High Tech Industrial Representation, Inc.
  • Home
  • Products
    • AB Tools
    • AKS Teknik
    • Champion Tool Storage
    • Eppinger
    • Jergens
    • Kyocera Precision
    • SGS Precision Tools
    • K-Tool
    • Masa Tool
    • Oemeta Coolant
    • Preferred Abrasives
    • Techniks Tool Group
  • About
  • Journal
  • Services
  • Promotions
  • Contact
  • Home
  • Products
    • AB Tools
    • AKS Teknik
    • Champion Tool Storage
    • Eppinger
    • Jergens
    • Kyocera Precision
    • SGS Precision Tools
    • K-Tool
    • Masa Tool
    • Oemeta Coolant
    • Preferred Abrasives
    • Techniks Tool Group
  • About
  • Journal
  • Services
  • Promotions
  • Contact
Search

Don't Take Your Retention Knobs for Granted

2/24/2021

0 Comments

 
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.
Retention knob pull stud casues of failure
​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. 


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 Pull Studs 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.

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. 
Percentage of Load on a Retention Knob Thread
Percentage of Load on each thread of a Retention Knob.
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.
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.
Retention Knob Metal Fatigue
These are the two weakest points of any retention knob.
Styles of Retention Knob for Rotary Toolholder
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. 
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 vs. 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%.  
LMT Fette - Thread rolling with F2 Rolling head on CNC lathe
Fette F2 Thread Rolling Head
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.

Shown here is a Fette F2 head cold forming a thread. Note how the three roller forms center and maintain near perfect concentricity of the pull stud shaft.


​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.
By comparison, cut threads interrupt the grain flow creating weak points.
MegaForce Retention Knob features
Photo courtesy Mike Roden at Fette Tool. www.turningconcepts.com

MegaFORCE Geometric Design Features

Picture
Features of Techniks MegaFORCE Retention Knobs
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 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.  

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 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.
​
Picture
MegaFORCE Technical Specifications
  • 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. 

  • 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. 
Broken Pull Stud
  •  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.

Indication Marks on Pull Studs ​is NOT Normal

Damaged Retention Knob
Damaged Retention Knob. photo courtesy Haas Automation. Click photo for original content.
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.

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

Machinist's Guide to Toolholder Maintenance: Part One

7/11/2018

0 Comments

 

General Overview

Modern CNC machines feature high-capacity tool changers that automatically swap toolholders in and out of the spindle as needed, by means of a high speed swing arm or a rotary carousel.
Periodically, toolholders should be examined for wear and if necessary replaced to maintain cutting performance. 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.

Many operators do not know why it is necessary to replace their tooling, or
have the experience to tell when it is time to do so. Determining if toolholder components need to be replaced is not a difficult task, but does require that the operator knows what to look for.

This article will cover the criteria
used to evaluate collet and nut style tool holders, describe when and why it is necessary to replace them and the implications of not replacing them
Techniks CAT40 ER 32 Collet Chuck Nut
A typical CAT40 ER 32 Collet Chuck with a ER32 Collet and a Collet Nut

Sizing Toolholders

A typical size description of a toolholder is CAT40 x ER 32. The “CAT” refers to the flange type, “40” is the taper size, and “ER 32” is the type and size of collet that fits into the pocket.

The other dimension to be aware of is the “Gauge
Length”. This refers to the distance the toolholder extends from the face of the spindle (see diagram.)
Sizing Roatary Toolholders

Parts of Toolholder

A general understanding of collet toolholder components and their functions is important.

There are four main parts to
a toolholder, which can also be called a a collet holder or collet chuck.
  1. Retention Knob/Pull Stud
  2. Taper
  3. Flange
  4. Collet Pocket

On toolholders that are end mill holders, shell mill holders or drill chucks the those portions below the flange are different than depicted here.

Parts of a Steep Taper Rotary Toolholder
Parts of a Steep Taper Rotary Toolholder

Pull Studs / Retention Knob

Retention Knobs, which are also called Pull Studs, are extremely important because they keep the toolholder in the spindle. Using worn pull studs or using the wrong pullstud for your machine may cause the toolholder to suddenly fly out of the spindle during operation, causing an unsafe situation for the machine operator

The retention knob screws into the top of the taper of the tool-holder. Some pull studs are hollow, to permit coolant to flow thru the toolholder.

When in use, the retention knob is held by the clamping set inside the spindle which pulls the hold
er up into the spindle mouth. A spring-loaded draw bar pulls the holder into place.
CAT40 - ANSI-C 45° retention knob / pull stud

Taper

The taper is the conical shaped area of the toolholder that enters the spindle when changing the tool. An 8 degree taper automatically centers the tool into the spindle. The taper is accurately ground to a tolerance of .0002” for both the taper tolerance and outside diameter tolerance.

There is a measurement, AT, of toolholder tapers that designates AT1 through AT8. Most all manufacturers specify an AT3 Taper tolerance as most spindles at made to an AT2 tolerance. Higher speed toolholders will hold a tighter, (AT2, AT1) tolerance.

Some
toolholders like HSK have a shorter taper than BT or CAT style.
Taper Tolerance

V- Flange

The v-flange is the part of the toolholder that the automatic tool changer locks onto when moving the tool from the tool changer to the spindle and back again. The flange is visually identified as the “V” groove found on the outer most diameter of the toolholder. Cutouts in the flange help orientate the holder in the spindle.

Note how HSK taper (right) is a dual-contact taper. Meaning that it is flush with the gauge
line of the spindle face, creating dual contact between the flange of the holder and the spindle face, and the taper itself and the spindle mouth. Dual contact increases tool-holder rigidity for improved performance especially at extended gauge lengths. Techniks DualDRIVE toolholders provide dual contact on V-flange (BT, CAT) spindles
Picture

Collet Pocket

Picture
The last part of the toolholder is the collet pocket, into which the collet is inserted before being secured by various types of collet nuts. The collet pocket, internal 8 degree taper,  should hold the same tolerance as the taper as they work together to control runout
0 Comments

How to Check Runout on a Rotary Toolholder

2/14/2018

0 Comments

 
Watch this video and learn how to check runout (total indicator runout or T.I.R.) to check your CNC machine tool holders performance. 

Good T.I.R. (less than .0002") keeps cutting tools cutting smoothly and prevents cutting tools from wearing out prematurely.

Tool holders do wear out over time and if good T.I.R. cannot be achieved either the collet, nut, or holder may need replaced
0 Comments

    News & Applications

    Learn about the latest breaking news and applications that we're working on right here!

    Archives

    June 2022
    February 2021
    February 2020
    September 2019
    April 2019
    March 2019
    February 2019
    November 2018
    September 2018
    July 2018
    June 2018
    May 2018
    April 2018
    February 2018
    September 2016
    February 2014
    July 2012

    Categories

    All
    3D Printing
    Back Counterbore
    Balax
    Ball Lock
    Case Study
    Collet
    High Tech
    IMTS
    Inspection Certificate
    Jergens
    K-Tool
    Kyocera SGS
    Maintenance
    Mark Hurst
    Martin Trunnion
    Masa Tool
    Miniature Gages
    Prototyping
    Pull Studs
    Retention Knobs
    Rotary Toolholders
    Runout
    Steel 4140
    Taper
    Techniks
    Threadfloer
    Thread Forming
    Thread Gages
    Todd Pakiz
    Trunnion Table
    Video
    Visualization
    Workholding
    Z- Carb
    Zero Point System

    RSS Feed

ABOUT
JOURNAL
CONTACT

High Tech Industrial Representation, Inc.

16610 Legacy Lane
​New Prague, MN 56071
612-712-4847
​[email protected]
© 2024  High Tech Industrial Representation, Inc.  All Rights Reserved
web design by Rapid Production Marketing
  • Home
  • Products
    • AB Tools
    • AKS Teknik
    • Champion Tool Storage
    • Eppinger
    • Jergens
    • Kyocera Precision
    • SGS Precision Tools
    • K-Tool
    • Masa Tool
    • Oemeta Coolant
    • Preferred Abrasives
    • Techniks Tool Group
  • About
  • Journal
  • Services
  • Promotions
  • Contact