Hard turning boosts gear blank production

Bore and face machining of case-hardened gear blanks is now completed in one operation instead of two since hard turning replaced grinding and eliminated inter-operation handling.

Turning of gear blanks as well as spacers and shafts in their case-hardened state has started to replace grinding at the Bradford factory of Hindle Gears, with the result that bore and face machining is now completed in one operation instead of two, handling has been eliminated at this stage in the process and overall cycle time has been halved.

Said technical manager, Chris Verity, "After researching the technology, we invested last year (2004) in our first Okuma lathe from NCMT, a Space Turn LB300-M lathe, due to the Japanese manufacturer's reputation for building rigid machines capable of good hard turning performance." Verity adds: "It is the first lathe on this site with driven tooling, which adds versatility to soft machining applications as well and further reduces set-ups".

"Cutting the number of separate operations is critical for us to keep production costs down, as cycle times tend to be short so unproductive load/unload times are long by comparison." Hindle Gears was established in 1930 and has a reputation for supplying high quality gearboxes, gear sets and other power transmission components for diesel engines used in power generation and some defence and marine applications.

However, like many British manufacturers, the family-owned company is coming under increasing pressure from competition in low-wage countries, notably China, India and the Czech Republic.

So adopting lean production techniques, which includes reducing cycle times and the number of set-ups, is paramount to the company's future prosperity.

Over 90% of gears produced at the Bradford works are of hardened steel.

The traditional manufacturing route is to (1) soft turn the bore, faces and OD of a billet, (2) shape, hob or shave the gear teeth, (3) case harden, (4) grind the bore and one face on a specialist bore grinder, (5) grind the other face on a surface grinder, and (6) grind the teeth.

By contrast, the hard turning route using CBN tooling replaces two of the operations - (4) and (5) - with one operation on the Okuma lathe whereby the bore and both faces are machined in a single set-up.

This is achieved on the 3-axis LB300-M, without the expense of investing in a sub-spindle machine, by using a boring bar with a rear-facing tool to turn the reverse face.

Not only is the total cycle time halved, but accuracy is also improved by eliminating tolerance build-up.

Parallelism between the faces of 10 microns total is routinely held, which provides better location for subsequent grinding of the teeth and contributes to smoother running of the finished gear.

Clearly one 3-axis lathe costs significantly less than two grinding machines, so capital outlay is reduced by hard turning.

Furthermore, the lathe is also able to turn the OD of shafts, so is more flexible than a grinder unless an even more expensive universal model is bought.

An example of the savings achieved by mill/turning sawn steel billets in the soft state using carbide tooling is the production of gear blanks for a vacuum pump, each of which requires bore drilling and face turning followed by manual component reversal for second operations.

These comprise face turning, edge chamfering and drilling and tapping three through-holes using the live tooling.

Previously the part was produced in three operations - two on different lathes and a separate set-up on a small machining centre - so some handling has been eliminated and overall cycle time has been reduced as well.

Off-line programming of all CNC machines at the Bradford plant is assisted by Edgecam software with DNC links to the machines.

Batch sizes vary widely from one-offs to one thousand, sometimes even two thousand.

That the Okuma joins a shop full of other major suppliers' lathes is in part testament to the installation of a twin-pallet, horizontal machining centre from the same supplier in 2002.

It allowed Hindle Gears to evaluate the quality and reliability of the equipment and technical help from NCMT, both of which Verity considers very good.

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