Boosting Machine Spindle Speed For Micromachining Applications

Small-diameter micromachining tools require very high rpm for sufficient cutting speed and material removal rates. This air spindle provides 80,000 rpm and an untethered compressed air interface to allow automatic changeovers using the machine's ATC.

Article From: 5/1/2007 Modern Machine Shop, ,

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This air spindle

This air spindle can be changed out using the machine’s ATC and stored in a carousel pocket as would any other toolholder. The compressed air that powers the spindle is delivered through a stop block installed on the machine’s spindle face.

This aluminum part

Air-powered spindles allow conventional machine tools to achieve spindle speeds as fast as 80,000 rpm for micromachining applications. The ribs on this aluminum part measure 0.0025 inch thick and 0.157 inch tall. They were machined at 70,000 rpm and 59 ipm using a 0.02-inch-diameter end mill.

Small-diameter end mills and drills used for micromachining require very high rpm to achieve sufficient cutting speed and material removal rates. There are different ways that shops can add high-speed capacity to allow them to machine micromolds, electronic devices, medical parts and other such tiny components. One option is to purchase a dedicated, high speed machining center. A more economical alternative is to use an auxiliary, high speed spindle that installs and operates in a conventional machine’s locked spindle.

Versions of these auxiliary spindles use compressed air as the power source. However, some of these air spindle designs do not lend themselves to automated machining operations. That’s because air lines must be attached manually each time the spindle is loaded into the machine. The RBX air spindle from Big Kaiser Precision Tooling offers adjustable speeds ranging from 30,000 rpm to 80,000 rpm for tools having diameters smaller than 0.06 inch. It also provides an untethered compressed air interface to allow automatic changeovers using the machine’s ATC. When not being used, the air spindle is stored in a tool carousel pocket as would any toolholder.

The RBX is compatible with VMCs that have CAT 40, BT 30 or HSK-A63 spindle tapers. Compressed air is piped through the machine and delivered to a stop block that installs on the machine’s spindle face. When the RBX is loaded into the machine, a position pin slides into the stop block to provide a sealed path for compressed air to reach the air spindle. The air supply is controlled (turned on or off) by M-code, and spindle speed is determined by the air pressure. For example, 60 psi of air pressure delivers 66,000 rpm, while 70 psi will generate approximately 71,000 rpm.

Accurate dynamic runout at high rpm is just as important for micromachining as fast spindle speed. The RBX uses a microcollet system that provides a maximum runout of 1 micron at the nut. The collet nut design has no flats or slots, so vibration at high rpm is reduced (a custom ratcheting wrench that grips the entire periphery of the nut is used to change tools and collets). While the RBX accommodates tool diameters smaller than 0.06 inch, it can clamp shank diameters as big as 0.159 inch. Its microcollets are sized in 0.004-inch increments.

In addition, excessive spindle Z-axis thermal displacement can reduce micromachining accuracy. The RBX’s turbine drive has 1/6 the thermal displacement of a conventional spindle during use, the company says.

Micromachining also presents difficulties recognizing when a cutting tool has broken. Because the RBX air spindle generates less than 65 dB, the noise generated by the cutting tool can be heard giving operators a better chance of determining if a tool has broken.

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