Use a Saw to Upgrade Your Cutoff

Rotary-saw cutting (RSC) is a cutoff method that replaces single-point cutoff in all turning applications. It is especially useful in high-volume applications, and most RSC units are used on screw machines and other types of automatics. In a limited number of applications, RSC can even be used for grooving and forming.

Shops in the United States have been cutting off stock with RSC since the 1960s. The method has steadily gained in popularity due to advances in technology that now allow it to be used in almost all cutoff applications. RSC can cut nearly any material. On steels, it can be used to cut anything from 12L14 to bearing and aeronautic steels. It can also cut aluminum, brass, plastic, and rubber. Irregular shapes, such as hex, square, and pinion stock; interrupted cuts; and thin-walled tubing are also easily cut with RSC (Figure 1). RSC's broad versatility allows it to be used in a wide range of high-volume manufacturing situations. RSC units are helping to produce parts for such industries as aerospace, hardware, computer, plumbing, automotive, and toy manufacturing.

To maximize profitability on turned parts, shops should consider RSC first for cutting off because of its many advantages over single-point tooling. When RSC units are set up, used, and maintained properly, they offer faster cutting times, closer tolerances, better finishes, less kerf, easier tool changes, better tool life, and a wider range of applications. Customarily during cutoff, either the workpiece rotates and the tool is stationary as in single-point cutoff operations or the tool rotates and the workpiece is stationary as in cold sawing. During RSC, these actions are combined by rotating the workpiece and the tool simultaneously. Using a rotating saw in turning applications dramatically reduces chipload, because the saw is always cutting a convex shape. This means that only one tooth is cutting at a time, and it is removing only a small amount of material. Low chipload is the key to RSC's success and is responsible for its benefits.

RSC Setup 
RSC is performed with a self-contained motorized attachment that drives a circular saw. This setup mounts in the standard cutoff position of the host machine, where conventional cutoff tooling typically would be installed. The RSC attachment is wired directly into the host machine so that the rotation of the saw is switched on and off with the main spindle of the host. The manufacturers of RSC units design the units and their mounts to fit on specific machines. Some manufacturers furnish turnkey packages, which include the installation of the unit and all the necessary adjustments. Common host machines include Acme-Gridley, New Britain, Conomatic, Davenport, Wickman, Gildemeister, Warner & Swasey, Brown & Sharpe, Traub, and Index screw machines and Bardons & Oliver and Modern cutoff machines. The RSC attachment can also be adapted to fit other turning machines and custom machines.

The saw must be positioned perpendicularly to the workpiece as with single-point tooling. The centerline of the saw and the centerline of the workpiece should be the same. In other words, the axes of both should be at the same height above the cross slide. However, cutting on center is not as critical with RSC as it is with single-point tooling. The saw's perpendicularity to the workpiece can be adjusted with a stepped aligning key. Screws set the longitudinal adjustment of the unit.

Shops will achieve the most desirable RSC dynamics by cutting off with an opposed cut, in which the saw and the workpiece are rotating in opposite directions at the point of intersection. A climb cut, in which the blade and workpiece are rotating in the same direction, is recommended only when cutting high-carbon materials with HSS saws or if the parts cannot be ejected easily from an opposed cut.

For the best results, shops must use the proper coolant and carefully control coolant flow. Because there is very little heat buildup during RSC, the process requires coolant more for its lubricating qualities than for its cooling qualities. The operator must make sure there is a strong coolant flow directed to the point where the saw enters the material. The coolant flow must be distributed equally to both sides of the saw to promote even saw wear and to easily flush chips from the workpiece as the saw rotates through the work.

When using HSS saws to cut ferrous metals, it is necessary to use an oil coolant with a heavy concentration of active sulfur. This will effectively prevent galling at cutting temperatures under 350° F. Water-soluble coolant is appropriate for use with carbide saws or when cutting aluminum or brass. When using RSC, operators can use feed rates per revolution of the work that are 1.5 to four times the rate they might use with a single-point tool. This makes it possible to reduce cutoff times and increase the number of pieces cut per hour. The low cutting pressure RSC produces allows the saw to cut completely through the piece for a clean, smooth cut, eliminating the cutoff nib almost completely and significantly reducing burr. The result is a better quality part and the reduction or elimination of the need for secondary operations. Furthermore, tool and workpiece deflection are negligible with RSC. This allows operators to hold close tolerances on length, flatness, squareness, and other dimensions. In addition, they can repeat these tolerances within tenths and maintain part dimensions over many hours of cutting.

This precision is possible even when the feed rate is several times what would be used with single-point tooling, and it typically can be achieved without supporting the part unless the part is long or heavy. With RSC, deep cuts can be accomplished without a breakdown tool and without sacrificing cycle time. As a result, the shop has one less tool to maintain, and it gains an extra tool station on its machine. In addition, RSC greatly increases the range of applications that can be gang cut. The negligible saw and part deflection minimize the problems usually encountered when attempting to gang cut with single-point cutoff tools. The saws themselves act as guides to maintain the parts' perpendicularity with the cut, and thus, the cut parts are easily ejected.

Changing saws is easy and takes only a few minutes. The operator needs to make sure the new saw is securely clamped and is running in the proper direction and that all arbor and support spacers are in good condition. Once the blade is mounted, the operator should check its trueness with an indicator while the blade is rotated. A blade that is out of true may need replacement, or it may indicate the need to clear dirt or nicks from the clamping components. Height adjustments are rarely necessary, because the height setting of the blade repeats every time. During saw changes, the operator occasionally may need to adjust the blade's location setting but only to maintain the tightest length tolerances. Location adjustments are made by modifying the spacer arrangement or moving the attachment.

Saw Selection
To gain the full benefits of RSC, a shop must be careful to select the appropriate saw blade for each application. When the proper blade is used, saw life is five to 10 times better per grind than single-point tool life. Because of the light chipload, the interrupted nature of the saw cut (each tooth cuts for only a brief moment at a time), and the side clearance on saws, a rotary saw cut builds up very little heat. The heat stays in the chip, which is immediately carried away from the workpiece and saw by a strong coolant flow. Because of the low cutting temperatures, the RSC process is far less demanding on a saw than the cutoff process on a single-point tool, and superior tool life is the result. Blade manufacturers offer a number of options covering a variety of characteristics, including saw material, saw thickness, saw diameter, number of teeth, tooth angle, and type of surface treatment. Shops must carefully consider these options and decide which combination would be most appropriate for their applications.

Saw Material
Saws are available in solid carbide and HSS. The surface speed of the job will usually determine the blade material to use. If the machine is set for a surface speed that is appropriate for carbide tools, then a carbide blade must be used for the RSC operation. Heat-sensitive materials also require the use of carbide saws. When an operation is running at HSS sfm rates, HSS saws can be used. Although carbide blades cost up to six times what HSS blades cost, shops will find them more economical to use. The carbide blades cut more cleanly, producing better surface finishes and less burr or reduced nib. Cutting with carbide saws subjects the equipment to less wear and tear, and carbide blade life is five to 10 times better than the life of an HSS saw.

Saw Thickness
Generally a shop can perform a cutoff operation with a blade that is one-third as thick as the single-point tool they might use (Figure 2). With RSC, the kerf lost between pieces averages 0.045". Aggressive cutting, deep cuts, and larger diameter materials might require a thickness greater than this rule of thumb suggests, while a narrower saw can be used to cut softer materials, smaller diameters, and thin-walled materials. Stock loss with RSC's narrower kerf is 50% to 75% less than that lost by conventional cutoff methods. As a result, shops can obtain more parts from one bar of stock. The higher yield per bar reduces a shop's stock purchases, stock handling, and machine stockups. Also, because kerf is reduced, shops using RSC have less waste to contend with. More importantly, the process produces scrap that is easier, safer, and less costly to handle. Rather than the long, sharp coils a standard cutoff process produces, RSC produces tiny flakes, because each saw tooth cuts only small portions of the stock (Figure 3).

Saw Diameter
The correct diameter of the saw is determined by the distance to the cut, the depth of the cut, the configuration of the tooling zone, and other attachments. Saws for RSC range from 21/2" to 6" in diameter. The average blade diameter used is 4". In general, smaller blades perform better, so the blade diameter should not be significantly larger than the diameter needed to perform the operation. However, a little extra (about 0.300") on the blade's outer diameter is desirable to allow for regrinds.

Number of Teeth
Shops must carefully consider the number of teeth to select, because this feature will have a significant impact on the blade's performance. A coarse tooth configuration (for example, 90 teeth on a 4"-dia. saw) is better for cutting 3/4"-dia. or larger workpieces. A coarser configuration is also appropriate for workpieces with a hole. Finer configurations (a 4" saw with 130 or more teeth, for example) are best for small diameters, cutting to center, and thin-walled tubing.

Tooth Angle
The last consideration for the geometry of the saw is the cutting angle on the cutting edge of the tooth. Cutting angles reduce burr and nibs and produce a better surface finish. However, shops should use the smallest angle possible that will still produce the desired results, because larger cutting angles could cause the tool to wear faster. As a starting point, a shop using carbide saws might select a saw with a 7° cutting angle. Shops using HSS saws might start with a 10° angle.

Surface Treatment
Many surface treatments are available, including treatments such as kalite sodite, liquid nitride, titanium nitride, and titanium carbonitride. Surface treatments increase tool life and produce better surface finishes. To cut high-carbon steel or 52100 bearing steel stock, shops should use a TiN- or TiCN-coated blade. In general, the treatment that might be used on a single-point tool for a given application will probably be appropriate for a saw blade of the same material.

Troubleshooting RSC
It is imperative for shops purchasing RSC units for the first time to get on-site startup assistance and training, operating specifications, and ongoing technical assistance from their vendors. The vendor also should be willing to provide new operating specifications whenever the application changes. Some machine operators are still unfamiliar with RSC, and they need the guidance a vendor can offer. The vendor can also help users pinpoint the cause when a problem arises. New RSC users often mistakenly assume that a problem is related to the RSC equipment and, therefore, they do not bother troubleshooting the host machine and its environment.

Most common problems can be avoided if the machine operator is properly trained. When problems do arise, the operator should follow all the fundamentals he or she might follow while troubleshooting any other chip-cutting operation.

Problems frequently lead to poor saw-blade performance or the blade's failure. Operators should remember that when saws fail, as when other tools fail, there is always a very specific cause. The least helpful course of action for the operator to take is to continue with a new saw without first identifying and eliminating the cause for failure. A number of problems can cause blades to break or chip before they become dull. Operators with broken or chipped blades should check to make sure the blades are clamped securely and rotating in the right direction. Other items to check include the condition of the arbor and support spacers and the alignment of the saw stroke to the workpiece. They should also ensure that there is not excessive endplay or backlash in the sawing unit, that the saw has a reliable source of power, and that the necessary adjustments have been made if the diameter of the blade being used is not the same as the previous blade's.

In addition to the blade and RSC unit, the operator should thoroughly examine the host machine. The machine should be supplying a continuous flow of properly aimed coolant, and it should allow the part to be cleared away from the saw once it is cut off. Pickoff attachments, when used, should be properly adjusted, and the interface from other tooling should not obstruct the RSC unit. The operator also should make sure the collet and spindle are tight. The slide movement should be checked to make sure it is smooth and that it offers repeatability from high to low in the cutoff stroke. Other problems to look for include power interruptions, bar-loading malfunctions, and incompatibilities between the speed, feed, and the gullet of the saw. The operator should also see if material conditions have changed. Hardness or hard spots in the workpiece may require a different saw or a temporarily modified operation.

However, operators shouldn't wait for a blade failure to look for ways to improve the operation. They should also evaluate blade life and consider changes that might increase blade performance (in terms of the number of cuts per grind they get from the blade). Most of the factors that negatively influence blade performance are the same as those that affect the performance of other chip-cutting tools. These factors include poor alignment of the tool to the cut, inappropriate blade geometry, excessive workpiece speed (sfm), excessive feed per workpiece revolution, poor coolant selection, inadequate coolant maintenance, incorrect coolant application, excessive host-machine vibration, and material deviations.

Of course, the operator will also want to consider those aspects of blade performance that affect the quality of the cut. Compared to the cut made by a single-point cutoff blade, the cut a saw makes is typically of a much higher quality-and it is made with one-third the width while the tool is fed at two times the ipr. Among the improvements a shop will see with RSC are better surface finishes, enhanced flatness, and tighter length tolerances. Also, there will be minimal burr and no nib. For jobs that require even higher quality, tradeoffs may be required from time to time. To optimize the finish, the shop will need to use fine-tooth saws with a cutoff angle. However, these saws require the operator to reduce the feed per revolution of the workpiece. Similarly, when thinner saws are used to maximize material savings, a lighter feed rate may be necessary.

When comparing RSC to standard cutoff methods, users should keep in mind that much time has been and is being spent on improving the performance of single-point tools. Despite this, a shop that invests in a little training and effort is virtually guaranteed superior results with RSC. All companies that are currently using single-point tools for cutoff should consider upgrading their turning machines with RSC. The switch will be especially advantageous to companies in the following situations:

• When a shop wants to significantly reduce its production and material costs on an existing job;
• When a shop is bidding on new jobs; 
• When the application involves cutting irregularly shaped materials, exotic materials, or thin-walled tubing;
• When a shop wants to expand its cutoff capabilities; 
• When there is a need to monitor and adjust for SPC requirements;
• When a shop is currently doing secondary operations; and 
• When a shop needs to lower its job bids to stay competitive.

In any of these situations, the RSC concept, put into practice through the proper use of the sawing equipment, will maximize a shop's cutting results and keep its turning applications profitable.

About the Author
Friedhelm (Fritz) Greulich is president of Watkins Manufacturing Inc., Cincinnati, OH. Watkins is a technical member of the Precision Machined Products Association.