CNC Collet and Chuck Choices

      CNC pros compare the advantages of different tool-gripping technology. November 26, 2007

Question
Most of our collets for our CNC are what you call range collets, where each one can be used for a variety of bits. (12-13mm range for 1/2" and 6-7mm for 1/4".) Recently a tooling salesman was insisting we should have dedicated collets (1/2" collet for a 1/2" bit). Supposedly this will be easier on the spindle, and increase tool life, among other things. Can anyone confirm this, or is it just another pitch?

Forum Responses
(CNC Forum)
From contributor M:
From my experience, range collets should not be used. Buy either metric sizes for metric shanks, or inch sizes for inch shanks.



From contributor J:
We use both range and size collets. Haven't had any problems.


From contributor A:
Why are you using collets in the first place? You should start thinking about hydro chucks, Tribos, shrink fit chucks. No collets to replace; run out on these tool holders is extremely low. They also last a lot longer. My operators love the hydro chucks because tool changes are very quick and very little cleaning is required. Changes are in seconds, not minutes.


From contributor R:
First, the tooling salesman was right. I would go into the technical aspects of this, but I can simplify it by asking you one question... Given how much you paid for that router, do you really want tools and tool holders that are "close enough"? If it were my machine and my money, I would not take a chance on something that might work out, might not... and might tear up my equipment.

Contributor A hit on something. The PS2000 chucks that we [Leuco Tooling] carry as well as the Tribos chucks are simplicity in action, as well as being highly precise. These not only make setup a great deal faster, but also have running tolerances that are greatly superior to the standard collet chucks.



From contributor B:
Hydro chucks? A bit more info, please. I took a quick look at the Techniks site, so have a general idea. Seemed they were specifying use in wet or oily environments, though. Is general wood cutting overkill for these? Cost?


From contributor R:
A hydro chuck is simply a chuck that uses hydraulic pressure to mount the tool. It works the same as hydro lock moulder heads, but somewhat backwards.

You will have a tool holder (for specific sizes only, the "close enough" approach will not work here) and when the tool is set to a specific height, you turn a lock screw which will create pressure in a chamber surrounding the bore in which the tool is set, the bore will contract, thereby locking the tool in place very securely.

Physics dictates that the pressure applied will be evenly distributed if given even points of distribution. This means that the same amount of pressure will be applied over the entire shank of the tool, thereby holding it with equal amounts of pressure as well as centering the tool as accurately as possible.

Just as a note: If one were to try to mount a 12mm shank tool in a 0.500" (12.7mm) hydro chuck, it may return to its pre-pressurized state, but most likely, it will go beyond the locking point and not return to a useable state, at least as far as the 0.500" tool is concerned.

There are a few other options as well. Keep your applications in mind when shopping for tool holders. Standard collet chucks do have a place, but you really need to make sure you use the right collets for the tools you are holding. I wouldn't use anything that was close enough unless I had a real production emergency.



From contributor B:
Before this thread I'd never heard of hydraulic tool holders. Between your explanation and my quick look at the Techniks site, I now at least have an introductory understanding of the concept... and it's pretty slick. I do like the idea.

I use ISO30 tool holders with ER32 collets specifically sized to the bit shank (typically 1/4", 3/8" and 1/2" along with a scattering of other sizes). I cut solid wood typically held in place with vacuum pods. My primary tools are a 3/8" downcut spiral 2-flute HSS bit on a 1/2" shank and the same in carbide as an upcut spiral with a 3/8" shank. Both have a 1 1/4" cutting length. I also frequently use a HSS upcut spiral 1/2" bit with a 1/2" diameter x 1 1/2" long cutting length. We rarely break bits... They usually wear out first.
So, do the hydraulic holders work with ISO30 setups? If not, then that is the end of this for me.

If they do, though, then what would be the typical comparison cost to a tool holder, collet and nut? What is the longevity factor? I know I at least am delinquent in disposing of collets, holders, and nuts that may be beyond their useable life, and probably others out there tend to do the same.

Finally, in a standard collet and nut system, the idea is that as the collet tightens, it becomes perfectly parallel with the shank of the bit as it closes in, thus properly holding the bit in place. What is the oil in the hydraulic holder pressing against that then presses against the bit shank? It would seem there must be some sort of pressure plate involved that matches the bit shank radius.



From contributor R:
It sounds like you have your bit requirements in hand and know what you need and have it in house. Good.

The way it works is the bore where you put the tool is, in reality, a thin steel wall machined to be slightly larger than the O.D. of the router bit. When the bit is inserted, you turn the locking screw and it pressurizes the grease chamber, forcing that thin steel wall to compress inward. The only moving part is the screw itself. When the screw is released, the pressure is released, and the wall flexes back to its original shape.

Yes, is does come in ISO30 style. I won't show pricing here as I think it would be in bad taste, but I can tell you that our ISO30 hydro chuck costs about 40% more than our ISO30 collet chucks.



From contributor B:
Thanks. What happens to that thin wall if the bit spins? Or is the hydraulic pressure great enough that a bit could not spin in these chucks as they occasionally can in a standard collet? This obviously isn't an every day thing, but if it can happen, it will happen... as I'm sure you know. I generally understand hydraulics, so your explanation is just fine. It's a simple and neat system.


From contributor R:
Simple and neat, indeed. And very efficient. What happens if (when) it spins? You buy a new chuck. There is some hope of buffing the bore, but 99.9% of these that come in for inspection have been spun and they are, for lack of a better term, toast. At that point they are non-serviceable.


From contributor C:
Most of the responses are correct. The more accurate your collet is, the better the overall tool life and quality of cut will be. You can take the hydro one step further and use a shrink fit system. This is even more rigid, which is another thing that you are looking for. Most of the range collets are actually just metric collets. The 1/2" you have is more than likely a 12-13mm collet.

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