Electric screwdrivers like the ones featured in figure 6.15 & 16 offer fast, virtually effortless methods of driving screws. Screwdriver bits of different types and sizes are available to suit both slot and Pozidriv (Superdriv) screw heads. Table 6.2 shows the different sizes of Pozidriv points to suit the appropriate screw gauge.
6.8.1 Method of use
These screwdrivers have a special clutch mecha- nism and depth setting device, which enables a screw to be driven to a predetermined depth, after which the screwdriver bit stops turning. It should be noted that in most cases, both clearance and
Depth sensing nose cone and bit holder
Forward & reverse rotation switch Trigger switch (variable speed)
Palm grip handle
Point of entry for cable and cable sleeve
Fig 6.15 Electric screwdriver
Adjustable nose stop to suit screw
sizes Forward &
reverse rotation switch
Point of entry for cable and cable sleeve
Screw feeder guide
‘Autofeed’ screwdriver in use Screw depth
adjustment knob
Fig 6.16 Electric screwdriver with automatic feed
Table 6.2 Pozidriv & Superdriv screw gauge in relation to the screwdriver bit point size
Screw gauge size 3–4 5–10 12–14 16⫹
Driver point no (size) 1 2* 3 4
pilot holes (fig 12.7) should be pre-bored before driving any wood-screw.
Figure 6.16 shows a screwdriver with auto- matic feed. Screws are fed to the driver via a strip of plastic into which they are temporary attached.
Note: because cordless drills also double up as power
screwdrivers they are more commonly used – partic- ularly on site work (see section 7).
6.9
Sanders
There are many different types and styles of sanders associated with bench work. We shall be looking at three of these, namely:
● belt sanders
● random orbit disc sanders ● orbital sanders
6.9.1 Belt sanders
Designed and constructed to tackle heavy sand- ing problems with minimum effort. Figure 6.17(a) names the main parts of this machine. The endless abrasive belt is driven by a motor- driven rear (heel) roller over a front (toe) belt- tensioning roller and then over a steel-faced cork or rubber pressure plate on its base – this is the part that makes contact with the workpiece. Dust is discharged via a suction-induced exhaust into the dust bag.
a Method of use – It should be permitted to reach full speed before being gently lowered on to the workpiece – allowing its ‘heel’ to make contact slightly before its ‘toe’, to avoid any kickback.When contact is made, there will be a tendency for the sander to move forward and force the workpiece backwards, due to the gripping action of the sanding belt (see fig. 6.18), so both the sander and workpiece must be held firmly at all times.The surface finish produced by the sander will depend on what grade of abrasive belt has been used. Figure 16.17(b) shows a belt sander being used with a sanding frame attachment. (See section 5.10.2; table 5.6 and fig. 5.86).
Note: the dust bag must remain attached to the sander while the motor is in motion, otherwise dust and particles of grit will be discharged directly from the exhaust tube at an alarming rate and could result in serious injury.
6.9.2 Orbital sanders and finishing sanders Figure 6.19 shows the main parts of a heavy duty machine. The abrasive sheet is attached to the Sanders 177
Rear drive roller
Stop Direction in which the
workpiece will travel if not restrained Direction the
sander will travel if not controlled
BENCH Work piece
Toe Heel
Fig 6.18 Securing the workpiece being sanded Fig 6.17 Portable belt sanders
Dust collection bag
Front handle
Sander belt
Pressure plate Front (toe)
tension roller
(a) Main components
Enclosed to rear (heal) drive pulley
Adjustment for belt tracking Cable sleeve Cable Rear ‘D’ handle and trigger switch Vented motor housing
base plate either via clips, levers, or are ‘Velcro’ (depending on the design and dust extraction system), the orbital diameter can be from 1.5 mm to 5 mm, with rotation from between 12000 to 1400 rev/min (RPM). These may also be known as 1⁄
2, 1⁄3 or 1⁄4, sheet sanders (see fig.
6.20). Palm sanders (fig 6.20) are smaller sin- gle handed versions.
Method of use – the self-weight of the sander
is usually sufficient pressure – anything other than light pressure could result in scratching, clogging the paper, or even the body orbiting while the pad appears to remain stationary. To obtain the best results, the appropriate grade of abrasive paper must be used to suit the job. This often means starting with a coarse grade of paper, then reducing the grade until the desired result is achieved (see section 5.10.2, tables 5.6 & 5.7 and fig. 5.86).
6.9.3 Random orbital disc sander (fig 6.21) Single and double handed machines are avail- able, capable of producing excellent finished results by working with an eccentric (off-centre) action whilst rotating. The base is circular with a flexible sanding pad, which, like the abrasive disc which is attached to it, is perforated allowing machined dust to be drawn into a collection bag or extraction system. Self-adhesive sanding discs with peel-off backing, or ‘Velcro’ type backed sanding discs are usually available.
Method of Use – application methods will be similar to all finishing sanders.
6.9.4 Dust extraction
As with all mechanical-sanding operations, dust can be a serious health hazard, and, even though all belt sanders and orbital sanders are fitted with a means of dust extraction (extraction methods do differ between models), an approved mouth and nose mask must be worn by all oper- atives and those in close proximity, whenever sanding operations are being carried out.