Collet chuck diameter VS tool shank size

[hacklordsniper]

Does a end mill with 6 mm shank go into 7-6mm collet or 6-5 mm collet?

 

[Jasonb]

I would use a 6-5mm size.

J

 

[MuellerNick]

I assume, you are talking about ER-collets.
Preferably, a 6 mm shank goes into a 6…5 mm collet.
But you could use a 7…6 mm collet. But with the mill's diameters of 3, 6, 8, 10, 12, 16, 20 ... mm, it is economic just to buy these sizes with their nominal diameter being the same (nominal diameter = bigger diameter).

Do NOT insert a bigger shaft than nominal size into an ER collet!


Nick

 

[mu38&Bg#]

The cheap Chinese ER collets I have, marked say 7-6 have a resting size smaller than 7. That is, to use a 7mm tool the collet must expand. This is inconsistent through the sizes in the set as well. My name brand collets are, like Nick says, at the upper limit at rest, so putting a 1/8" tool into a 1/8" Lindex collet, grips as soon as the nut is snugged. Cheap ER collets I've found are pretty bad, so I have good ones for milling.

 

[MuellerNick]

The cheap Chinese ER collets I have, marked say 7-6 have a resting size smaller than 7.

Aside from the attribute "Chinese", it doesn't mean that much. If a collet falls to the floor, it already might get -seemingly- undersize.
I don't buy cheap collets (well, I do have a few) because of their runout. That doesn't do any good to a milling bit. If the runout get near (or above) the feed per tooth, one tooth of the cutter doesn't cut but only rubs and wears quicker.

OTOH, if you collet holder / spindle has a runout, you can compensate that with a cheap collet by rotating it to the position with the least runout. hahaha


Nick

 

[mu38&Bg#]

Nick, these were like this from day one. Unless dropping on the floor was a factory option free of charge. I bought a cheap ER set to get started, but quickly learned they weren't good enough. Only reputable brands from now on.

 

[MuellerNick]

Unless dropping on the floor was a factory option free of charge.

Haha!
The undersize might have to do with the manufacturing process.
AFAIK, they are completely machined with the slots already cut. To prevent collapsing while machining, there is a ring at both ends that keeps the collet at the same diameter and that is NOT slit. The last step is to remove both rings. It might happen, that the collet then collapses a bit. Might have to do with not carefully tempering it.
But I can only guess.

I buy Haimer collets and can be sure to get top quality.

Nick

 

[hacklordsniper]

Sorry for not saying im using ER25 collets. My problem is if I use end mill with 6 mm shank on 7-6mm collet screwing the nut all the way and my end mill is still not held strong. I tried using 6-5 mm collet but i can only tighten the nut 1 thread so i managed to strip the thread on collet chuck.