A case study
A medium type of manufacturing unit had a range of shafts, spindles, as a major part of their production. The diameters, some times in stepped sizes, were ranging from 20 to 35 mm and lengths between 100 mm to 300 mm. These shafts often had single cross holes of 3 to 5 mm dia. The batch sizes used to be rather small to demand or justify separate drilling jigs for the cross holes for every item. Kailash Singh, who joined there as the Machine Shop Superintendent studied the full range carefully and made an innovation. He prepared one set of accessories which would cater to all the varieties of drilling works. The set consisted of:
1. A strong well ground base plate with tenon piece in a full length - tenon slot on its top surface at the centre.
2. A sliding V-block hardened (Rc 50) and well ground with tenon slot at its bottom surface to match that on the base plate.
3. One tunnel like bush holding frame sliding in the same tenon. This consisted of central liner bush.
4. A set of drill bushes with OD matching the liner bush and ID matching the drills to be used.
5.A set of clamps, stoppers, nut bolts, etc.
By making proper selection he could prepare within 2 hours a drill jig to suit a batch order with any dimensional specification within the known range. Extending the idea further, you can have a large range of parts to assemble what is called as sectional or modular jig or fixture. After completing the production batch we have to dissemble the fixture and preserve the spare parts for any other requirement to come. Many small and medium scale industrialists are averse to broaching. They have genuine reasons:
1. It is not a versatile machine.
2. It is a very costly investment.
3. A broach is not a standard item like a drill or a tap. It has to be tailor-made for each item specifically depending on its work material and various geometrical parameters. It is made by specialised companies.
4. A broach has to be sent for resharpening every now and then to a specialist who demands exorbitant charges. Also, transportation means loss of time and money.
While these points are certainly valid, some ingenious methods and set-ups can still make broaching viable and you can reap the benefits of this technique.
(a) A sophisticated broaching machine is not always a pre-requisite. A simple standard hydraulic press or even a manual arbour press can be conveniently used for push broaching.
(b) One major limitation of push broaches is their slenderness due to which they tend to buckle; this can be overcome by having a limited length which in turn means less number of teeth and less material removal. We can have the work done by two or three broaches of progressively rising sizes rightly called as first, second and third pass, etc.
(c) We can even pass the same broach twice or thrice by adding shims (packings) of calculated values.
(d) It is quiet common to broach two or three pieces at a time. In fact, especially if the component thickness is too small, say below 10 mm, the broach is designed to cut two or more pieces held together one above another well organised at the pilot stage of the broach. This helps the self-guiding process. As a thumb rule 3 to 6 teeth should be in contact.
In various machining practices, there is always a danger of an operation being carried out on the wrong side of a job if the job also enters the fixture say, in the upside-down position equally well. In such cases, the designer has to anticipate such errors and make such provision that such a wrong placing or usage does not happen at all. Sometimes two or three components have mostly similar dimensions and appearance. Thus a tool may get used on a wrong job. Or a batch of wrong jobs may get placed in a wrong or common fixture and subjected to wrong operation. Suppose there are two similar jobs A and B but A has 2 holes and B has 4 holes. If common fixture with 4 holes is used, we should use a blinder type plate while using the fixture while drilling job A so that only 2 proper bushes are open.
In tool room practice, we have to manufacture many items to meet some particular need. Do not look to the immediate need narrowly. Make the piece a bit more versatile by using slightly larger size, adding a few tapped holes here and there, especially before hardening. How much extra work you should do depends on situation and how much you can spend. It also depends on the urgency of the present piece in hand In fact, you can meet the urgency of the user by making a soft piece to meet the urgent need and then, as and when possible add more features (value addition) and then harden the piece. Surely the time spent today will help you some other time in urgency as good investment. The V-block in Figure 4 explains the point.