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INDICATION MARKS ON PULL STUDS ​IS NOT NORMAL

5/25/2021

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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.
Picture
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.

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To Balance, Or Not To Balance? Toolholders, That Is

3/16/2021

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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.
An argument can be made for balancing almost every tool put in a machine. In the world of rotating tools, small changes to an assembly, like a new cutting tool, collet, nut or retention knob, can put an assembly out of tolerance.

​Therefore, it stands to reason that any unbalance could translate to the part, tooling and/or machine spindle in harmful ways. You’ll hear the case for balancing every single tool based on the 
long-standing ISO 1940-1 standard.
over-balanced-tool-holder
Balancing a toolholder several times causes the toolholder to become excessively modified. It's OVERBALANCED
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.
In other words, if you’re rough milling with a heavy radial cut, the different forces will start bending the tool. When that happens, the cutting forces and all the feed forces will be substantially higher than whatever the unbalance forces might be. If that’s the case, it’s not that you take the unbalance force and add it to the cutting force and find your adjustment. 
Big Kiaser New Baby Chuck and Mega New Baby Chuck are balanced for High speed machining
Big Kiaser New Baby Chuck and Mega New Baby Chuck are balanced for High speed machining. The Precision collet is guaranteed to produce a maximum runout of only 1 micron at the collet nose.
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.
BIG KIASER Mega ER Balanced holders
All the components of Big Kaiser's Mega ER Grip Series - Body, Collet and Collet nut - Are all balanced for high speed machining

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.
Auto-balancing boring heads are designed specifically for the high-speed finishing I mentioned earlier, where unbalance force can be greater than cutting force. Our EWB boring heads, for instance, have a small internal counterweight that moves in direct proportion with each adjustment. Because the weight is carbide, it’s three times more dense than the steel in the tool carrier and is maintained inside the head’s symmetrical body.
Picture
Autobalance boring heads, Series 310 EWB, maintain perfect balance throughout the work range due to the integrated counter-balance mechanism. Even at maximum speeds, balanced tools guarantee vibration-free boring, resulting in increased productivity and high precision.
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.
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Don't Take Your Retention Knobs for Granted

2/16/2021

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​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. 
1 Retention knob pull stud casues of failure

Retention knobs go through thousands of tool changes which means that they are subjected to the very high pulling forces from the spindle’s drawbar.

This force can be up to 2300 ft. lbs. for 40 taper toolholders and up to 5000 ft. lbs. for 50 taper toolholders.
​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 threads on your retention knob will stretch slightly when load is applied and the loading borne on each thread is different.

When you apply a tensile load on a threaded pull stud, the first thread at the point of connection sees the highest percentage of the load.
Percentage of Load on a Retention Knob Thread
Percentage of Load on each thread of a Retention Knob.
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?  ​
For the same reason we put corner radiuses on end mills, sharp corners are a common area of failure for any mechanical device.

​The same holds true with your pull studs:  The sharp angles on the head of the retention knob and at the minor diameter of the threads are common locations of catastrophic material failure.
Retention Knob Metal Fatigue
These are the two weakest points of any 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.
Styles of Retention Knob for Rotary Toolholders
Styles of MegaFORCE Retention Knobs

MATERIAL

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

5. Cut thread vs rolled thread retention knob
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%.  
6. LMT Fette - Thread rolling with F2 Rolling head on CNC lathe
Shown here is a Fette head cold forming a thread. Note how the three roller forms center and maintain near perfect concentricity of the pull stud shaft.
In cold forming, the thread rolls are pressed into the component, stressing the material beyond its yield point. This causes the component material to be deformed plastically, and thus, permanently.

There are three rollers in the typical thread rolling head that maintain better concentricity by default than single point cutting of the threads.

Also, unlike thread cutting, the grain structure of the material is displaced not removed.
Rolled threads produce grain flows that follow the contour of the threads making for a stronger thread at the pitch diameter which is the highest point of wear. 

The cold forming process also cold works the material which takes advantage of the nickel work hardening properties of 8620.
7. Fette Turning Concepts Thread Rolling Magnaflux
Photo courtesy Mike Roden at Fette Tool. www.turningconcepts.com
By comparison, cut threads interrupt the grain flow creating weak points.

MEGAFORCE GEOMETRIC DESIGN

MegaForce Retention Knob features vs standard pull stud
Overall Length
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.
Blended Radii
With the new Techniks MegaFORCE pull studs, stress risers of sharp angles have been eliminated through the blended radii on the neck where the gripper engages under the head of the pull stud.
9. Techniks MegaFORCE Pull Studs
Ground Pilot
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.
MegaFORCE Technical Specs
  • Material: SAE8620
  • All knobs are case carbrized, hardened, and tempered to:
    • Case depth: 0.025” – 0.030”
    • Surface hardness: HRc 56-60
    • Core hardness: HRc 44 minimum
Torque Specs
The following are the guidelines for torquing your pull studs according to Techniks.
  • BT 30 36 ft. lbs.
  • ISO 30 - 36 ft. lbs.
  • 40 taper - 76 ft. lbs.
  • 50 Taper - 100 ft. lbs.

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.
  • Regularly inspect retention knobs for signs of wear. Wear may appear as dimples or grooves under the head or visible corrosion anywhere on the retention knob. 
Picture
  • If the retention knob demonstrates any signs of wear replace it immediately
  • Make sure to properly torque the retention knob to the manufacturer’s specifications. Use a torque wrench and retention knob adapter to ensure proper torque. 
  • Overtightening can overly stress the retention knob leading to premature failure and can cause the tool holder taper to swell leading to a poor fit between the machine spindle and the tool holder.
  • Apply a light coat of grease to the retention knob MONTHLY to lubricate the drawbar. If you use through-spindle coolant (TSC), apply grease to the retention knobs WEEKLY.

Special thanks for Greg Webb at Techniks and Mike Roden from Fette Tools/ Turning Concepts, for providing technical insights. 
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    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
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    Authorship

    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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