Plastic Turned Parts (PTP) was the first company in the UK to install a Citizen CNC sliding-head turn-mill centre incorporating the machine manufacturer’s patented Low Frequency Vibration (LFV) machining technology (www.citizenmachinery.co.uk).

As a result, PTP has totally eliminated swarf control problems when machining plastic components.

Managing director Jonathan Newis said: “For us, the development of LFV has transformed our turn-milling operations, so we can now confidently leave the Citizen machine running without having to continually interrupt the cycle to clear troublesome ‘birds’ nests’ of plastic swarf.

“LFV has been especially effective on deep-hole drilling applications, as we have eliminated any clogging of the drill flutes.

“Previously, we would often set the machine with two — or even three — identical drills, but we can now go straight to depth with a single tool. The process leaves the flutes completely clean.”

PTP installed the L20-VIII LFV from Bushey-based Citizen Machinery UK Ltd (www.citizen machinery.co.uk) in January, following a visit to MACH 2016, where it saw a demonstration of the technology on the Citizen stand and promptly placed the order.

Mr Newis says: “The result of applying LFV is so impressive that we are planning to purchase a second machine— one with a smaller 16mm capacity — as soon as it is available, thereby allowing us to more effectively machine smaller components.”

Based in Watton-at-Stone (near Hertford), PTP employs five people — all skilled in machining plastics. Established in 2004, the company has grown by 20% over the last two years.

It has eight Citizen CNC sliding-head machines with capacities up to 32mm, and a Miyano BNA-42S fixed-head turn-mill centre — supplied by Citizen and installed in 2013.

Also in the machine shop are fixed-head lathes with a 65mm-diameter bar capacity and a 250mm-diameter chucking capacity. A vertical CNC mill provides additional milling capability.

Diversity of products

A wide range of components is produced for customers in the marine, paint spraying, bearing, food, motor-sport, telecommunications, automotive, valve and medical sectors. Indeed, a major long-term medical contract won by Mr Newis is the production of 6,000 ear grommets a month for babies that suffer from ‘glue ear’.

These parts are micro-machined on a Citizen K16 from 5mm PTFE bar; they are turned down to a diameter of just over 1mm and have a 0.75mm-diameter hole drilled through.

In addition to PTFE, the plastics-based materials machined include Acetal, Delrin, Nylon, PVC, polypropylene, polyethylene, Nylatron, PEEK and graphite-filled PTFE — plus other high-performance plastics, including variants of glass-reinforced types. Typical batch sizes are in the range 500-10,000.

Mr Newis said: “We provide a specialist service and have developed the skills required to machine these demanding materials. Polypropylene, for instance, can react totally differently under cutting conditions and will even behave almost like chewing gum.

“Some plastics can soften or even melt, while others ‘grow’, deform or spring back under cutting conditions.

“Machining polyethylene can be a nightmare, with swarf wrapping around the tools and often melting on drills. With LFV, the material readily chips, allowing us to run unattended for several hours, and freeing our setter/operators to do other things during the day.

“This is a massive advantage for a small company. Historically, reinforced plastics ‘kill’ tooling, so when processing these materials, the machines had to be continuously monitored. That is not the case with LFV; indeed, the capability of the new Citizen is transforming our business.”

Patented process

The patented LFV process is unlike conventional ‘ultrasonic vibration’ machining and took Citizen in Japan some three years to develop. It is not only achieving significant improvements when cutting plastics (as PTP has found), but also exotic materials, plus difficult-to-chip ferrous and non-ferrous materials such as copper.

The LFV process also allows the depth of cut to be increased, surface quality to be enhanced, and both tool life and spindle up-time to be increased. The process can be applied to very small diameters and thin-walled components; it can also be used for turning, facing, eccentric and interrupted cuts, drilling and even thread cutting.

The process, which is fully programmable, is activated through a G-code that allows on-demand application anywhere in the cutting cycle.

The servo axes of the machine drive system are ‘oscillated’ in the direction of the tool feed in phases involving tens of microns; these oscillations are also synchronised with the rotation of the spindle, interrupting the cut and breaking the swarf into smaller pieces.

Furthermore, LFV minimises built-up edge on the tool tip, which adversely affects tool geometry and can result in premature failure.

Darren Evans, who is responsible for setting PTP’s Citizen with LFV, said: “We can maintain what is effectively a wiper action, as the tool ‘re-cuts’ the material, and this helps to create a superior surface finish.

“We can even program the LFV to ‘tailor’ the ideal length of swarf through P1 and P2 codes that set the frequency of oscillation.

“For us, this is a very important benefit, as ultra-small chips of plastic would just build up and clog the machine.”

PTP tends to run the machine spindle at 4,000rev/min, with feed rates of 0.03mm/rev (P1) or 0.05mm/rev (P2). P1 is used for general swarf breaking and P2 for small-diameter turning and drilling.

“We use both carbide and HSS tools, and a further bonus of using LFV is that we have reduced our use of form tools, as geometries are now generated quickly and easily using standard single-point tooling.”

With plans for further LFV machine installations, Mr Newis says he is confident of being able to move the business forward.

“A major problem with plastics is variation in the material, which can become a major headache on certain jobs. We now have the ability to easily influence the application to meet our needs.”