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 know 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."
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."
Getting coolant through the toolholder and to your cutting tool can be accomplished in two ways with Parlec and Techniks toolholders. Coolant is delivered from the spindle by two methods:
Through-Flange/ DIN B Coolant Delivery
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".
Through Flange/ Form B is an available standard for many tools and available as a standard modification for most toolholders.
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
We have had several inquiries regarding steep taper rotary toolholders specifications. Below you will find all of the technical reference information related to V-Flange Tolling Tapers and dimensions.
CAT V Flange Taper Specifications
About the ASME B5.50 – 2009 Standard
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.
BT Taper Specificationsƒ
BT-40 Shank is also known as: JMTBA MAS-403 "BT", JIS B 6339 - 1986, JIS B6339 - 1992, and ISO 7388/1 - 1983.
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.
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.
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.
This article will discuss the following types of causes and types of wear.
Checking For Spindle Mouth Wear
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.
Taper Wear / Fretting
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.
Fretting is believed to be caused by imperfect mating between tooholder taper and spindle, creating vibration and heat which develops the fretting. It is visible as small copper colored pits or marks on the taper. This is evidence that the toolholder is becoming worn. Fret-ting is easily mistaken for rust, but it is not. Once noticeable fretting develops the toolholder should be replaced. New toolholders that quickly develop fretting, or toolholders that stick in the spindle, may indicate a spindle that needs to be reground.
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