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Thread: Tool holder stiffness
09-28-2013, 03:05 PM #1
Tool holder stiffnessCame 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.
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 TypeModal StiffnessDyamic Stiffness
Shrink-fit0.890.065 Collet Chuck0.750.155 Hydraulic0.530.196 Milling Chuck0.520.184Surprising 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.
Thoughts for tooling selection time....
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09-28-2013, 03:43 PM #2
I think it's fairly common for machinists to under tighten ER nuts. Rego Fix torque specs seem a bit lower than the Technics, but still higher than you'd think.
I wonder why they don't supply longer wrenches if they expect you to tighten the nuts so much.
Best bet is to use a tourque wrench and take the guess work out. I know Rego and Nikken both make tourque wrenches and nut adapters for the various styles.
09-28-2013, 03:46 PM #3
09-28-2013, 05:48 PM #4
Techniks distributes a torque wrench that takes various end tools, which are (surprize surprize) designed to fit various collets.
I was at some kind of demo where a mold-making expert was demonstrating the wonders of an Okuma milling machine, and it came up in conversation that he felt that (a) shrink fit holders do indeed ring like a bell, but are wonderful for long reach, fine finishing, etc. (b) various sorts of milling chucks (sino-R, hydraulic, etc.) are better for roughing. So the conclusions of the report pretty conventional wisdom to me. Whether that CW is really correct, I cannot say.
09-29-2013, 01:06 AM #5
Here are a few suggestions for attacking cutter chatter.
1 Irregular flute/cutting tooth indexing. Cutting teeth if equally spaced promote mass/compliance resonences and harmonics. If exitations arrive in phase with the previous they add to the circulating energy in the resonance. If the cuttting teeth indexing were randomized a trifle (consider vari-pitch teeth in cut off saw band stock), phase relationships would be disrupted to some degree.
2 Vary spindle RPM +/-5% over the short term (5 seconds for example). Continual speed change has the effect of de-phaseing resonance exitation.
3 Reduce material cutter engagement to the degree possible. Feeding an endmill into an inside corner is a prrime example. Soon as the cutter engages the full fillet more deflection/exitation promotes resonances. Instead circular intepolate the inside radius to minimize the length if engaged cutter.
4 Reduce width of cut. The reason roughing cutters are so successful is that any one tooth engages about 1/3 the stock. The chips come off narrow and fibrous rather than long spiral needles. Finishing endmills can be relieved to interrupt their cutting edges. If deftly done and the currer runs true the finish will be the same but feed rates can be increased.
5 Discourage resonances with damping. Any part of the resonant system is an opportunity for damping. I used to mount cutting tools on gasket material shims when working particularly nasty material like dish shaper nickel aluminumbronze castings The tiny bit of dampin has a profound effect on tool resonance. A heavy ring hung on the extended end of a boring bad, a lead hammer or a length of chain draped over a long shaft - the opportunities and expedients are endless.
In the constricted world of CNC where tools are tightly packed, clearances are scant, and machine elements move abruptly and rapidly discourages old timey resonant reduction tricks but clever programming and tool selection may alleviate much of it.
09-29-2013, 06:56 AM #6
To the OP.
I have seen this table in techincs catalogue a while ago.
There are few things that are not there.
How about gripping force and where is the lonely set-screw type endmill holder?
The truth that hurts is that it could beat all those chucks and collets in almost all of the parameters except for maybe runout.
But runout is only important when finishing.
When i am roughing i dont care that my endmill is runing out half a thou because deflection alone may be 5 thou.
Set-screw holders gripping force is much better than that of any other comon type of chuck. Because of physical connection that does not rely on friction the only way to spin the tool inside the tightened holder is to snap it off. This is why some MFGs add either sidelocking features or square shanks etc to their collets and shrink fit holders.
We could argue based on facts if the table was complete.
Unfortunately it is not and OP's source is not a very reliable too.
Disclaimer i am speaking from my experience only.
Edit: i dont think anyone will notice one thenth runout on a 1/8 endmill, and i dont think 0.0005" runout is noticable on a half-inch rougher as well
09-29-2013, 07:37 AM #7
When I see the greater dynamic stiffness of the chucks I have to think it is a result of a larger diameter toolholder shank rather than any improvement in the tool holding mechanism.
When you are doing serious hogging, the bulk of a collet nut makes it more difficult to get coolant into the cut to flush out the chips. In close quarters deep work the nut also makes it more difficult to get the toolholder inside the work so longer tools have to be used.
As far as runout, I always preferred to get runout down to 20% to 25% of the feed per tooth. This removes tool diameter from the discussion. A little runout has about the same effect as Forrest's unequal pitch spacing on the teeth on harmonics.
09-29-2013, 10:03 AM #8
Collet chucks should come with a wire feed welder so you can put a spot weld on the endmill before you load it into the chuck so it cant pull out.
that or a sledge hammer to get it torqued up nice and tight.
Then you reduce feed rates and speeds to maximize the time to do the job. why bite off .250" when you can take 10 passes at .025"? I want to charge my customer by the mile of table travel VS. the job.
I just love to spend two days milling on one parts because the POS collet chuck is the weak link, its not like there are pallet loads of work that needs done too.
I would buy a collet chuck if I had a 1 HP mill and did lots of aluminum. Till then I will stick with endmill holders.
freakin collets are just a tiny step up from a drill chuck.
09-29-2013, 10:32 AM #9
I have not found collet chucks to be a pullout issue for the tools I use. I don't do huge parts, so my roughers are usually 1/2" max. I run those 1/2" roughers(In steel/iron) until my 10hp vmcs are at 100% spindle load + without them moving at all. They will do that all day, everyday without issue.
I personally hate sidelock holders. I don't like having to grind flats, have had endmills that either don't want to go in, or come out. I wouldn't use them unless I had to.
John Welden liked this post
09-29-2013, 10:46 AM #10
A subscription to Playboy keeps my tool stiff as all get out