Stockport-based Renthal, a world-leading producer of accessories for motorcycles and mountain bikes, is changing its rear chain-wheel (sprocket) manufacturing process from CNC drilling followed by a sequence of ‘manual’ operations to ‘virtually’ one-hit production on a pair of Brother machining centres.
Renthal is famous for manufacturing aluminium alloy handlebars, which are not only popular in the ‘after-market’ but also fitted as OEM components on Honda, Kawasaki, Suzuki and KTM moto-cross bikes. Rear chain-wheels are also a core product, and it was to these in particular that Renthal turned its attention in 2011.
Rear chain-wheel blanks, which are made from aluminium, have outside diameters in the range 150-280mm and are plasma-cut from sheets that are 6, 7 or 9mm thick. They are transferred to a gear planer to have their teeth machined around the periphery and chamfered.
While these preparatory operations are still performed, the new process route differs markedly thereafter, with the remaining operations completed in a single cycle on both pallets of two Brother TC-R2B three-axis moving-column turret-type machining centres — supplied in January this year by Kenilworth-based Whitehouse Machine Tools Ltd (www.wmtcnc.com).
With the established method of manufacture, which is still in operation alongside the new cell, chain-wheels are machined in four stacks of six on a different make of machining centre that uses indexable-insert drills to quickly produce around three dozen holes; these are for lightening the chain-wheels and for fixing them to the motorcycle.
From here on, counter-sinking, deburring and other jobs are done as individual operations. This method of finishing the many millions of holes was both time-consuming and costly. The minimum batch size was 24, and the production time was 5-6min per component.
Single-piece production
The new method of manufacture being phased in is based on single-piece CNC production. As each hole is now profiled individually, very fast machines were needed to minimise idle times and maximise metal removal — hence the choice of the Brother TC-R2Bs.
These machines have a 420 x 320 x 305mm work envelope, a 16,000rev/min spindle speed, a fast servo-driven 14-position tool turret with a chip-to-chip time of 1.7sec (including deceleration from and acceleration back to maximum spindle speed), and 50m/min rapids.
Moreover, pallet change (in less than 3sec) can take place at the same time as rotary-table movement, tool change and linear-axis movements, paring even more from the overall idle time.
After machining, the holes are chamfered by plunging or interpolation; the component is fixtured to allow a dovetail cutter to pass through the holes and produce a similar chamfer on the reverse. Even the part number, which was formerly hand-stamped, is CNC-engraved. The cycle time is now in the range 3.5-4min — a reduction of around one-third compared with the traditional method of manufacture.
Production engineering supervisor Steve Grainger says: “The new production route is not only more economical in terms of time and labour, but has also freed up an enormous amount of space for other projects. We have gained another level of accuracy in rear chain-wheel production by doing away with repeated handling and re-fixturing, as well as variable hand finishing.
“We used to have to machine the chain-wheels 24 at a time and hold a large number of them in stock, which was costly and took up a lot of space, whereas now we can make them quickly and economically to order.”
Novel pallet exchange
Renthal’s first experience with Brother dates back to 2007, when it bought a TC-R2A. Production engineering manager Adam Kirby adds: “We were drawn to Brother because — at the time — it was the only twin-pallet machining centre in its class on the market on which the pallet simply rotates without lifting.
“This means that there is virtually no possibility of swarf becoming trapped underneath, avoiding the repeated pallet alarms we can get with other makes of machining centre. The machine proved to be as reliable as predicted, which gave us confidence in the machine builder, all of whose models use the same method of pallet exchange. So two years ago, when we embarked on a big push to expand in-house CNC production, we decided to standardise on this supplier for prismatic machining equipment.”
Next to be installed were a Brother TC-R2B and a TC-324 in 2010. These machines are taking over production of most new ‘core’ work, as well as sharing the machining of ‘non-core’ products with existing machining centres on the shopfloor. Such components include motorcycle handlebar levers and, more recently, parts for top-end cross-country and down-hill bicycles.
The fourth Brother machine, installed in 2011, was a TC-S2D-0 with a 700mm X axis rather than the standard 500mm travel. It was bought specifically for research and development — a function that until then tied up production machines.
A Nikken two-axis tilting rotary table was supplied, so that the machine can simulate manufacture on all previously installed Brother equipment as well as the latest (the seventh) machine, installed in March this year; this has a four- and five-axis capability.
Designated TC-32BN-QT, it is Brother’s top model, with 70m/min rapids, a 40-tool ATC, 1,000psi high-pressure through-tool coolant and an FSE swarf management system with back-flushing rotary-drum filtering to less than 50µm. On one pallet is a Kitagawa two-axis table that gives five-axis functionality; on the other pallet, there is a Kitagawa trunnion that provides an additional CNC axis.
The machine currently produces 10 types of handlebar lever from aluminium billet in three operations instead of the previous six. The plan is to machine other components as well, including Renthal’s new Duo Stem for high-performance mountain bikes.
The two-piece design presently needs 14 machining operations, which will be halved on the TC-32BN-QT. Production costs will be lower, not least through the use of fewer fixtures, which are all designed and manufactured in-house.