How do you bore? The boring bar consists of a spindle, indexable insert and an anchor wherein the spindle is also anchored to the machine. The length by which the spindle extends from its anchor is called the overhang. Most end users seek to limit the overhang as much as possible since the longer the spindle extends from the anchor; there is a greater risk of deflection which ultimately leads to insert failure and chatter. You are machining with a flagpole so if the flagpole is longer, the weaker it is.
The operators of machines generally make an acceptable judgment on the overhang by getting the ration of the overhang with the diameter of the bar. This ratio should be as large as possible. Most deem this ratio at four to one, overhang to diameter. Accordingly, the ratio can also go as high as 10 to one but that would really be the worst case scenario. There are boring milling machines that have bar diameters between 5” and 6”. Most machinists try not to overhang as much as 36”. Experts also say that an eight to one ratio can also be achieved without having any problems at all. The diameter of the bar on a particular machine will determine the amount of horsepower needed. A 5” spindly will need roughly 50 to 70 horsepower and as you have larger diameter, you also increase the needed horsepower for the hob.
The insert is where the rubber meets with the road on a boring machine. The specific type of insert used is determined by whether the end user is finishing or roughing. On the finishing side, you make specific wiper geometries which are referred to as knife-edge according to the solutions leader at Sandvik Coromant, Tony Williams. You also get a clean and sharp cutting edge which allows the boring tool to pass between the bore at a much higher feed rate and still generate a smooth and nice surface finish. According to Williams, a very popular type of material of the boring mill insert is cemented carbide. This type of insert coating can be determined by the type of work piece material which a shop is machining. It is to be noted however that aluminum oxide is an ingredient that is almost always used when coating.
History of Boring
The history of boring machines dates back a century ago. MAG Giddings and Lewis built its first ever boring machines in the year 1905. The greatest change in boring mills came about when CNC controls were first introduced and applied to the machineries in 1950s. The boring mills started as mechanical machines which were gear driven. The operator throws levels in order to engage clutches and in order to change feed rates and even to crank large wheels in order to move an axis. Today, the boring mills have totally evolved to CNC controlled machineries.