The Importance of Float During Micro Reaming

On Swiss-type lathes, precision micro holes are often created using the same series of operations as holes with sizes that might be considered more conventional. They’re typically drilled, bored for straightness and reamed for diameter roundness. Of these three micro-holemaking operations, reaming presents particular challenges in terms of tool alignment with the hole. After all, no CNC machine is blessed with perfect positioning accuracy.

Parts that require tiny holes with precise roundness include micro valves and components for pneumatics and fuel delivery systems. For these and related applications, Genevieve Swiss, a manufacturer of tools and accessories for small-parts machining, offers carbide micro reamers in diameters as small as 0.0236 inch. The company says a floating holder should be used for such small reamers to compensate for machine positioning inaccuracies. A small amount of float helps the reamer follow a hole’s path should that path deviate from the centerline slightly. It also prevents the reamer from tapering or “bell-mouthing” the hole.

The orbitally floating holder from Genevieve Swiss allows for as much as 0.009 inch of off-center float as a reamer follows a drilled hole. A proprietary bearing design returns the reamer to the center position upon completing each reaming operation, ensuring part-to-part consistency. In addition, float sensitivity is adjustable to accommodate reamers of different diameters.

While the company’s micro reamers are also suitable for milling machines, using a floating holder in a live spindle requires a slightly different approach. Because the holder floats, rotating it in the spindle can cause an imbalance that makes the reamer travel off-center. Some shops avoid this by beginning spindle rotation only after the reamer makes very slight contact with the hole to be reamed.

The various micro reamers that Genevieve Swiss offers feature a precision diameter tolerance of +0.00 inch to -0.00012 inch, which is essential when reaming is performed on a tiny scale. The reamers are made from K15 carbide (various optional coatings are available) and have left-hand helical flutes with a right-hand cutting geometry. According to the company, helical flutes provide better chip evacuation than straight flutes during micro reaming. Chip evacuation during micro reaming is important to prevent chatter and to generate a quality finish. Micro reaming is typically performed with flood oil coolant delivery, the company says.