Tapping goes high tech.
Thus, it may come as a surprise that tapping has gone high tech and now has answers to most tapping dilemmas. Pay a little more for a high-tech tap, and you can get 10, 20, or even 100 times more threaded holes per tool while reducing machining time and boosting quality. There are a lot of new twists in tapping. Best of all is the growing recognition that thread quality is important to product quality and, therefore, worth learning about. For just about any given situation, there's a tap available now that can offer significant improvement. It's just a matter of tailoring its composition, geometry, coating, and other subtleties to the task at hand.
For starters, here's a brief description of basic tap categories and how each works:
Fluted taps cut threads with the chamfered end of the tap, starting with the core-hole diameter and moving outward to the final thread form. Conventional taps have a relieved (noncircular) thread form that can cut beyond the initial full-form cutting thread, and thus can be resharpened, much like a drill. For through holes, the chamfer is ground with a cutting rake that throws the chips ahead of the tap into the core hole. For blind holes, the rake is reversed to throw chips up through the tap flutes. Depending on the material being cut, the fluting helix can be angled low for hard materials or high for soft.
Leadscrew taps are unrelieved (circular) so that, beyond the cutting chamfer, the rest of the thread form has no cutting flutes and acts like a leadscrew, driving the fluted cutting end through the hole. For deep-hole threading, channels can be added to allow lubrication to get down to the threadcutting area. Once the initial threads have been formed, the leadscrew drives the cutting edge, controlling pitch and concentricity tolerances better than a conventional tap.
Thread-forming taps cold-form the metal in the core hole into final thread form without cutting. In most materials, this produces a cold-working benefit over a cut thread (roughly 10 Rc points) in addition to eliminating chip-removal and chip-wear problems for the tap. The resultant thread is never larger than the tap itself. (In badly skewed core holes, the thread-forming tap will follow the hole axis and break, whereas the threadcutting tap will follow its own axis and produce oval threads.) For thread forming, core-hole diameter is critical, a the tap requires more torque. A uniform tapping speed is also critical to get uniform metal flow. Because metal is extruded from pitch diameter to the crest of the formed thread, thread-form percentage always varies, depending on tapping speed, lubrication, and actual core-hole diameter.
One solution to holding thread form is a special Reime tap, made in Germany and offered here by Evergreen Engineering Assoc, Troy, MI. Explains Evergreen's Fritz Fiesselmann, "The Reime SpanlosSKR tap has an aggressive thread-forming section in the chamfer and a reaming device on the extended tip of the tap to trim the thread crest on withdrawal. With this tap, we can guarantee that you will hold thread-form percentage and get a finished thread with no feather edge or anything to contact the root of the screw."
Most tap manufacturers now offer something for difficult-to-machine materials. At Vermont Tap & Die, Lyndonville, VT, a big winner has been its cobalt taps for materials above Rc 35. These taps combine three variables: composition (a high-cobalt, high-speed steel with good powder-metal microstructure), a proprietary geometry, and Tin coating. "What's different today," explains Ted Henderer, chief engineer, "is that these taps are now standard tools. Their high-vanadium, high-cobalt powder-metal microstructure offers unsurpassed wear resistance, hot hardness, and uniformity. This means higher cutting-edge toughness for longer and more consistent tool life, important factors in untended machining."
The cost premium of the cobalt taps over conventional HSS taps is about 50%. Because the benefit far exceeds that, these taps are seeing increased use to combat the increased use by industry of more difficult-to-machine materials.
Henderer notes that adding coatings to taps helps in most cases, yet can be detrimental in others. "In some cases, it doesn't show a huge benefit, usually where cutting speed is extremely slow. When the tool runs at the proper speed, tool life is three to five times that of an uncoated tool." The key is to optimize the other cutting variables first to get this benefit.
Although the use of coolant flow through the tap is increasing in popularity, it still represents only 1% of the taps Vermont sells today, Henderer reports, but that should accelerate as machine tools with spindle-coolant capability become more widely used.
Center-hole lubrication can be provided either straight-through for blind holes or branched to deliver coolant to critical flute areas. There is some disagreement over what pressure is necessary. Obviously, you can't use more than a given machine tool has available, usually less than 100 psi. Yet, some tap makers argue for much higher pressures to blast chips away from their taps, while others say the coolant is there for lubricity and cooling, and it's better to leave chip removal to tap geometry and fluting angles.
As in other cutting situations, carbide taps are making an appearance, mostly for the more sophisticated user who has already mastered advanced cobalt taps. For the unsophisticated, carbide taps can be a disaster-broken long before they have a chance to wear out.
Carbide taps can run at much higher speeds-approaching drilling speeds, and properly used, can yield dramatic improvement. They're best for materials with short chipping characteristics and tensile strengths of 120,000 psi or less. More so than other taps, they should have a chamfered core hole to avoid unnecessary impact.
The manual alternative
"Tapping on CNC machines is obsolete," says Walter Shanler, President, Walden International Machinery Corp, Monsey, NY. He feels performing tapping operations is prohibitively expensive on high-cost-per-hour CNC machine tools, considering the total cost of programming, indexing, positioning, toolchanging, and tapping times.
His solution: a manually operated pantograph tapping arm for the machine operator to use for these functions while waiting on the next part to be machined. Shanler has demonstrated that off-line manual tapping can do the same job in half the time at 1 / 12th the cost for the fully burdened machining center ($91 /hr or $3.03/hole). He figures the pay-back on an investment of $6000 in a tapping machine occurs in 2800 holes (typically, a matter of a few weeks.)
Shanler is also challenging dedicated fixed-spindle tapping machines with his tapping arms. "Here, the machine's spindle feed or quill must match the tap's feed-rate to make an accurate thread. When the tap bottoms out, some type of return spring pulls the tap out of the hole, and, in softer materials, this tension can stretch or strip the top threads."
With Shanler's tapping arm, you don't rely on either operator feel or tapping-station stroke. The operator merely introduces the tap to the hole, and the tap supplies all the thrust. When it bottoms out, a clutch slips, cutting stops, and the operator either hits a reverse button or an attachment does this automatically. Clutch slippage in the tapping stroke tells the operator that the tap is dull and needs replacing, and is a key benefit in avoiding tap breakage. Positioning speed is another advantage of the movable arm over a fixed tapping station. It's much faster to bring the tap to the hole than position the part under a fixed tapping spindle, even with small parts.
The thread-mill alternative James Hartford, vice president and general manager, Advent Tool & Mfg Inc, Mundelein, IL, is a leading proponent for thread milling as an alternative to tapping, even in thread sizes as small as V4". "With a thread mill of a given pitch, you can create any diameter, any helix angle, in a single cutting pass: one three-axis Z move with helical interpolation."
Which is thread milling's biggest advantage-and biggest handicap-programming that move. "It's now a relatively simple thing to program," he claims. "four lines of code with a general Fanuc controller. Also, when the mill wears, you can modify pitch diameter with cutter compensation, something you can't do with a tap. "
Thread milling is also better on blind holes, he points out. "We can go within half a thread of the bottom on blind holes with a single pass versus the need for several tapping operations. Chips are also much smaller, and you're not creating stringers that tend to pack up the tool." Advent can provide through-the-tool coolant as an option, and thread-mill resharpening.
While it may be easier today to program thread milling, there still aren't many who feel confident about getting three axes moving at the same time or know how to ramp in and out of cuts, Hartford admits. "So I train them."
"People use thread milling for a higher quality thread when they've had a hard time tapping or maintaining thread finishes in materials such as stainless, Inconel, Hastelloy, or Monel. We're never going to replace tapping--just relieve certain problems. The small job shop with a 10-hp machining center which couldn't drive a 1.25" tap can use a thread mill and get a high-quality thread at one third the horsepower a tap would take.
"We're selling a lot of small thread mills-small NPTS, 1/4-20s, 3/8-16s. Our tool costs more than a tap, but the cost per hole is either the same or less. Tool life is much like you would get with an end mill. One user got 26,000 tapped holes on one regrind, but more typically, it's 400 to 500 in 3/16" stainless."
With the increasing demand for both higher speeds and coolant-fed tapping, Tapmatic, Irvine, CA, a maker of units for CNC horizontal and vertical machining centers, is coming out with a unit that can deliver both. "There have been previous nonreversing tapping tools that you can run coolant through," says Mark Johnson, Tapmatic chief engineer, "but they required reversing the machine spindle. There are also rotary-seal options to deliver coolant to the tool for drilling, but not for taps."
Tapmatic units offer axial compensation--tension on the driving stroke to allow the tap to follow its own lead into the hole and compression to initiate reversal and the tap's return. They also have a "hard start" unit with only return tension that controls thread depth more accurately. This approximates rigid tapping where the spindle drives the tap directly with no interceding tapping unit.
"With axial compression on the tap as it enters the hole," johnson explains, "it takes more pressure to get it started. That uses up some of the machine's feed, and the tap goes to a different depth. That's why we recommend a hard start in a lot of cases where a conventional tension/compression tap holder would give you variation in thread depth for a fixed in-feed distance."
On final withdrawal of the tap from the workpiece, isn't there always some tension and some danger of loosing threads? "That's true in really soft materials," he replies. "You have to be careful about the stiffness of the tension spring. It's not a problem for most situations."
Rigid or floating?
But isn't that the argument for rigid tapping-control of entry and exit with no danger of stripping threads? "Right," he laughs, with some disbelief, "provided you can match feedrates. One of the benefits of a self-reversing tool over rigid tapping is that the machine spindle is always going the same speed and in the same direction. With rigid taping, you're continually slowing that spindle down, stopping, and reversing, and major portions of the tapping cycle are not at the ideal cutting speed, and this reduces tool life. Also, the smaller and faster-accelerating tapping unit uses less machine time.
"So, the tapping unit gives you greater speed, tool life, and spindle life, and I'm sure you'd much rather wear out a tapping attachment than the machine spindle. Rigid tapping works best for tapping a few holes at slow speed."
But no clutch feedback?
Unlike the simpler manual tapping units that use clutch pressure to tell the operator when to replace dull taps, Tapmatic's tapping units for CNC machines do not. "We use a positive drive because we feel a clutch on a CNC machine isn't such a good idea. If it's not set right and slips, without an operator controlling tap feed, you could easily crunch taps. It would be more troublesome than helpful. On a drillpress operation, the operator sees right away to stop feeding."
So how do you handle the CNC situation? "By controlling depth so that the tap doesn't hit bottom in a blind hole." The operator has to monitor tap wear by either gaging feedback that threads are being cut undersize or simply counting a standard number of threaded holes pertap.
The right cutting fluid
Tapmatics's Johnson says a key reason for tap breakage is trying to push thread-form percentages too high--80%--in depths of three or four times tap diameter. "Cutting fluid is also critical. A good cutting fluid will give much better tap life and thread finish."
Is there a difference between the ideal metalcutting fluid and the ideal tapping fluid? Yes," he replies, "drilling and tapping both need a good lubricant, and tapping needs a higher-pressure delivery. Thread-forming taps also require good lubricity, more so than tap cutting. Yet, most times, people use water-based coolant for both operations, when tapping would be better with oil.
"We offer a special dispenser with cutting fluid that can be used in conjunction with machine coolant. You use an M-code to turn off coolant and turn on this unit to shoot out cutting oil for the tapping operation. It can be used where you need the very best thread and tap life. It provides a significant improvement over coolant's lubricity and will boost tap life and thread finish."
That suggests that supplying coolant through the tap is less than optimal? "True, it's not as good as cutting oil, but much better than external coolant because it can be directed at the cutting edge and used to flush out chips."
Peter Matysiak, president, Emuge Corp, Northborough, MA, acknowledges that users can become cynical about salesmen's claims of remarkable improvements in tap life. "By tailoring the tap closely to very specific tapping situations, we can exceed a conventional tap's life by a factor of 20 to 50 times, at a cost increase per tool of three or four times."
The key to high-tech taps is using the most advanced tool steels, tap geometries, and tap-grinding machinery, combined with special hardening and tempering surface treatments. Based in Germany, Emuge stocks as many a 240 different taps in a given thread diameter to cover all the tapping situations they envision.
To illustrate the benefit of tailored taps, Matysiak cites an example. "One of the most popular taps in the auto industry is a low-tech, traditional tap in the 5/16" and 3/8" size range that gets 1000 holes per shift used in nodular cast iron. We took a tap from our inventory, specifically manufactured for that type of use, and it gave them 54,000 holes. It cost three to four times more."
Just as coatings can be used as a Band-Aide for a poorly applied tap, Matysiak feels, coolant through the tap is now being used in some cases to make up for poor lubricity and chip-removal geometries.
He also argues with those claiming a formed thread is better than a cut thread. "That may be true comparing a roll-formed thread with a thread cut with a conventional tap, but not with threads cut with advanced geometries--they're just as perfect as rolled threads."
Mr Matysiak sees continuing improvement in tapping speeds, and feels that even high-speed reversing can be done by the machine spindle.
What do you consider fast today? "We work with tapping speeds ranging from 5 sfm in a very hard stainless, Inconel, or titanium up to 150 sfm in aluminum. That's normal and not considered particularly fast-just the speeds we recommend for general tapping. We have developed special taps that can go to 250 sfm under ideal conditions and even higher."
Will tapping and drilling speeds ever approach each other? "No, I don't think so. You can always blow chips out of a hole faster than you can cut a fine-edged groove. So both will continue to increase."
Mr Matysiak is enthused about measuring tapping torque in the tap driver to detect tool condition. "Torque measurement is one of the best tool-detection methods, and we're working on that." Emuge offers mechanical-slip devices that detect clutch slip and uncompleted stroke, but these are, he feels, only interim solutions. Electronic torque monitoring could add tap-breakage detection and monitoring of tap wear.
The ultimate tap?
Emuge offers a full line of carbide taps, although they are not yet an accepted tap among their customers in Europe. Carbide offers the potential of higher cutting speeds and lifetimes of 100 to 500 times the life of a conventional tap at about ten times its cost, Matysiak says. "A 1/4" carbide tap might sell for between $125 to $150. But we recommend them only for very controlled situations. We have demonstrated recently in an automotive application that an advanced cobalt-steel tap can approach 80% to 90% of the life of a carbide tap. We feel that most of the answers to tapping problems today lie with advanced steel taps. After perfecting that, the next step might be to a carbide tap. Beyond this, our research people are already working on experimental taps, including a diamond-film-plated ceramic tap that might be the ultimate tap, but we don't know yet."
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|Title Annotation:||process of drilling holes in metal parts|
|Publication:||Tooling & Production|
|Date:||Aug 1, 1991|
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