BF Shaft Coupling for Motor and Ballscrew Shafts

A BF shaft coupling is a small flexible connector that joins a stepper or servo motor shaft to a ballscrew or linear guide shaft on a CNC machine. It typically handles light to moderate torque loads found on desktop and mid-size CNC routers, plotters, and engraving machines. It is one of the most common transmission parts on router tables, engravers, and lathe conversions.

This part suits technicians and machine builders who need a reliable link between drive and load without machining a custom adapter. It fits industrial retrofits and hobbyist CNC builds alike, since installation only needs two sets of Allen screws.

Features of the BF Shaft Coupling

The BF shaft coupling is machined from an aluminium alloy body with a slotted flexible section at its centre. This keeps the part light while still transmitting enough torque for typical CNC motor and ballscrew combinations, flexing just enough to absorb small angular and parallel misalignment between the two shafts.

Clamping is done with hex socket set screws on each side, so the coupling can be removed without disturbing the motor or the ballscrew bearing blocks. Because the two bores can differ in diameter, always check the motor shaft size and the ballscrew shaft size before ordering.

• Aluminium alloy body for low weight and adequate torsional stiffness
• Flexible centre section that absorbs minor shaft misalignment
• Hex socket set screws for tool-free clamping and removal
• Available in different bore combinations for motor and ballscrew shafts
• Reduces bearing and motor wear caused by rigid, misaligned connections

Where it’s used

A BF shaft coupling is typically found between a NEMA stepper or servo motor and a ballscrew on router gantries, Z-axis lifts, and linear axes. Smaller versions also appear on 3D printer-style linear axes and on rotary indexers that use a stepper motor to drive a leadscrew. It also appears on engraving machines and CNC conversions, wherever a rotary motor drives a threaded or ball-type shaft.

Besides new builds, it is a frequent replacement part when a machine develops backlash, noise, or positioning drift from a worn coupling. For a wider range of motion and transmission parts, see the CNC parts shop.

What to check before buying a BF shaft coupling

Before ordering, measure both shaft diameters with a caliper rather than assuming they match, since motor and ballscrew shafts often use different sizes on the same machine. Also check the overall length and outer diameter of the coupling body against the available space.

A common mistake is buying a coupling based on the motor’s torque rating alone. The coupling also has to fit both physical shaft diameters, so confirm both bore sizes, not just the motor’s frame size. It also helps to note the coupling’s overall length, since a longer body may not fit within a tight motor-to-bearing gap.

Installation and maintenance

Install the BF shaft coupling by sliding it onto both shafts, aligning the motor and ballscrew as closely as possible before tightening. Tighten the set screws on one side first, then position the second shaft and tighten evenly, following general guidance such as the overview on Wikipedia’s coupling page.

During maintenance, periodically check that the set screws remain tight, since vibration on a CNC axis can loosen them. Inspect the flexible section for cracks or play, and replace the coupling if you notice backlash, unusual noise, or visible wear.

FAQ

What does BF mean in a BF shaft coupling? BF refers to a common aluminium flexible coupling style used to join motor and screw shafts on CNC equipment.

Can I use a BF shaft coupling with different bore sizes on each side? Yes, it is commonly sold with two different bore diameters, one for the motor shaft and one for the ballscrew shaft.

Does a BF coupling correct shaft misalignment completely? No, it only absorbs small misalignment; the motor and ballscrew should still be aligned as accurately as possible.