Collet choice? ER-20, ER-16, ER-32, or...?

[cnczoner]

I have decided to get a set of collets so that I don't have to use that drill chuck anymore for "non-standard" drill bits (like the sizes for #4, #6, #8 tap holes, etc). Max size I need this for is just shy of 1/4".

I have pretty much decided to pick up an ER-20 collet holder (to fit my R8 spindle) and a set of collets, because I was thinking I might pick up the Tormach TTS some day, and I wouldn't have to purchase a whole new set then. But how far to either side of the specified size can I use each collet for? As an example, a #6 tap drill is #36, which is 0.1065". Most people sell ER-20 collets in increments of 1/32", and the closest two are 3/32" (.09375") and 1/8" (0.125"). Can I use a 0.1065" bit on either of those?

Another option is to use a metric collet. 0.1065" = 2.705mm, so it's closer to 3mm, and I can get an ER-20 collet for that. Is this necessary?

BTW, I said " most people sell ER-20 collets in increments of 1/32" " above, because I can also find these in 1/64" increments from Tormach, but it's a large expensive set, and I don't need a lot of those right now. Also, if I ever had a problem with one, they don't sell individual replacements.

Is there a better option for my needs, or is this example fully acceptable usage?

Cheers,
-Neil.

[Geof]

The ER collets, 16 and 32, both can accomodate 1mm variation, which is a bit more than 1/32"; I guess the ER20 can do the same.

This means a 7mm can grip down to 6mm; etc.

[cnczoner]

Thanks. To clarify, "1mm variation" means +1mm or -1mm correct (not 1/2mm on either side). Wonder why there would be 1/64's then, perhaps just better to be closer matched I guess...?

[DareBee]

IMO the smaller sized collets won't stretch quite that far, but a .105 drill will easily work in a .125 collet.

[Geof]

Originally Posted by cnczoner Thanks. To clarify, "1mm variation" means.... -1mm correct ....

Correct, they can close up 1mm although as Darebee points out on the small sizes they do tend to bind.

Don't try to make them 'open up' to take something slightly larger than their largest size, it can be very difficult to get out again.

[cnczoner]

I just spreadsheeted ("spreadshot"?) the sizes I use regularly, and the closest collet size I would need for these. Looks like my max deviation is 0.012", so I think 5-6 collets of these specific sizes will meet my needs. Thanks.

[DareBee]

As Geof said above

Don't try to make them 'open up' to take something slightly larger than their largest size

You can get away with a few thou over but you will have difficulties with putting .010 over in.

[cnczoner]

Hmmm... I read that wrong then. I took "largest size" to mean specified size + the variation, and figured a 10-thou to be well within that largest size. So this changes things a bit, but still doable. Max deviation now is 28- and 29-thou for the #10 taps. I don't use those often, but perhaps that's the reason to hunt down a 11/64" collet someday. Revised table...

Thanks,
-Neil.

[DareBee]

Use the collets where you can and get some drill chucks for the rest.
I suppose you could also get a few spare collets and bore grind or EDM them to your requirements.
I have also turned down the shanks of drill bits that have needed to go into collets.

[cnczoner]

Funny thing here is that I started off (another thread a while back) asking about drill chucks that have almost the same height as R8 collets, because I only have 5" of CNC'ed Z travel (the quill). That's when I was informed of collets.

I was wondering about re-sizing/drilling collets, as I saw some "emergency" r8 collets in catalogs, but haven't seen any emergency ER collets. I might consider doing this someday, but for now it's not a big need.

Cheers,
-Neil.

[g-codeguy]

As previously stated, don't use undersize collets. Don't put a 1/8 drill in a 3mm collet. One thing the guys at work will do is to use something like a '0' center drill, and then tighten the pi$$ out of the collet. Then when I go to use that collet, I have to throw it in the garbage. There will be a step in the collet at the point the center drill was inserted to. You can't get a drill of the correct size to go past this step.

I've never heard of such a thing as an emergency ER collet. Bet it would be fun putting in all those spacers so you could bore it to size.

[AppEng]

Originally Posted by Geof The ER collets, 16 and 32, both can accomodate 1mm variation, which is a bit more than 1/32"; I guess the ER20 can do the same. This means a 7mm can grip down to 6mm; etc.

Incorrect! Standard precision ER type collets have 1mm TOTAL variation; meaning the collet will go approximately .015" bigger or collapse .015" smaller, although technically you should always use the exact size for accuracy and collet life.