Electronic Gearbox in CNC Hobbing Machines: How It Replaces Change Gears and Improves Flexibility

The electronic gearbox is the defining technology that separates modern CNC hobbing machines from the mechanical hobbing machines they replaced. On an older machine, the precise speed ratio between the hob and workpiece — the ratio that determines the number of teeth generated — was set by physically installing a specific set of change gears with the correct tooth count combination. On a CNC machine with an electronic gearbox, this ratio is set by a parameter in the part program. The implications for production flexibility, setup time, and gear accuracy go far beyond the obvious reduction in changeover time. This article explains what the electronic gearbox actually does, why it produces better gears than mechanical change gears, and how to evaluate the quality of the electronic gearbox implementation when comparing machines.

What Change Gears Were and Why They Were a Problem

On mechanical hobbing machines, the relationship between hob rotation speed and workpiece rotation speed was set by installing a specific set of change gears — typically four to six gear wheels with specific tooth counts — in the machine’s drive train. The ratio of tooth counts set the hobbing synchronisation ratio, which determined the number of teeth generated on the workpiece. Changing from a 35-tooth gear to a 42-tooth gear required the operator to physically remove the existing change gear set, calculate the correct tooth count combination for the new gear, select the appropriate gears from a change gear set, and install and verify the new set — a process that could take 30 to 90 minutes per changeover.

Beyond the time cost, change gears introduced mechanical imperfections into the synchronisation. Every gear mesh in the change gear train introduced backlash and periodic speed ripple at the gear mesh frequency. For the course DIN classes typical of mechanical hobbing machine production (DIN 8 to DIN 10), these imperfections were acceptable. As gear accuracy requirements tightened in automotive and aerospace applications, the accuracy limitation imposed by change gear transmissions became the primary constraint on achievable DIN class.

What the Electronic Gearbox Actually Does

The electronic gearbox in a modern CNC hobbing machine is a software function in the CNC controller that maintains a precise, continuously updated speed ratio between the B-axis (hob spindle) encoder and the C-axis (workpiece table) position command. The CNC reads the current B-axis angular position thousands of times per second and calculates the required C-axis angular position at each control cycle update — then commands the C-axis servo drive to maintain that position. The “gearbox ratio” is simply the mathematical relationship programmed in the part program:

C-axis position = B-axis position × (Number of Hob Starts) / (Number of Gear Teeth)

For a 1-start hob cutting a 35-tooth gear, the ratio is 1/35 — the workpiece rotates 1/35 of a turn for every full rotation of the hob. For a 42-tooth gear, the ratio becomes 1/42. Changing between these is a parameter entry — one number in the CNC program — with no mechanical change required.

How the Electronic Gearbox Improves Gear Accuracy

The electronic gearbox does not just replace change gears with software — it actively produces higher gear accuracy than change gears are capable of. The reasons are fundamental to how each system maintains the synchronisation ratio:

No Backlash

A change gear set has backlash at every gear mesh — a small but non-zero angular clearance between meshing gear teeth. Under the reversing load conditions of hobbing, this backlash allows the driven gear to dwell momentarily at the reversal point before the driving gear re-engages, producing a brief synchronisation error at load reversal frequency. The electronic gearbox has no physical backlash — the servo drives follow the commanded position directly, and any positional error is corrected within microseconds by the servo feedback loop.

No Speed Ripple at Gear Mesh Frequency

Each gear mesh in a change gear train produces periodic speed variation at the frequency of the mesh (speed × tooth count). Multiple gear meshes produce multiple superimposed ripple frequencies. The electronic gearbox eliminates all gear-mesh-frequency speed ripple from the synchronisation signal. The remaining speed ripple comes from encoder quantisation (typically negligible on modern high-resolution encoders) and servo bandwidth limitations — both of which are far smaller in magnitude than gear mesh ripple in a physical change gear train.

Instant and Repeatable Ratio Change

The electronic gearbox ratio can be changed between parts in the time it takes the CNC to process a new program block — typically less than one millisecond. In a production cell with multiple gear variants, this means the machine can switch from variant A to variant B between consecutive parts without any operator intervention, provided the hob geometry is compatible with both gears. In practice, multi-variant runs with variants sharing the same hob are common in automotive production — helical gear families where the helix angle and module are constant but the tooth count varies.

Helix Angle Control Through the Electronic Gearbox

The electronic gearbox extends beyond the basic B/C synchronisation to control the A-axis helix angle setting and the X-axis approach motion. On older machines with physical change gears, the helix angle had to be set by rotating the hob slide manually and locking it in position — a process that introduced angular setting errors and required re-verification after each setup. On CNC machines, the helix angle is a program parameter that the A-axis servo positions automatically to the commanded angle. The electronic gearbox maintains this angle throughout the cut, including during the dynamic conditions of axial feed and radial approach.

Multi-Start Hobs and the Electronic Gearbox

The electronic gearbox also enables straightforward use of multi-start hobs — hobs with two, three, or more thread starts rather than one. Multi-start hobs cut faster because the workpiece advances by one tooth per hob start per revolution (rather than one tooth per revolution for a 1-start hob), increasing the axial feed rate and reducing cycle time. The synchronisation ratio for a multi-start hob is simply multiplied by the number of starts:

C-axis/revolution = Number of Hob Starts / Number of Gear Teeth

A 2-start hob cutting a 35-tooth gear produces a C/B ratio of 2/35 — the workpiece rotates 2/35 of a turn per hob revolution. On a mechanical machine, switching from a 1-start to a 2-start hob required recalculating the entire change gear set. On a CNC machine with electronic gearbox, it is a single parameter change in the program.

Electronic Gearbox Quality: What to Look for When Comparing Machines

Not all electronic gearbox implementations are equal. The quality of the synchronisation depends on the CNC control update rate, the servo drive bandwidth, and the encoder resolution on both B and C axes. Key questions to ask when evaluating a machine’s electronic gearbox: