Custom Precision Gears for Machine Tool Spindles

Customer Background#

A CNC machine tool manufacturer whose machining center spindle box requires high-precision gears, requiring smooth transmission and high positioning accuracy. Inside a spindle box the gear runs at high speed, and its transmission error shows up directly in the positioning accuracy of the machine. The accuracy of the gear therefore has to be held not only on a single part, but from piece to piece in series production.

Technical Requirements#

• Module M3, number of teeth 72, material 20CrNiMo
• Accuracy DIN 4 grade, tooth surface Ra≤0.2μm
• Dynamic balancing G2.5 grade

The module and tooth count are fixed by the spindle box design, so the gear is a custom part rather than a catalogue item. DIN 4 grade together with a tooth surface of Ra≤0.2μm sets the level of the grinding process, and the G2.5 balancing grade adds a second requirement that is not covered by the tooth accuracy itself: the mass distribution of the finished gear has to be controlled as well.

Solution#

1. Material optimization: 20CrNiMo has stable hardenability and controllable deformation, which is what makes a DIN 4 grade part repeatable after heat treatment.
2. Gear grinding process: worm grinding wheel gear grinding, consistently achieving DIN 4 grade on the tooth flank and on the tooth surface roughness.
3. Dynamic balancing: dynamic balancing inspection is performed on each gear, so the balance grade is verified piece by piece instead of on a sample only.

Results#

• Accuracy consistently DIN 4 grade, tooth surface Ra0.15μm
• Spindle runs smoothly, machining accuracy improved
• Already supplied for 500 machine tools

The gear is now a regular item for this spindle box, and the same process is used for repeat orders of the same module and tooth count.