This pattern maker uses magnetic workholding to secure parts for machining that traditionally aren't held by the "invisible" clamping method. The technique it uses also allows it to repeatedly position a part in any one of its machines.
Sometimes while solving one problem, a solution to another dilemma serendipitously appears. That was recently the case for Anderson Pattern, in Muskegon Heights, Michigan.
According to Mark Nielsen, machining manager, the more than 70-year-old pattern and moldmaking company began its journey to lean manufacturing 1 1/2 years ago. Since setup time reduction is central to any lean push, the company decided to switch from traditional mechanical workholding clamps to electropermanent magnetic milling chucks to hold billets and castings in its various vertical and horizontal machining centers. Its goals were similar to other shops that have integrated magnetic workholding—quicker change-overs, better repeatability and more spindle uptime thanks to access to up to five sides of a part in one setup.
Some of the company's large components fit the conventional profile of parts most commonly held by magnetic workholders—big billets with ample surface area for which the magnet can clamp to 12 tons per square foot. Others parts, including smaller billets and castings, did not provide enough surface area for the magnet to grip. Examples include wheel molds with 1-inch-wide annulus on their bottom surface or pattern blocks that have material removed from their backside for reduced weight.
Parts such as these are prone to twist on a magnet during cutting operations, especially when cutting forces are directed parallel to the face of the magnet (which occurs when milling about a part's periphery), versus down toward the magnet (as in a drilling operation). In fact, an axial cutting force only 1/18 of a magnet's total pulling force on a part can cause that part to twist out of position, according to John Powell, Ph.D., president of WEN Technology (Raleigh, North Carolina).
But where there is a will, there is a way. And in learning how to prevent small parts from twisting on magnets during cutting, the company happened upon a universal part positioning system for all of its machine tools and its two coordinate measuring machines (CMMs).
Invisible Workholding
Magnetic workholding chucks take advantage of commonly known magnetic principles. These magnets have north and south poles across which flux, or magnetic energy, flows. When a ferrous workpiece is located between magnetic poles, the flux enters the part and induces polarity in the part (opposite the magnet), causing an attraction between the part and magnet.
Electropermanent magnets are the magnetic workholding technology of choice for machine tools, and with good reason. These units use permanent magnets that are only energized after a quick shot of electricity is delivered by a remote controller that is temporarily connected to the magnet. The controller's cable is then disconnected, and the magnet will continue to hold the part until de-energized by another zap of electricity from the controller. Cable-free workholding is especially helpful on horizontal machines with pallet changing capability. Anderson Pattern uses a number of electropermanent magnets in both vertical machining centers (VMCs) and on tombstones used in horizontal machining centers (HMCs).
The key is to deliver the proper amount of magnetic flux that will hold the workpiece but not extend past the part's surface, which would magnetize the outside of the part. Chips would then stick to the part and be re-cut, reducing both tool life and surface finish quality. This is done through the use of pole extensions, which are cubes or strips of steel that may be screwed or placed on top of the magnet's individual poles to raise the workpiece off of the magnet. Pole extensions prevent damage to the magnet during workpiece machining, but also allow fine tuning of the amount of magnetic flux delivered into the part through the adjustment of the contact area between part and pole extension.
Pins Position, Prevent Sliding
Anderson Pattern's facility is divided into roughing and finishing areas. Billet blocks for patterns typically are first rough machined, then delivered to the finishing area, where four-axis contour milling is performed on the top of the block. After contouring, the blocks are returned to the roughing area for additional periphery machining work. When experimenting with its first workholding magnet, the company found that reference points which were added to the top of the block during finishing were off by up to 0.020 inch after re-installation on the roughing machine. This caused the tool to take a bigger than expected cut, yielding higher cutting forces that caused the blocks to twist out of position on the magnet.
Dowel pins (like the one circled) not only allow repeatable workpiece positioning on this shop's machine tools and CMMs, but they also allow parts with limited surface to be securely held by magnetic workholding tables.
Because the first process for each part was skimming the bottom of the blocks flat, Dr. Powell suggested moving the reference points from the top of the part to the bottom. The reference points, accurately machined holes to receive mating dowel pins, would be replicated on pole extensions screwed to the magnet's poles. The part could then be located on the pole extensions with dowel pins, which not only prevents parts from sliding, but also provides a way to repeatedly position a part in every machine tool and even the shop's two Brown and Sharpe CMMs.
Workpieces are much more likely to twist on a magnet than be lifted off of it. This wheel mold has only a 1-inch wide annulus for which a magnet can grip, but dowel pins used in select bolt holes will prevent the mold from twisting on the magnet during machining.
Anderson Pattern makes its own pole extension rails by machining steel stock flat and square, then adding holes to allow them to be bolted to the magnet, as well as dowel positioning holes. Not all parts will receive dedicated dowel pin holes. In some cases, existing holes are reamed to a tight tolerance to accept pins.
Anderson Pattern doesn't use magnets for some finishing operations in which the cutting forces are directed downward. In these cases, steel rails with locating holes for dowel pins will be used to quickly position the blocks in the machine. The blocks are then secured to the machine's table with standard strap clamps.
Magnetic workholding chucks may not be necessary for finishing operations that do not exert a significant amount of axial cutting forces. This block is secured to rails with standard strap clamps. The dowel pin holes that were used for roughing this block on a magnet are also used to position the block on the rails for finish machining.
Magnets are also used on tombstones used in a number of the shop's HMCs. Billets are first secured to the magnet and probed to determine their position. What would be the bottom surface of the block is then skimmed flat, and locating dowel pin holes are drilled. The part is then flipped over and located on the dowel pins holes for initial roughing operations.
Anderson Pattern plans to implement magnets on most of its machines. It has even developed an idea for a four-sided tombstone that would have magnets on the two wide faces and standard T-slots on other two faces for parts that require standard clamping methods.
Mr. Nielsen points out one other magnetic workholding benefit that is a key part of lean manufacturing—standardization. "Workers had their own way of clamping and setting up a part to be machined," says Mr. Nielsen. "Even if the parts were the same, each person would do it a little differently." The magnets deliver the same clamping force regardless of who energizes the magnet.
Tips For Magnetically Holding Small Parts
The ideal part candidate for magnetic workholding is one that offers a large, flat surface area that the magnet can grip. However, there are ways to safely hold small parts and oddly shaped castings on a magnet, even on a tombstone. Dr. Powell offers a few tricks. 
Hang on. Small parts and castings can be secured on a tombstone for horizontal machining. The photo at the right shows a casting that has had initial operations to machine one face and finish some precision holes. For subsequent operations, it is installed on a magnetic chuck that is mounted on a tombstone. Pole extensions attached to the magnet have both locating pins and longer hanger pins. The castings can be slid over the hangers, located on the pins and then firmly secured once the magnet is energized.
Think sideways. Most folks envision magnetic workholding only in a vertical sense. That is, they only think in terms of a part being pulled toward the magnet's face. Consider a part shaped similarly to a section of box tubing. A part with such a thin perimeter would lack the surface area for a magnet to pull, and its narrow walls would not allow the use of dowel pins. That's when you turn the magnetism on its edge. This is done by driving magnetism through pole extensions and into the part's side and end walls. The side grip will pull the part into the corner created by the two blocks, preventing the part from twisting when being machined, while the downward pull (though limited) prevents the part from being lifted off of the magnet.
Maintain balance. Stray magnetism can be an issue when holding small parts on a magnet, as it can retain chips in the cutting zone. To prevent the outside of the part from being magnetic, the part should first be located on the magnet so that equal amounts of surface area contact the magnet's north and south poles. The contact area between part and pole extension should be adjusted so that the amount of flux traveling from pole to pole is sufficient to hold the part, but won't break through the part surface. Also, small parts should be used on magnets with narrow poles; larger parts should use magnets with wide poles.
Secure sheet. Imagine clamping thin sheet that requires a chamfered perimeter and chamfered opening in its center. Traditional mechanical clamping methods would have a tough time supporting the sheet near the cutting edge in the middle of the part, and result in significant vibration problems. In addition, re-clamping would be necessary to allow access to the entire perimeter. On the other hand, magnetic pole extensions could be located close to the cutting edge to support the sheet, prevent vibration and allow complete machining in one setup.
Self-shim. For parts that may be warped, bowed or deformed, self-shimming magnetic pole extensions are useful. These pole extension cubes are cut at a 45 degree angle and reassembled so that the mating faces can slide on each other. When installed under a bowed workpiece, the top half of the pole extension will slide up and attach itself to the bottom of the workpiece. Once the magnet is energized, the part will be secured.
Hold aluminum. Non-ferrous parts can be held by a magnetic milling chuck by first installing the part in a vice and then securing the vice to the magnet. The downside is that because the part is installed in the vise, machining access will be limited to only three sides of the part. (end)
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