12 Drilling

The drilling machine, because of its simplicity and the early need for drilled holes, is usually the first machine to be used in the metalwork room.

When starting to drill the following hints will be useful:

1. Check that you have the correct drill and be sure the chuck is properly tightened.

2. Secure the work either in a hand vice or a drilling vice.

3. Take precautions against drilling into the vice or drilling machine table when (a) using a hand vice by resting the work on a piece of accurately planed hardwood or, (b) by using packing strips when using the machine vice. A typical machine vice is shown in figure 1. A hand vice is shown in Chapter 3, figure 21.

4. Check that the speed is correct for the drill and the material you are drilling.

5. Bring the drill down to the centre punch mark and press hard enough to start the drill cutting and keep it cutting—do not let it rub, as this overheats the drill tip and makes it blunt. Ease off the pressure when the drill is about to break through because at this stage it tends to "grab" the work.

6. Use a coolant if applicable to the material you are drilling.

 

CENTRE PUNCHING

This must be done accurately on the marked out cross lines. When a large hole is to be drilled the work should be marked out as shown in figure 2 with the outside circle to the finished diameter. Drilling is then started with a small "pilot" drill and then opened up with successively larger drills. If the small drills run out of true it can be seen against the smallest marked-out circles then corrected by filing out with a round file until finally the largest drill just splits the dot punches on the outside circle.

 

HAND DRILLS

Hand drills (fig. 3) are used for making holes up to 5/16" diameter. They are useful for making holes in work which cannot be taken to the drilling machine.

Electric Hand Drills

These do a similar job to the hand drill and have maximum capacities between 1/4 "—1/2" diameter. Figure 5 shows a stand on which an electric hand drill (fig. 4) can be held for use as a sensitive drilling machine.

 

DRILLING MACHINES

 

In the small workshop these are either bench drilling machines or pillar drilling machines. Bench drilling machines have a capacity up to J* and should stand on a sturdy bench. The pillar drill (fig. 6) is bolted to the floor and has an intermediate table which can be raised or lowered and on some models tilted. Taper shank drills can be inserted into the taper socket spindles of these machines when the chucks are removed (fig. 7). If the taper shank drill is too small for the spindle socket the difference in size can be made up with a sleeve (fig. 7). These are made in single steps, e.g. 1 to 2, 2 to 3, and so on. Others are available which step up from 1 to 3 or 1 to 4. These sleeves should be tapped into place with a hide mallet and removed by lightly hammering a drift into the slot provided. The speed of the drill can be altered by adjusting the position of the vee belt on the pulleys.

 

TWIST DRILLS

Twist drills are available in H.S.S. or carbon steel. Carbon steel drills are cheaper than H.S.S. but must be run at half the speed. However, the smaller sizes of these, with proper use, will last almost as long as the H.S.S. kind.

The speed of the drill should be calculated in the same way as for turning and the coolants recommended for turning also apply to drilling (see Chapter 13). It is important to know whether a drill is made from high speed steel or carbon steel if it is to be run at its proper speed. Often drills are marked either H.S. or C.S. but if no such marking is visible the sparks must be observed when it is being ground.

Twist drills are made with either parallel or taper shanks. Sizes up to about 1/2 " have parallel shanks and above this morse taper shanks. Straight shank drills are usually called "jobbers drills". Figures 8 and 9 show the twist drill nomenclature in accordance with the British Standards Institution.

Drills are made in number sizes from 80 to 1 and in letter sizes from A to Z (see table at back), also in fractional sizes from 1/64" to 3", and metric sizes from 0.5mm to 65mm.

The drill commonly used in the small workshop is ground to the angles shown in figure 10. The nomenclature is given in figure 11. The relationship between a lathe tool and a twist drill is shown in figure 12.

 

Slow Helix drills used for brass, gunmetals and phosphor bronze (the rake angle is decreased).

Quick Helix drills used for soft metals such as aluminium and copper (the rake angle is increased).

Multi-flute Core drills may have 3 or 4 flutes and are used for enlarging existing holes, particularly cored holes in castings. These drills are strong because the flutes are not deep and therefore the web or core is relatively thick.

In the small workshop these special drills are seldom used, but the tip of an ordinary twist drill can be modified to a certain extent to suit the material being drilled. When drilling brass almost no rake is needed and the drill helix angle is ground at the tip as shown (fig. 14). Cast iron can best be drilled with a drill ground as shown in figure 15.

Sharpening Twist Drills

When accuracy is required in drill grinding it is done with a drill grinding attachment as shown in figure 16. This type of attachment is essential for large drills but drills below 1/2" can be ground with a reasonable degree of accuracy by the offhand method. When doing this it is helpful to use a new drill as a model in order to obtain the correct angles and clearances: but practice is necessary to achieve proficiency in this type of freehand grinding. Some faults in drill grinding are shown in figure 17.

 

FLAT DRILLS (figure 18)

These can be made from silver steel or carbon steel in the school workshop. They must be hardened and carefully tempered usually to a light straw.

PROCESSES

Countersinking

This is done with a countersinking cutter (fig. 19).

Counterboring

This is done with a counterboring cutter (fig. 19) to accommodate a cap screw or cheese head screw. Counterbores made from H.S.S. may be bought or they may be turned in the workshop from silver steel or plain carbon steel, and the flutes filled. Care must be taken with hardening as cracking may occur where there is a change of section. Temper to a mid-straw colour and sharpen with a slip stone.

Spot Facing

This is done with a counterboring cutter on rough or uneven surfaces, but only deep enough to provide a square seating for a bolt head (fig. 19).

Reaming

This is a secondary process which is done when smooth accurate holes are required. Reaming may be done using a hand reamer (fig. 20) which has a parallel shank and is turned with a tap wrench, or with a machine reamer which has a morse taper shank and is held either in the tailstock of the lathe or in the drilling machine. Adjustable reamers (fig. 21) can be slightly enlarged after each pass through the hole until the correct size is obtained.

Whichever reamer is used care must be taken not to turn them backwards but keep them rotating in the cutting direction both in entering and withdrawing, otherwise the swarf tends to become wedged behind the cutting edges causing them to chip. Reaming should be done slowly and cutting fluid used when applicable. Holes to be reamed should be drilled out to about 0005" to 0010" below size for holes up to 1/2" and between 1/64 " and 1/32 " below size for larger holes.