The Perfect Tooth Trace

  • New tooth-trace modification options using the electronic helical guide function on the LSE 200-500
  • Superpositioning the various machining motions also allows you to combine a range of tooth-trace modifications
  • Possible meshing interference between the cutter and workpiece flanks during chip removal is reduced

A lightweight, compact gearing system design frequently results in difficult-to-access machining locations. These include, for example, tiered shafts, internal or external gear teeth next to collars, where for design reasons there is only limited space (often less than 5 mm) for tool withdrawal. Machining such gear teeth is a job where gear shaping is the only option.

Cases of gear teeth, where gear shaping is the only option

Cases of gear teeth, where gear shaping is the only option

The requirement of gear system design engineers to undertake load-dependent toothtrace modifications on these gear teeth has increased steadily over the last few years. To date tooth-trace modifications, such as helix crowning cβ, tooth trace angles fhβ or conicity and end relief could only be undertaken on shaped gear teeth using a specially designed back-off cam (B4-axis, see pictures). Where taper angles are greater, the socalled taper shaping process can only be actioned by tilting the entire machine base (B5-axis). Given the two-flank shaping process, the above-mentioned modifications on both tooth flanks are symmetrical. This does not permit any degree of optimization in relation to differing demands on drive and coast flanks.

Greater Tooth-Trace Modification Design Flexibility

Feasible tooth-trace modifications using an LSE machine

Feasible tooth-trace modifications using an LSE machine

Both symmetrical and asymmetrical tooth-trace modifications can be undertaken by using the LSE 500 gear-shaping machine’s electronic helical guide function in combination with single-flank shaping. Superpositioning the various machining motions also allows you to combine a range of tooth-trace modifications.

Symmetric lead crowning:

  • Possible with back-off cam in double-flank cutting
  • Possible with CNC-movement in single-flank cutting

Symmetric lead taper:

  • Possible in the micron range with back-off cam in double-flank cutting
  • Possible up to several degrees with column tilting in double-flank cutting
  • Possible with CNC-movement in single-flank cutting

Asymmetric lead crowning and lead corrections:

  • Only possible in single-flank cutting
  • Special CNC-movement necessary

Enhanced Gearing Quality at Marginally Longer Machining Times

The increased flexibility provided by single-flank gear shaping means that machining times are a little longer compared to conventional two-flank gear shaping. The increase in machining times is between 5 and 20 percent, depending on the application. This small percentage increase is down to the fact that during this Liebherr-developed process sequence, not all machining operations have to be single-flank. Thus rough machining and semi-finishing as well as finishing the first tooth flank is performed in two-flank mode. Finishing the second tooth flank is the first operation to be performed in single-flank mode, by in-feeding the cutter in rotatory fashion.

Another positive effect of this final single-flank machining operation is that possible meshing interference between the cutter and workpiece flanks during chip removal is reduced and gearing quality is enhanced. In particular the s-shaped profile-shape flaw, which occurs in gear shaping and which is caused by different chip thicknesses on the leading and trailing flanks, can be significantly reduced or even completely eliminated.

CNC-axes on a state-of-the-art gear shaping machine

CNC-axes on a state-of-the-art gear shaping machine

Simple Programming Incorporating Correction Options to the Exact μm

Tooth-trace modifications (Figure XY) can be undertaken by simply entering target data, e.g. 5 μm helix crowning for the left and right tooth flanks, into the control system.
All the necessary computations to create the modifications are then performed by the machine control system. After machining and gear teeth inspections of the first workpiece, the machine’s control system enables the operator to correct the measurement criteria to the exact μm, e.g. helix crowning cβ. The relevant measurements can be entered in a special correction window in the control system directly from the gearing diagram.

“Who Needs What?”

The electronic helical guidance function on LSE machines enables tooth-trace modifications to be performed simply. Longer machining times are certainly acceptable where small batch sizes or prototype production or individual component manufacturing are involved. In these cases the additional costs and longer delivery lead times for special cams can be avoided. Where large batch sizes are involved, the difference in cycle times will ultimately determine how cost-effectively the required modifications can be performed.

Shaping tooth flank modifications
2-flank, conventional 1-flank, electronic helical guide
Very effective double-sided chip removal Modifications can be easily programmed
High stroke rate (DS/min) feasible Symmetric and asymmetric modifications feasible
Taper gear-teeth shaping Excellent tooth-trace quality due to correction options
Facilitated by pivoting machine base Accurate to the exact μm (DIN 1-3)
Symmetric modifications (S-contour) Low chip formation leads to superior profile quality
Very good tooth-trace quality (DIN 1-4)
Specially designed back-off cam required Marginally longer machining times
Asymmetric modifications not feasible Reduced stroke rate (DS/min)
Profile quality (S-contour) can be influenced by meshing interference

Source Liebherr Group