Reduce set-up times on sliding head automatics

Renishaw and Citizen Machinery will show how to dramatically reduce set-up times and build confidence in 'lights out' machining of complex parts on CNC sliding head automatic lathes.

Renishaw and Citizen Machinery will be showing what the companies desc ribe as a revolutionary new approach to setting and controlling complex machining processes on CNC sliding head automatic lathes at the UK's MACH 2008 machine tool show.

Renishaw has been trialling a new approach to on-machine probing in its own machine shop and reported dramatically reduced set-up times and confidence in 'lights out' machining of the most complex components.

Renishaw told manufacturingtalk that it has used Citizen CNC sliding head automatics for over 20 years and has nearly 30 such machines in its modern machining facility at Stonehouse in Gloucestershire, UK.

The machines include ten Citizen M32 types, which feature powered tooling on the turret and platen.

The arrangement enables cross machining on the main and opposed spindles.

This capability is used to produce the increasingly complex parts.

Renishaw said that its product designers are specifying more milled profiles and asking for fewer, smaller, more elaborate parts.

The company is to meet its efficiency and cost targets by using the in-cycle gauging system on tCNC sliding head automatics.

* Reducing set-up times - principal manufacturing engineer at renishaw, Paul Maxted, explained: "As the feature count and level of milled features has risen, we've experienced longer set-up times as our setter-operators have tried to optimise the process.

Although we have automated set-up and process control on our machining centres, on the Citizens we have traditionally made a complete part, checked it on a co-ordinate measuring machine (CMM), interpreted the results and then updated process parameters".

He continued: "This is obviously manually intensive and can be prone to human error.

It's not unusual for us to take five to ten attempts before we get all the features in tolerance.

Set times are typically several hours, with the worst examples taking even more than an entire shift to complete.

These long set times impacted on our productivity and caused us in some cases to run larger than ideal economical batch quantities".

* Automated tool offset updates - with the tooling stations on the Citizen machines already extensively used, Maxted developed a mounting bracket on top of the platen to carry a probe having a special 'T' stylus.

Unlike probes mounted to a fixed headstock, the platen mounted probe can move in all three linear directions.

This flexibility allows routines to be carried out that check part position and verify part size, including cross and axial features.

The probe and stylus configuration also allows comprehensive measurement of all drilled, milled and turned features on main and secondary spindles.

Instead of machining the first part complete, each tool is brought out and performs its work.

A rapid probing routine follows that directly measures the tool's performance and updates the tool offsets without operator intervention.

Roughing and finishing tools are measured and individually updated so that each is optimised.

* Tooling performance - Maxted explained some of the other benefits of probing on sliding head machines: "It's invaluable to measure each tool's performance there and then, on the machine.

Firstly it's automatic, but just as importantly, we get a better understanding of the cause of process variation and can therefore better control its effects".

He said that roughing and finishing tool performance is inter-related, so it's vital to control each to get the optimum process precision.

Maxted explained that when you measure offline, you immediately lose the association of the part with the machine tool as you measure everything from a part datum - if that's wrong then the other feature positions will appear wrong too.

In contrast, on-machine probing gives you full traceability and the means to address problems at source.

Although probing lengthens a cycle time to make the first part, what happens next makes all the difference.

A second part is made using the new process parameters to prove that the process is set correctly, and then production can start.

This compares to numerous manually intensive 'measure, interpret, update, re-machine' loops using traditional methods.

Probing has given set-up reductions of several hours in some cases, reducing setting costs and boosting productivity, said Renishaw.

* In-process control - the other major benefit is realised once production starts.

The probe is used to measure each tool on a sample basis, with the frequency determined by the tolerance that must be achieved.

With each source of variation under independent control, and with fully automated checking and immediate feedback, genuine 'lights out' production can be achieved.

What's more, said Renishaw, the in-process control reduces the level of part-to-part variation and keeps processes on centre in the face of tool wear or thermal drift, resulting in improved process capability.

Maxted reported: "Early indications from production testing have confirmed that the anticipated improvements in process capability will be achieved on turned and milled features, with Ppk figures up to 2.00".

He added: "Such figures have traditionally given us the confidence to run unmanned, matching the high level of automation that probing has given us on our other classes of machines".

The technology demonstrator can be seen on a M32 machine on the Citizen Machinery stand at MACH 2008.

Manufacturers making complex parts on sliding head machines who need improved process performance and traceability shouldn't let this new technology slip them by, said Renishaw.

* Renishaw at MACH 2008, NEC, Birmingham, UK, April 21-25, Hall 5, Stand 5140.

* Citizen Machinery at MACH 2008, Hall 5, Stand 5260.

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