Page 40 - CNC West Oct-Nov 2021

THE ONE TENTH = 10% RULE AND THE EFFECTS OF RUNOUT
Article & Photos Supplied by Big Kaiser
Runout is one of the sneakier issues machinists and tool managers must deal with. Put simply, runout is variation in the diameter of a cutting
tool at certain points as it rotates. It’s almost impossible to recognize in-process and difficult to diagnose after the fact. The potential sources are numerous and different.
When it comes to the holder, the lower the ratio of taper-to-taper contact, the more runout. (We machine holders to sub-micron tolerances, achieving at least 96% contact.) The same goes for the point of contact between the cutter and the holder.
The standard collet angle, for example, is 16 degrees, which provides an appealing clamping range. However, a chuck at 12 degrees will make more contact and offer more control of the tool. Solid nuts also introduce more risk of runout in collet assemblies. The last few degrees of wrench pull twists the collet, while a bearing nut maintains low torsion for smooth, concentric clamping.
The size of the tool assembly plays a role; the length- to-diameter ratio and mass are going to affect force as it spins. The larger the tool, the more resistant it is to
runout. With tools 3/4”-diameter tools or larger, runout of 0.0005” may not impair tool life. However, with smaller tools runout may need to be much better.
We’ve also found, in our own research, that the cutter material reacts to runout differently. We tested nearly identical carbide and HSS drills with the same runout and found carbide to be more sensitive to runout.
Another variable that’s even more difficult to control or diagnose is that cheaper, mass-produced holders and cutters can have extra runout essentially built in. It’s just not easy to make high-performance holders and cutting tools. Holders, specifically, need to deliver clamping force, concentricity, rigidity and balance for high speeds. A cheap OEM pullstud made of weaker metal is susceptible to deformation that can affect centerline alignment too.
That said, even the best holder cannot deliver superior performance in a worn or imprecisely ground spindle. Keep those gage bars handy, especially if you’re doing high-speed work when centrifugal forces increase exponentially.
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CNC WEST October/November 2021