Choice of an electric drill will largely depend on: a type of work,
b volume of work, c size of hole,
d type of material (wood, metal, soft brick or blockwork etc).
Figure 6.8 (a) shows a typical palmgrip drill. Figure 6.8 (b) shows a heavy duty four-speed ‘D’ back handled rotary drill.
Ideally, an electric drill should be adjusted to rotate at a speed to suit both the material (work- piece) and the hole size (drill bit). Provided the drill is powerful enough and the correct drill bit (see table 6.1) is used, it is possible to bore holes in most materials (including soft masonry). 6.4.1 Drill Cutting Speeds
A drill bit should rotate at its most effective cut- ting speed, otherwise it could overheat, quickly become dull, or even break. Cutting speed is often misquoted as revolutions per minute (rev/min), which only denotes the number of times the chuck revolves every minute (the drill speed). To determine a drill bit’s cutting speed (edge speed of bit), the distance it travels every revolution must be known.
As can be seen from figure 6.9, the distance covered by one revolution will vary according to the diameter of the bit.
Therefore, to find the cutting speed of a drill bit – which is usually quoted as metres per sec- ond (m/s) – we use the following formula:
Cutting speed (m/s) ⫽ π ⫻ Diameter (m) ⫻ Drill speed (rev/s)
where π(‘pi’) ⫽ 22/7 or 3.142.
For example:
For a drill bit of 6 mm diameter in a drill with a working speed of 1500 rev/min the distance around the outer edge (circumference) of the drill would be: π ⫻ Dia. or (πD)
1 First let us change the diameter from millimetres to metres by dividing by 1000 so: 6 ⫽ 0.006m
1000
Resulting in; 3.142 ⫻ 0.006 ⫽ 0.018852m 2 Now convert RPM to number of
revolutions (rev) per second(s):
Divide the spindle speed (RPM) by 60 to reduce to seconds: Spindle speed (RPM) 60 1500 ⫽ 25rev/s 60
3 The cutting speed at metres per second (m/s) can now be determined by using : 0.018852m ⫻ 25rev/s ⫽ 0.47m/s
3.142 ⫻ 6 1500
or ⫻ ⫽ 0.47m/s
1000 60
When drilling holes in metals a guide to the rec- ommended ranges of cutting speeds is shown in table 6.1.
Drilling holes in wood usually requires about twice the cutting speed for metal, but, because of the many hard and abrasive wood-based materi- als now in common use, consideration should be given to the material’s composition before choosing a bit or its speed.
Drill manufacturers usually recommend the appropriate rev/min to suit both the drill bit size and the material. It is a good idea to take a copy of these and attach it to (ideally inside) the drill carry- ing case.
Note: As a general guide, the harder the material
or larger the hole the lower the rev/min.
Clip
Back vent Palm
grip handle
Point of entry for cable and cable sleave
Point of entry for cable and cable sleave Trigger switch
(variable speed) Keyless chuck
Speed selector
Specification plate Trigger
switch 'D' grip handle Side handle Heavy-duty keyed chuck (a) (b)
Fig 6.8 Rotary electric drills
X X
NOTE: If the drill speeds in RPM are constant a point on the outer edge of the larger diameter drill will travel further in the same time as shown at X
D⫽diameter of drill bit
X⫽Distance covered by the cutting periphery (outer edge of the drill in one revolution)
D D
Fig 6.9 Drill bit cutting speed
Table 6.1 Guide to recommended cutting speeds
Material Cutting speed (m/s)
Aluminium 1.00 to 1.25
Mild steel 0.40 to 0.50
Cast iron 0.20 to 0.40
An example for drilling Mild steel and Masonry Drill Dia mm rpm 3 mm 2000 6 mm 1100 10 mm 750 12 mm* 550
*When drilling larger holes it is generally recom- mended to first drill a smaller pilot hole.
6.4.2 Method of use
How the drill bit is applied to a workpiece will vary according to the workpiece’s size and shape, but it is usually a two-handed operation. It is therefore essential that the work is securely held by a clamp or is such that it will not be affected by the pressure needed to drill the hole. Pressure should be regulated to allow the bit to cut into the material, to keep the hole clear of waste par- ticles, and also to avoid breaking through the under side. Insufficient pressure could result in both the bit and workpiece being overheated, due to heat being generated by friction.
6.4.3 Attachments and variations on type Although most manufacturers supply a large range of drill attachments, only those which aid the drilling process should be considered suit- able for use by the trade; for example,
a angle-drilling (fig 6.10), b drill stand (fig. 6.11), c mortiser (fig. 6.12).
a Angle drilling – figure 6.10(a) shows a rotary right angle drill. This type of drill is ideal for getting into awkward corners or spaces between floor joists etc. A right angle drill attachment like the one shown in figure 6.10(b) may be available.
b Drill stand – permits the correct amount of pressure to be applied and leads to greater drilling accuracy. Figure 6.11 shows a bench drill stand being used with a chuck-and-drill-bit guard in position. It is mandatory to fit and use such a guard to comply with the Health and Safety at work Act 1974 and current regulations.
c Drill Stand and Mortiser – the chisel mortiser works on a similar principle to a
drill stand. A special bit drills a hole at the same time as the chisel is paring it square. Figure 6.12(a) labels the main parts, and figure 6.12(b) shows it in use; note how the cable from the drill is kept well away from the work bench. This attachment is very effective – mortise holes of up to 13 mm ⫻ 13 mm can be cut every time the handle is brought down.
Electric drills (rotary) 173
(b) Using an angle-drill attachment
Fig 6.10 Angle drilling
(a) 'Bosch' Purpose made angle drill Note: A cordless angle drill is also available
Keyed chuck
Chuck key and holder Point of entry for cable
and cable sleave Drill body
These have a dual purpose, they can be used for rotary drilling or, by the turn of a knob or lever, they can be transformed into an impact drill which with the use of a special tungsten-carbide- tipped drill bit can bore holes into the hardest of masonry or concrete. The hammering action is achieved in this case by a built-in ratchet mech- anism. Impact frequency can be as high as 48,000 impacts per minute. Some models with both clockwise and counter clockwise rotation, electronic speed control and slipping clutch device may be capable of being used as an improvised screwdriver.
Figure 6.13 labels the main parts of a palm grip type of percussion drill. This type of drill can be used for drilling holes in masonry up to 20 mm in diameter – eye protection must be
Fig 6.11 Bench drill stand
Fig 6.12 Chisel mortiser and stand