Making the cut

Choosing the proper blade for the application, optimizing its use, maintaining the saw, and using a cutting coolant all pay productivity dividends.

Choosing and Using a Blade

Sawing pipe and tube stock is fundamentally more difficult and more complicated than cutting barstock or billets. As saw teeth penetrate tube walls and the hollow inside diameter, the shock of the interrupted cuts tends to deform or break the blade's cutting edges.

The vibration or chatter of the interrupted tube and pipe cuts shortens blade life, disturbs workholding fixtures, and spoils the accuracy of bundled workpieces. The abrasive, work-hardened chips that collect inside a pipe may reduce blade life 20 percent and increase downtime with frequent blade changes.

Binding or pinching blades between tube cuts can jam the machine and damage saw blade edges. Preventing these conditions, which is essential to productive, profitable cutting, is an enlightened approach to using band saw blades.

Cost per Cut, Not Cost per Blade. The truly accurate measure of band sawing cost-effectiveness is more than the cost of the saw blade spread over the number of cut parts. Blade manufacturers calculate overall cost per cut by factoring labor, sawing machine burden, and blade costs. The result amortizes the total cost of the job over the number of acceptable finished pieces.

Machine costs include the total value of machine time based in part on local labor costs. For instance, when sawing pipe or tube, bundle-cutting cost calculations should allow for setup time. Is it more cost-effective to cut a bed of four tubes side by side or a bundle four pieces across and four high? The first job might take 10 minutes to load, while the bundle takes half an hour and requires nesting fixtures.

Blade costs spread purchase price over the number of pieces cut, and they factor in the cost of unproductive downtime for blade changes. Compared with annealed barstock, hot-rolled steel pipe typically reduces cutting speeds 20 percent and reduces blade life 15 to 20 percent.

Carbide or Bimetal?While carbide blades cost more than bimetal blades, they can open expensive production bottlenecks with faster cuts and up to five times the cutting life. They can also cut difficult materials without exotic equipment. Carbide blades, for example, cut heat-treated alloys harder than Rockwell C 45. The best blade for tube or pipe cutoff is one that minimizes cost per cut and maximizes productivity.

Blade Construction, pitch, tooth geometry, and set. Choosing the right saw blade construction, tooth geometry, pitch, and set is nevertheless more complicated than blade selection for solid stock. Even with the same construction, not all blades are created equal. Ground-tooth blades are more uniform than milled blades. They facilitate chip formation to accelerate metal removal and resist wear better to last longer at higher cutting speeds.

Selecting the optimal blade-for example, when cutting very hard materials, choosing a blade with milled teeth rather than one with ground teeth-can increase cutting speed by approximately 60 percent.

Ground-tooth blades also can cut faster than milled-tooth blades. For example, changing from a milled blade to a ground-tooth blade for cutting 4.5-inch AISI 52/100 steel pipe with 0.5-in. walls increases cutting speed from 7 to 11 square in. per minute. It also provides about 40 percent better cutting life.

• The choice of band saw blade pitch generally is based on workpiece size. Yet unlike solid workpieces of constant width, a 20-in.-diameter tube with a 0.5-in.-thick wall has a length of cut that varies dramatically from the beginning to the middle of the workpiece.
  
  A rule of thumb calls for a pitch that keeps at least three teeth in contact with the workpiece at all times to dissipate chip loading forces and prevent tooth breakage. Thick-walled pipe greater than 2 in. in diameter calls for relatively coarse pitch blades with few teeth per inch.
  
  By comparison, thin-walled tubes or pipes require fine-pitch blades with more teeth per inch to prevent tooth breakage and wear. Bundled tubes or pipes call for variable pitch blades that nullify vibration in stacked workpieces.
  
  
• Tooth geometry, including rake angle, clearance angle, and gullet depth, is also optimized for different applications. Thin-walled tubes are best cut by regular tooth profiles with 0 to 6 degrees' rake. Positive rake teeth from 5 to 15 degrees create a cleaving action more suitable for sawing thick-walled pipe and solid shapes.
  
  
• Tooth set-the left and right displacement from the steel backing-creates clearance for saw blades to travel through their cuts unrestricted. It can promote chip evacuation and coolant flow at the cutting point. Single pipes are cut effectively by blades with a regular or neutral set. In contrast, bundles of pipe are best cut by blades with a heavy or wide set that creates a wider kerf and minimizes pinching.

Optimizing Blade Use

Getting the highest performance from a blade requires a combination of proper machine settings and monitoring the blade, chips, and cut quality.

Full Speed and Feed. To maximize productivity and optimize blade life, band saw operators should exploit the full machine feeds and speeds recommended by the blade's manufacturer. Running a blade slower or at low feed pressure to reduce wear is false economy. In today's high-productivity shops, a good rule of thumb calls for blades to last about eight hours. If blades last longer than a work shift, you can probably increase your cutting speed to decrease your cost per cut. The key to productivity remains throughput, not blade cost.

Proprietary software is available to help band saw operators choose the right blade and determine machining parameters for maximum productivity when cutting pipe and tube. The most advanced programs prompt users to enter material properties, workpiece dimensions, and machine characteristics. More than simple catalog tables, today's sophisticated algorithms select the best blade and calculate cutting data to optimize productivity.

When software aids are not available, band saw operators sawing tube and pipe can estimate cutting data from catalog charts. However, chart data is based on cold-finished, fully annealed solid barstock. Generally, optimum cutting rates for tube or pipe stock are less than those quoted for solid shapes.

Blade manufacturers' technical representatives can help define a window for efficient tube and pipe cutting. Using that window, band saw operators need to look, listen, and exercise their judgment to maximize productivity.

Look and Listen. For an experienced operator, the appearances of the band saw blade, cut chips, and finished workpieces tell tales about machining data and machine condition.

Blade teeth broken or stripped after a cut often are signs of excess feed or inadequate coolant. Scratches or scoring on the blade backing may warn of impending fatigue failure and call for blade guide replacement or other machine maintenance.

Chips that resemble the numeral 6 or 9 (commonly called sixes and nines) are evidence of a properly chosen blade cutting productively. Chips or cut pieces that turn blue with heat discoloration, or smoke rising from the workpiece, tell band saw operators to slow down. Just as irregular chips can warn of blade wear, a crooked cut can be a clear call for blade replacement.

With the band saw blade properly tensioned and the feeds and speeds properly adjusted, the sawing machine should cut without vibration or chatter. Harmonic sounds like a guitar strum can signal machine troubles. They can warn that blade tension is too low, or the distance between blade guides is too wide.

Harmonics also can indicate problems with nesting fixtures that bundle tube or pipe. They also can warn of incorrect blade pitch or cutting data, or a coolant mixture too lean for the cutting job at hand.

More than cutting solids, sawing tube and pipe requires careful attention to band saw machine settings and maintenance.

Maintain the Machine

Blade tension is one essential part of technically optimum cutting and generally should stay constant throughout the cut. It is determined both by the band saw machine and the selected blade. Blade specialists typically specify 30,000 pounds per sq. in. of tension on machines using bimetal blades 1 1/4 in. wide or wider. Manufacturers usually specify the right tension for narrower blades. While higher-than-usual tension may be appropriate for some hard-to-cut workpieces, it can reduce blade life. Inadequate blade tension can result in a crooked cut. Like all band saw parameters, the right blade tension is the one that provides the best combination of throughput, cut quality, and blade life.

An effective machine inspection examines idler and drive wheels, guides, blade tracking, chip brushes, coolant, clamps, and feeds and drives. In addition, rigid, stable setups are especially important to prevent chatter when sawing bundled tube and pipe.

Cool Cuts

High-quality coolants are essential to reduce heat, extend blade life, and prevent thermal distortion in most band saw jobs. They are particularly important when cutting tube and pipe, because sawing heat can warp thin tube walls and abrasive swarf can accelerate blade wear.

While straight or undiluted oils may be necessary for some difficult-to-cut materials, fluid manufacturers generally recommend dilutions to optimize performance.

Equally important, putting cutting fluid into the cut is necessary to maximize lubrication and heat dissipation. Splashing fluid all over the saw does little for the workpiece. The ideal coolant solution depends on the sawing job and, like other process parameters, may require expert assistance.

Håkan Hellbergh is R&D Center manager and Dallas Stuck is business development manager for Bahco North America Inc., Box 2036, Scranton, PA 18501-2036, 800-828-9893, ext. 244, fax 570-877-5687, hakan.hellbergh@bahco.comor dallas.stuck@bahco.com, www.Bahco.com. Bahco North America Inc., a wholly owned subsidiary of SNAP-ON Inc., was formerly known as Sandvik Saws and Tools Company. It manufactures hand tools and saw blades.