farmers drill has the same clearance as a twist-drill but the flutes are straight. It is used mostly in drilling brass and soft metals or in drilling cross holes or castings where blow holes may be found, as it is less likely to run than the twist-drill.

Oil drills have the advantage of the cutting edge being kept cool and of the chips being forced back through the grooves which reduces friction to a minimum. They are used for all kinds of drilling, mostly deep hole work. In cast-iron drilling air is sometimes used to blow out the chips and keep the drill cool. They are generally used in a screw or chucking machine or a lathe fitted for this work. Where the drill is held stationary and the work revolves, the oil is pumped to the connection and flows through the holes in the drill as in Fig. 11.

Where the drill revolves

FIG. 10.

Shell Drill

as in a drill press, the oil is pumped into a collar which remains stationary while the drill socket revolves, as in Fig. 12. An oil groove around the socket and holes through to the drill connects with the holes in the drill itself. Other types are shown in Figs. 13 and 14. The latter is used mostly in screw or chucking machine turrets where the oil is pumped into the center of the turret and into the large hole in the shank of the drill. The hollow drill shown in Fig. 15 is used for deep drilling or long holes and is used in a lathe or some similar machine fitted for the purpose. It has a hole lengthwise through the shank connecting with the grooves of the drill. The shank can be threaded and fitted to a metal tube of such length as desired. The outside of the drill has a groove the whole length of the body. The lubricant is conveyed to the point of the drill on the outside through these grooves, while the hollow tube admits of the passage of oil and chips from the point. In using this drill the hole is first started with a short drill the size of the hole desired and drilled to a depth equal to the length of the body of the hollow drill to be used. The body of the hollow drill acts as a packing, compelling the oil to follow the grooves and the chips to flow out through the hollow shank.

Three and four groove drills are used for chucking out cored holes or enlarged holes that are first drilled with a two-groove drill. They are much better than a two-groove drill for use in cored holes or to follow another drill. The ends of the drills, Fig. 16 and 17, indicate that they are not made to drill from solid stock but for enlarging a hole already made.

[blocks in formation]

The following tables give standard drill sizes in various ways, each being very convenient for certain classes of work:

[merged small][graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][ocr errors][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][ocr errors][ocr errors][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][ocr errors][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][ocr errors][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][ocr errors][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed]
« PreviousContinue »