This article first appeared in Practical Engineering 1940 Vol1 No2. The article is therefore correct as of 1940 The article discusses the design of new types of Engineering equipment available at the time.
While a knurled head will afford a grip sufficient to manipulate a small or medium-sized slip bush, larger ones are often operated by means of a tommy bar. This bar may be a loose one or fixed into the head of the bush. Where the latter arrangement prevails it can be made to provide the means of retention.
It may be in certain forms of jigs that the use of projecting catches as a means of securing slip bushes is precluded on account of the necessity of preserving unbroken flat surfaces with the bushes removed. In such instances it is usual to accomplish the locking of the bush by means of a pin inserted under the head and to incorporate the catch in the liner.
The Use of a Bush
Bearing in mind the necessity of having the guiding surface for the cutting tool situated as close to the work as possible, it may be that one or more holes are located at the bottom of a cavity. Where this occurs and the depth of the cavity is abnormal in relation to the length of a suitable standard drill, the bush is made to accommodate an extension shank. It should be pointed out that even where a standard drill can be used, the mouth of the bush should be relieved by counterboring in all cases where the design dictates that its length needs to be above the normal.
Screwed bushes are sometimes used to serve the dual purpose of guiding the cutting tool and securing, or assisting to secure, the work in the jig. This arrangement is not to be recommended, but where it is employed the thread must not be relied upon to locate the bush. A plain portion in the liner or insert for this purpose permits the thread being made an easy fit. The end of the bush is coned out when it may be used to locate and hold a cylindrical boss on the work to be drilled. Where a bush of this description is employed the opposing surface of the jig would be flat if the boss be on one side of the work. It may be, however, that the hole to be drilled is to pass through the end of a drop-forged lever having a boss on either side, in which case the screwed bush can be opposed by a fixed female coned bush to locate the under-bossed portion. By the omission of the coned end it will be apparent that the bush can also be used for holding flat work.
Methods of Holding Jigs
?The manner in which a jig is held or secured to the machine table while the operation is being performed will largely depend upon the size of the job, or, rather, that of the drill or drills necessary to carry out the work and the type of machine available for the purpose. Thus, for a job that requires a series of small holes drilled by a sensitive drill, the jig can be provided with a handle for hand steadying.
With a similar machine and where the job is one that calls for one small hole only in each piece, the risk of drill breakage can be greatly minimised by securing the jig in the correct drilling position on the machine table by clamping. Reverting to the first example, the jig can be permanently fixed to the machine table during use where an adjustable multi-spindle machine or multi-spindle drill head is available for the purpose.
Work of a heavier nature which has to be carried out on a multi-spindle machine, to overcome the necessity of making three or four tool changes during the performance of a cycle of operations, necessitates the moving of the jig from spindle to spindle on a common table. Here it is sometimes possible to arrange for the jig to slide between long, parallel strips bolted to the machine table.
?Jigs intended for use on pillar or radial drilling machines must be provided with bases to bolt on, or other provision made to clamp them down to the machine table. Where the drilling has to be carried out in more than one plane it is often advisable to mount the jig proper by means of trunnions in a substantial frame which is bolted to the machine table. The various bushed surfaces of the jig are then brought into correct relation to the drill spindle by means of indexing mechanism.
Special purpose, or adjustable multi-spindle machines for drilling holes in different planes simultaneously, would naturally require to have the drilling jig rigidly fixed in position.
Types of Drilling Jigs
?Drilling jigs may be divided into three main types, namely, plate jigs, built-up jigs, and cast box jigs. Any jig is, of course, in reality a built-up jig, as seldom does it consist of only one piece, but the terms mentioned refer mainly to the form which the jig body assumes.
In countless instances a flat plate of metal suitably bushed and provided with some form of clamping device and suitable location points will provide the necessary jig. Other jobs demand that the jig be built up from steel or cast iron or a combination of both materials.
?While jigs may be built up from flat steel by a system of screwing and dowelling into a box-like formation, this practice is not one to be recommended for those of large proportions and where the metal forming the bodywork has to be left in a soft condition, as constant use will most certainly render such jigs inaccurate. The more satisfactory method for such jigs is to utilise a casting made from a pattern in which are incorporated the necessary bosses, projections and stiffening webs as may be necessary to provide rigidity.
Methods of Construction
A plate jig can consist of a flat bright steel or machined cast-iron plate into which hardened steel feet are screwed in. The height of these feet requires to be such that they will raise the clamping arrangement used clear of the machine table. Where this method of construction is used the means of location is usually secured by pins or suitably contoured plates screwed to the underside of the plate. A properly made casting for a similar jig can have stepped surfaces and abutments for locating points cast integral, when there will be less likelihood of pieces working loose and causing variations in drilling.
One form of built-up jig which provides a rigid construction is where the top and bottom or bush-carrying plates are separated and secured by four or more pillars. At the ends, these pillars are shouldered down to pass through holes drilled in the corner of the plates. Hardened steel cap-nuts screw on to the shouldered ends to secure the pillars to the plates and form feet. Needless to say, for accurate results the distances between the shoulders on all pillars need to be exactly maintained, as does the overall length of the nuts. While of simple construction this form of jig will, with certain modifications, serve as a basis of "jigging" many jobs.
This is but one of the many examples of built-up construction, but it is one that is typical of the class in general. Wherever the building-up process is employed, reliance should never be placed entirely on the use of slotted-head screws. Bolts afford a better medium of securely holding the parts together, but the disadvantage of such a method is that bulky heads may cause interference. Hollow-head cap screws provide a neat and effective alternative. The heads of these screws are small in comparison to those of bolts, and require to be seated into counterbores in the same manner as cheese-head screws, but unlike which they can really be tightened.
This being so, reliance should never be placed on the screws alone for maintaining the parts of the jig in position, and therefore all important parts at least should be additionally secured by dowelling.
Jigs of the cast-box type require careful designing with a view to providing sufficient strength without unnecessary addition to the total weight and also as to the ease of machine-ability of the jig interior. Actual examples of cast-box jig construction will be given later. One important point relating to jig-feet in general is to see that they are so disposed and of sufficient size not to drop into or foul the table slots or holes in the machine table on which the jig is to be used.