Figure 4 shows a similar operation being carried out on a pair of crank webs. The blanks for the webs have been sweated together and buttons fitted at the required centre distance. In this case the important dimension is the distance between the centres of the holes and the button positions can be set using a micrometer – no need for gauge blocks. This application illustrates the use of the long button in the set to facilitate access when two holes are required close together. Once both holes had been bored the outside profile of the webs was completed.
The examples shown so far have been for machining holes from a solid work piece. There are many occasions when a hole already exists - in a cored casting for example - and it is necessary to locate the job accurately in order to machine the bore. Two examples of this situation are shown in Figures 5 and 6.
Figure 5 shows a typical cored cylinder casting being set up for boring. The outside of the casting has been machined to finished sizes to provide suitable reference faces. A mild steel plate with a 6 BA tapped hole in its centre has then been glued to the casting using super glue and a button set up with reference to the machined surfaces.
The job is then set up in the lathe, in this case in a four jaw chuck, and once adjusted until the button is runnng true the steel mounting plate and button can be broken off the casting and the bore completed. The superglue provides adequate strength for the setting up operation but is easily broken off when this is complete. In this particular case the casting was left over length until the bore was completed. A light skim was taken over the end after finishing the bore and the cylinder block subsequently mounted on a spigot to machine the other end to finished length.
Figure 6 shows a similar procedure applied to the machining of an eccentric strap. In this case the requirement is to ensure that the bore of the strap is centered on the split line of the strap. The casting for the strap has been cut in half and after machining the cut faces to clean up the halves have been soft soldered together and the outer faces machined to clean them up.
A steel carrier plate has been prepared with edges corresponding to the split line of the strap and a mounting area for a button. The button is set up with its centre on the split line and the carrier plate glued to the prepared casting using superglue, with the reference faces on the soldered joint. The assembly is then set up in the four jaw chuck for boring and the carrier plate broken off. The outside profile of the strap is subsequently finished and the clamp bolt holes machined with reference to the finished bore.
So much for positioning holes accurately in components during the manufacture of single items or very small batches. If large numbers of identical items are to be produced it is worth making jigs to ensure consistency and minimise time. I do not intend to elaborate on this matter here since every job will present its own requirements.
There is one aspect of jig use which it is worth mentioning, however. It is almost inevitable that at some stage we find that a job has progressed to an advanced stage and we have overlooked a hole or a modification to design has been decided on which requires an accurately positioned hole. A typical example which I have encountered myself is the need for a hole (or holes) in the frames of a locomotive when the job has reached an advanced stage of erection and it is impractical to mark out in an accurate manner or to transport the assembly to the drilling machine.
In such a case there are almost certainly other holes in the vicinity of the new requirement which have been accurately positioned at an earlier stage of manufacture. A jig can be made up from a piece of plate with two or three holes to match the existing holes in the frame and the new hole accurately located relative to them.
Alternatively the jig may be made to reference from an edge or other feature from which the position of the new hole can be accurately defined. This plate can be machined by coordinate positioning so that it accurately resembles a small area of the frame and clamped into position thereon using bolts through the existing holes or, if this is not convenient, using dowel pins and a toolmaker’s clamp. The new hole can then be drilled with a portable drill. The procedure is illustrated in Figure 7, which shows the hole for the mounting of a drain cock operating lever being drilled at a late stage in building. In this case the temporary drill jig has been referenced to the top edge of the frame and an existing hole and is held in place with a toolmakers clamp. I have used this procedure on numerous occasions. It gives guaranteed accurate results with a minimum of hassle.
So much for positioning our holes. Next time we will give some thought to making and measuring them!
