Much the same way temperature and load changes require tire pressure adjustments on a car to ensure even performance and a smooth, steady ride, so too are such adjustments needed when it comes to controlling vibration during BTA (Boring and Trepanning Association) deep hole-drilling operations. Dampeners installed at specific intervals along the length of BTA drill tubes can be adjusted to reduce the negative effects of harmonic resonance or vibration in deep hole drilling.
As a side effect of deep drilling—holes with a 10:1 depth-to-diameter ratio—vibration or chatter from the interaction of the cutting tool and the workpiece is more easily transferred to the tool tip as the length-to-diameter ratio increases. Such machines are capable of drilling to depths of 100 times the diameter or greater, and with BTA drill tubes as long as 30 ft (9.1 m) spinning at high rpm, vibration can derail hole concentricity, degrade surface finishes or worse without some way of controlling and minimizing it.
To counteract the adverse effects of vibration and resonance, dampening assemblies comprised of heat-treated components and precision bearings are clamped around the BTA tube, holding and supporting it while still allowing smooth drilling rotation. Dampening assemblies are available in two broad categories: standard and hydraulic. Standard units are manually adjusted prior to drilling via tensioning locknuts on each end of a tapered collet for a durable and cost-effective solution.
Hydraulic dampening systems have a higher cost due to their increased complexity, but they offer multiple advantages over standard dampening assemblies. While the latter can only be adjusted prior to drilling, hydraulic vibration dampeners allow the application or release of clamping pressure while drilling. Such flexibility is extremely helpful in deep hole drilling, as harmonics tend to change at different depth-to-diameter ratios and/or the workpiece material may have hard spots that change vibration characteristics mid-operation. Additionally, when multiple dampening devices are used, hydraulic systems can be adjusted to changing conditions by allowing the release of pressure from individual dampeners when they are no longer needed.
A higher, more precise level of control is also possible with a hydraulic unit. Micro-adjustments for fine-tuning can be accomplished with more precision than that obtained with a standard dampening system’s manual adjusting threads, and operators can make adjustments quickly without the use of hand tools. Process documentation and clamping set-point repeatability for future setups are also easily achieved by simply recording hydraulic pressure gauge readouts.
Overall control of hydraulic dampeners is maintained via a manual screw pump or powered hydraulic unit. A manual pump offers a lower price while providing a hydraulic system’s advantages over standard dampeners. But for the highest level of control and repeatability, a central hydraulic unit allows operators to actuate dampening at the machine control with the hydraulic pressure settings becoming part of the program on CNC-equipped machines. This is the highest level of control—and repeatability—in vibration dampening technology that UNISIG has installed on B-Series deep hole-drilling machines.
Whatever the application, high depth-to-diameter ratios and difficult workpiece characteristics virtually assure vibration in deep hole drilling. However, adverse effects can be avoided when shops employ dampening systems that will support the drill tube and allow operators to tailor the amount of dampening needed for the job. As with maintaining tire pressures, the right dampening system will provide a smooth and productive deep hole-drilling process.
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