. This subject is a natural follow on from the last Magnet Society event held in Derby titled ‘Computer Simulation of Novel EM Devices’. We do not make our own product range. We are sub-contract manufacturers, so we are working from customer specific drawings or concept ideas. We will advise on material suitability and availability, how viable parts are for manufacture, component tolerances and stack assembly methods. We will manufacture quantities from just a few prototype laminations loose or stacked, right through to volume production. We produce laminations from 1mm thick down to 0.1mm, and material grades of 3%-6% silicon electrical steels to cobalt and nickel iron grades.
????????This subject is a natural follow on from the last Magnet Society event held in Derby titled ‘Computer Simulation of Novel EM Devices’. We do not make our own product range. We are sub-contract manufacturers, so we are working from customer specific drawings or concept ideas. We will advise on material suitability and availability, how viable parts are for manufacture, component tolerances and stack assembly methods. We will manufacture quantities from just a few prototype laminations loose or stacked, right through to volume production. We produce laminations from 1mm thick down to 0.1mm, and material grades of 3%-6% silicon electrical steels to cobalt and nickel iron grades.
The first things we consider with any lamination enquiry are quantity, component materials and tolerances. Sometimes a requirement is just to prove a design or theory, so volume production is not a consideration. More often though the requirement is for a small number of prototype laminations or stacks which will be built and tested. There will usually be a few customer design changes and further parts and testing before a product is classed as production ready.
Some customers have a commercial requirement and know their market, other don’t, so this can be a difficult area to know how to best quote for volume production as manufacturing methods and costs can vary quite significantly depending on quantity. Some motors remain relatively low volume and remain being manufactured by the wire erosion process, never converting to tooling. This is not a problem to us. You need to be pretty certain of final design before committing to the tooling route for volume as modification once built can be time consuming and expensive.
Consideration of how the stacks are held together has to be primary. Options are – mechanical cleating, welding, bonding or loosely stacked. Most electrical steels are supplied with an insulation coating and their grades can have an effect on punching, welding and bonding. Some laminations once pressed may require annealing, so insulation choice here is important as some coatings will not withstand the process. If bonding is the chosen stack assembly method then pre-bonded material cannot be considered if annealing is required as the bonding agent will be destroyed in the process. The annealing process needs to be considered carefully if laminations are to be post-bonded as some types of annealing will produce oxide ‘flaking’ which is not good for bonding.
Designers sometimes choose a material grade from charts to suite their application and have overlooked or are unaware that it may not be commercially available. Electrical steel production mills for instance want to sell not surprisingly, tonnes of material. They are reluctant to sell less than a tonne of material. This is where stockists come in. They buy from the mills in volume and then sell on to the trade in lesser quantities but they will only stock what they can sell. This means some grades they will not re-stock if sales have been poor. Particularly with small laminations, even in volume the material requirement is quite small. Prototypes more so may be just a few kgs. are required. Electrical steels thinner than 0.35mm are not stocked in the U.K. We have to buy from the mills in Europe, so you can start to see the material supply problems we can encounter. We do try to hold some stocks but we can only cover some eventualities.
Wire erosion is an electrical discharge machining (EDM) process. This basically is an electrical spark that is created between and electrode and work piece. Wire erosion uses metallic wire, usually brass, between 0.1 and 0.3mm diameter and follows a programmed contour through the work piece. This cutting process takes place in a dielectric of de-ionised water. Conductivity, spark, cooling and flushing of debris are carefully controlled. Accuracy and surface finish determine whether a single cut or secondary skim cutting is required. Wire erosion is one of the most accurate cutting processes available, with tolerances possible to just a few microns and surface finish to 0.2Ra.
Once we have determined the lamination material to use, we consider the finished lamination stack height. We then look at the layout configuration and decide on a suitable blank size. Blanks are cropped, rolled flat and de-burred and stacked to a suitable height for wire erosion and clamped between steel top and bottom plates. Fixing positions and wire start holes are drilled through, and the stacks bolted together. Should the stacks be pre-bonded material, then the stacking is still clamped together but then heated in an oven and cured, meaning bolting through is not required. This stacking process needs great care to ensure stacks are completely flat, parallel and without air pockets. A poorly produced stack gives problems further down the cutting route and assembly routes. We aim to wire cut stacks where possible, around the 50mm high mark. This is a good compromise between cutting speed, finish and tolerances. Sometimes we need to tag components in the stack to ensure there is no movement and that they remain in place. Our wire erosion department is temperature controlled and we have a CMM measuring facility in the department. When cutting to micron tolerances, it is important that material being cut has acclimatised to the conditions both for cutting and measuring. Once the parts are cut, we need to consider the cleaning of parts. Laminations being made from ferrous materials have a tendency to rust after being subject to the de-ionised water for long periods. We need to combat this before this rust can start to stain and take a hold. Where possible we clean in ultrasonic tanks to ensure parts are thoroughly cleaned prior to inspection and packing.
Pressing or stamping of laminations is the most cost effective way to produce these parts in volume. There are several tooling methods that could be applied to laminations but the most common being progression. This is where the material usually in coil form, is fed through several progressive stages and its final pitch being a finished component. In some instances parts can be produced at several thousand per hour. The type and size of the power press that accommodates the press tool will depend on the tonnage required to make the part.
The decision to press laminations is usually based on volume, life of product and the ability to re-coupe the tooling cost outlay in a given amount of time. A simple, small lamination tool could be as low as a few thousand pounds but a very complex tool with high volume carbide dies that can skew and stack and is expected to make millions of parts, could be nearer 100,000 pounds. We would consider our company to be in the low to medium volume production. We run lamination tooling from batch runs of 1,000 to several hundred thousand. The life of a product can be a year to more than twenty years, so volumes over time can be quite substantial. This is why it is important to have an idea of volumes or life of product so the correct punch and die materials are chosen at quotation and then design stage. Component material usage again is important and the design will make this as economically viable as possible. This is where it is important to have discussed at prototype stage the complexities of a particular parts manufacture and design out these potential problems wherever possible. Our smallest pressed laminations are around 10mm diameter and we usually limit our sizes to 350mm when making them as one piece. However we do make segmented laminations where several parts are produced to make up a diameter. This approach is cost effective with materials as there is no centre hole wastage. As I have mentioned earlier, some lamination materials and their insulation coatings cut better than others. Pressing these materials is best done with a light lubricant but laminations are generally required to be supplied fairly dry. Vanishing oils applied sparingly are usually acceptable. Pressing without lubricants increases tool wear and leads to more frequent regrinding of the punches and dies to keep components free of burrs. A secondary operation for degreasing should be avoided if at all possible as it can increase costs. Components can be removed from the tool by conveyor or fall onto stacking fixtures under the tool and press. Parts can be supplied loose or on wires or rods to ease secondary assembly times but consideration has to be taken into their vulnerability to damage.