HAS IT ever occurred to you how much of our time in the workshop is spent making holes? Just think about it. Everything we make requires holes of some sort for fixings, bearings, cylinder bores, bushes, pipe fittings – one could go on. The list is endless! I thought that this time we would give some thought to the subject of holes and see what demands their making places on our resources.
First of all, what requirements have to be met to satisfy the specification for a hole. It will have two main features – its position and size. In many cases there will be additional features to the specification such as the finish, a blind end or stepped bore or a thread.
We will start by considering the features common to all holes, their position and size. First of all, position.
The traditional and simplest way of locating a hole is to mark out with a rule and scriber and centre punch the position. The centre punch mark may then be used directly to start a drill. This procedure, while simple, is probably the least accurate method of positioning a hole.
The accuracy of the procedure may be improved in a number of ways. First of all, the marking out can be carried out using a height gauge on a surface plate, the gauge being set to reference some feature on the job from which the hole is dimensioned. The resulting scribed lines will then be accurately located.
The centre punching operation is improved if the initial punch mark is made using a ‘prick’ punch i.e. a punch having a much smaller included angle than a normal centre punch. The prick punch can be located to the scribed lines by ‘feel’, running the point along the line to find the crossing point. A light tap starts the mark and the position can be checked with an eye glass. If there is any error the mark can be ‘drawn’ by angling the punch and tapping in the required direction. Once accurately positioned a normal centre punch can be used to enhance the mark.
The drilling procedure is critical in maintaining the accuracy achieved in the marking out. The drill point must locate in the centre punch mark. It is, therefore, necessary to use a drill with a small enough point for this to happen.
By far the best approach is to start the drilling with a centre drill with a suitable size pilot point. If the final size of the hole is more than about 1/8 inch drill a pilot hole of that size and open up progressively to the required finished size. This procedure reduces the risk of the drill wandering off course as machining proceeds.
When drilling large holes it is usual to scribe a circle of hole diameter at the marking out stage and centre pop four points on the circumference. This provides a visual indication of any drift of the drill position as opening up proceeds.
The procedure described usually results in a reasonable degree of positional accuracy, but leaves something to be desired if the hole is required to be located within better than about five thou. If this sort of accuracy is required more sophisticated techniques are required.
The most obvious of these techniques is the use of co-ordinate positioning. Typical examples might be drilling the required holes in an engine bed or drilling the numerous holes required in the frames for a locomotive.
The remarks above all assume that the hole to be produced will be drilled. It may be that the size of hole concerned if too large to be drilled, either because of the nature of the work piece or the facilities available. If this is the case a start can be made with the largest drill thought to be prudent and this opened up to the required finished size by boring.
This process is illustrated in Figure 1 which shows a large hole being bored in a boiler shell for the dome bush. This hole was positioned with reference to the outside of the barrel and the smoke box end by the coordinate positioning technique.
The hole was started with a centre drill and opened up progressively to ½ inch diameter by drilling. This was thought to be the largest drill which it was prudent to use in this case because of the nature and thickness of the material. It was then opened up to the required size using a boring head as shown. The picture shows a job being carried out on a milling machine but it is worth noting that a boring head provides a useful method of machining a hole in the lathe where a job is too large to swing in the machine for boring in the conventional way. The boring head is then mounted on the mandrel and the job supported on the cross slide or a vertical slide.
There are many occasions when accurate positioning of a hole is required but co-ordinate positioning is not an option. One of my favourite techniques in these circumstances is to use toolmakers buttons. These useful devices are rarely seen these days but they offer a very simple and accurate method of setting up a job in either the lathe or milling machine.
A typical set of buttons (made some 60 years ago during my apprenticeship days) are shown in Figure 2. Each button consists of a small cylinder hardened and ground to an accurate diameter with a fairly large hole through the middle and provided with a fixing screw and substantial washer. The drawing shows details of the buttons in the photograph but if you are a metric convert convenient metric dimensions can be substituted. For home workshop use a set of buttons made from silver steel and used in the unhardened condition would be perfectly adequate. The diameter of the buttons is not important so long as it is known accurately.
So, how are they used? The idea is to mark out the position of the hole to be produced as accurately as possible and drill and tap a hole to suit the button fixing screw (6BA in the case of the buttons illustrated). A button is then secured to the job using its screw and before fully tightening the screw the position of the button is adjusted to the exact location required for the hole using whatever measuring facilities most suit the application. The job is then set in the machine using a DTI. This may be in the milling machine or lathe depending once again on the application.
