Saw Change

Technological advancements in contour bandsawing - bimetal blades, carbide-tipped blades, new tooth selections, and cutting fluids - can greatly ease a troublesome operation. But many shops have yet to take advantage of these developments.

Until recently, saw suppliers tended to overlook contour cutting. They brought few changes to the contour-cutting process. But those making the cuts continued to encounter new and enduring challenges in contour cutting that demanded new solutions. Now, recognizing the need for blades that last longer and can stand up to more difficult work materials, suppliers are introducing new tools and cutting methods to the industry.

Contour cutting can be defined as making nonsquare cuts through material less than 2" thick on a vertical bandsawing machine using a narrow blade (between 1/16" and 1/2" wide). The lion’s share of contour cutting is performed on workpieces less than 1" thick. Contour cutting is preferable to milling for material removal, because it saves money and material. The large pieces of material or drops that are wasted as chips through milling are saved in the cut and can be used for other jobs.

Most contour cutting involves cutting a pattern to make tools used by die shops, machine shops, toolrooms, and mold shops. Other common practitioners of contour cutting include automotive- and extrusion-die manufacturers, as well as foundries.

Foundries perform contour cutting mainly in piece-work operations, such as the cutting of castings or gates and risers. Because piece workers depend on piece- work quotas to earn bonuses, they are quick to recognize the value of more expensive blades. They inevitably choose bimetal blades (blades with HSS edge material electron-beam welded to a flexible backing material) or carbide-tipped blades over carbon blades (carbon-steel, one-piece blades that are less resistant to heat and wear). Operators doing piece work need to cut fast and don’t want to have to push a blade harder to overcome premature wear. Speed suffers if the blade is not heat- and wear-resistant, so higher-quality bimetal blades are preferred. A good bimetal blade also dramatically reduces downtime caused by the frequent blade changes needed with less fatigue-resistant blades.

Mold- and diemakers prize accuracy over speed. They must cut slowly because the patterns being machined are so detailed. Here, too, a high-quality bimetal blade gets the job done more easily and efficiently.

The Blade

The increasing incidence of contour cutting on abrasive workpiece materials has sparked demand for carbide-tipped blades, which are ideal for sawing nonferrous materials, aluminum, graphites, fiberglass, and other highly abrasive materials. Carbide-tipped blades feature carbide welded to a high-strength alloy backing. They resist wear better than bimetal or carbon blades in nonferrous and some ferrous applications. Also, they have a triple-chip tooth grind that eliminates set collapse, improving the surface finish on the workpiece. Pattern shops are using carbide-tipped blades to cut aluminum and composites, because the blades leave an excellent finish in the cut. This eliminates the need for further finishing work, such as heavy sanding.

Many shops still use carbon blades for contour cutting, even if the shops have switched to bimetal or carbide-tipped blades for other jobs. There are three main reasons why shops have held so tenaciously to carbon blades for contour cutting. First, carbon blades are less expensive than their bimetal and carbon-tipped counterparts. Second, carbon is made in a 1/16" blade width, whereas bimetal and carbide-tipped blades are not made that narrow. Some shops need a narrower blade for certain applications. Third, shops are used to carbon blades and are wary of switching. After all, carbon blades do the job. However, they also wear out more quickly because of their low hardness. In fact, a bimetal blade lasts 10 to 15 times longer than a carbon blade, depending on the material being cut. And when abrasive workpiece materials are involved, carbide-tipped blades outlast any other blade.

Bimetal blades are two-and-a-half times more expensive than carbon blades, while carbide-tipped blades can be up to five times more expensive than carbon. The price turns many away. But the extra cash outlay is more than offset by bimetal’s longer blade life, accuracy of cut, and elimination of secondary finishing work. Carbon-steel blades are preferable only when used in applications that cannot accommodate a bimetal blade.

Edge materials for bimetal blades include matrix, M-42, or M-2 HSS. A bimetal blade with an M-42 edge offers the highest heat resistance of any blade material. It is often the blade material of choice for tough materials such as die steels. Carbon, meanwhile, has the poorest wear factor of blade materials accepted industrywide. With a tooth hardness of Ra 92, carbide-tipped blades last much longer than either bimetal or carbon on abrasive materials.

A Tight Radius

The hardest part of contour cutting is achieving a tight radius. If this challenge proves impossible, it usually means the cutting method is wrong. The operator may either be using a blade that is too wide, or the workpiece may be too thick. Sometimes the problem is the blade material being used. A carbon blade, for example, may be ineffective cutting a difficult-to-machine material. A bimetal blade would be preferable. If the workpiece simply cannot be contour sawed, milling the piece may be the answer.

If an operator is cutting a thick piece of material with a blade that’s too narrow, the blade will deflect, because it lacks sufficient beam strength to meet the material. The amount of deflection experienced varies with different workpiece materials and force being applied to the blade. Tougher materials cause a blade to deflect more than mild materials. When deflection occurs, the operator should choose a wider blade.

If an operator cannot achieve a tight radius, he should choose a narrower blade. However, if the problem stems from dragging - that is, the blade cannot pass easily through the work material or drags through it, making an uneven cut or freezing altogether - there is a way to salvage the blade. There is a little-known trick metalworkers can borrow from woodworking to prevent dragging. While the blade is running, hold a stone or fine file up against the square back edge of the blade to round the corners. By rounding the corners in this way, you will reduce the drag as the blade goes around the turn.

Kerf, Tooth Selection, and Weld

Cutting a tight radius requires a very narrow blade that will provide an adequate kerf or pathway through the material. The kerf is determined by the tooth set. ("Set" refers to the bending of the teeth to the left or the right to allow clearance of the back of the blade.) If the set is insufficient to clear the path, the kerf will be insufficient and the blade will be unable to work itself through. Friction will build up on the blade, causing the blade to freeze as the operator attempts to put a radius on it.

The wider the kerf, the freer the blade will turn. That’s important in contour cutting, which requires the ability to twist and turn the blade. Most blade manufacturers supply a reference chart showing the suggested minimum radius for the width of each blade, from 1/16" to 1/2". Follow the chart recommendations - it is unlikely that you will achieve a desired radius using a wider blade than is recommended for that radius. A bandsawing specialist can recommend the proper tooth set.

The blade’s tooth selection is also significant. Tooth selection refers to the number of teeth measured over 1". For example, a 10/14 variable tooth indicates a pattern of 10 teeth/inch mixed with 14 teeth/inch. The variable-tooth 10/14 is suited to a wide variety of jobs in a machine shop. One of the newest available tooth selections is the 14/18 wavy, which provides a better surface finish than coarser teeth such as 6/10 or 8/12. Fine, variable-tooth blades offer more cutting action, and they combine an acceptable cutting rate with the ability to create a good-size chip without burring thinner materials. Coarser teeth are more aggressive and will pull at the material, sometimes stripping the blade’s teeth.

The enduring question for those doing contour cutting is whether to buy coil stock or prewelded blades. Users who perform pocket cutting will need to make the weld on-site. Otherwise, it’s best to go to the distributor for welding. Here’s why:

First, when a company purchases coil stock, there is often an odd length left over. There is nothing to do with a 10' remnant but to throw it away. The buyer knows he is wasting material at the time of purchase.

Second, the price of the weld is cheap insurance. The distributor is responsible if the weld breaks and will usually provide a replacement blade. A shop doesn’t get a free replacement blade if its own weld doesn’t hold up. The low cost of a weld is worthwhile insurance.

Third, and most important, is the fact that the user can expect a better weld and therefore a longer lasting bandsaw blade if the blade was welded by the manufacturer. A strong weld reduces the possibility of tooth strippage. The blade will also be straighter. A strong weld and a straight blade enable the teeth to work together as designed, preventing unnecessary problems.

Old and New

While contour-bandsawing machines have not changed dramatically in recent years, some companies have refurbished older machines to improve their contour-cutting operations. Some foundries are adding updated guides and fresh tires to old gooseneck-style saws made in the 1950s. With these redressed machines, foundries can employ newer bimetal and carbide-tipped blades to do the cutting. Older machines can’t handle carbide-tipped blades without refurbishment due to their inherent vibration and rough performance.

These refurbished machines are actually better than the fabricated metal machines, because they have a whole casted head. These sturdy machines can be bought at auctions for a low price.

Micronizing

Micronizing lubrication, which dispenses droplets of fluid directly into the cut through a small applicator, dramatically improves contour-cutting efficiency. This fluid provides excellent cutting and sliding lubrication, extending blade life at minimal cost. Pneumatically powered, the unit attaches easily to the machine. Yet contour cutting is often still performed dry, simply because many are unaware of the cost-effective benefits.

The mess and maintenance associated with flood lubricants do not apply to the micronizing applications recommended for contour cutting. And, in response to environmental concerns, new fluids are nontoxic, nonflammable, and water-soluble. Unlike oil-base fluids, newer fluids leave no messy, oily buildup and actually keep the working environment clean.

Micronizing applicators provide excellent control of lubrication to reduce fluid waste while improving cutting and prolonging tool life. Micronizing lubrication in contour cutting is especially cost-effective when cutting cross-sections 1/2" thick or thicker. Here the reason for fluid use is lubricity rather than cooling. (Cooling is important when cutting large cross-sections where heat builds up and wears the blade.) Lubrication promotes effective chip flow when cutting aluminum.

Besides Bandsawing

Other methods besides bandsawing are making small inroads into contour cutting. EDM and laser cutting bring high precision and speed to contour cutting. But these methods have not caught on largely for one reason: cost. Capital investments and operating expenses are very high for both methods. Most shops prefer bandsawing for their contouring work. Another drawback is the extensive setup that EDM and laser require, which demands more time and greater cost. In addition to being a much lower-cost operation overall, bandsawing demands less setup time. This is important to shops doing contour cutting sporadically (as in toolrooms) rather than continuously.

Machinists who machine contoured parts have never forgotten or abandoned contour bandsawing. And now their suppliers are again giving the technology the attention it deserves.

About the Author

Ann Rooke is communications coordinator for American Saw and Manufacturing Co., East Longmeadow, MA.