Came across this information on the CNCcookbook page (all Credits given)
Seems like collet systems fare quite well as far as stiffnes goes. No hard numbers for "grip" (pullout resistance) Perhaps there are relationships to be made.
Seems like collet systems fare quite well as far as stiffnes goes. No hard numbers for "grip" (pullout resistance) Perhaps there are relationships to be made.
I read a fascinating analysis of toolholder efficacy in a graduate thesis out of the University of British Colombia ("Mechanics and Dynamics of the Toolholder Spindle Interface") that raised some good data I hadn't seen before:
- HSK toolholders are as much as 4x stiffer than equivalent sized CAT40 holders. This is mainly due to the dual-face contact of the HSK design.- The thesis compared the performance of milling chucks, shrink fit, hydraulic chucks,and collet chucks. The most important characteristic for finishing operations is modal stiffness. In order of best to worst performance, here is how the different toolholders ranked:
Tool Holder Type Modal Stiffness Dyamic StiffnessShrink-fit 0.89 0.065Collet Chuck 0.75 0.155Hydraulic 0.53 0.196Milling Chuck 0.52 0.184Surprising that the lowly collet chuck performed nearly as well as finicky shrink fit tooling and quite a bit better than more expensive hydraulic and power chuck-style holders!However where roughing is concerned, the Dynamic Stiffness is important for suppressing chatter. For maximum material removal rates, we want to maximize Dynamic Stiffness. Here the Collet Chuck also performs pretty darned well, and it is in hogging out lots of material that the hydraulic and milling chuck style holders start to come into their own. The shrink-fit performs poorly because the shrink fit doesn't dampen the vibrations, it just holds the tool very very tightly. Makes you wonder if it doesn't make it more likely to ring like a bell when held so tight?Other toolholder thoughts:- Consider balancing your toolholders. It's a requirement for rpms over 10K, but it will smooth vibrations even at lower rpms. Obviously tools like flycutters are inherently unbalanced to start with, or must they be? Perhaps a design that is balanced will produce a smoother result.- Consider runout. A holder with a lot of runout injects a lot of vibration that will ruin surface finish and ultimately break cutters. Smaller cutters are more sensitive to runout. A thousandth of an inch is a lot of runout for a 1/8" cutter to deal with as it is effectively jerking the cutter around in the cut as it rotates.- Collets often have less runout than setscrew holders. Some complain they don't hold as well. A compromise would be to use set screw holders for larger shanked tooling (say anything over 1/2") and collets for smaller shanks. OTOH, for those that claim the collets don't hold (tools are getting sucked out), others are claiming the toolholder and collet aren't clean, or the nut isn't torqued down tightly enough. The specs for ER collet nut torque are pretty high, so be sure to give the spanner wrench a good tug. Alternatively, I have been using ball bearing nuts which take surprisingly less torque because they have a ball bearing interface with the ER collet instead of trying to spin against the collet as it tightens.- Integral shanks are almost always stiffer than inserting a shanked tool into a toolholder.- Various specialized holders are available such as flatback drives that seat the toolholder against a precision spindle face for more support than the taper alone could provide.
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Thoughts for tooling selection time....
cheers