Model Engineers’ Workshop – August 2019

August 2019 33

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Long Drill

Supersize Me!

I

get interesting commissions from time to

time and this was something completely

diff erent. My self employed electrician

neighbour Andrew asked for my help with

special tooling to help him install lighting at

a local church.

The request for the work had been made

by the Diocesan Advisory Committee

for the church to conceal the armoured

cabling supplying a set of vandal resistant

outdoor lights and contain it in a trench

in the churchyard by running it out neatly

through a wall.

No problem you might think, but not in

this case. The wall was in the bell tower, a

Grade II listed structure, built about 700

years ago and estimated to be six feet thick

in dense sandstone!

Background

As well as this being a very diff erent sort

of workshop challenge, the job had now

acquired a fl avour of pioneering work along

the lines of the method used to drill the

long holes for pre-stressing bars in the

supports built under York Minster in the

late1960s to underpin the central tower.

The estimate of thickness proved to be

pretty accurate, at the death the wall was

1.7 metres thick [ 5’ 7”]

Andrew is an IMechE engineer and had

already fi gured out a way ahead for the job;

we discussed his thoughts and agreed on

some modifi cations to the basic principles.

Accordingly, I was presented with a new

Bosch 25mm diameter SDS drill bit of 1

metre working length and another shorter

drill with the same SDS chuck fi itting end.

It was the biggest they had at the very well

equipped professional tool hire and sales

organisation locally. On asking for their

thoughts they said the job couldn’t be done

without expensive guidance equipment to

get a hole to meet in the middle.

Bearing in mind that we are talking

here about a 25mm hole and not tunnel

boring, Andrew’s ideas seemed perfectly

reasonable to me; the recommended dual

drilling approach was clearly a nonstarter

anyway using hand held drilling methods.

He is also a stubborn Yorkshireman and

such a challenge was like waving a red rag

at him, he was not going to be defeated by

such negative thinking!

With the job fi nally done and dusted I

now know rather better and will add more

information further on.

For those unfamiliar with the term, SDS is

an abbreviation for Special Drilling System

which very eff ectively combines rotation

with rapid axial hammer action. A typical

drill bit is iIlustrated in photo 1.

The induction brazed tungsten carbide

tip is ground to a centred pyramidal shape

and not sharp edged like a traditional drill;

it works by battering the material into small

chippings that are swept away as they form

by the rotary action.

The tooling

The tool was made up from the 1 metre long

SDS drill with its drive end inserted into a

close fi tting socket into a 1 metre length

of 16mm diameter En1 black steel bar I had

ordered in for the job.

At the other end of the bar, a similar

socket was made to contain the drive end

Brian Wood extends a masonry drill to 2 metres in length

1

2

A typical SDS drill

The joint preparation