Cut a hole in a part; cut a part from the hole... that's what one job shop operator is doing using a novel hole cutter that gives him more slugs than chips.
The plant operates 24 hr/day, 6 days a week. The NC department is an off-shoot of the main operation in town which specializes in parts with rolled threads that can be produced on six-spindle automatic screw machines.
The NC department consists of three CNC chuckers (6", 8", and 12" Nakamura's) and a small CNC Bridgeport mill (Series One). In addition, it contains a fairly typical complement of support equipment and a larger than usual amount of modern inspection equipment--Roger is very much a believer in statistical process control and uses it to good advantage in keeping his product quality at a high level.
And, like any serious businessman, he believes in increasing productivity and reducing costs whenever possible. This is what led him to investigate the production capabilities of the Hougen Rotabroach.sup.TM., an annular (trepanning) cutter that can replace twist drills and similar tools in a variety of hole making operations, Figure 1.
The solid center
The Rotabroach offers faster cutting, lower horsepower and thrust requirements, and other benefits. The tool's two most important features as far as M K Chambers is concerned are an increase in productivity and the solid center slug that can be used to produce smaller parts in the line, or recycled at a scrap value much higher than that for ordinary chips.
For example, Roger is currently using two different sizes of cutters. Within the production family of 30 parts being manufactured, there are eight opportunities to use the slug from one holemaking operation as raw material for an additional, smaller part. Formerly, using a twist or NC drill, this material was converted to chips with very little value, at a considerable cost in power and time.
The original bar sections, pierced blanks, slugs, and finished part samples, associated with using the two cutters are shown in Figure 2. Both blanks are 2.170" long. The larger blank is 2.875" OD. The machined hole is 2.362" and results in a slug diameter of 1.841". The smaller blank has a 2.375" OD. Cutting the 1.820" dia center hole produces a slug with a 1.430" dia.
On the larger blank, the previous holemaking operation was performed with a twist drill at 800 rpm. The 0.010 ipr feed rate was equivalent to 8 ipm and 495 sfm. This resulted in a metal removal rate (chips) of 35 cu in/min.
The Rotabroach hole cutting operation is now performed at 350 rpm with a feed rate of 0.0378 ipr. This is equivalent to 13.23 ipm and 216 sfm. The effective metal removal rate for this operation is 58 cu in/min (chips and slug); a 66 percent increase over drilling.
At 8 ipm, drilling required about 16 sec compared to roughly 10 sec for the trepanning cutter. However, the present operation does not necessarily represent a time saving of the full 6 sec because the slug must be manually removed from the cutter before indexing the turret. Roger is currently working on ways to get around this.
When a surplus of slugs is created, Roger reports they are worth more as scrap than chips. He recently sold a quantity of slugs at $0.31/lb compared to $0.20/lb for chips.
About the tool
The trepanning tools were originally developed for, and have been used extensively in, the stell fabrication and erection industries, and other applications using magnetic drill units. Their use in production and machine-tool applications has grown considerably during the last three years.
The fact that the tool does not cut the center (or slug) area means that the machine tool does not have to supply the power to convert this material into chips. This saved power can be used to increase hole size, feed rate, or both. Figure 4 shows the percentage of power required to drive typical Rotabroach cutters in comparison to twist drills.
The center area of a hole also represents the zone of least efficient cutting. This is because the surface speed of a drill is zero at its center. As a result, the center is not actually cut by a drill, but deformed outward to a point where it can be cut. The force required to displace this metal must be supplied by the machine as thrust on the drill. Fixturing must also be able to withstand the thrust.
Since a Rotabroach is not cutting this area, required thrust is dramatically lower. Also, because the teeth do not extend to the tool's center, a greater number of teeth can be used allowing increased feed rates. In addition, the tooth is able to cut at or near its optimum speed.
Figure 5 shows a comparison of the thrust required for various size holes at a particular feed rate. Thrust is more difficult to predict than horsepower because it is affected differently by speed and number of teeth. However, it is evident from the graph that the required thrust is much lower than for a drill. In fact, it is seldom a factor in selecting equipment.
The uncut center area of a hole provides other benefits, and some limitations. The first limitation is that the remaining slug must be disjoined from the part. This usually means that the cutter is limited to throughhole operations, although intersecting holes or grooves can also free the slug.
In addition, the slug, once separated, must be handled. This poses some problems for systems and people accustomed to handling only chips. Cutting action allows production of partial holes, overlapped holes, and holes on inclined surfaces, which are difficult to do with drills because the angular point tends to wander.
Every cutter has an axial hole through the shank to allow cutting fluid to flow through the center of the tool. Adequate flow is important to cool and lubricate the cutting edge, help evacuate chips, keep the slug from expanding, and help eject the slug.
A coolant system capable of delivering at least 5 gpm at a minimum pressure of 25 to 50 psi is required. Many newer machines are available with through-the-spindle, high-pressure coolant supplies. For other machines, spindle adapters with coolant inducers can be obtained from several sources.
Recommended feed rates are generally in the range of 0.001" to 0.005"/tooth. The major factors affecting feed rate are desired hole finish, horsepower available, and quality of the set-up (including rigidity and coolant flow). Lower feed rates usually yield better hole finishes and require less power. When power, set-up conditions, and hole finish are within acceptable limits, heavier feeds usually will provide better overall results in terms of both productivity and tool life. Also, at lower feed rates, on holes deeper than 2", the thinner chip sometimes has more of a problem pushing its way up along the flutes and out of the hole. When this occurs, a heavier feed usually will solve the problem and help prevent tool chatter.
Use of Rotabroach cutters in production is relatively new and operating parameters, while not complex, must be controlled for best results. For instance, spindle and workpiece rigidity are more critical than with a twist drill. Ample coolant flow through the tool to the cutting zone is important. Choosing the proper tool and selecting proper cutting speeds, feeds, and power requirements are also important.
Since an annular cutter requires considerably less power than a drill, the main factors that affect spindle horsepower are tool condition, machinability of the work material, efficiency of the machine tool, and rate of metal removal.
These factors, except for metal-removal rate, are the same for all types of hole cutters. Because the Rotabroach creates the hole by cutting only a groove, the actual removal rate is a small percentage of the effective metal-removal rate.
Charts in Figure 6 show both removal rates for the heavy duty model at various feed rates. Actual chip metal removal rate is used for power calculations. For drills, effective metal removal rate is also the actual rate for power calculations.
For rough calculations using mild steel, the old rule of thumb of 1 hp/cu in/min may be accurate enough. Using this approximation, the chart may be read directly as horsepower. For most applications, however, a machining handbook should be referenced for information regarding the unit horsepower factors for various materials, machine efficiencies, and appropriate formulas.
In determining the horsepower available from a particular machine, the efficiency of the spindle drive and the method of speed control must both be taken into account. Many newer machines use variable speed AC or DC drives. While variable speed controls provide infinite speed selection within their range, they often do not deliver full power at lower speeds. Several typical 20-hp vertical machining centers, for example, are rated at full power only down to several hundred rpm in low range. Although power/speed information usually is included in the machine's operating manual, the power reduction at lower speeds is frequently overlooked.
Suggested speeds for cutting various materials with the HSS Rotabroach are listed in Table I. It usually is best to start at the slower speed, then increase the speed if experience indicates this is practical. The most economical speed for the cutter depends on a number of variables, the more important of which are: composition and hardness of workpiece; hole depth; condition of machine tool; desired finish; and cutting fluid effectiveness.
Certain exotic alloys have a work hardening tendency if proper cutting procedures are not followed, i.e., uninterrupted uniform feed pressure, ample lubrication, and proper sfm. Many of these alloys cannot be drilled, but can be cut with a trepanning tool.
The patented "Hougen-Edge" geometry is generated by alternately lowering the inner end surface or outer end surface on successive teeth, Figure 7. This provides a cutting action, in which, at a given feed, each tooth is taking a thicker chip, but only on a portion of the inner and outer cutting edge. Neither edge cuts a chip as wide as the flute depth.
Stock sizes of the tool can produce through holes up to 3-1/16" dia in materials up to 6" thick. Larger diameters and greater depths are available as specials. For more information, circle E39.
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|Author:||Dobbins, Donald B.|
|Publication:||Tooling & Production|
|Date:||Mar 1, 1985|
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