Using a tension-compression or floating toolholder helps eliminate the negative effects of misalignment and feedrate/spindle rpm variations when tapping. Image: Sandvik CoromantClick image to enlargeby Kip Hanson

Thread milling offers better size control, process predictability, and lower cost per piece than tapping and other thread cutting methods

In 1918, mechanical engineer Franklin Day Jones—one of the authors of the first Machinery’s Handbook—published a treatise on thread cutting methods. A copy from the New York Public Library has since been scanned by Google and is available for download on the Internet Archive. You might want to grab one for your shop’s reference library, because it seems not much has changed over the past century of thread machining.

Okay, maybe that’s an unfair statement. Modern CNC machinery and cutting tools produce threads faster and more accurately than ever imagined by Jones and his contemporaries. Yet the fact remains that Jones’ treatise describes in great detail the thread rolling, tapping, single-pointing, and other common threading processes still in use today, most of which were invented long before Jones was born.

Multi-flute indexable thread mills reduce tooling costs compared to solid carbide tools while still maintaining good production levels and part accuracy.   Image: Seco ToolClick image to enlargeOne of these is thread milling. In Jones’ day, thread milling was already half a century old, and even then promised higher quality and greater efficiency than competing methods. For some reason, though, thread milling never took off as it could have—or perhaps should have—and today enjoys but a small piece of the thread making pie.

Tap, tap
“If I had to guess, I’d say 90 per cent of shops still use taps for the majority of their internal threading,” says Brian MacNeil, milling products and application specialist for Sandvik Coromant Canada. “In many of these cases, a thread mill would be a better choice.”

There are many reasons for tapping’s popularity. It is a mature process, understood by most anyone who’s spent time on the shop floor. Programming a tapping operation on a lathe or machining centre is an easy task, requiring but a single line of code per hole. And when used with tension-compression or floating style toolholders, taps are fairly forgiving of alignment problems and slight feedrate or rpm discrepancies.

Another consideration is tooling cost, something that receives unfair scrutiny in most shops, considering that most industry experts agree cutting tools account for just three per cent on average of machining costs overall. Despite this, a purchasing manager or shop owner might compare the eight to tenfold higher price of a solid carbide thread mill vs. a tap and decide to take the lower cost route.

Yet MacNeil points to a growing trend that may finally tip the scales in thread milling’s favour: unattended machining. “You can turn your back on a thread mill,” he says. “If you come back twenty minutes later and there’s no thread mill left in your toolholder, you can usually just replace it and pick up where you left off. Break a tap, however, and you’re probably looking at a scrap part, or an expensive EDM operation to remove the broken tool.”

Thread mills are available in a number of geometries, including full-form tools able to machine the entire thread in one or two passes, to multi-teeth tools that circular interpolate the thread, moving from top to bottom (or vice-versa) until the entire thread has been machined.  Image: Sandvik CoromantClick image to enlargeMacNeil notes a number of other advantages that make thread milling a shoe-in for high value parts, difficult materials, and lights-out or lightly attended machining:

  • Thread mills create small chips that are easily washed out of a workpiece and away from the tool. This virtually eliminates the long stringers common with tapping operations, chips that may wrap around the tool or workpiece and create the potential for recutting, an event that will almost certainly destroy most taps.
  • Size control is much easier with thread milling. With tapping, an over or undersized thread means a call to the tooling supplier for the next H-value tap, which in some cases might be a special order. Adjusting the thread size with a thread mill takes nothing more than a quick offset adjustment and you’re back to making good parts.
  • One size fits all…well, almost. Unlike tapping, which requires a specific cutting tool for every thread size you need to produce, a single thread mill can cut multiple thread sizes—for example, a 6 mm x 1 mm thread mill can cut an M 8x1.0, an M 30x1.0, and any other thread with a 1 mm pitch. The same holds true with imperial threads, pipe threads, Whitworths, and even Acmes. This reduces tooling inventory and saves time during setup.
  • Tapping a hole is faster than thread milling, but the difference is a matter of seconds, and can often be made up through a carbide thread mill’s greater tool life compared to high speed steel (HSS) taps. Carbide also ventures into materials where HSS dares not tread, machining superalloys and hardened steels with impunity. “‘I once thread milled 400 M8 holes in 54 Rockwell H13 steel, and still had a usable cutter left at the end of the job,” says MacNeil. “High speed can’t compete in work like this.”

Turn it up
Similar arguments can be made against single-point threading. Don Halas, threading and grooving product manager at Seco Tools Inc., Troy, MI, says single-pointing can cause problems with chip control on a lathe, and is much slower than thread milling, requiring at least five passes and often far more to complete a thread.

Despite this, Halas says thread milling remains in the minority of thread making processes. That’s partly because there are no canned cycles as with single-point threading or tapping, so thread mills are considered more difficult to program (even though most CAM systems now support thread milling). And in the automotive industry, where large amounts of aluminum and magnesium are machined and every fraction of a second counts, threading with roll form taps still reigns supreme.

Rigid tapping on the main spindle or on live tool spindles is a common option available on many live-tool equipped CNC lathes today. It can be a good alternative to thread milling, especially on smaller threads. Image: Haas AutomationClick image to enlarge“The biggest opportunity for thread milling is in hardened materials and high-temp alloys,” he says. “We can machine up to 70 Rockwell steel, something that is very difficult or outright impossible with other methods. Because of this we’re starting to see more shops using it, especially in the aerospace and die-mold sectors, where difficult materials are the rule.”

Halas says once a shop begins thread milling, they seldom turn back. “Sometimes it’s their first attempt, other times they tried it ten years ago and couldn’t get it to work. Either way, it’s comes down to comfort level. I recommend chucking up a block of wood or plastic and trying it out. This avoids scrapping out an expensive workpiece or breaking a cutter, and gives everyone a chance to get on board with thread milling. Once they do, they’re usually hooked.” SMT

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