Micromachining, cutting where the volume of chips produced with each tool path is very small, is not a high-speed operation in relation to chip load per tooth. Rather, it involves a high spindle speed relative to cutter diameter. The part may be physically larger, but details of the part require ultra-small profiles achieved only by micromachining. In other words, micromachining is not limited in scope to only miniature parts.

TOOLHOLDING
In medical work, where tight tolerances are standard, dynamic runout; the measurement of the spindle at high speeds, performed using laser or capacitance resistance technology, and balance must be controlled to deliver and maintain viable tool life.

A holder with 0.00060" runout accuracy produced nearly two-thirds fewer holes, only 800. In this scenario, the shop could save hundreds of dollars a month in carbide costs – as well as labor costs due to less tool changing – by making one smart tool holder choice.

Holder attributes that can boost production include symmetrical design, a perfectly concentric collapse of the collet around the cutter, and a ball-bearing raceway nut with precision-ground threads.

CHALLENGES
While these characteristics are good rules of thumb, things change fast in this field and, like our customers, we must adapt as trends emerge.

Batch sizes are getting smaller. Bone screws, for example, were typically run on multi-axis, Swiss-type lathes where the same tools and programs ran for days at a time. Traditionally, prototyping in this arrangement was not an option because of the complexity and time involved in programming and setup. Today’s need for customized sizes demands flexibility and quick changeover to remain productive.

We are investing a large portion of our research and development (R&D) in tackling this challenge. We are working on hydro-clamping tool holder systems that could make the decades-long approach of using ER collets obsolete. It would make it possible, for example, to perform a simple drill change on a gang slide in seconds.

COOLANTS
Another trend in medical manufacturing being driven by the U.S. Food and Drug Administration (FDA) is clean machining without the use of water-soluble coolants.

We are focusing on two features:

• Holders that remain completely sealed to outside atmosphere
• Very small delivery holes in collet faces or clamping nuts that properly restrict gas flow
  

TOOLING
Tool considerations also must be taken into account to keep up with the demanding medical field. Better results often cannot be achieved by simply increasing spindle speeds or using smaller tools; a deeper understanding of cutters is necessary.

We consider tools with diameters <3mm to be micro tools. These aren’t simply smaller versions of their macro counterparts. They have geometric considerations all their own. For example, the 1mm Sphinx drill can run at 80xD. But this is only possible because the cylindrical shaping extends further down the tool, closer to the tip, to facilitate pecking and maintain strength.

Tool carbide should be ultra-fine grain (nano or submicron grain size) to ensure high abrasion resistance and good toughness. Coatings are valuable too, but it’s important to understand how coatings can negatively impact micro tool performance. Micro tools have extremely fine surface finishes and sharp cutting edges. Coatings can fill in valuable space – a flute on a drill, for example – needed for proper chip evacuation, which is critical in these applications.

Coatings must be ultra-thin (<1µm) and smooth; our experience shows that misapplied coatings result in poor tool life due to breakage; the coating reduces cutting edge sharpness, increasing torque force on the drill. When coating is necessary, consult with the cutting tool manufacturer to provide this directly.

Chips and small tooling naturally do not get along well. Compensating for low spindle speeds with tools that have more flutes support an ideal feed rate, but chip evacuation may suffer. Determining the appropriate chip load – as close to the cutting edge as possible – allows operations at the highest possible spindle speed, accelerating the cycle and improving surface finish.
Optimal conditions exist when the chip load is relatively equal to the cutting edge radius.

Many micro end mills are designed so the cutting edge radius has a positive rake angle to create a shearing action. A chip load less than the cutting edge radius often results in a negative rake angle where the tool rubs rather than cuts. This increases the force required and generates more heat which can result in built-up edges and poor tool life. A chip load significantly bigger than the cutting edge radius often leads to premature failure because the tool is not robust enough to withstand such forces.

MACHINE TOOLS
Micromachining requires machine tools with very high sensitivity, fine resolution in the feed axis, and very precise spindles capable of high speed with low dynamic runout. For micro-drilling operations, specialized micro machines are best.

Micro milling machines are suited for small tools and small workpieces. They are characterized by spindle speeds faster than 50,000rpm using small HSK tool holders such as HSK-E32, E25, or E20. With the right holder, tool runout can be controlled to less than 1µm (0.000040") at the cutting edge, ensuring sub-micron accuracy.

In medical micromachining, understanding each piece of the equipment puzzle is critical. It’s also important not to make assumptions based on other tools or parts you may have worked with, especially in more standard sizes. Invest the right time and energy in gearing up for the next medical job and you’ll get more parts done right faster.

DEPTH OF CUT COLUMN
by Jack Burley, President and COO at BIG KAISER Precision Tooling Inc.

Since its institution in 1940, the G2.5 balance specification has been widely accepted across the industry; i.e., “it’s how things have always been done.”

However, machines were much slower 80 years ago. Back then, the most advanced machines would have spun larger, heavier tools at a maximum speed of about 4,000 RPM. If you applied the math from those days to today, you’d get unachievable values.

For example, the tolerances defined by G2.5 for tools with a mass of less than 1 pound rated for 40,000 RPM calculates to 0.2 gram millimeters (gm.mm.) of permissible unbalance and eccentricity of 0.6 micron. This isn’t within the repeatable range for any balance machine on the market.

Similarly, application-specific assemblies, for operations like back boring and small, lightweight, high-speed toolholders, can’t be accurately balanced for G2.5.

Machine tool builders rely on an outdated number, too, often basing spindle warranty coverage on using balanced tools at very specific close tolerances. While it’s true that poorly balanced tools run at high speeds wear a spindle faster, decently balanced tools performing common operations won’t wear spindles or tools drastically and deliver the results you’re looking for.

OSG Corporation has announced the release of the ADO-MICRO small diameter coolant-through carbide drill series for stable and high efficiency drilling in small diameter deep-hole applications.

Poor chip evacuation is a common complication in small diameter deep-hole drilling. Micro sludges can be easily accumulated around the outer periphery of the cutting tool, which is a key cause of abrupt tool breakage.

The ADO-MICRO features a unique double margin geometry with an extended flute and shortened end margin to enhance chip evacuation capability.

In addition to the outstanding chip ejection performance, the double margin configuration supports the straightness stability of the tool and reduces rifle marks on the inner surface of holes.

Furthermore, the ADO-MICRO features a pair of large oil holes and employs a hollow shank design to allow large coolant flow volume for trouble-free chip evacuation.

The ADO-MICRO is coated with OSG’s original IchAda coating that provides excellent surface smoothness in conjunction with high abrasion resistance and heat resistance to enable small diameter tools to achieve long tool life.

​With the ADO-MICRO’s unique tool geometry and IchAda coating, non-step drilling is made possible even for deep-hole applications, enabling high processing efficiency.

The ADO-MICRO is suitable for carbon steel, alloy steel, stainless steel, cast iron, ductile cast iron, aluminum alloy, titanium alloy and heat resistant alloy. The ADO-MICRO is available from diameter 0.7 mm up to 2 mm for drill lengths 2xD and 5xD, and diameter 1 mm to 2 mm for drill lengths 12xD, 20xD and 30xD.

Got an application you want to try this on? Give us a call!