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.

The Challenge 
The challenge was to achieve both hard and soft cost savings, as well as time spent on drilling flanges from one side, chamfering the backside and creating a controlled chamfer on the top surface which was a secondary process.

The Solution
We designed an indexable drill with our rear cutting carbide deburring insert along with a fixed pocket chamfer insert to create the precision top chamfer that was required. The spade drill insert also had a special feature that allowed it to chamfer the top side of the bottom hole that was pre-drilled.

The Results
​Four operations were completed in one pass.

The EZ Burr Burr-Free Drill drilled the hole, deburred and chamfered the back all in one step, thus eliminating the secondary step of chamfering the bottom surface on the top hole.

This saved time as well as money, plus the tool life of the Burr-Free Drill achieved a more consistent result and had a longer tool life than the previous method.

Tech Tips: Hannibal Carbide

If you are ordering a special drill, here is the nomenclature you should be familiar with when preparing to write your specifications.