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Should a 3-4 mm collet clamp OK on a 3mm drill bit.

mancavedweller

Aluminum
Joined
Aug 29, 2012
Location
Melbourne, Australia
My boss was not too happy with me at one point today.

When I set up the tooling in the cnc lathe, I put a 3mm drill bit in a 3-4 mm collet. Yes I did tighten it enough.

Anyway when it came to drilling the part the drill pushed back in the collet. I told my boss and he acted like I'd done something really stupid, by using a 3-4 mm collet instead of a 2-3 mm collet.

I simply assumed if a collet says 3-4 mm, it is capable of clamping on a 3mm part, otherwise why the hell would it say 3-4 mm.

I've clamped plenty tools that are at the lower end of a collets clamping capability and never had an issue. And I'm talking about carbide insert milling bits on D2 tool steel.

From this point onwards, to cover my arse, I'll never use a collet at the lower end of its clamping spec, but do any of you guys think I did anything stupid with the aforementioned 3 mm drill bit.
 
Unfortunatly, some collets may not tighten well enough if they are not made correctly. However, most of the time it's going to be the drill shank that is the problem as some have the size stamped on the shank...that can prevent the collet from tightening correctly also because it's a raised edge.
 
Not stupid, but maybe a bit of a rookie error. A couple of good explanations above. Also consider that when you clamp something at the bottom of the clamping range, you are only contacting at a few small points. When you are clamping something near the top of the clamping range, you get much more contact area.
 
Actually.. as you have not YET even specified WHICH of several dozen families of collets?

We have to guess, based on the range alone, that ER is a prime candidate.

But even so "not only".

If you need to do this "a lot", see if that Boss will pop for half-millimeter sizes, or a few limited-collapse collets for the sizes used most often.

They can not only grip better - they can last longer doing it from the reduced distortion in their own metal they have to accommodate to deliver a given grip at a given collapse.
 
Not stupid, but maybe a bit of a rookie error. A couple of good explanations above. Also consider that when you clamp something at the bottom of the clamping range, you are only contacting at a few small points. When you are clamping something near the top of the clamping range, you get much more contact area.

Theoretically, anything under nominal is point contact, both around the tool shank and the taper.

I really don't like using ER collets for anything but their nominal size. Unavoidable for drills, but for everything else it's a no-no.

OP, fwiw I've given every single one of my guys the same bollocking at some point or other. I expect your boss just wanted it to sink in so you don't do it again.
 
Thanks for the input everyone.

The collets will be either ER16 or ER20.

Yes I am indeed a rookie.
I've got my own manual lathe, "Bridgeport" (clone), built my own CNC plasma table, and most of what I know is self taught out of interest. I've only had this job in the machine shop for about 3 1/2 months now. I've told my boss before I took the job that I'm just a back yard guy that has a strong interest and whatever I know is from books, Google, Youtube, etc.

Don't see my boss being interested in buying more collets. Example we have this carbide insert lathe tool I detest. The support underneath the carbide has been ground away (for sharpening LOL) by about...........wait for it...........5mm !!!!!!!! So the insert is cantilevered out from what's left of the base. I was gobsmacked when I first seen it, yet it's commonly used in the shop. Every single time I've used it the insert tilts down under the cutting pressure then the edge chips. So all I can do it take multiple light cuts but then the job takes too long. I just can't win, the boss says there's nothing wrong with the tool. I think you'll see if he doesn't chuck a tool like that in the bin and replace it, he won't be investing in better collets.

I just wish my boss would educate me (like you guys have just done) instead of scolding me like I knowingly did something dumb.
 
I really don't like using ER collets for anything but their nominal size. Unavoidable for drills, but for everything else it's a no-no.

+1 My ER (40 and 20) are fitted to damned near everything in the shop but the overhead light fixtures.

ONE SET each, and in "inch" of collets give me "last resort" grip-anything-round-not-so-badly-after-all coverage. And I treat ER as a "last resort".

All else works better on at least one count, if not several, and I'm not short of those many other options.
 








 
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