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Subcontractor produces fully CNC machined mountain bike
North Bucks Machining (NBM) has produced a fully CNC machined mountain bike, improving on conventional designs by removing the weaknesses of weld seams on the joined hollow tubes. The NBM design is built around solid aluminium billets and i-beam technology to scallop out the sides of the frame to reduce mass, with rib thicknesses machined down to 5mm while retaining structural integrity.
A family owned business, NBM was established by Dave Palmer in 2011, with the bike being the brainchild of son Stewart, a biking enthusiast. The bike design consists of three core features, the main frame, the seat post and the swing arm that connects the main frame to the rear wheel and suspension. To emphasise the level of machining required, the main frame is machined from a 100kg aluminium billet down to a 4.5kg frame. The seat post starts as a 15kg billet and ends as a 250g part and the swing arm is machined from a 50kg billet down to a 2kg part. This leaves an aluminium frame of less than 7kg from an overall billet weight of 165kg.
Using Open Mind’s Hypermill, the frame is machined in 40 hours, the swing arm in 18 hours and the seat post in 15 hours. However, with just two frames produced to date, time is not the key issue for NBM. “At this early stage, the machining time is not the key issue; its the surface finishes,” says Stewart. “Once this frame is machined, it has no secondary hand finishing, it’s ready for final assembly or shipping. It’s all about delivering an aesthetically perfect bike to the customer.”
To this end, Hypermill has genuinely been in a class of its own. As Stewart mentions: “The project wouldn’t have been possible with our previous CAM software. Without Hypermill and the 5-axis Quasar machine, it would have been impossible. But what sets Open Mind’s Hypermill apart is its finish machining cycles. Hypermill automatically groups together the Z-level finishing and ‘profile finishing’ cycles to blend perfectly between two surfaces and the two separate cycles. This is critical for our ambitions of the perfectly finished frame.
“Furthermore, the frame has a lot of pocket machining, so naturally there is a radius left at the bottom of each pocket by the cutting tool. To resolve this, Hypermill has a ‘pencil milling’ feature that can isolate all the remaining fillets for re-machining. The machine then selects a small tool, which removes the excess material in the corner of each pocket.”
Whilst surface finish is more critical than cycle times, Stewart is aiming to reduce the machining time by implementing Open Mind’s new Hypermill Maxx machining package for rough machining. Marketed by Open Mind as the next generation in CAM software for cutting hours from machining cycles, NBM has run tests of Hypermill Maxx and found the cycle time reduction to be extremely positive. Just one of the roughing cycles on the main frame has been reduced from two hours 20 minutes to less than one hour 30 minutes, a 40% reduction. It is estimated that if the part was run on a machine with a spindle speed in the region of 15 to 18,000rpm, as opposed to the existing 8,000rpm, the cycle time would fall close to 30 minutes, a massive 80% reduction on the existing run-time.
With the project in its infancy, Stewart has ambitions of further enhancing the design of the frame to reduce the overall weight from 6.75kg to approximately 5kg. As an engineer with a pedigree in F1 design and production, getting the frame to shed a few grams as it progresses will be second nature. At present, if you are a mountain biking enthusiast, you can contact North Bucks Machining in the heart of the UK motorsport valley Milton Keynes to get more details. The company will look to retail the three-part frame and rear suspension unit in the region of £7000 or a completely built bike with a top specification at approximately £10,000.
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