Micro holemaking presents unique challenges
by Ed Robertson
Production efficiencies dictate doing more with less. Fixtures and workholding systems can accommodate families of parts; tools and inserts can accomplish numerous functions such as milling, turning, and holemaking; and multi-tasking machining centres with various spindles, axes of motion, rotating worktables and the like, all offer the promise of greatly improving production efficiencies and lowering throughput times and costs.
But parallel to the unceasing pressure to gain efficiencies is the growing specialization of modern parts. Fuel injectors with different performance characteristics, aluminum enclosures for PCs and other electronic equipment, and delicate medical instruments, all require drilling micro holes as part of their production process. And micro holes are a specialty job with their own set of challenges.
How do you define a micro hole and the best way to go about making it? Some tooling suppliers define a micro drill as being 1 mm (0.039 in.) in diameter.
Others make equipment that regularly produce holes 90 per cent smaller (0.10 mm or 0.004 in.). And there is the realm of laser-produced holes for microprocessors and other specialty applications that are in the range of four-millionths of an inch.
For purposes of this discussion, we’ll take the middle way, looking at holes in the range of 0.38 mm (0.015 in.) in diameter. Material (ferrous, non-ferrous, plastic), number of holes, hole depth, and need for hole finishing, are important initial considerations.
If the material is steel, electrical discharge machining (EDM) is a consideration. Any electrically conductive material can be a candidate, including steel alloys, stainless alloys, tool steel alloys, aluminum alloys, and elemental metals. And because EDM small hole drilling involves no contact with the part and no force during the drilling process, the usual challenges of drill lead off and drill breakage are non-existent, and parts undergo no stresses due to drilling forces.
But if your material is not electrically conductive, a specialty approach may be in order.
Lighter machines, faster speeds
Numerous CNC equipment providers will tout their vertical machining centres as all the machine you need for making holes, even approaching the micro range. The problem is, it might be too much machine.
Datron Dynamics, a subsidiary of Germany-based Datron AG, based in Milford, NH, specializes in production equipment for micro-range holes and other applications.
“The use of micro tools is becoming more and more prevalent; efficient and cost-effective use of these small tools requires both the foresight to employ equipment specifically designed for them, and a willingness to deviate from standard machining practices,” says Stephen Carter, marketing manager.
This is due primarily to the fact that the spindles on conventional CNC equipment cannot achieve the higher rpm speeds required for small diameter tools. Even if they can, it puts undue stress on the equipment by constantly red-lining the spindles. For example, a conventional CNC machining centre running tools smaller than ½ in. in diameter at 10,000 rpm or less will result in unfavourable feed rates and costly tool breakage.
Tool breakage is often blamed on operator error, incorrect machining parameters, or the nature of small tools. The reality is that it’s due to the force of a conventional machine’s heavy spindle and its inability to reach the high rpm speeds required to effectively evacuate chips from the cutting channel.
High frequency spindles with speed ranges up to 60,000 rpm are ideal for milling, drilling, thread milling and engraving using micro tools. During the machining process, the tool continually carves a chip out of the work piece. The generated heat develops approximately 40 per cent from friction on each side of the tool, and 20 per cent from the deformation of the chip. Therefore, about 60 per cent of the heat is inside the chip. High speed machining tries to evacuate the bulk of the heat with the chip.
Better hole quality is based on cooler tooling, lower machining forces, and vibrations. High spindle speed reduces the chip load to less than 0.005 in. Such a low chip load reduces significantly the forces between the tool and the material. High speed/low force machining yields less heat, reduces tool deflection, and allows machining of thinner-walled work pieces. This results in cooler machining, superior surface and edge quality, better accuracy and, as a by-product (of low force), easier workholding—since modular vacuum tables can be employed, particularly with thin flat substrates.
A small tool with intricate geometry turning at a high rpm calls for a cooling and lubricating agent with lower viscosity than water. Lower viscosity is needed because the coolant needs to make it to the cutting edge of the tool despite the high spindle speeds involved. The low evaporation point of ethanol makes it an extremely efficient cooling and lubricating agent for high speed machining operations.
Plus, while conventional flood coolant is petroleum based and needs proper disposal, ethanol evaporates.
There are rules of thumb for high speed machining.
“First of all, avoid red-lining your spindle, as this increases wear and tear on it and significantly reduces its lifetime,” says Carter. “Machine with maximum half the tooling diameter in Z. Machine with a smaller step-over but with higher feed rates. And finally, move fast and evacuate the heat with the chip.” SMT
Ed Robertson is a contributing editor and manufacturing journalist based in the Detroit, MI, area.