Saturday, December 09, 2006

Sliding head lathes excel in second-operations

Three sliding head CNC automatics, recently added to a machine inventory of some 100 single- and multi-spindle lathes, have already brought in more new work for the factory owner.

Wilco Manufacturing, Birmingham, specialises in the sub-contract manufacture of precision turned parts. Its policy is to offer customers as diverse a service as possible in three main areas: complexity of component produced, from a simple dowel to a complicated turn-milled part; batch sizes it is prepared to undertake, which currently range from 500- to 500,000-off; and delivery service, either kan-ban, schedule or discrete orders. With further diversification in mind, the company has supplemented its 100 or so single- and multi-spindle cam automatics, sprints and fixed-head CNC lathes with three sliding-headstock, multi-axis turning centres from Star Micronics GB.

In addition to broadening the scope of its manufacture, the other main driving force behind the acquisitions was to eliminate costly secondary operations performed on conventional drilling, milling and capstan machines in order to reduce manufacturing costs, improve product quality and become even more competitive.

So, when evaluating the available sliding-headstock lathes on the market, particular attention was paid to the power and speed of the driven tool stations and of the sub-spindle for reverse end-working.

It was this that led to the selection of Wilcos original Star SR-20RII lathe in October 2003, followed in autumn 2004 by the purchase of two further, identical machines.

Compared with the other short-listed sliding-headstock lathe, the Star offered 25 per cent more power on the five driven cross-working tools, which are rated at 1.2kW, as well as a 60 per cent higher top speed of 8,000 rev/min.

The machine also offered a 1.5kW(cont)/2.2kW(15min) sub-spindle motor, double the power available on the competitors machine.

Consequently the Star lathe is easily capable of drilling and tapping holes up to 8mm diameter/M8 in the back of components.

Another advantage was that the sub-spindle and four powered reverse end-working tools, one more than on the competitors model, could work totally independently of main spindle operations served by the platen.

In all, eight of the 28 cutters in the working area of a standard SR-20RII can be used in combination with the sub-spindle, allowing considerable machining versatility.

Furthermore, in respect of the front end-working tools used with the main spindle, four were provided on the Star machine, one more than on the competitors model, and again these operations may be carried out independently of sub-spindle operations.

All this may seem a little obscure, but as Martin Lane, Managing Director of Wilco points out, such enhancements in machine design translate into shorter cycle times and in some cases the ability to finish-machine a component in one hit instead of having to perform a second operation, resulting in lower manufacturing costs.

Often, Wilco has to centre, drill and tap a hole in the back of a component as well as turn the end face or O/D.

So if only three stations are provided for end-working, one of the operations would have to be carried out using a rear-facing tool mounted on the back of the front end-working attachment or in the cross-working post.

As these are both linked to the turning platen, no simultaneous machining in the main spindle would be possible and productivity could be lost.

An example of where a considerable productivity improvement has been made on the Star machines is in the manufacture of a 5/8in A/F hexagonal steel connector for the automotive industry.

Wilco used to produce it in four successive operations on a single-spindle automatic, a magazine-loaded auto and a drilling machine, followed by manual deburring.

The sum of the previous cycle times was 95 seconds and manual handling was required between the separate operations.

The same component is now machined completely in one automatic cycle, in half the time, on one of the Star lathes and with improved repeatability, 24 hours a day.

Moreover, the absence of inter-operation handling has eliminated potential damage that was previously caused on occasion to the 3/8in UNF male thread at one end, eliminating scrap or rework.

Another example that shows a big improvement is an hydraulic component produced from 0.400in diameter ground mild steel bar.

This part also used to be manufactured in four set-ups: turning, drilling and tapping a 2-56 UNC thread at one end on a single-spindle automatic; reversing the component, forming an end taper and drilling a 4.5 mm diameter blind bore from the other end; milling a 1.5mm deep by 1.5 mm wide slot along the length of the component; and cross-drilling a tiny 0.8 mm diameter hole to meet the initial hole made for the 2-56 UNC tap.

The 93-second total cycle time has been cut by over a third to 58 seconds on a Star lathe and manual handling has once again been eliminated.

Batches of typically 2,000-off are produced and the lead-time has been cut from four weeks to one, dramatically improving customer service and reducing work-in-progress.

Wilco Manufacturing obtained a DTI grant through a Regional Selective Assistance Scheme to help purchase the three Star lathes.

It enabled Lane to telescope a five-year plan to acquire the sliding-headstock lathes into one and a half years.

The original idea was to transfer work requiring multiple operations from the conventional machines to the first Star lathe.

In practice, however, when customers heard that sliding-headstock capacity was available, new business rolled in so quickly that it filled the machines capacity.

Lane confirms that between the first Star machine coming in and the installation of the other two, in excess of GBP 100,000 of new work was won as a direct result of having sliding-headstock turn-milling capability.

Furthermore, the second and third machines are currently sampling new orders from back inquiries and if these also come to fruition, a further GBP 150,000 of business will have been won partially as a consequence of having bought the first Star.

All three machines are fitted with a full length FMB automatic bar magazine and are mainly employed for producing stainless steel, steel and alloy components in cycle times ranging from 18 to 90 seconds to a general tolerance of +/-0.015 mm, which is easily held.

All work going through the machines tends to be high accuracy, complex components that would traditionally require multiple secondary operations, so the powerful cross machining and reverse end-working capabilities of the Stars are nearly always employed to extract maximum benefit from the machines.

Whereas the cam automatics are only operated during the manned day shift, all the bar-fed CNC machines run overnight without operator attendance, contributing significantly to Wilcos productivity.

The company was established in 1940 as a subcontractor serving the automotive industry, principally the Rover Group.

By the mid-1990s, over 90 per cent of the company's turnover was in the automotive sector.

With the BMW takeover, the trend at that time was for component and sub-assembly production to be outsourced to other countries such as Brazil, and for overseas-built engines to be imported rather than made here, all of which reduced the amount of work available to UK subcontractors.

Lane resolved to diversify into a number of other areas including electronics, leisure and hydraulics, the latter now accounting for 15 per cent of the companys GBP 1.4 million turnover, while the proportion of automotive work has dropped to 60 per cent.

The recent entry into sliding-headstock turning will assist the company to spread its work more widely over different industries, further reducing its exposure to the automotive sector.

It will also place Wilco in a better position to compete with the increasing number of low-wage manufacturing nations on the world scene, notably China, India and Eastern European countries.

Photographs herewith, captioned: 1.

The three Star SR-20RII sliding-headstock lathes on the shop floor at Wilco Manufacturing, Birmingham.

2.The machining area an SR-20RII, showing simultaneous front and reverse end-working in progress on a steel hydraulic component, which is now produced complete in 58 seconds.

3.The steel hydraulic component seen from both ends, showing the operations in both faces, the longitudinal groove and the 0.8mm diameter cross-drilled hole, which would be inefficient to machine without 8,000 rev/min live tool stations.