APRIL 2003 / VOLUME 55 / NUMBER 4

‹ BY BRAD LEWIS, ASSOCIATE EDITOR ChallengeChallenge onon TapTap Shops and toolmakers discuss the challenges of tapping.

hen it comes to challenging op- erations, tapping ranks among the toughest. Unlike other met- W alworking operations, rapidly removing metal and imparting a fine surface finish are not the sole ends of the operation. When tapping, the point is to produce a thread that must mate with a fastener according to a given standard. In addition, when other tools have finished cutting, tool removal is fairly simple. But for tapping, removing the tool can consume almost as much time as cutting the thread. All in all, this can make tapping a slow and tedious, but ultimately necessary, operation. Beyond these everyday concerns, other factors add to the challenge of tapping. Primarily, they can be divided into two groups: material-related and operation-related. For both, the selec- tion of the right tool can mean the dif- ference between tapping many holes per tool, or one, or none. To that end, let’s take a look at what some machin- Drill Co. Twist Precision ists and toolmakers have done when rial can be a costly proposition. A day tools hard, but tough. confronted by such challenges. spent watching a tool shatter in the cut According to Dan Gajdosik, engi- can take the wind out of any shop neering manager for Besly Products Workpiece Headaches owner’s sails. Toolmakers are cog- Corp., South Beloit, Ill., the primary Tapping a difficult-to-machine mate- nizant of this and strive to make their difficulties in tapping such materials arise from the heat generated in the cut Eventually, Brown settled on the and the workpiece material shrinking HSS Exotap tool from OSG Tool & Die around the tool. Titanium is particularly Inc., Glendale Heights, Ill. With the Ex- susceptible to this problem, he said. otaps, Brown said the number of holes When tapping these materials, he ad- per tool increased to four. vised machinists to look for a tool that “The taps cost around $12 to $15 has a generous relief in the profile of apiece, which worked out to be about the thread and the tap’s back taper. $3 or $4 per hole. This is kind of high, These prevent the tool from seizing. but in this kind of material, that was In addition, Gajdosik said that there not excessive. But to be paying almost are issues surrounding the strength of $20 per tap and have it last one hole, the tap’s substrate. that was unacceptable.” “Many times, the tap’s crests start to Tapping with a carbide tool, further- roll over, or become dull quickly, due to more, was simply not feasible. “On the workpiece’s high hardness and parts like these, you don’t want to go strength,” he said. In those cases, he with carbide, because it’s too hard. For recommends using premium high- drilling the hole, it’s OK, but there’s a speed-steel taps, as they have a higher lot less forgiveness with carbide,” toughness and red hardness and resist Brown said. tool deformation and wear. This didn’t mean carbide taps were When designing a tap for hard mate- out of the picture for this job, however. rials, Paul Motzel, tapping applications Though Exotaps taps are HSS, a sec- specialist for Guhring Inc., Brookfield, ondary chasing operation was done by Wis., said a low rake angle is critical. hand with a carbide tap. According to The angle lends more support when Brown, each part was removed from cutting. the fixture and given a quick thread One of the most difficult-to-machine chase with the $60 carbide tap. materials is Stellite 31, a nickel-base su- Finding the right tool was not the en- peralloy. Mike Brown, owner of Com- tire solution to the problem, however. puter Integrated Machining, Santee, Brown said using the right lubricant Calif., won a contract to produce a fan was nearly as critical a factor. On the blade for an aerospace motor. About 25 advice of a machinist friend, he blades were needed for each motor. The changed from a tapping lube to a tap- job required producing a 10-32 UNF-2B ping wax from Castrol. The wax was thread in a through-hole for a jackscrew applied by hand with a brush prior to to release the blade from the motor. each tapping run. From the start, the company was In another example of tapping diffi- aware of the material’s strong “mem- cult-to-machine materials, Dan Welter, ory,” or tendency to contract once ma- foreman for Diamond Tool & Die Inc., chined. In anticipation of the shape- Oakland, Calif., was given the job of memory problem, Brown first pro- tapping a “super-austenitic” stainless duced an oversized hole. “The print steel called AL-6XN. The material is called for a hole 0.159" in diameter. So popular for parts that convey liquid, we drilled a 0.161"-dia. hole, going up such as pumps, and is particularly re- by about 0.002"—not a lot. That way, sistant to chloride corrosion. the tap had less work to do,” he said. The material contains high levels of With the first taps tried, however, chromium, nickel and molybdenum Brown said he could only tap one or two (20, 24 and 6 percent, respectively). holes before the tool shattered. “The taps With its relatively high hardness at 88 made a very high-pitched noise, like HRB, the material tends to work- brakes were being put on, and we just harden, said Welter. knew something was wrong.” The part was an ink pump for a print- Finding the right tap, one that would ing press. In it, Welter tapped six 4-40

last more than two holes, was a matter through-holes, three on each side of the & Die Tool Diamond of trial and error. Brown said the taps he part, with a 0.250" depth of thread. Diamond Tool & Die uses Prototyp’s Triple- was experimenting with cost $18 each The job first required him to drill the S tap to thread holes 3 diameters deep in and lasted only one or two holes. holes to depth in one operation, fol- 304 stainless steel. lowed by tapping. At first, he tried this 0.0001"-thick coating is a deposit difficult-to machine materials. Accord- using standard, HSS, TiN-coated taps. of ductile, solid lubricant placed on the ing to Mathias Armbruster, manager of However, the material’s hardness was surface of the tap, which imparts a bur- technical sales development for Proto- too great and the taps simply shattered. nished surface. As a result, less torque typ, which is a part of Sandvik CTT’s After trying about six different HSS is required to drive the tap, which helps sales organization, the high helix of the taps, he gave up on standard thread cut- prevent welding, loading and galling tool allows the tap to reach those mul- ting as a solution to this problem and between the tap and the workpiece. tiple diameters. “The high helix curls began exploring alternatives, such as the chips tight and prevents them from roll-form tapping. Until recently, roll- Deep Tapping While Blind ‘birdnesting’ around the shank,” he form taps were considered appropriate Workpiece material is not the only said. only for soft materials. Welter found obstacle when tapping. Sometimes, the In addition to supplying the tools, roll-form taps to be the answer. mechanics of the operation can be sometimes toolmakers are closely in- He applied an OSG roll-form tap tricky, such as tapping blind holes. volved with solving problems for ma- coated with ElektraLUBE. OSG says Dave Sibinski, process engineer for chinists. Sai-Kin Eng, senior engineer Roberts Automatic Products Inc., Chan- for Firth Brown Tools Inc., Cambridge, The following companies hassen, Minn., said one of the biggest Ontario, assisted on a job requiring an contributed to this article: challenges in tapping blind holes is re- 18mm thread in a part with a 60mm- moving chips from the bottom of the deep hole. He said several factors make Besly Products Corp. hole so the tap doesn’t compress them. deep-hole tapping particularly difficult. (800) 435-2965 And Sibinski knows whereof he speaks, Coolant delivery is difficult in deep- www.besly.com because as a screw machinist, all the hole environments, because chips be- holes he taps in bar stock are blind. come congested in the hole and prevent Computer Integrated “Even though the print may not call coolant from reaching the tool/work- Machining Inc. for bottoming, it is a bottoming condi- piece interface. (619) 596-9246 tion,” Sibinski said. However, because the material had www.cimsd.com As he sees it, the problem with tap- good elongation properties, the cus- ping blind is providing room for the tap tomer was able to apply a roll-form tap. Diamond Tool & Die Inc. to go deep enough for the print re- Since a roll form tap produces no chips, (510) 534-7050 quirements. “If you’re drilling beyond the hole size, typically near the pitch dtdjobshop.com/diamond.htm the cutoff and into the next piece or bar diameter of the thread, had to be held end, there’ll be concentricity prob- more closely than that of a thread cut Firth Brown Tools Inc. lems,” he said. with a conventional tap. (519) 621-2610 And if you’re drilling a deep hole To help coolant delivery, a coolant- www.firthbrown.com blind, he added, there is the danger of through tap was selected. In addition, ending up with a concentricity problem multiple oil grooves and a special coat- Guhring Inc. that gradually worsens. ing aided lubricity. This, in turn, low- (800) 776-6170 At Diamond Tool & Die, Welter ered operating temperatures and kept 5 www.guhring.com needed to tap one ⁄16-18 blind hole with the tool’s edges from softening. Taken a 1"-deep thread per part in 304 stain- together, Firth Brown helped achieve a Prototyp less steel. Tapping 3 diameters deep, he 50 percent reduction in tool life. (800) 877-3745 said he needed a rigid tap that could do Though we have scarcely covered the www.prototyp.com the job in one shot and provide effective range of potential challenges that await chip removal. Roll forming the opera- machinists, the tapping problems cov- Roberts Automatic Products Inc. tion was quickly ruled out. ered here offer insight into how day-to- (800) 490-9875 Prototyp, Crystal Lake, Ill., provided day issues are resolved. Clearly, a www.robertsautomatic.com the needed tool: Its Paradur Triple-S, strong measure of patience is required which is part of its ECO line of taps for to meet the challenges of tapping.

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