Introduction to High Speed Horizontal Machining Center
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At least there is room for maneuvering when milling. Part of the speed increase can be spent on lighter and more numerous cuts, loosening the load on the entire process.
Not with drilling. Regardless of whether the drill is made at conventional speed or at "high speed", one pass still means one hole. The fast or slow toolpath is the same. That is why all challenges associated with a slower process become more pronounced as the spindle speed and feed rate increase. The most important of these challenges is the removal of heat and chips when the tool penetrates deeper into the hole.
Here the term has a definite definition. "High speed drilling" refers to drilling at spindle speeds high enough to allow a penetration rate of three to ten times higher than a conventional rate, depending on the material of the workpiece.
However, the strategies used by companies can also be used to achieve fast drilling according to a broader and more accessible definition: drilling faster than before. If the machine tool offers spindle speed and feed currently not used in drilling operations, it is possible to achieve the same tool life and tool performance as at present at higher penetration speeds, simply by making appropriate changes to other elements of the process. By choosing the right tool holder, drill bit, coating and coolant delivery system, the store may be able to unlock potential productivity that has never been used before.
Almost all the suggestions in this article can be about drilling at any speed. However, they acquire new significance as the speed exceeds conventional values. Every part of the process worth paying attention to conventional speeds probably deserves more attention - and more investment - as the speed increases.
This is especially true for coolant. Cooling through the spindle in combination with drills offering internal channels for supplying coolant directly to the tool tip, can extend the life of the drill at almost any speed. However, in most of today's high speed drilling applications, high pressure coolant flow through the tool is critical to removing potentially catastrophic chips and heat.
It may change tomorrow. Several companies in the United States are experimenting with reducing the need for coolant during high-speed drilling. They follow the leading European manufacturers facing significantly higher costs of utilizing the used coolant. In some parts of Europe - and perhaps in the United States - rapid machining will be increasingly used with minimal or no coolant consumption.
Any store that implements rapid drilling also sets up to drill dry this way, regardless of whether this position is attractive today. High-speed drilling and dry drilling are complementary processes presenting similar engineering challenges. Thus, by adopting some of the techniques for high-speed drilling found in this article, a shop can also be preparing itself for the transition to a machining process with a much smaller appetite for coolant.
One of the most important requirements for fast drilling is also one of the most basic. Any qualified mechanic knows how to check for alignment errors - or runout - on the cutting edge of a drill. As speed increases, it becomes even more important that the centerline of the drill is aimed.
The key reason why this has to do with the tool material. At lower speeds, stores can drill efficiently using tools made of high speed steel, which provides relatively high strength and bending strength. However, higher-speed applications require carbide, and perhaps ceramics - both of these elements sacrifice some of this strength to wear and heat resistance at higher cutting speeds. Uniformly distributed cutting forces due to low runout may be necessary to achieve acceptable tool life for these brittle materials, especially when the spindle revs are shifted by five digits.
An effective drilling process at this speed should have a total runout of no more than 20 microns. This error, measured after loading the drill into the tool holder and spindle, is the sum of runout in four process elements: machine spindle + spindle / tool holder interface + tool holder / tool interface + tool itself. If the workshop uses an existing high speed drilling machine and does not plan to replace the spindle, then the first two of these error causes are out of control. However, none of them will probably be the most important source of runout on a relatively new machine. Spindles offered for most of today's machine tools, even relatively inexpensive, usually offer a coaxial alignment and frame tolerance sufficient to leave sufficient space on this 20-micron margin. The drill itself will also not contribute significantly to this error. Cutting tools today are being manufactured to ever-tighter concentricity tolerances.
This makes the tool holder clamp the most important source of runout. In fact, the wrong tool holder can cause more than 20 microns of error in itself. Some say they have tried many standard collet and side lock holders and have found a wide range of crimping concentricity from supplier to supplier. In general, they came to the conclusion that a top quality tool holder with side locking can keep runout error up to around 8 microns, a top quality collet holder can hold up to around 11 microns, but some types of both types can add 30 microns to the total error. The third alternative is the hydraulic chuck, which uses the oil tank to equalize the clamping pressure around the tool. The company said that these holders are able to provide reproducible clamping concentricity in the range of 3 to 9 microns.
If the store has the luxury of choosing a spindle for high-speed drilling, it is recommended that the interface of the tool holder be HSK. The HSK connection offers a small, predictable runout error. About 12 to 15,000 rpm comes the time when HSK is necessary to get the most out of the drill.
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