The Fundamentals of Long-Hole Drilling
Monday 12 October 2009
In the first part of the fundamentals series, John Lovatt takes a look at long-hole drilling
This factual series is for all skill levels, from beginners to the more advanced. To make things easier, we will also be putting together a glossary of terms and catalogue for future publication.
One may ask if long-hole boring is, well, boring? Or, is drilling boring? Are they the same thing and how do they co-exist? With no clear boundary between the two expressions, and doubts that anything would be gained by a deep philosophical debate, it has been decided that drilling, as a term, should apply to the making of relatively small diameter or short holes, and as such, will justify a stand-alone article in this series later on.
This article shows the deep hole boring (for the electric wiring) of the middle section of a standard lamp.
The operation features a basic fixed hollow boring centre (bore size to match the auger), in conjunction with a shell auger, also known by other names such as a standard lamp auger, split-nose auger or simply, a nose-bit.
Internationally, the most common auger size for this type of work is 8mm (5/16in) diameter x 750mm (30in), excluding the wooden handle. Long-hole boring is deemed tricky by many, however, in truth, by using a methodical approach, you will be able to achieve results very easily - and consistently.
We have laid out the process in five fundamental stages, boring the hole down a spindle to help you sort out the main points. Due to the overall length of this set up, the illustrations have been truncated.
Starting the project
1. Make centre point indentations in both ends of the workpiece - this is best done with an engineering centre drill (slocombe bit), as it provides a positive 60 degree v-location matched to the headstock and tailstock centres. Mount the workpiece between the drive centre and the live tailstock centre and turn to shape. Depending on how good your accessories are, it might pay to leave the final finishing cuts until the boring is completed
Getting ready for boring
2. Remove the live centre from the tailstock and replace with the hollow boring centre. This can either be a 'dead centre' variety or a better option is the 'hollow revolving centre'.
Fitting the hollow tailstock centre
3. Form a circular indentation in the workpiece with the hollow centre by slackening the centre pin set screw so that it slides back inside the centre as you advance the tailstock quill. When a reasonable circular indentation has been made, remove the hollow centre and extract the centre pin, remembering to put the said pin somewhere safe. Return the hollow centre to the tailstock and relocate in the workpiece
4. With the lathe running at a slow speed (about 200rpm), feed the auger through the tailstock with the gullet (hollowed section) in the 'down in' position and engage the workpiece, taking care not to damage the cutting nose as it is inserted. It is a good idea to put two marks on the auger handle to indicate 'gullet up' and 'gullet down'. Start to cut by pushing forward in a pecking action whilst slowly rocking the auger. If the auger is sharp, this does not require more than moderate pressure. Rotate the auger into the gullet up position, and withdraw to empty out the shavings. Continue until you have penetrated about 25mm (1in) or deeper than the length of the 8mm (5/16in) diameter counterbore drive centre pin - this is to avoid hitting the pin with the auger later on. Do not attempt to penetrate more than about 25mm (1in) at a time or the shavings will bind in the gullet and lead to a broken auger - or worse. This is not an operation to rush. Simply continue until you have reached just over half way. At this juncture, flip the piece end for end and centre bore the remaining half of the piece.
Shell auger and sharpening
This type of auger is a hand-forged tool with specialised characteristics, which make it the most reliable and practical for this work type.
For the cutting face, only sharpen the front surface of the cutting tip with a small flat lap or slip stone (preferably smooth grade diamond or ceramic). Ensure the flatness of the surface is maintained and the curved top left is untouched. This should only take about three firm, downward strokes with the auger held steady.
For the shell, use a similar grade lap or slip of a suitable diameter and restore the cutting edge of the shell by making horizontal strokes whilst the auger is held steady. Leave the opposite trailing edge untouched.
Hollow revolving centre
Using a live hollow centre avoids the need to lubricate the workpiece. Some live hollow centres feature a centre finder and pull out centre tip which obviates some of the remounting work and helps to maintain good working concentricity. This is a typical centre for this type.
Alternative long-hole boring bits
Long-hole standard lamp augers, or split-nose shell augers, as featured on the preceding pages, remain unchanged in design from their 19th century origins. They are still hand-forged and finished the same way today, which involves nine separate manufacturing stages.
The cutting action is effective in severing the wood fibres, whilst the auger advances with virtually no side thrust effect, thus maintaining the bore on the axis of the workpiece.
They remain the most practical, informed choice and are available
in a variety of diameters from 6.5mm (1/4in) to 22mm (7/8in). However, the most popular size for lamps, or for any other items requiring the passage of electric wires, is 8mm (5/16in) diameter. Virtually all hollow tailstock centres and counterbores available today are only available in that size.
A plain shell auger is sometimes referred to as a spoon bit. This lower cost alternative is machined from a solid bar, so some reduction in cutting performance may be experienced.
Twist augers have a conventional twist drill end or a straight section cutting lip. They are effective but it can be difficult to achieve initial entry and maintain your work on axis.
The gun drill, the more common half round (or single cutter) version, is considered by some to be superior to any other kind of tool in producing a very accurately made straight hole. However, practical considerations weigh heavily against the general adoption of this tool because unless you are going to produce long holes
on a commercial basis, the cost of setting would present a problem. For example, high tool cost requires a constant compressed air supply. Also, a custom made device would need to be constructed to replace the conventional tailstock, etc.
The twin cutting form of a gun drill provides faster cutting and is more suited to materials like wood, but still requires material-specific, multi-angle surfaces to be machined.
Other helpful boring bits
A saw-tooth bit has ripsaw-style teeth which cut through the peripheral wood fibres. This allows the two cutters to progress more quickly through the wood with less force than what would be required with a Forstner bit. This design is effective from about 7mm (1/4in) diameter up to around 75mm (3in) diameter. The sharpness of the entry rim and internal bore finish makes them a popular choice for many operations.
A Forstner bit has a slower cutting rate but this is offset by the cleanness of the cut and the smooth finish that can be achieved. It is a sound first choice for good quality work. The available range is similar to that of saw-tooth bits.
A spade bit is a cheap, crude, but effective cutter. For some work, the cutting faces can be ground to a shallower angle and the point length reduced. It can also provide a source of material for the production of specialised boring forms.