Ver el sitio en Inglés
Buscar el siteo:
 

News and Events: Technical Articles and case studies



Boring in the Fast Lane
Publish Date - May 1998

Source: Manufacturing Engineering

By: Jack Burley


 

Boring in the fast lane


KPT Kaiser's Ser. 310 in action, boring at 4000 sfm (approx. 3800 rpm).

Though boring tools represent a relatively small share of all tools involved in most machining operations, their impact on overall productivity can be very high. Precision boring is one of the final operations on a workpiece that has acquired value each step of the way through the machining process. Scrapping a part at the finish-boring stage is a heavy loss. For years manufacturers ran boring operations at slow speeds to minimize the chance of boring oversize or out-of-round holes. Because speeds were moderate, users did not think they could justify the cost of buying balanced boring tools.

As the number of machines and applications going to high-speed operations to increase productivity soared recently, so has pressure to improve the balance of boring tools and to add boring heads capable of high-speed operation. The enhanced performance capabilities of today's machining centers, combined with a new generation of balanceable and automatic balancing boring heads, now make it practical to achieve cutting speeds unheard of a few years ago.

KPT Kaiser's automatic-balance boring heads run at cutting speeds as fast as 18,000 rpm for small diameters (1.260­1.654´´ [32­42 mm]) and up to 7000 rpm for larger diameters (3.346­4.134´´ [85­105 mm]). A four, five, or six-fold increase in boring speeds drops machining time significantly, but the improvements in surface finish, tool life, spindle bearing life, and bore quality, along with the reduction in cutting forces, may be even more important. These new tools make high-speed finish boring one of the hottest technologies in manufacturing.

How Fast Is Fast?


This automatic-balance boring head, KPT Kaiser's Ser. 310, precision-bores an aluminum air compressor cylinder housing with a cast iron sleeve using silicon nitride inserts.

High speed for boring tools may not mean high-rpm spindle speed. Generally, we consider high-speed finish boring to be running at faster than 1500 sfm (457 m/min) or above 8000 rpm. An extremely wide work range of diameters and workpiece materials gives a range of rpm values. For instance, a manufacturer of small aluminum connecting rods with a wrist pin bore of 0.625´´ (16 mm) may want to run at 2000 sfm (610 m/min). Calculating rpm as surface feet × 3.82 divided by diameter, we get 12,224, which falls well within the "high-speed" definition for spindle speed.

A manufacturer of large cast iron engine blocks with a piston bore of 4´´ (102-mm) diam who wants to run at 2000 sfm, however, will find the application running at 1910 rpm. To run safely at this speed, the tool must be constructed so that very high centrifugal forces cannot cause moving parts to shift or come apart. The tool's locking system must secure the insert, the insert holder, and the cartridge inside without affecting adjusting precision and ease of use.

Nonferrous workpiece materials like aluminum and magnesium are most commonly associated with high-speed boring. Surface speed has no limits for the insert of choice, polycrystalline diamond (PCD). Cubic boron nitride (CBN) or silicon nitride inserts bore certain types of cast iron at high speed. In addition, you can bore steel and stainless with cermet grades reliably at speeds up to 2000 sfm. Even with the right insert, however, balancing is the key to success, and tool imbalance is the challenge.

Imbalance Is the Enemy

Tool imbalance degrades boring quality. When an unbalanced tool rotates at high speed, it creates high, nonsymmetrical centrifugal forces which produce vibration. The higher the imbalance and speed, the higher the frequency of the vibrations. They cause the tool to chatter, which reduces surface finish quality and tool life. Also, an unbalanced boring tool used with a loose spindle creates the condition known as "out-of-round" bores.

You can calculate the centrifugal forces of an unbalanced tool from the imbalance amount and the speed of the tool. For example, a finish boring tool with 400-g*mm imbalance running at 2000 rpm will produce a force of only 17.5 N (3.9 lb-force). The same tool with the same amount of imbalance running at 18,000 rpm will generate 1421 N (317 lb-force), dramatically affecting bore quality and performance. The amount of imbalance is constant and can be corrected. However the nonsymmetrical centrifugal force that results increases exponentially to the square of the speed.

The single-cutter finish boring tool has the largest potential for imbalance of any tool used in a modern machining center. Its unbalanced mass (the tool carrier) projects away from the body of the tool in only one direction. Most single-cutter boring tools perform finishing operations. To be cost-effective, their range of travel must serve the requirements of many different bores. As the tool carrier extends further from the centerline to bore larger diameter holes, imbalance increases. Correcting for imbalance on single-cutter tools is difficult because the imbalance is large and ever-changing.

Defining Imbalances


This small-diameter boring head with a manual balance djustment ring uses a carbide insert to bore 0.5´´-diam holes in aluminum at a speed of 14,000 rpm (1830 sfm) and feed of 0.003 ipr (42 ipm).

The three common types of imbalance in boring are static, couple, and dynamic. You can overcome or alleviate all these imbalances with the right strategy.

Static Imbalance. Static imbalance occurs in short boring tools with a length:diameter ratio of less than 3:1. It's the most common imbalance we encounter in boring tools and has the most significant effect on tool performance. You can correct for static imbalance in two ways: you can add weight 180º opposite the imbalance area, or you can remove weight at 0º in line to the imbalance mass. You must use these compensating methods at a linear position at or near the source of the imbalance. Single-plane balancing will measure and correct for static imbalance.

Couple Imbalance. If the compensating offset is some distance from the source point, the compensation will occasionally cause another type of imbalance, known as a couple. When equal imbalance amounts in two planes are exactly 180º out of phase and separated from each other, a couple can cause a wobble in the boring tool, but its effect on operations is often negligible. To avoid creating couples, we recommend correcting static imbalance as close as physically possible to the tool's source of imbalance.

Dynamic Imbalance. In longer tools, when the principal axis of inertia does not intersect the shaft axis, and length:diameter ratios exceed 4:1, dynamic imbalance typically occurs. Most tool assemblies with length:diameter ratios greater than 4:1 require dynamic or two-plane balancing. The cause may be a combination of couple imbalance plus static imbalance, where the static imbalance is out of phase with both of the couple imbalance masses.

Fighting Imbalances

Eccentricity. The eccentricity (e) or shift of the tool axis up, down, or sideways from the centerline position will cause significant imbalance. To avoid centerline displacement of the tool's taper to the spindle taper, toolmakers must manufacture the tapers of shanks for high-speed operation to a tolerance of AT3 or better, must precision-grind outside diameters, and must ensure that internal coolant holes do not deviate from centerline.

ANSI Requirements. The specifications of the ANSI B5.50 (CAT taper) make the drive keys intentionally offset to each other. Though intended to prevent operators from inserting an incorrectly oriented single-point boring tool into the machine, this offset requirement creates an imbalance that can cause problems at high speed (CAT50 produces an imbalance of almost 200 g*mm). To compensate, a pocket in the "high" side drive key of equal mass to the imbalance mass is milled in all CAT taper standard tools during soft machining.


Boring bars in Kaiser's modular boring tool system can be assembled in varying lengths and diameters. An integrated counterbalance mechanism allows precise alance of the tool for any component and bore diameter

Modular tooling connections may increase imbalance because of the hex in the connection screw and the lost mass in the socket of the male connection. Drilling an offset hole 180º to the screw may compensate this imbalance, the way an imbalance in an end mill holder is corrected.

Imbalance in single-cutter boring tools can be quantified down to a few mg of weight because the balance parameters are calculated from known constants within the same boring system. The amount of imbalance is a factor of the mass of imbalance in grams multiplied by the radius in millimeters. You can therefore determine the specific compensation needed for each boring head at any bore diameter and correct it automatically.

Small-diameter single-cutter boring tools pose special balancing problems because the boring bar is offset from centerline and can be either carbide or steel. Because boring bars come in a variety of diameters and lengths, the balancing procedure is even more complicated. The higher density and weight of carbide bars compared to steel bars demands significantly different compensations. To balance the boring head and its boring bar, which is set for bore diameter, accuracy demands manual balancing based on balance data tables.

To offer a completely balanced tool in conjunction with the new balanceable and automatic balancing heads, we prebalance shanks, extensions, and reductions by correcting the imbalances created by the design of the CAT standard taper and the modular connection. These prebalanced modular components also improve operations when using twin-cutter roughing heads.

Balance Quality


Of all tools in a machining center, the single-cutter boring tool has the greatest imbalance because the tool carrier projects away from the tool body in only one direction. The further it extends from centerline to bore larger diameter holes, the more the imbalance. The EWB heads precision-balance themselves automatically throughout the work range of the boring head. Imbalance for each head does not increase when the diameter setting changes.

Modern cutting materials such as polycrystalline diamond (PCD) and cubic boron nitride (CBN) perform best at high speeds. Balanceable tooling gives the optimum surface footage required for peak production and high finish quality. The International Standards Organization (ISO) defines application criteria in terms of a balance quality grade, or G. Although the goal is to balance to the closest quality possible, it can be very difficult to obtain optimal G-quality balance with adjustable tools that use indexable inserts.

Some imbalance is always tolerable because no tool is truly "perfectly balanced," just as no part is really dimensionally perfect. Application, weight, and rotational speed define the allowable imbalance of any rotor, spindle, or toolholder. The grade of insert chosen for a balanced tool with replaceable inserts can greatly alter balance quality.

Carbide or cermet boring inserts, identical in size and shape, will fit into the tool and can do the same work. The difference is weight. For example, TCMT110208 carbide inserts weigh 1.2 g, compared to 0.6 g for the same cermet insert. If we calculate the imbalance for each of these inserts at a diameter of 100 mm, we get 60 g*mm for carbide and 30 g*mm for cermet, a difference of 30 g*mm. We developed automatic-balance boring heads to be used with carbide inserts because they are the most common in high-speed boring (PCD and CBN-tipped inserts are fabricated onto carbide inserts).

What seems like a minor change of inserts can impact balance quality significantly when running adjustable tools with replaceable inserts. Manufacturers of dedicated high-speed boring tools must completely assemble them with all components adjusted for precise bore diameter, then use a balancing machine to balance them precisely.

Balanced heads are the best predictors of optimum performance. Balanced boring tools increase not only cutting speed but also penetration rate (feed). Tools last longer and chatter and vibration are eliminated, so that users see freer cutting, better surface finish, improved bore quality, and extended spindle bearing life.