The article aims at establishing the fact that drilling is something more than just removing metal from the workpiece.
Drilling is an ancient operation; ever-present around the world, in almost all machines Numerous developments in this apparently traditional operation have taken place. Regular maintenance is a chief requisite to make sure that the drilling operations are smooth and hassle-free. In drilling, the cutting edge is not visible to the operator. Hence, keeping track of the interaction between the cutting tip and the workpiece, as the former pierces through the latter, entails some form of detection such as feed force - torque or power monitor. There is another alternative to the use of the monitor. This is a post-operation check; wherein the chip formation, colour and the drill bit are examined. Whereas turning and milling facilitate direct overseeing, drilling, with the rapid rise in penetration rates, facilitates the use of modern techniques for monitoring the progress of the operation.
Engineering is not always a precise science or art that can be defined in exact formulae. It requires experience to develop skill, methods and knowhow accumulated over the years. This approach applies very much to drilling and that is why the hands-on approach, combines with the theory of drilling, can help solve most applications with regard to tools and machinery. Looking back at the history of making holes, there has been very little progress from the middle of the nineteenth century and onwards. Spade and twist drills were eventually developed and later made of high speed steel. It was not until the latter half of the 1970s, that there was a complete break from convention, when indexable inserts came into the picture. A revolution had begun!
With penetration rates almost ten times that of twist drills used at that time, the indexable insert drill provided the basis for drilling to reach the standards of turning and milling in NC machines. By operating at much higher spindle speeds, feed forces were reduced to around half of that of the twist drill. Drills moved from the earlier straight-flutes configuration to the current spiral-flutes one. The change in configuration enabled easy chip disposal, as the operation progressed. The merit of the first indexable insert drill was mainly that it could produce holes in a much shorter time. The surface finish was, however about the same as the twist drills. For holes requiring a higher quality finish and closer tolerances, boring bars and reamers were used. To start with, this was a roughing tool.
Other drivers for development
With the onset of development, several other driving factors began to influence innovations and improvements. Among these were the need for increased speeds and the demand for better quality. While talking of quality, parameters like straightness, surface finish and tolerance are to be considered. With these considerations, a two-step approach is recommended. The drill should be capable and of high quality; having attained this, it should be designed to facilitate chip evacuation. Transporting the chips away from the cutting edge has always been very critical in drilling. The chips, besides, in transit should not impair the hole quality. Jamming of the chips in the flutes, will cause wobbling and throw the drill bit out of balance. Achieving correctly formed chips and getting them out without damaging the surface is the way to achieve quality assurance and smooth production in modern drilling with high penetration rates. This brings us back to the previously talked-about change in the configuration of the flutes. Removing chips from the hole being drilled was the main criterion of the first high-penetration rate drills. Sandvik Coromant for example, did not introduce the helical flute until it could be made deep and with enough room for optimum chip evacuation, without compromising on the rigidity of the drill for it to maintain stability.
The U-drills have coolant holes which follow the twisted flute design to enable the chip space to be made deeper towards the centre-line of the drill. This enables chip removal without damaging the whole surface. Having ensured reliable chip evacuation, cutting speeds can be increased further with the use of cemented carbide grades. Using moderate feed rates, even long stringy chips can be streamed out of the holes. This is advantageous when dealing with materials like stainless steel and low-carbon steels.
Setting means precision
In its balanced condition, the drill can be set accurately to achieve close tolerances and high surface finish. The stationary indexable insert drill can be offset radially, providing it with the possibility to produce holes with diameters different from that of the nominal values and to high levels of accuracy. Stationary drills apart, rotating drills can be held in adjustable holders, which allow precision setting for the cutting edge.
Maintenance is essential
Using drills to their optimum best involves religious maintenance activities. Maintaining cutting tools takes a lot of time that could be spent on productively running machines. Often compromises are made as regards suitability of the tool selected from the inventory, leading to inferior performance. This unproductive time usually gets worse the longer it goes on. Many workplaces soon accumulate this and that, over a longer period of time, often leading to the frustration of not finding a suitable drill bit or drill, for the drilling operation. Before the drilling operation is commenced, reviewing the tool inventory is a very good start. Often there are historic pieces that need not be kept. Tools and inserts that are outdated and hardly ever used. A qualified assessment and an overview of operations should lead to a proposal that provides the right balance of dedicated and all-round tooling.
Inserts are always to be checked for the following parameters:
- cutting edges for damage or abnormal wear
· insert-clamping screws for condition
· Use the keys provided to give the right force while tightening
· Insert seat and shim for any damage or wear
· Clean seats, shims and lubricated screws
· Condition of the tool-body contact faces
· Tool overhang
· Condition and the amount of the support of tool-clamping
- Tool holding suitability for strength, stability and accuracy
Thus, quality, speed, chip disposal and above all, a religiously followed maintenance schedule are among the factors that govern the drilling operation. Effective lubrication is indispensable, when you wish to protect the drill bit and the workpiece. And it can emphatically concluded that drilling is something more than just removing metal from the workpiece.
|Posted : 10/26/2005|