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."
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