AUGUST 2001 / VOLUME 53 / NUMBER 8

cover story

❿ BY MIKE RESTALL The Ins and Outs of

ThreadThread MillingMilling Carboloy Inc. Advantages of and operating guidelines for thread mills.

hread is the process of pro- place a number of taps. thread-milled requires no addi- ducing internal or external threads When thread milling, the operator can tional cleanup. Ton a stationary workpiece with a change the pitch diameter of a thread by Plus, the flexibility of modern machine specialized cutter. Holes smaller than making a CNC programming change. allows the thread mill to cut from 1 ⁄4" in diameter can be thread-milled with Thread milling is also useful for appli- either the bottom up or the top down. In a solid cutter. Insert-type thread mills cations in which keyways, side holes or a blind hole, from the bottom up 3 are practical for ⁄4"-dia. and larger holes. other voids interrupt the area being avoids recutting chips and marring the Thread milling requires a machine ca- threaded. The entry and exit burrs are , or damaging the cutter. pable of helical interpolation—the abil- minimal when thread milling, compared Thread mills run at high speeds re- ity to move simultaneously in three to other threading methods. Often, the quire less power than taps to produce . The cutting moves in a circu- lar pattern—X- and Y-axis move- ments—and at the end of one complete revolution around the circumference of the thread, the cutter has advanced—Z- axis movement—into the workpiece a distance equal to one thread pitch. Thread milling offers a number of ad- vantages over other threading methods. It is obviously better when a tap would be impractical or the workpiece is un- suitable for on a . And be- A machine capable of helical interpolation—motion in three axes—is required for cause a single thread mill can thread thread milling. With both internal and external thread milling, the tool moves in a cir- holes with different diameters, it can re- cular pattern (X and Y axes) while simultaneously moving into the hole (Z-axis). good threads. Tapping requires a lot are usually carbide. Solid cutters are Material Chip Load Per Surface Speed of horsepower at the and on Tooth (sfm) made of HSS, solid carbide or have steel the Z-axis servomotors, which are bodies with carbide tips brazed onto much smaller than the spindle mo- Aluminum 0.0015”-0.0060” 800-1,200 them. Through-coolant styles are avail- tors. Also, CNC machines usually 0.0020”-0.0065” 400-700 able and, as with any cutter, a suitable have low torque at low rpm—a 1045 0.0010”-0.0016” 320-420 coating extends thread mill life and may problem when tapping on machin- 4140 0.0008”-0.0009” 260-290 allow higher cutting speeds. ing centers. Thread milling makes it 316 ss 0.0008”-0.0018” 200-320 The simplest cutter is the laydown in- possible to cut threads on lower- sert in a threading bar. It produces the 420 ss 0.0012”-0.0015” 240-280

horsepower machines, saving time M. Restall best thread form but is slow, since the and money. Recommended starting for cutter has only one effective tooth and Considerations that must be ad- TiN-coated thread mills with indexable inserts. must make a complete revolution around dressed when deciding whether or the circumference of the thread to pro- not to thread-mill include thread diame- cut with taps than with thread mills. duce a thread length of one pitch. Tools ter, thread pitch, depth of thread, percent with several sets of threads ground into of thread engagement, workpiece mate- Tool Materials, Styles them and multiple edges produce rial and hardness, and tool-to-machine Thread mills can be made from a va- threads faster, because the thread length rigidity. In general, small-diameter riety of materials, depending on the per revolution is increased and multiple 3 holes—up to ⁄4" in diameter—are better workpiece material. Thread mill inserts edges allow higher feed rates.

Advantages of thread milling Company specializes in ‘superbolts’

hreading operations are much more uperbolt Inc. manufactures a generate tension on the bolt or Tdemanding than straight turning S patented, mechanical, multi- threaded stud, as do conventional hex operations because the cutting forces ple-jackbolt tensioner in diameters nuts when tightened with a wrench. 1 are considerably greater in threading, from ⁄2" to 30". The internal threads in the cylinder and the cutting edge of a threading in- Superbolt® products replace con- blank, as well as the threads for the sert is much smaller and weaker. Various ventional hex nuts and are easier and jackbolts, often are thread-milled. threading methods are available, in- safer to install. Any diameter stud or The threading is performed on ver- cluding tapping, single-point turning, bolt can be tensioned with the use of tical centers. Machines die threading, thread grinding and hand tools. with 40-taper spindles thread small thread rolling. The tensioners consist of a short, and medium-size units, and 50-taper Another method for cutting threads is thick-walled cylinder with internal spindles mill the threads in bolts 12" thread milling, which offers the follow- threads. Additionally, the ends of the in diameter and over. Generally, the ing advantages: cylinder provide a flat surface that is cycle time for thread milling varies ■ One thread mill can be applied to drilled and threaded for a series of with material and the thickness of the cut many ID thread sizes, whereas a hardened jackbolts. multi-jackbolt tensioner. specific tap is required for each ID In service, the threaded cylinder is The production process begins thread size. screwed into position on a bolt. As when Superbolt receives made-to- ■ One thread mill can be applied to the jackbolts are tightened, they order, heat-treated, extremely thick- cut many OD thread sizes, whereas a wall tubing in 16' to 20' lengths. After specific die is needed for each OD the material hardness has been veri- thread size. fied, a cut-off section of tubing be- ■ Thread milling is more accurate comes the basic element for a Super- than single-point turning, tapping or bolt tensioner. die threading. After initial machining to dimen- ■ Thread milling is also faster than sional requirements, the thread single-point threading, tapping or die milling begins and generates threads threading, because the thread is cut that conform with ANSI B 1.3M-1992 with several teeth in one pass. System 22 and System 23 standards. A right-hand thread mill only rotates Superbolt produces threads with a in a right-hand direction, but by using differential analysis of less than

up or down milling and changing the Superbolt 0.001", which is the difference axial feed direction, right-hand and Superbolt thread-mills its mechanical, between actual and functional left-hand threads can be cut externally multiple-jackbolt tensioners, which pitch diameters, according to ANSI 1 and internally. are available in diameters from ⁄2" System 23. —Information supplied by Carboloy to 30". —M. Restall To cut threads longer than the tool length when using a multiple-tooth cut- ter requires that the tool move down into the workpiece the length of the cutter minus one thread pitch. The overlap- ping of the threads at the end of the cut- ter allows for the continual production of good threads.

Toolholders, Tools The length of thread that can be cut successfully depends on the rigidity of the machine and toolholder. Rigidity is critical for producing high-quality threads. Producing the best threads re- quires applying the largest-diameter cut- Superbolt ter that fits into the holder while allowing Thread mills can cut internal and external threads in one pass. the cutter to engage the workpiece and enter and exit the cut properly. Machines There should be no runout in the versally used style, but not necessarily with 50-taper spindles are capable of cut- tool/toolholder setup or any possibility the most rigid. Cutters with built-in ting the majority of threads used today. of the tool slipping. Thread milling CAT tapers offer more rigidity, but at an While large-diameter cutters that ac- holders—or any of the new-generation extra cost. Bolt-on cutters for arbors are cept more inserts allow faster cutting, hydro-grip or shrink-fit holders—work available in larger diameters. there is a greater chance of the tool well with straight-shank tools. When using insert-style thread mills, moving away from the work and, as a are not recommended. it is important that the inserts are ground result, cutting unacceptable threads. Straight-shank tools are the most uni- accurately and have a positive seating

Thread milling equations

n a linear milling operation, feed per tooth depends on the ratio of times. If you must use the straight in-feed method, reduce the feed per I the radial DOC (Ar) to the cutter diameter (D) and the average chip insert by one-half until the desired DOC is achieved, then circular-mill at thickness (Hzm). When thread milling, the is rotating recommended feed rates. through the helix—not a flat . Therefore, the radial DOC must be Finally, calculate the rate of table travel (ipm) at the cutter diameter calculated prior to calculating the actual feed per tooth and the rate of (IPMD) and centerline (IPMCL). table travel. IPMD = Sz ✕ rpm ✕ N To determine Ar, first calculate the major (D1) and minor (D2) diam- i eters and plug the dimensions into the equation: and (D - D) ✕ 1 D1 = D2 + (2 thread depth) IPMCL = xxxxxxxxx x IPMD D1 For example, if the depth of an internal thread is 0.048" and the minor diameter, or hole diameter, is 3.187", then the major diameter is: Continuing with the example, if the rpm is 1,241 and the thread mill 3.187" + (2 ✕ 0.048") = 3.283" has one insert (Ni), then the ipm at the cutter diameter is: IPM = 0.012 ✕ ✕ For an external thread: D 1,241 1 = 14.892 ipm ■ Subtract twice the thread depth from the part diameter to find the And the ipm at the cutter centerline is: minor diameter. ■ The major diameter equals the part diameter. (3.283 - 1.20) IPMCL = xxxxxxxxxxxxx ✕ 14.892 = 9.45 ipm Use the results of the previous calculation to determine the radial DOC. 3.283 ( D12 - D22) For an external thread, the following formula is for determining the Ar = xxxxxxxxxxxxxxxx speed at the cutter centerline: 4 x (D1- D) Therefore, if the cutter diameter is 1.20", the radial DOC is: (D1+ D) IPMCL = xxxxxxxx x IPMD D (3.2832 - 3.1872) 1 Ar = xxxxxxxxxxxxxxx = 0.074" 4 ✕ (3.283 - 1.20) Therefore: (3.283 + 1.20) Next, use the following formula for average chip thickness to calculate the IPMCL = xxxxxxxxxxxxx ✕ 14.892 = 20.34 ipm 3.283 actual feed per tooth (Sz). Sz = Hzm ✕ √D/Ar Equations : A = radial depth of cut IPM = ipm at cutter centerline With an average chip thickness of 0.003", the feed per tooth is: r CL D = cutter diameter (inches) IPMD = ipm at cutter diameter Sz = 0.003 x √1.20/0.074 = 0.012 ipt D1 = major diameter (part OD for (at the insert) external threads) N = number of inserts When milling an internal diameter, avoid plunging the cutter in the ra- i D2 = minor diameter (hole ID for Sz = feed per tooth (ipt) dial direction to the full DOC. Insert chipping can occur and tool life is internal threads) likely to be shortened. The preferred technique is to increase the DOC Hzm = average chip thickness gradually up to its full value, traveling around the inner diameter 1.25 (inches) —Information supplied by Carboloy method. Shimmed cutting tools offer through-the-spindle coolant. Coolant di- the incremental cut into the material some protection in the case of a ma- rected to the cutting edge improves tool through a specified arc of the circle at chine crash. life and flushes chips away. Also, the the beginning of the machining cycle. At Superbolt Inc., where we thread- action of the makes smaller Ramping in eliminates plunge cutting mill both internal and external threads chips and prevents creation of the to full depth, which can lead to start- in AISI 4140, we apply tools with a neu- “bird’s nest” that often occurs when ing-point chatter and premature cutter tral rake. Softer materials, like alu- stringy materials are threaded. failure. Failure to ramp in properly usu- minum and alloys, re- ally results in chipped cutter teeth. quire high-positive-rake angles. The Programming In general, the objective is to cut the best tools for our applications are 3"- Properly chosen programming para- thread in one pass. However, in cases dia., multiflute thread mills. They ac- meters lead to the production of the best where the thread pitch is coarse and the cept the 20 to 30 styles of inserts needed threads. As such, many thread-mill material is tough to machine, it may not for the variety of threads we produce manufacturers offer software that se- be possible to get an acceptable thread (see sidebar, page 31). lects the tool and allows the operator to in one pass. With materials such as 4140, in the write the program by selecting parame- The surface finish produced by thread hardness range of 25 to 30 HRC, we find ters on the CNC’s screen. milling should be better than that pro- that 6 threads per inch is about the coars- The operator inputs information such duced by tapping and, in many cases, est pitch that can be cut in one pass on a as thread type, size and pitch, and equal to single-point cutting on a lathe. 40-taper machine with a multiple-insert whether the threads are internal or ex- As in any thread-cutting operation, poor cutter having a diameter less than 1.5". ternal, right- or left-hand. The software stability in the setup leads to chatter. A Threads that are coarser than 5 threads also processes information about cut- scalloped appearance on the threads in- per inch, greater than 2" in diameter and ting conditions, such as chip thickness dicates the feed rate is too high. have thread lengths exceeding 2-times (chip load) and surface footage. the diameter should be produced on a The better software allows users to About the Author machine with a 50-taper spindle. specify, in degrees, the amount of ramp- Mike Restall is the production manag- When possible, it’s beneficial to use in and ramp-out. Ramping in prescribes er for Superbolt Inc., Carnegie, Pa.

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