Milling magnets offer the convenience of fully accessible five-axis machining in one setup with the benefits of cost reduction and tolerance improvement.
Parts that previously took a day or more to machine can now be finished in a couple of hours indicating how far machining practices have come in the past 15 years. Faster machines and new cutting tool developments have enabled companies to become more competitive through increased productivity.
In many cases however, 1920s style clamping methods are still employed and although the machines are faster, set-up time has not changed significantly. Today, many machines have the ability to machine in up to five axes, yet most conventional fixtures do not give total access to all five faces of the workpiece.
Magnetic workholding, with its unique properties, is the next significant step in increased productivity. Users of magnetic workholding estimate that set-up reduction averages 50% through reduced number of setups and reduction in the overall set-up time. In addition, most users estimate that payback takes less than a year.
Until recently, the use of magnetic workholding technology had been basic, but new innovations like powerful permanent electro-magnetic set-up blocks and new, more powerful permanent milling magnets have greatly increased their usefulness, particularly with moldmakers.
What Are Magnetic Chucks ?
The three types of magnetic chucks are permanent, electro-magnetic and permanent electro-magnetic.
Permanent magnets switch on and off through the alignment of magnets within the chuck to the mild steel surface on top of the chuck. These magnets are restricted in their clamping force by one’s ability to overcome the magnetic resistance in turning the chuck on and off. Historically, permanent magnets were only used for grinding and EDM work due to the limited clamp forces resulting from magnetic resistance.
Electro-magnetic chucks use a constant supply of electricity through a coil to create a magnetic field perpendicular to the coil. Electro-magnets can generate large magnetic fields, but if power is lost, so too is the magnetic clamping – making them impractical for use on pallet changers and machining centres. It is important to also note that electricity through an electro-magnet generates significant amounts of heat within the chuck and on the surface.
Permanent Electro-magnetic workholding has come a long way in the past 10 years, especially with the advancements in neodymium magnets. Neodymium iron boron (NdFeB) magnets are smaller and more powerful than other types of permanent magnets, therefore magnetic fixturing is more powerful and more compact. In addition to neodymium, permanent electro-magnets also use alnico magnets as the magnetic switch so that the magnet can be turned on and off via a pulse of electric current. This switching cycle takes less than a second as permanent electro-magnets only require electricity to turn the magnet on and off ….no electricity is required to maintain the magnetic field.
Whats New ?
In 2001, two new types of milling magnets entered the market, magnetic setup blocks and permanent milling magnets.
Magnetic Setup Blocks
As milling magnets turned the workholding industry upside down, so too will permanent electro-magnetic setup blocks forever change conventional fixturing as they combine the holding power of a milling magnet and the flexibility of a setup block into one unit.
Magnetic setup blocks are the latest advancements in permanent electro milling magnet technology. They are designed to offer greater flexibility than conventional milling chucks without sacrificing holding force.
The introduction of magnetic setup blocks gives the shop greater flexibility and profitability because each module offers the same clamping force as a pair of balanced poles on a milling chuck. As a bonus, if a job requires fewer modules then the remaining modules can be used on another machine and because they are permanent electro-magnets one controller can fire infinite setups.
Permanent Milling Magnets
The other new item to debut in 2001 is a permanent milling magnet that offers significant holding power at a reasonable price, typically 3.5 times more powerful than traditional grinding magnets making them ideal for small VMCs and knee mills. Knee mill capability is extended to handle larger workpieces, such as those that overhang the table, and multiple parts. A permanent milling magnet does not require electricity, it can be easily moved and has the ability to utilize top tooling such as pole extenders, back parallels and magnetic angles.
Magnetic clamping offers as many benefits for the small shop as it does for the volume production machine shop. Magnets can offer full access to all five sides of a workpiece in just one setup. In addition, the use of magnets makes programming easier as there is no need to coordinate clamp locations with the program.
Magnets reduce vibration, therefore tooling is free to run faster and longer. Through the use of top tooling or modular poles, a block of steel can be faced, edged and drilled through in one setup. This greater access to the workpiece means that a machine can run longer unmanned (lights out) which results in higher productivity.
Milling on magnets can benefit both VMC and HMC operators through setup reduction and improved safety, plus it reduces the risk of loosing your qualified datum by reducing the number of setups.
Magnetic setup involves simply loading parts, indicating parts and energising the magnet which in itself takes less than one second of electricity. In comparison, many injuries during setups are a result of over-reaching when tightening a clamp, or from trying to support work whilst tightening a clamp.
Other advantages of milling on a magnet include consistent clamping force. Regardless of the strength of an operator, the clamping force of a magnet is constant.
Most milling magnets offer pole extenders to raise the workpiece off the magnet surface to allow for through drilling as well as milling.
And the new individual magnetic setup blocks can be located anywhere under a workpiece and used as a milling magnet or magnetic work support with conventional fixturing.
Magnetic workholding is the next significant step in cost reduction and quality improvement.
Alpha Workholding, USA, provided content for this article.
Romheld Australia distributes Alpha products and a complete range of precision workholding equipment.