edited by Bernard Martin
It’s been estimated that a tool with a run-out of 50% of the tool’s chip load will reduce its tool-life by 40%. That means that a 1/8” tool with a 0.00019” chip load per tooth will lose 40% of its tool-life with a run-out of less than 0.0001”.
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.
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.
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.
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.
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%.
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.
Magnetic Particle Tested
Each 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.
indication marks on Pull Studs
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.
- Through the Spindle
- Through the Flange Entry (DIN B)
Through Spindle Coolant Delivery
Through holes are standard in all Parlec and Techniks toolholders. Solid and through hole retention knobs are available to accommodate coolant and non-coolant applications. This is the most common method of coolant delivery.
The coolant is delivered from the spindle through the knob and exits through the cutting tool. Coolant through knobs are sold separately.
Combined with solid retention knobs "Through the Flange" holes go through the flange to deliver the coolant from the spindle.
This is sometimes referred to as "DIN B" or "Form B".
Form B convertible or AD/B (BC) is available in many sizes. The AD/B (BC) style can be used as either through the spindle , as supplied, or converted to Form B, through the flange. Flange entry is enabled by removing two screws
- All Parlec and Techniks Rotary toolholders meets or exceeds ASME B5.50-2009 specifications and all current specification updates except where improvements are made for high speed operation.
- All Parlec and Techniks CAT tooling incorporates ParSymmetry for balance and chip hole for identification.
This Standard pertains to the standardization of a basic tool holder shank and retention knob for computer numerically-controlled machining centers with automatic tool changers. The requirements are intended to provide tool holder interchangeability between machining centers with automatic tool changers of various types.
The dimensions for cone-angle control are in accordance with the International Standard ISO-1947. This Standard will improve the understanding of the “CAT” toolholder, its associated components, and nominal operational values. It unifies the principle components of the basic machine tool holder interface—toolholders hank and spindle receiver geometry, pull stud, and conical taper information--into a single-source reference, providing instant access to information.
This new information also eliminates ambiguities and establishes absolutes for all aspects of the toolholder/spindle interface.Intended forthose involved in the design, manufacture, use, or maintenance of steep-taper (7:24) toolholders and their ancillary components.
- All Parlec and Techniks Rotary toolholders meets or exceeds JIS B6339-1986 specifications. ƒ
- Taper is toleranced so that any error increases rate of taper only. Refer to Manufacturing Specifications for tolerance specifications.
The spindle interface JIS B 6339 as the traditional interface for milling spindles distinguishes itself through it robust design. Its field of application ranges from fine machining to heavy duty roughing. The tool holder is pulled in the milling spindle with the help of an additional pull stud.
The centering takes place via the taper contact. Therefore, the JIS B 6339 interface is primarily suitable for applications with a spindle speed of up to 12,000 rpm in an unbalanced condition.
New operators should be taught how to properly evaluate toolholders so they can recognize when toolholders need to be replaced to prevent premature cutting tool failure, or even expensive damage to the spindle.
Determining if toolholder components need to be replaced is not a difficult task, but does require that the operator knows what to look for.
A worn out holder will not provide good accuracy and will quickly wear out your cutting tools. Worn tooling causes poor surface finish, and may damage your spindle.
- Spindle Mouth Wear
- ATC Alignment Issues
- Taper Wear / Fretting
A worn spindle can cause runout issues that affect tool-holder accuracy and reduce cutting quality and productivity. This is a condition known as bell mouthing. If toolholder issues can be eliminated by bench checking T.I.R., then the source of the problem is often a worn out spindle mouth. A trained professional will be required to check and repair bell mouthing.
Check the taper for signs of wear or damage where it contacts the spindle mouth. Any problems with the taper will have a direct effect on machining accuracy. If there are any imperfections on the taper, the toolholder should not be used. If noticeable marking is evident on the taper a condition called fretting may be occurring.
Fretting happens when two steel parts (holder and spindle mouth) are rubbing against one another.
Once a toolholder is fretted it can pass the fretting to other spindles. A spindle with fretting can pass the fretting to other toolholders. Fretting in this sense if akin to sexually transmitted diseases and it should be considered just a seriously.
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