The installation of a Citizen L12-VII CNC sliding-head turn-mill centre, which can be quickly re-set between being operated with a guide bush for long-shaft work and without a guide bush for shorter parts, has allowed the precision small-part sub-contractor Tenable Screw to win a major European contract to supply 300,000 short parts a year.

Commercial director Nigel Schlaefli said: “The machine significantly increased our competitiveness on this component for the security industry, allowing us to cut cycle times by some 15% — largely due to shorter idle times. Having now completed the first batch run of 30,000 parts, the machine is helping to repay our investment in it by achieving a consistent round-the-clock 24/7 operation.”

This capability was confirmed by machine shop supervisor Paul Waldron: “Throughout the batch of brass components, we never had to input a single offset. Moreover, using the machine in non-guide bush mode achieves material savings by reducing the length of the bar remnant that would have resulted if we used the machine in the normal sliding-head guide-bush configuration. This gave us an additional 12 components per bar length.”

The brass security component is 8.85mm in diameter x 6.85mm long; it also has a 5deg taper over half the length, leading to another 5deg angle (cone) on the back face. The front face also has a flat-bottom bore 5mm in diameter x 5mm deep; there is also a slot 0.8mm wide x 0.86mm deep across its face — and an off-centre hole that is 1.02mm in diameter and cross-drilled from the outside diameter.

The part is produced in a single 21sec cycle. First, the stock material is fed out; then, using a boring tool, the part is bored and chamfered from solid. A spot drill and drill (from each side) produces the 1.02mm-diameter off-centre cross-hole, and the 0.8mm slot is milled. The bore is then finished to deburr the cross-hole breakthrough, the OD is skimmed, the slot is deburred, and the part-machined component is parted-off for transfer to the sub-spindle, where the 5deg angle on the OD and face are then machined.

Machine re-setting

Having completed the first batch, Mr Waldron re-set the machine, putting the guide bush assembly in place to produce a longer component. This took him just 45min. “It is very straightforward, because the change-over has been carefully thought through.” He highlights the use of captive screws to hold the covers, the fact that the tooth belt drive is very accessible and the fact that the nose cap holding the collet assembly is easy to fit. “Moreover, there is a sealing ring behind the collet assembly that prevents oil and swarf contaminating the mechanism and the back of the machine. Without the guide bush, the maximum part length that can be machined is 30mm; with guide bush installed, it is 135mm.”

The seven-axis Citizen L12-VII from Bushey-based Citizen Machinery UK Ltd (www.citizenmachinery.co.uk) has a 27-tool capacity, a 15,000rev/min main spindle and a 10,000rev/min back spindle. Each driven tool position has a maximum speed of 10,000rev/min (to accommodate small diameters), and the rapid-traverse rates are 35m/min.

Furthermore, even greater flexibility is provided by the option of a three-spindle driven angled unit that is mounted on the gang tool-post. This provides a “double-ended tool approach” with an adjustable angle between 0 and 30deg to the centre line.

A privately owned sub-contract business, Tenable Screw is reckoned to have the largest installed base of CNC sliding-head machines in the UK; moreover, 40 of the 54 are Citizen machines. The company, which had sales of £7.5 million last year, employs 110 people at its three sites: 50 at the Merton (South London) headquarters; 20 in Coventry; and 40 in Marlborough, Wiltshire.

Significant in Tenable Screw’s operation is that 10% of the workforce are now apprentices. Managing director Simon Schlaefli said: “The skills are no longer readily available, so we have committed to training our own. Much of their time is spent as ‘buddies’ working with setters, which gives them a tremendous insight into production techniques.”

Merton has two machine shops; one is for sliding-head machines, the other for fixed-head machines, such as the latest Miyano BNA-42DHY turn-mill centre installed by Citizen Machinery UK last year as part of Tenable’s £1 million spend on equipment.

At the time, the company also purchased another Citizen CNC A20 sliding-head machine. The Marlborough site is focused mainly on high-volume production using special-purpose rotary-transfer and multi-spindle machines, while Coventry is home to some 50 multi-spindle machines and also undertakes second-operation machining.

Comprehensive capability

At Merton, production ranges from prototype work to fulfilling contracts requiring 2.5 million parts a year. Here, nine of Tenable’s Citizens are running 24/7 producing fuel injection components for a German company.

Nigel Schlaefli said: “With the number of sliding-head machines installed, we have the flexibility required to respond to customer emergencies, and we can find gaps between set-ups to help with prototype development, for instance. Also, the level of versatility we have from different machine specifications helps when delivery lead times are tight. Overall we will re-set six to eight machines each day.”

Tenable Screw was late in adopting sliding-head technology, installing its first Citizen in 1998; and such were the levels of support and ‘hand-holding’ in the early days that Citizen has become its principal machine brand.

Simon Schlaefli said: “We now have five high-specification Citizen M32 turn-mill centres, and we regard this model as the most versatile sliding-head machine currently available. We tend to spend around £1 million a year to keep our production capability very competitive.

"This year, we exhibited at Southern Manufacturing in February and are at MACH 2014 at the NEC. Based on past exhibition experience, we will convert more enquiries to help fill our spindle capacity.”