If you work in the metalcutting, signmaking or cabinet making manufacturing industry, the term “collets” is already very familiar to you.
There are many types of collets used in many different industries and applications. This article is focused on collets used in rotary tool holders found in CNC milling machining centers and CNC Routers and also used in CNC Lathes and Swiss Style CNC's.
Let's cover the basics:
What are Collets?
Collets are the critical connection between the cutting tool and the tool holder, also called a collet chuck. Most collets are round, cone-shaped, and slotted. Collets encircle the cutting tool shank to evenly distribute holding power around its center bore.
Before getting too deep into the technical aspect of collets, It's going to be helpful to anyone new to the use of collets to understand the basic anatomy of collets and of a collet chuck system.
How Collets Work
The tapered collet base is made to fit into the collet pocket of the collet chuck body. The free release locking tapered (16°included, 8° per side) design of the collet base and collet pocket allows the collet to be centered in the pocket as it is pushed in by the collet nut on the lead face during setup.
This centering effect enables the collet to achieve a high degree of accuracy (concentricity); much more than drill chucks and side-lock style end mill holders.
As he collet nut is tightened down on the collet, it is pushed into the pocket collet chuck pocket. The slots in the collet allow the I.D. bore to collapse and apply clamping pressure to the cutting tool shank. It's essentially a spring that is compressed tight around the shank of the cutting tool such as a drill or end mill.
The result is a very strong and rigid clamping force on the cutting tool. Since the collet base is tapered to match the collet pocket, tool runout (T.I.R.) is reduced.
Total indicator runout (TIR) is a term often used in manufacturing, especially when dealing with rotating parts such as cutting tools, particularly endmills and drills. TIR is defined as the difference between the maximum and minimum values measured across an entire rotating surface about a reference axis.
As the title implies adjusting screws, also known as back-up screws, stop screws and preset screws, are not just a simple set screw. They are a screw with a purpose--three actually.
The first is to provide a fixed stop for a cutting tool to rest against during tool changes. This allows an operator to save time as they do not have to pull out a ruler, setting jig, etc. to reassemble the cutter into a holder.
A secondary purpose of the adjusting screw is to assist the tool holder in keeping the cutter from being pushed up into the holder if the cutting loads increase to the point where the tool may slip up into the holder.
The third is to offer sealing for coolant-through tools.
1. Expected repeatability of cutting tool length
When an old cutter is swapped out and a new one put in its place, the repeatability of this process will vary based on a few parameters such as cleanliness and the OEM cutting tool overall length tolerances.
Cleaning the clamping bore or collet of a holder provides better runout repeatability which should be old news to everyone, but if old coolant and contaminants are not removed, they would get jammed between the end face of the shank and the adjusting screw, affecting the length setting.
Cutting tool overall length tolerances may also vary from one OEM to another. We have seen them range from ±.3mm to ±.5mm (±.012” to ±.019”). Others may be tighter or looser.
Most modern machining centers come with tool length offset measurement systems which will provide the final precise gage length of a tool assembly. With the rough position provided by the adjusting screw, the machine operator can continue working and does not need to worry about tool clearances and stick outs.
2. Forms of adjusting screws
The clamping mechanism of the holder also affects the length repeatability. Both hydraulic chucks and milling chucks are radial clamping systems, whereas a tapered collet is drawn down into a taper by a threaded nut. This draw down causes the cutter to be drawn down as well.
For this we have two types of adjusting screws: HMA/HDA solid type and NBA rubberized type. The solid type is a one-piece steel construction part, whereas the rubberized type has a rubber padded conical pocket that absorbs the axial travel of the cutter shank as the collet is clamped.
3. Option for adjustable reduction sleeves for MEGA DS/HMC
Milling chucks also have a second type of adjustment screw option that can be built into the back end of a reduction sleeve. As cutting tool diameters get smaller, the length of the shank also gets shorter.
As such, the end face of the shank may not reach the HMA adjusting screw when installed it the body of the holder. The AC Type Collet adjuster screws into the back end of the reduction sleeve where the shank the tool can easily be reached.
4. Warning on holders that cannot support adjusting screws
It is always recommended to consult the tool holder catalog or technical documentation to ensure that a holder can support an adjusting screw. Some holders are very short or have very deep internal features that may not allow for the use of any adjusting screw. In those cases, a depth setting ring or collar on the shank of the cutting tool may be an acceptable alternative.
Caution should be used on shrink-fit holders. Thermal expansion/contraction occurs in all three axes, so as the body of a shrink-fit holder cools down it will draw the cutter down jamming onto the adjusting screw. This could lead to damage to the screw, the holder or the cutter.
The four critical requirements for tool holders are clamping force, concentricity, rigidity, and balance for high-spindle speeds. When these factors are dialed in just right, there’s nearly no chance of holder error and considerable cost reduction is achieved thanks to longer tool life and reduction of down-time due to tool changes.
Easier said than done, our experts shared some of their best, quick-hitting advice for top tool holder performance in different situations.
1. Balance holders as a complete assembly
Long-reach milling has some unique demands; when setting up this type of job, always balance tool holders as a complete assembly. While many tooling providers pre-balance their holders at the factory, it’s often inadequate, especially for long-reach applications.
2. Holder damage can go from bad to worse quickly
Wear and tear on holders can be costly in the end, but there are ways to protect against it. Inspect and care for your holders. 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.
3. The rule of thumb on holder dimensions
Looking for affordable ways to avoid vibration? Start by opting for a holder with a combination of the largest diameter and shortest length possible.
4. Rigidity can harm tapping operations
What many don’t realize about tapping operations is that a perceived strength of collet chucks—their rigidity—can actually be detrimental. Rigidity does very little to counteract the dramatic thrust loads imposed on the tap and part, exacerbating the already difficult challenge of weathering the stop/reverse and maintaining synchronization.
5. Balancing is crucial to five-axis machining
Five-axis machining introduces a whole new set of tooling challenges. While important in any type of machine, balance may be of most importance in full five-axis work. A well-balanced holder helps ensure the cutting edge of the end mill must be consistently engaged with the material in order to prevent chatter and poor surface finish quality.
6. Consider spindle speed requirements when choosing between shrink-fit and hydraulic holders
If you have to choose between shrink-fit and hydraulic holders in a long-reach application, consider the spindle speed required. If a hydraulic chuck exceeds its rated RPM, fluid is pulled away from the holder’s internal gripping gland, causing loss of clamping force. But when used within its recommended operating range, a hydraulic tool holder offers superior runout and repeatability. On average, a good shrink-fit holder has about 0.0003-inch runout, while a hydraulic chuck offers 0.0001 inch or better.
7. Don’t overlook the tool’s effect on holder performance
The cutting tool affects holding ability more than most machinists and engineers realize:
8. Not all dual-contact tooling is the same
Anyone in the market for BIG-PLUS dual-contact tooling should consider this simple statement: Only a licensed supplier of BIG-PLUS has master gages that are traceable to the BIG grand master gages and have the dimensions and tolerances provided to make holders right. Everyone else is guessing and using a sample BIG-PLUS tool holder as their own master gage—a practice that any quality expert will advise against.
Look for the marking: “BIG-PLUS Spindle System-License BIG DAISHOWA SEIKI.”
9. You may have a BIG-PLUS spindle and not even know it
You’d be surprised how often we hear from our certified regrinders or engineers in the field about folks that didn’t realize their machine had a BIG-PLUS spindle—the message can get lost in the supply chain or during the sales process.
The easiest way to know if an interface is BIG-PLUS is to place a standard tool into the spindle and see how much of a gap there is between the tool holder flange face and spindle face. Without BIG-PLUS, the standard gap should be visible, or about 0.12 in. If it is BIG-PLUS, the gap is half of this amount, or only 0.06 in. These values change depending on 30 taper, 40 taper or 50 taper sizes, but the gap is visibly less than usual.
10. Use positive offsets during holder setup
It may be how it’s traditionally been done but touching off holder assemblies in each machine to establish negative tool offsets based on the zero-point surface—the vise, machine table, workpiece, etc.—is not the most efficient process. We think the choice is pretty clear: adapting machines to a single presetter so they can receive positive gage lengths is superior to using all types of machine-specific negative offsets.
This is a change to “the way things have always been done” that can be met with some resistance, but in the grand scheme of things, it’s a relatively small and simple step that makes life much easier. It’s a relatively low-cost opportunity to introduce more standardization of holder setup to the shop floor.
Holders are the bridge between the machine and the part. That’s a lot of pressure—literally and figuratively. It’s important to select, care for and use holders carefully from the day they are purchased until they’re tossed into the recycling bin.
From collet chucks to coolant inducers, BIG KAISER is North America’s source for standard-bearing tool holders that guarantees high performance. Explore the full lineup.
by Bernard Martin
There have been some who claim that drawbar gripper fingers and/or ball marks that appear on retention knob head after several tool changes is normal.
It is NOT.
THAT IS FALSE.
According to Haas CNC, ball or gripper marks on the edge of the pull stud indicate that the drawbar does not open completely.
If you see these indication marks you should check your drawbar and replace these pull studs immediately.
DEPTH OF CUT COLUMN
by Jack Burley, President and COO at BIG KAISER Precision Tooling Inc.
It’s time for machine tool builders and machining companies to shelf the long-standing ISO 1940-1 standard in favor of ISO 16084:2017. Not only is balancing tools rarely necessary, it can also be risky.
A lot of conflicting information has circulated over the years about balancing tools. As an author of the new standard for calculating permissible static and dynamic residual unbalances of rotating single tools and tool systems – ISO 16084:2017 – allow me to clear some things up and, hopefully, make life a little easier for you.
Since its institution in 1940, the G2.5 balance specification has been widely accepted across the industry; i.e., “it’s how things have always been done.”
However, machines were much slower 80 years ago. Back then, the most advanced machines would have spun larger, heavier tools at a maximum speed of about 4,000 RPM. If you applied the math from those days to today, you’d get unachievable values.
For example, the tolerances defined by G2.5 for tools with a mass of less than 1 pound rated for 40,000 RPM calculates to 0.2 gram millimeters (gm.mm.) of permissible unbalance and eccentricity of 0.6 micron. This isn’t within the repeatable range for any balance machine on the market.
Similarly, application-specific assemblies, for operations like back boring and small, lightweight, high-speed toolholders, can’t be accurately balanced for G2.5.
Machine tool builders rely on an outdated number, too, often basing spindle warranty coverage on using balanced tools at very specific close tolerances. While it’s true that poorly balanced tools run at high speeds wear a spindle faster, decently balanced tools performing common operations won’t wear spindles or tools drastically and deliver the results you’re looking for.
While it’s true that poorly balanced tools run at high speeds wear a spindle faster, decently balanced tools performing common operations won’t wear spindles or tools drastically and deliver the results you’re looking for.
A Little Lesson About Forces
This all begs the question: When do you need to take the time to balance holders? I would argue that tools require balancing only if they’re notably asymmetrical or being used for high-speed fine finishing. Here’s a rule I’ve long followed: If cutting forces exceed centrifugal forces due to unbalance, high-precision balancing isn’t needed because the force required to balance the tool will most likely be less than cutting forces.
At that point, aggressive cutting – not unbalance – is going to damage the spindle.
Unbalanced tools are also blamed for issues that turn out to be misunderstandings about a machine’s spindle. I’ve visited shops with new high-speed spindles that had trouble running micro tools over 15,000 RPM. They rebalanced all the tools on the advice of their machine tool supplier, but to no avail. It turned out the machine was tuned for higher torque and higher cutting forces. Before going to the effort of balancing toolholders, work with your machine builder to understand where a spindle is tuned.
Not only is balancing tools rarely necessary, it can also be risky. Our inherently asymmetrical fine-boring heads are a good example. Because we balance them at the center, a neutral position of the work range, you lose that balance if you adjust out or in.
To adjust, you’d typically add weight to the light side, which can be a problem for chip evacuation and an obstructor. Or you can remove weight from the heavy side, but that means you have to put some big cuts on the same axis of the insert and insert holder, ultimately weakening the tool.
In longer tool assemblies, common corrections made for static unbalance can also cause issues. It happens when a toolholder is corrected for static unbalance in the wrong plane; i.e., adding or removing weight somewhere on the assembly that’s not 180 degrees across from the area where there’s a surplus or deficit.
Once the tool is spun at full speed, those weights pull in opposite directions and create a couple unbalance that often worsens the situation.
A Cautionary Tale
If you do go down the balancing road, you’d better know where you can modify tools, what’s inside, how deep you can go, and at what angles. Whether you’re adding or removing material on a holder, I highly recommend consulting the tool manufacturer for guidance first.
As a cautionary tale, consider a customer who was attempting to balance a batch of our coolant-fed holders. Based on the balancing machine, the operator drilled ¼-inch holes at the prescribed angle into the body of the holders. Not realizing what was inside, he drilled into cross holes connecting coolant flow and ruined several holders.
Tooling manufacturers are doing their part to avert disasters like this. For most, simple tools like collet chucks or hydraulic chucks are fairly easy to balance during manufacturing. We account for any asymmetrical features while machining and grinding holders and pilot each moving part, ensuring they’ll locate concentrically during assembly. These measures ensure the residual unbalance of the assemblies is very, very low and eliminate the need for balancing.
Decades of the same standards have conditioned us to think a certain way about balancing tools. While it seems logical that every tool must be balanced, it’s just not the case: Many issues attributed to unbalance aren’t caused by unbalance, and the risks of balancing every single tool often aren’t worth the reward.
Save your balancing time and resources for high-speed fine finishing. If you do have work where balance is crucial, consider how the tools you buy are balanced and piloted out of the box and/or consult your partners before making any modifications.
by Bernard Martin
Retention Knobs are the critical connection between your machine tool and the tool holder and they are the only thing holding a steep taper tool holder in the machine’s spindle.
Techniks has recently introduced their MegaFORCE retention knobs that have some rather unique features when compared to standard pull studs. Before delving into the features of the MegaFORCE pull studs, let's review some things that you may not know, or think about, on a daily basis.
According to Haas, you should expect a service life of about 6000-8000 hours for a retention knob.
Most all rotary toolholder manufacturers state that you should be replacing your pull studs at least every three years.
However, if you're running multiple shifts, 24-7, making lots of tool changes, making very heavy cuts with long reach or heavy cutting tools, and/or have ball lock style grippers instead of collet type grippers used on the retention knob, you will probably need to replace your studs at least every six months.
Given the spindle speeds that we are running at to remain competitive, retention knobs are not an item that you want to take a chance on breaking. I can tell you firsthand that 5 pound toolholder with a drill in it flying out of the spindle at 23,000 RPM is not something you want to experience.
METAL FATIGUE: WHY THEY FAIL
Pull studs encounter catastrophic failure as a result of metal fatigue. The metal fatigue can be caused by a number of reasons including poor choice of base material, engineering design, machining process, poor heat treatment, and, sometimes, they have just met or exceeded their service life. We're going to dig into each of these reasons below but first let's look at some threading fundamentals.
The load on each subsequent thread decreases from there, as show in the table. Any threads beyond the first six are purely cosmetic and provide no mechanical advantage.
Additional threads beyond the sixth thread will not further distribute the load and will not make the connection any stronger.
That is why the length of engagement of the thread on a pull stud is generally limited to approximately one to one & a half nominal diameter. After that, there is no appreciable increase in strength. Once the applied load has exceeded the first thread's capacity, it will fail and subsequently cause the remaining threads to fail in succession.
RETENTION KNOB DESIGN
Repetitive cycles of loading and unloading subject the retention knob to stress that can cause fatigue and cracking at weak areas of the pull stud.
What are the weak areas of a standard retention knob?
The most common failure point for a retention knob is at the top of the first thread and the underside of the pull stud where the grippers or ball bearings of the drawbar engage and draw the toolholder into the spindle.
Remember, bigger Radii are stronger than sharp corners. More on that soon.
Not all retention knobs are made from the same material, however, material alone does not make for a superior retention knob. Careful attention to design and manufacturing methods must be followed to avoid introducing potential areas of failure.
Techniks MegaFORCE retention knobs are made from 8620H. AISI 8620 is a hardenable chromium, molybdenum, nickel low alloy steel often used for carburizing to develop a case-hardened part. This case-hardening will result in good wear characteristics. 8620 has high hardenability, no tempering brittleness, good weldability, little tendency to form a cold crack, good maintainability, and cold strain plasticity.
There are some companies making retention knobs from 9310. The main difference is the lower carbon content in the 9310. 9310 has a tad more Chromium, while 8620 has a tad more nickel. Ultimate Tensile Strength (UTS) is the force at which a material will break. The UTS of 8620H is 650 Mpa (megapascals: a measure of force). The UTS of 9310H is 820 Mpa. So, 9310H does have a UTS that is 26% greater than 8620H.
That said, Techniks chose 8620 as their material of choice because of the higher nickel content. Nickel tends to work harden more readily and age harden over time which brings the core hardness higher as the pull stud gets older. The work hardening property of 8620 makes it ideally suited for cold forming of threads on the MegaFORCE retention knobs.
It should be noted that some companies are using H13. H13 shares 93% of their average alloy composition in common with 9310.
ROLLED THREADS VS. CUT THREADS
A cut thread, image 1, has a higher coefficient of friction due the the cutting process, while a roll formed thread, image 2, has a lower coefficient of friction which means that it engages deeper into the toolholder bore when subjected to the same torque. You will notice that Cutting threads tears at the material and creates small fractures that become points of weakness that can lead to failure. Rolled threads have burnished roots and crests that are smooth and absent of the fractures common in cut threads.
Rolled threads produce a radiused root and crest of the thread and exhibit between a 40% and 300% increase in tensile strength over a cut thread. The Techniks MegaFORCE retention knobs feature rolled threads that improve the strength of the knob by 40%.
Also, unlike thread cutting, the grain structure of the material is displaced not removed.
By comparison, cut threads interrupt the grain flow creating weak points.
MEGAFORCE GEOMETRIC DESIGN
There are some claims that a longer projection engages threads deeper in the tool holder preventing taper swelling. While a deeper thread engagement can help prevent taper swelling, applying proper torque to the retention knob is an effective way to reduce taper swelling.
An over-tightened retention knob may still cause taper swelling regardless of how deep it engages the threads of the tool holder. Additionally, the longer undercut section above the threads presents a weak point in the retention knob.
There is a ground pilot, underneath the flange, which provides greater stability. The pilot means the center line of the tool holder and pull stud are perfectly aligned.
Magnetic Particle Tested
Each Techniks MegaFORCE retention knob is magnetic particle tested to ensure material integrity and physical soundness. MegaFORCE retention knobs are tested at 2.5X the pulling forces of the drawbar.
RETENTION KNOB BEST PRACTICES
In order to maximize the life of your retention knob and prevent catastrophic failure here are some technical tips to keep your shop productive and safe.
A machine’s spindle is one of the key links in the machining chain. In other words, if there are irregularities inside or at the face, they can show up on your part.
It makes regular inspection and spindle maintenance critical to getting the most out of your equipment and maintain process efficiency. These three accessories, the Dyna Contact Taper Gage, the Dyna Test Bar and the Dyna Force Measurement Tool, can help you perform this maintenance easily without eating into valuable spindle time.
Dyna Contact Taper Gage
Dyna Test Bar
With the help of a dial indicator, you can uncover any runout while safely spinning the spindle at a very low RPM and verify the parallelism of Z-axis motion.
Dyna Force Measurement Tool
The Dyna Force measurement tool provides a precise digital reading that reveals reduction in retention force in increments of 0.1kN.
by, Bernard Martin
As carbide end mills gain higher and higher speeds and metal removal rates there has also been a trend by round tool manufacturers to tighten up the tolerances on both the cutting diameter and the shank diameter to improve concentricity. At the same time, shrink fit holders have become more and more popular because they hold a tighter concentricity as well. To achieve this both the shank and the bore now have similar surface finishes and this has led to a problem The tools pull out in the cut.
Shrink fit holders are the most accurate for TIR as the toolholder engages completely around round shank tools with a bore tolerance of -0.0001" to -0.0003". As high performance end mills have tightened shank tolerances to the same range of -0.0001" to -0.0003" they have used finer and finer grain grinding wheels which give the shanks a 'shiny' appearance.
Shiny means that the superfinished shank has a lower coefficient of friction. So, although the TIR is tighter, the shank is more "slippery". End mills traditionally had surface finish of about 8 μin on the tool shank. But that's changed. It's been recommended that tool shanks used in shrink fit holders should not have a finish finer than 16 μin. for optimum holding power, but tell that to the guy who just superfinished the end mill to a super cocncentric tolerance that you don't want it looking that good.
Everyone knows that the last thing you want is for the end mill to slip in the middle of a heavy cut or on the finishing pass of a high tolerance part. These 'hi performance' end mills, often times have higher helix angles which are great for ejecting chips but also create a higher pull out force on that slippery shank. And reducing the helix angle is not the answer.
We already know that the gripping pressure is a function of the interference between the tool shank and the shrink fit toolholder bore. Most shrink fit holders have a already bore surface finish of between 12 μin. and 16 μin. So they are ground to a very high tolerance and have about the same surface finish as the toolholder shank.
End mill manufacturers and machinist have tried a variety of methods over the years to stop the tools from pulling out. This has ranged from grit blasting the shank to rubbing chalk on the shank, but most everyone in the industry has felt that the problem really needs to be addressed by the longer life toolholder rather than the replaceable cutting tool.
That's the problem that Techniks wanted to address. Techniks claims that their "proprietary non-slip TTG594 compound virtually fuses the tool shank with the shrink fit toolholder."
ShrinkLOCKED Toolholders eliminate cutting tool pull-out and provide 4X the friction drive force compared to un-treated shrink holders.
It’s not just a rougher bore finish that enhances the holding power. TTG-594 is a compound that has a much higher Brinell hardness than carbide so it can “bite” into the tool shank. But this does not affect the ability to perform tool changes.
Techniks arrived at their 4x the holding power comes from torsion testing vs. a standard shrink fit toolholder. They used a ¾” carbide gage pin in a standard holder and found the torque at which the tool will spin in the bore.
They then tested the ShrinkLOCKED holder using the same test.
According to Greg Webb, at Techniks,
"We actually could not find the point at which the tool would spin in the ShrinkLOCKED holder as we broke the carbide gage pins at 4x+ times the torque of the standard holder. The holding power is greater, we just have not found a way to measure this, so we kept our claims conservative at 4x."
Technical Support Blog
At Next Generation Tool we often run into many of the same technical questions from different customers. This section should answer many of your most common questions.
We set up this special blog for the most commonly asked questions and machinist data tables for your easy reference.
If you've got a question that's not answered here, then just send us a quick note via email or reach one of us on our CONTACTS page here on the website
Our technical section is written by several different people. Sometimes, it's from our team here at Next Generation Tooling & at other times it's by one of the innovative manufacturer's we represent in California and Nevada.
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