R8 round collet question

[wildcatter]

Hi guys, I have a milling machine with an R8 spindle and have been using round collets to hold endmills. I always wondered about the rigidity of these things but never saw the issue addressed anywhere till I saw a video that said a collet should only have .003 clearance between the endmill shank and the hole in the collet,otherwise the "finger" of the collet will only grip the shank with the very tip of the finger. This makes sense to me, but do you guys check this with every collet you use? I checked a 1/2" endmill in a 1/2" collet and had a nice fit. Then I checked a 3/8 endmill in a 3/8 collet and there was over .020 thousandths clearance and sloppy as all get-out. Does this mean my collet is junk? What do you guys do? Hope someone can enlighten me on this.

 

[benmychree]

End mills that I have are quite close tolerance for diameter, your collet is an unknown, is it simply sprung out and does it return to a close fit when it grips the tool shank? The fact that it is oversize when retracted may not mean much if it tightens up when clamped. Personally I like setscrew style end mill holders rather than collets.

 

[wildcatter]

The first thing that popped into my head when I saw how sloppy the fit was : " I should be using set screw type holders". You confirmed my feelings. I am going to see if I can find a way of checking collet contact with the shank, maybe layout blue or just snug the collet up with an endmill in place and see if there is any discernable wobble. maybe the fit is great once the collet is tight.

 

[benmychree]

The first thing that popped into my head when I saw how sloppy the fit was : " I should be using set screw type holders". You confirmed my feelings. I am going to see if I can find a way of checking collet contact with the shank, maybe layout blue or just snug the collet up with an endmill in place and see if there is any discernable wobble. maybe the fit is great once the collet is tight.

Good plan!

 

[MrWhoopee]

Reminds me of the employee who was dreaming up a hundred possible problems in order to avoid actually doing the job. Put the endmill in the collet and get back to work. If there's an actual problem (and there will be enough), troubleshoot it when it arises.

 

[benmychree]

Reminds me of the employee who was dreaming up a hundred possible problems in order to avoid actually doing the job. Put the endmill in the collet and get back to work. If there's an actual problem (and there will be enough), troubleshoot it when it arises.

Agreed! I was accused of that attitude oh! so many years ago, and perhaps somewhat guilty, but anticipating problems (within reason) avoids spoiling work; there is a fine balance here!

 

[mksj]

When I checked on this several years ago, the clamping range for R8 collets was something in the range of +/-0.002". These will vary by collet design and manufacturer. Most of my endmills and center finders are a snug fit into my Lyndex R8 collets, too looses and they will not clamp evenly. They all should be consistent in the dimensions/fit, something usually not seen in the generic Chinese collets. Also many will not fit the R8 guide pins, I had purchase a Chinese set from eBay many years ago, it got returned because the QC was terrible. This was 12+ years ago, maybe they have gotten better. The same holds for many ER collets at their clamping range limits, so better to get close fitting collets. Chuck up some precision ground rod or a end mill with a long shank and check the TIR.

R8 collet clamping range - is there a standard?

I searched for a published industry standard for clamping range for R8 collets. I had no luck finding one. As always, I found McMaster helpful. They specify a tolerance for what they sell of -.004" to 0". BTW, the 5C's McMaster sells are Lyndex so I would expect their R8's are of the same...

www.hobby-machinist.com

 

[wildcatter]

I'm not trying to avoid a job, but i'd rather take care of a problem before I spend a couple hours on a job, not after. I get you point about over-analyzing a job but this is a pretty basic issue. Still, any and all advice is greatly appreciated!

 

[jwmelvin]

I think your consideration is good. But if you are clamping a 0.375” tool in a 0.375” collet, that is the correct fit even though the collet may have a larger opening before it is tightened. You can count on the collets to be appropriate for the size they are marked with.

 

[C-Bag]

It has been a good thing for me to check problems as they come up because I won’t always remember to go back and understand if it really is a potential problem or just belly button gazing. Because of certain criteria like how precise do I need to be and how critical things are I’ve also gone to set screw style mill holders when I had mystery problems with holding tolerance. And it solved it. But I’m glad I have a selection of collets.