All about drills and bits – and more: Part 3 – drill bits continued – and some hints and tips

Last time we took you through some of the more common types of drill bits you may require for use around your home or workshop. Now we look at some more unusual bits, and add some hints and tips for you that we hope you will find useful…

26. Three examples of step bits, all with 6.35mmØ hexagonal shanks. This type of bit is also sometimes referred to as a unibit. These bits are used to enlarge holes in thin metals, plastic, fibreglass and other thin materials such as plywood using just the single bit. Note that the step bit in the centre has longer cutting edge steps, allowing it to be used on thicker materials. As you can see the step bit is cone-shaped with increasing diameter cutting edges often in increments of 1mm following from the tip. This means the same bit can be used to drill any diameter hole within the range of the bit. For example…

27. This step bit with a large smooth shank can be used to drill any hole in thin material in the following diameters: 4mmØ, 6mmØ, 8mmØ, 10mmØ, 12mmØ, 14mmØ, 16mmØ, 18mmØ, 20mm and 22mmØ.

28. This is a diamond-tipped core bit (8mmØ) that can be used on ceramic tiles, glass and other masonry materials. These bits are available is a range of sizes from 8mmØ up to 50mmØ and are very useful when used to make holes for plumbing pipes, for example in a tiled bathroom as it eliminates having to split a tile and nibble out a gap for a hole for a pipe – so no more unsightly split tiles. When using these bits it is best to keep the cutting edge cool with water, preferably trickled over the drilling spot, or at least by dipping the bit into water frequently. If the either of these two alternatives are possible, then some authorities suggest these bits can be used at up to 1000rpm, and if no water is available to cool the cutting edge, then a maximum of 250rpm is recommended.

29. This spear-shaped bit is designed for drilling into ceramic tiles, glass, granite and brick wall. Again, the tip is tungsten-carbide and stands proud of the shank. Size range is as follows: 3mm, 4mm, 5mm, 6mm, 8mm, 10mm and 12mm. Note that these bits should never be used with hammer action.

30. These taper-square shank augers are designed for used with a brace and only for use on wood. The threaded tip pulls the cutting surfaces into contact with the material. As they are used only with a brace, they allow very fine precision drilling of the holes. They are available in metric sizes ranging from 6mm to 25mm. Note that also available for use with a brace are screwdriver bits, countersink bits and awls.

31. This shows the parts of an auger bit.

32. For greater versatility, one can use an expansive auger bit. The provision of two blades allows for the boring of holes ranging, typically, from 15mmØ up to 45mmØ, but again, this can vary with the brand and model – some range from 22mmØ up to 76mmØ. Note that this example as a 6.35mmØ hexagonal shank, but there are variants available with taper-square shanks for use with a brace.

33. Countersink bits are available in the following sizes: 8mmØ, 12mmØ, 16mmØ and 19mmØ. Shown here is the 16mmØ version. As the name suggests, these bits are used to drill a shallow ‘V’-shaped recess for a CSK screw; this avoids the screw head forcing the wood fibres apart and potentially splitting the wood – quite likely when driving a screw into a thin piece or wood, or at the very end of even a thick piece of wood. Perhaps most commonly used on wood, these bits can also be used to countersink screw holes in plastics, non-ferrous metal, cast iron and alloy steel to accommodate screws with CSK (countersunk) heads. Note that these bits have 6.35mmØ shanks. They can also be used to drill deeper countersunk holes to accommodate the CSK screw and then a plug above the screw head to hide it for aesthetic reasons.

34. Sometimes referred to as screw diggers, but perhaps more often as pilot screw bits, these bits are ideal for drilling the pilot hole, the clearance for the upper section of the screw shank, and then the countersunk recess for a CSK screw. Shown here are the No. 6, No. 8, No. 10 and No. 12 bits. Note the inclusion of the Allen key, which is used to secure a replacement pilot drill bit in the event of breakage, and also for adjusting the depth of the pilot hole being drilled. These bits are secured with one or two grub screws (depending on the brand) – visible on the bits’ bodies. The depth of the pilot drill bit’s penetration into the material is adjusted by loosening the grub screw, adjusting the depth and then retightening the grub crew again.

35. A set of replacement pilot drill bits for the above screw diggers/pilot screw bits. Note that they have flattened shanks to ensure a secure fastening when the grub screw is tightened. Note also that there is quite a bit of the shank length that has been so ground flat, allowing for quite a lot of adjustment in drill depth, as mentioned just above.

36. These are Forstner bits and they are used to drill very smooth-sided holes with a flat bottom and in any orientation with respect to the wood grain. The only part of the drilled hole not smooth and flat is the small indent left by the central spur. These bits can be used to drill overlapping holes, angled holes – such as pocket holes for screws driven in at an angle – to secure the underside frame of a table to the tabletop, for instance – and even for cutting curved notches on the edges of a workpiece as these will cut whether or not the central spur is engaging the workpiece. This feature is unique to the Forstner bit.

37. In fact, as this image shows, these bits combine the actions of a hole saw – cutting or carving into the material – and drill bits, as in creating a hole in the material. Sizes are generally available as follows: 15mmØ, 20mmØ, 25mmØ, 30mmØ, 35mmØ, 40mmØ, 45mmØ, 50mmØ, 55mmØ and 60mm, with a drilling depth (depending on the brand and model) of about 50mm or so.

38. Perhaps one of the most common applications for Forstner bits is in cupboard hinges and the fitting of cup hinges, as shown here. The bit is used to create a flat-bottomed hole in the cupboard door for the hinge insert (on the right), which is secured in place with two chipboard screws. The other part of the hinge is attached to the cupboard body, also with two securing screws. This type of hinge can be adjusted in or out and left and right, allowing the door to be precisely aligned with its companion, or with the cupboard body.

39. This is a set of depth stops, also called drill stops, which are used to lock the drill bit – HSS twist drill, brad bit or even masonry and/or glass bit to a particular depth and no deeper. They are simply collars that are locked in place at the right spot on the drill bit with a grub screw and are often sold in sets to fit a range of drill diameters. Shown here is a set of four – 4mmØ, 6mmØ, 8mmØ and 10mmØ.

And now some hints and tips for you…

40. Oops! Using a spade fit for a large hole, you have drilled your hole in the wrong position in your wood workpiece. Here’s a simple fix… cut a length of dowel of the same diameter as the hole you drill and glue it into the hole. Allow time for the glue to set and then drill the hole again – and this time…get it right! Here we have shown a hole drilled with a spade bit, but the principle will work – if done properly – with many other drill types and sizes.

41. If you drill through very thin metal there is a chance you could end up with a ragged-edged hole. The solution is to sandwich the metal very firmly between two pieces of scrap wood and drill through both – and you can see from this image that the result is very clean.

42. When drilling through a tile, masking tape will keep the tip in the desired position. As an additional tip, if mounting anything on a tiled wall – say a towel rack in a bathroom – try to position it so that the drill holes are in the grouted grooves between the tiles, or even better, at the corners, where four tiles meet – as there is generally a little more space there. Doing this means that if you ever have to change or remove the item, the drill holes will be more easily concealed with grout – rather than being very clearly visible in the middle of an otherwise unmarked tile.

43. The bigger the hole saw, the more difficult it is to centre the centre drill and mandrel precisely. The trick is to drill a small precisely positioned pilot hole – in this case through a stainless steel sheet.

44. Now it is far easier to centre the drill saw – and complete the job with pin-point accuracy. Note the use of large amounts of oil to keep the hole saw from becoming too hot.

45. When drilling through metal, excessive heat build-up through friction can lead to the drill bit tip becoming too hot and then losing its temper (and not in a good way!). The solution is to make a little cofferdam of play putty and put in some cutting oil, or lubricating oil if the former is not available. It helps keep the tip of the bit cooler. The same idea works when drilling through glass – though there you can use a continuous trickle of water.

46. Here a mirror is getting the hole for an attachment screw bored using a spear bit. The trick is to use low speed, not use much pressure at all, ensure that the glass or mirror is on a perfectly flat surface – so there is no tendency to flex it (and therefore crack it) – and to not drill any hole too close to the edge or a corner. About 25-30mm is about the safe minimum. Note also here that no water was used – but the drill was run a very slow speed. And NEVER use the hammer action when drilling into glass – it will crack, or possibly shatter.

47. And there is the result – followed by a ‘Phew!’ of relief.

48. Drilling through the centreline of a pipe is a battle as the drill bit tip has a mind of its own. One solution is to trim a piece of scrap wood to exactly the pipe’s diameter and mark the centre point on that.

49. Then clamp the two into a vice – as shown – and drill through the wood at a dead accurate 90° and then through the pipe – the hole made in the latter will be in the centre, Mr Venter!

50. Here is a handy device you can make up with three lengths of wood and two screws or bolts. The illustration shows the basic parts. The only thing you really need to very careful of is that you drill the holes with extreme accuracy… with the holes in the dark wood arms (simply to make them stand out – obviously you can use the same type of wood throughout) absolutely centred on the wood. The three holes in the piece shown vertical (the pivot arm) must also be dead centre on the centre line and the middle hole must be absolutely on the centreline and absolutely centred between the adjacent holes. Note: The three holes in the pivot arm are shown same size here, but the middle hole can be much smaller – say to accept a 2mmØ or 3mmØ drill bit – so you can drill a pilot hole in the tube or pipe and then enlarge it as necessary.

51. Here is the completed unit – using 6mmØ gutter bolts to secure the arms to the pivot arm and a G-clamp to hold them firmly against the workpiece. Take note of the point made just above… the holes for the gutter bolts are 6mmØ, but the central hole in the pivot arm can be much smaller. In fact it is better if it is – all you want to do is make a pilot hole in the workpiece if necessary.

52. This is how it works… clamp the device firmly on to the workpiece – in this case a 45mmØ curtain rod – and drill the hole.

53. The device will work with any width of material over which the arms will fit… and the holes you drill into the workpieces will be dead centre every time. (But ONLY provided you drilled all the holes accurately… any error you make on the device will be transferred to each and every workpiece, but if the device you make is perfectly accurate, you will get perfectly accurate results each and every time.)

Hint: Do it in stages

When drilling into masonry, drill the hole more than once – say three times, working up from a small diameter drill, to an intermediate size and the diameter you want. You’ll take a little more time, but you will actually find that drilling the hole will be easier, and there will be less work for the drill itself.

Next month… hole saws and more!

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Mica Stores carry a range of power drills, hand drills and bits and accessories. To find your closest Mica and whether or not they stock the items required, please go to www.mica.co.za, find your store and call them. If your local Mica does not stock exactly what you need they will be able to order it for you or suggest an alternative product or a reputable source.