Beyond Tap Limits: Advice on Internal Threading
An interview with Marlon Blandon
Holemaking is said to be the most common of all machining operations. Thread milling a rotary machining operation that uses a milling cutter with a single V-shaped tooth or a series of such teeth to cut thread forms, is an underutilized and often misunderstood thread-making alternative. EMUGE-FRANKEN USA’s thread mill product manager Marlon Blandon shares tips on making the most of thread milling to extend tool life thanks to the benefits of flexible and accurate thread milling.
Q: What’s the most common mistake you see people make when thread milling?
A: We often see shops using an ER collet chuck to hold thread mills, but these lack the rigidity needed to withstand the higher radial cutting forces encountered with any kind of milling operation, threading included. Hydraulic or even shrink-fit toolholders are good options, but a mechanical milling chuck such as our FPC tool holding system is the best bet.
Q: What’s next on your list of thread milling dos and don’ts?
A: As with most milling operations, it’s important to avoid chip re-cutting, making effective chip evacuation key to predictable processes. Therefore, we generally recommend staying away from neat oils and other high viscosity cutting fluids during thread milling. Instead, I suggest using a generous supply of water-based cutting fluid or even an air blast to get chips up and out of the hole.
Q: Any other problem areas?
A: Programming the wrong feedrate is another big one. For example, a customer called me the other day to ask why he kept breaking tools. Once he sent me the program, I saw that he was grossly overfeeding the cutter, and a hole that should have taken close to a minute to thread was being done in just a few seconds. That’s because he was calculating his feedrate based on the centerline of the programmed arc instead of the actual tool diameter, which is much larger. The obvious takeaway here is to ask for help if something doesn’t seem right.
Q: What about single flute vs. multi-flute or full profile thread mills? From a productivity perspective, isn’t the latter of these the clear choice?
A: It really comes down to economy and versatility. Because you can cut any pitch thread with a single-flute thread mill, shops don’t have to stock as many tools. They can also just leave that tool set up in the magazine and call it up whenever they need to make some threads. That said, a multi-flute tool is much faster. Instead of making a couple of dozen passes with a single flute tool, one with the complete thread profile can produce any thread in two passes or less, geometry and material permitting.
Q: What do you mean by that last comment, geometry and material permitting?
A: Two things, actually. The first is that most full milling section thread mills — ours included — are designed to cut thread depths to twice the tool diameter. Given the high cutting forces encountered with thread milling, that’s about all you can expect in terms of threading length for a full profile tool. In addition, materials such as Inconel and hardened steels might require an extra spring pass or two to compensate for tool deflection. On the other hand, single-flute tools support much deeper threads, although with a longer cycle time.
Q: Some suggest that thread milling is safer than tapping. How so?
A: There’s always the possibility of catastrophic tool breakage when tapping, but in my mind, chip length is a bigger problem. I like to use an extreme example of a 2″-4 ½ thread, which is common in shipyards. Do the math and you’ll find that a single turn of a tap this size produces a chip just over 6” long. Now multiply that by a thread 2” deep and you’re looking at chips nearly 5’ in length. Granted, this isn’t as much of a concern with smaller threads, but even here, you’re dealing with long, stringy chips that are difficult to dispose of. Thread milling has none of that.
Simply put, thread mills allow you to go beyond tapping limitations.
