A fashion item that is safety critical.
Rimstock plc can produce up to 10,000 alloy wheels per week and export them to more than 35 countries worldwide. They have an impressive list of customers from luxury marques, such as Aston Martin and Bentley, to volume producers as well as specialist custom car firms such as Overfinch.
The company provides a comprehensive service to customers from initial design concept through analysis and modelling to finished production components.
The customer is heavily involved throughout the realisation of initial style and design concept phase. There is a vast library of design data to call on where the clients’ concept is put through finite analysis and 3D simulation to define expected loads and stresses. The customer can be provided with visual aid representation of a design or a physical resin model.
The designing process starts with the spokes, then the rim. Analysis of the design is carried out to determine resultant strength, stiffness, weight as well as possible weaknesses before committing to making the tooling.
When the tooling design is finalised the die can be produced quickly and efficiently from the models. Normally the tooling is a six part die set with upper and lower dies and four core dies.
The dies are set on a low pressure die casting machine which combines the die closing and opening mechanism above a refractory lined pressure vessel. Die coatings are very fine grading and the cores are coated with an insulating material.
Initially low pressure air above the molten metal bath forces metal into the die cavity at approx 10 to 15 p.s.i. This is followed by a second phase where the pressure is increased to 25 p.s.i. to ensure feed metal is available for a sound casting. The bath is de-pressurised and the casting is left for up to two minutes before being removed from the die.
The casting machine cycle time can vary considerably – from four to thirteen minutes depending on the complexity and weight of the finished casting.
The wheels are produced in a 10% silicon aluminium alloy and the base charge material is high quality imported ingot. The metallic charge balance is made up with in house returns. A shaft tower melting furnace is used as the prime melting unit with an optimum melt rate of one tonne per hour and a holding capacity of two tonnes. The actual melt rate is controlled to meet the demand from the die-casting machines which have a usable holding capacity of 300 kgs. Strontium fade does occur with time and ‘top up’ additions are made when required.
Depending on the specification of the wheel, approximately 50% of the wheels produced are heat treated in house to give the desired finished properties. A typical treatment cycle is to hold at 535deg C for 6 hrs down then to 150 deg C for 2 hrs.
The castings are subjected to real time x-ray monitoring with full computer control where the wheel hub is x-rayed first followed by the spokes and finally the rim. The quality control on wheels produced varies on the intended market from 100% for original equipment wheels to a predetermined level which is influenced by the style of wheel for the after market, with a count back on wheels processed in the event of a failure.
From x-ray monitoring the wheels are machined on CNC machining centres where a typical yield of 80% is achieved. The wheels are then 100% pressure tested.
Further quality checks that are carried out on an AQL basis include dimensional checks, testing for running true, radial strength, cornering strength, impact as well as a test for radial fatigue after 100,000 cycles. The company aim is to have zero defective wheels in the field. The testing and compliance regime of the company ensures they can comply with any recognised standard in the world.
The wheels can comply to Japanese or German standard and are independently audited by a test house to bear the relevant authority markings
Rimstock have invested in a purpose built non-chromate Pre-Treatment and Paint Facility with state of the art robotic handling systems. The plant applies environmentally compliant coatings with 1200 hour corrosion protection and the finished coatings exceed modern ecological legislations.
The company also provides wheels produced on 5 axis and 3 axis milling machines which are machined from bought in forged blanks where there is up to 2 hours machining on each wheel. Additional services offered by the company are an alloy wheel refurbishment service and a tyre fitting facility to further reduce onward quality issues. A distribution company that warehouses over fifty thousand wheels in a purpose built computer controlled storage facility is also part of the company.
Adrian concluded his informative talk outlining Rimstock’s commitment to the analysis and training of their staff as well as interaction with customers at all stages of the production life cycle.