You've got a brand new setup working on the floor; you've invested in new holders, cutters; the job's been expertly engineered and programmed; everything is going smoothly. Then, suddenly, something starts changing and nobody knows exactly why. All the fundamentals are the same: material, machine, tooling, temperature, etc. What gives?
There's a good chance the issue is with a tool holder system, one of its small, hard-to-reach, often-overlooked nooks or crannies. While it can be time-consuming to clean and maintain holders, doing so is critical to long-term process control. If you treat holders and accessories right, in and out of assembly, not only will performance improve, but overall costs will be reduced by avoiding costly spindle repairs and tool breakage.
Clamping Nut Thread Wear
Proper use and maintenance is especially important for three particular areas of the assembly that can experience process-affecting wear and tear: pull studs, collets/nuts and the tool holder taper. Let's examine these so you can identify and address issues early to prevent small tool holding imperfections from turning into bigger problems.
Though small commodities, as the single component that keeps a holder in the spindle -- weathering up to 7,500 lbs. of load in 50-taper arrangements -- pull studs are critical to protecting both your people and machinery. If you've ever experienced a break, you know how scary and dangerous it can be. What's more, if that tool comes out it's going to damage the spindle resulting in necessary repairs that can easily exceed $50,000.
As with any metal-on-metal contact, one is the wear part. In a tool holder setting, the backside of the pull stud assumes this role. Depending on the collet type, repetitive ATC (automatic tool change) can cause dents that lead to stress risers and potentially breakage. A testament to how important this is to consistent performance is that Big Kaiser actually changed all of its smaller-taper knobs to a high-purity tool steel.
Pull Stud Comparison
Regardless of the type of pull stud you're using, inspecting them every time you change the cutting tool is recommended, both for wear and to make sure they aren't loosening. Get the bad ones out immediately or you'll begin to see effects throughout the interface and the workpiece. Pull studs generally last about three years, so finding a way to track them, whether you use studs manufactured with date coding or develop a system of your own, is very helpful.
Finally, in addition to helping to maintain a consistent form longer, installing pull studs correctly ensures the taper won't bulge at the end and disrupt spindle contact. Use a removable thread locker and a torque wrench, but definitely not a cheater bar or hammer. Torque recommendations are:
When it comes to a traditional setup like solid clamping nuts with an ER style collet, it's all about friction. Collets and nuts are wear products too. The repeated twisting and sliding of the nut on the collet during clamping adds up. Likewise, since the nut's threads are what's actually pulling, the coating of the thread that helps maintain the proper relationship between the two can deteriorate.
What's the big deal, you ask? The nut and collet connection directly impacts runout and gripping force -- becoming other potential culprits of those unexpected, hard-to-explain changes in a cycle's performance. In testing, Big Kaiser found that after 500 clampings with a solid ER nut, runout accuracy changes by about 35 percent -- that 12xD drill you're using is now off center.
They also found after 500 clampings that gripping force can decrease up to 50 percent, increasing the chances of tool breakage. And the price of carbide is only going up.
The best way to prevent these issues is pretty simple: keep them clean. An ultrasonic tank with a precision part cleaner is recommended. A small bench-top tank is great for removing swarf, particles, coolant residue and oil. Immediately after cleaning, dip them in rust-preventative oil -- the longer the planned storage period, the heavier the oil. Since you must only ever work with a clean, dry collet, give them one more cleaning before returning them into cycle.
This cleaning method is not, however, recommended for bearing nuts since cleaning fluid can get caught in the bearings and cause premature failure. Then again, their design in and of itself addresses most of these common to ER styles.
The longer you wear a shoe the more it reshapes to adapt to your foot. Spindles and holders aren't much different. The problem for machinists is that holders rarely stay paired with the same spindle. As such, trauma on a holder or spindle -- dings, scratches, gouges, etc. -- can magnify quickly. One bad holder can spread its problems like an illness. If you're seeing disruptions like these on your holders, get them out of the rotation.
The most common type of taper wear is fretting -- the thin, rust-looking build-up that appears over time. Interestingly, it's not actually oxidation due to exposure; rather, it's caused by micro vibrations that push oil off the contact points resulting in metal-to-metal rub. Need another reason to get bad holders out of play? Well here it is: if there's fretting on the taper, you can bet it's on the spindle.
Good news is that this is almost always caused by pushing the tools too hard for the machine, fixturing or workpiece. If you see fretting, re-evaluating your processes and dialing back to the proper specifications for each part of the process would be advisable. As with collets and nuts, proper and thorough cleaning, with taper and spindle cleaners, also helps prevent fretting and other damage from starting in the first place.
That said, an easy way to nip this in the bud is with a dual-contact interface like Big Kaiser's BIG-PLUS®. It all but eliminates the root cause of fretting -- micro vibrations.
Of course, every part of the tool holding assembly wears in one way or another, further speaking to the importance of proper maintenance and use, but these three areas are especially sensitive. So, when a cycle mysteriously changes, don't overlook these three parts of the holder assembly. Better yet, instead of waiting for the we-aren't-sure-what's-going-on stage, take care of your holders.
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