Pre-machined and heat treated gear blanks for aircraft pumps are turned in their hardened state of 65HRC on an Okuma LB300 lathe to an 8 - 12 micron CLA finish in three minutes at the Midlands factory of a leading component supplier to civil and military aerospace programmes.
Previously, a grinding machine took three quarters of an hour to remove the same amount of stock - 50 to 75 microns - and leave a similar surface finish.
So dramatic have been the savings from hard turning that they have funded the purchase not only of the lathe, but also of a new Okuma grinder to replace an outdated model for subsequent machining of the component after nitriding.
The two machines, supplied by UK agent, NCMT, sit alongside a gear grinder and a cone grinder to form a pump gears finishing cell, which precedes a super finishing cell comprising special lapping machines.
The most important prerequisite for hard turning is a rigid lathe, combined in this case with CBN (cubic boron nitride) tips for machining CPM10V, which is a very hard, crucible powdered metallic material with a high vanadium content.
Part of the turning cycle involves interrupted cutting on the gear faces which precludes the use of coolant, so the journals are dry machined as well for convenience.
In any case when metal is cutting with CBN, all the heat goes into the chips, so coolant is not needed to control the temperature of the part.
Taking heat out of the component is also the reason behind the manufacturer replacing resin-bonded CBN wheels with the vitrified variety on the new Okuma GA25-T angle-approach grinder, which is equipped with the necessary high pressure coolant facility.
The grinding operation after nitriding previously took 15 minutes on the old grinder to achieve the high surface finish, whereas now the same result is achieved much more quickly, resulting in a further cycle time saving.
In replacing its grinder, the company opted for additional capabilities including continuous rotary dressing to ensure that the wheels are always in good condition.
Specified also was in-process gauging of the gear journals and faces using two separate sets of probes that operate sequentially during the cycle.
Working on the flow line principle rather than as a conventional cell, the system produces two internal gears in batches of six to 12 for about a dozen pump types.
Component size range is approximately 19 to 63mm in diameter, 13 to 51mm gear width, and one to eight inches overall length.
All machines are DNC-linked and component load / unload is manual.
The whole gear manufacturing cell based on hard turning is much more controllable and less skill dependent than the previous arrangement.
Programs now drive the processes, so the manufacturer can apply SPC techniques to the machines rather than to the operators.