# Collets and concentricity



## Julia (Sep 30, 2019)

I've been reading up and watching videos to try and learn some basics about machining (cheers blondihacks). I notice a lot of the times people use a collet chuck to hold an item, especially when they want to machine 2 ends of say a shaft, and they want to maintain concentricity. What I don't understand is how a collet chuck achieves this. Most collet chucks appear to be essentially a many jaw'd chuck, how can they be better at maintaining concentricity than say a 6 jaw chuck? 

What am I missing? 

J


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## RJSakowski (Sep 30, 2019)

A collet has a very limited range of operation.  5C collets typically are 1/32" or less, ER collets a little more.  Additionally, the surface that forces the closure of the collet is immediately adjacent unlike the cantilevered jaws of a three jaw chuck.  The three jaw chuck also uses a scroll to close the jaws and imperfections through manufacturing or wear will affect jaw position.  A six jaw chuck generally does a better job of capturing the part but issues with the cantilevering and scroll still apply. Chuck jaws also have various complex features which all have to be made to perfection in order for it to maintain concentricity.  A collet has relatively simple geometry.


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## chips&more (Sep 30, 2019)

RJSakowski said:


> A collet has a very limited range of operation.  5C collets typically are 1/32" or less,


Hi and welcome!
I’m not sure about a 5C collet being useful at 1/32” from nominal? Especially going bigger from nominal.
ER collets, rubber flex and some others can tolerate some variance from nominal.
The big difference is “holding surface area” of a collet versus a 3 or 6 jaw chuck.
Everything has a manufactures tolerance (and some are fudged!!!). So, using a collet and or 3-6 jaw chuck will have runout. I would highly recommend getting a collet chuck and or 3-6 jaw chuck that has runout adjustment. Good Luck…Dave.


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## mikey (Sep 30, 2019)

To add on to the above, consider that a collet chuck has a standardized internal taper that is supposed to be precision ground. This shape is supposed to be matched on the exterior of the collets that fit the taper. When the collet nut or draw bar locks the collet in place, all the collet does is collapse into this taper. There are no real moving parts in a collet; it just collapses down onto your part or tool and this maintains as much concentricity as the chuck and collet are capable of. In contrast, all non-set-true scroll chucks have radially moving parts and by design cannot be nearly as accurate as a collet. A 4 jaw independent chuck and set-true type scroll chucks, on the other hand, have the potential to be even more accurate than a collet.

An ER collet chuck and similar chucks are good for holding an already turned part or one with an accurately ground surface. In these cases, it can be pretty accurate. A 5C chuck can hold nominal stock with a smooth surface, like cold rolled stock or smooth extruded stock; it isn't nearly as accurate when holding this kind of material as it is when holding ground stock but it is fast to use and secure enough for work holding. Collet chucks are also good for holding threaded parts without damaging the threads.

For general work, a 3 jaw chuck is fine for any first operation (initial turning). For second operation work (holding a part that has already been turned), a collet chuck is fine. So is a 4 jaw independent or a set-true type chuck. 

Bottom line: buy a collet chuck, like an ER chuck, if you need to hold threaded parts or if you abhor the 4 jaw chuck. If you plan to do production runs with cold rolled nominal stock then a 5C collet chuck makes sense. Otherwise, collet chucks for the lathe are largely superfluous in a hobby shop.


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## ttabbal (Sep 30, 2019)

Yeah, but they are nice for doofuses like me that are always holding finished bits rather than planning ahead properly.  

That said, a 4 jaw independent, properly adjusted, is at least as good at holding things concentric as a good collet. And it can hold oddly shaped things. Adjusting it takes time and practice. I need a couple of minutes to dial mine in, but some people are able to pull it off with only once or twice around the chuck. 

My ER collet chuck has made me lazy about honing my 4 jaw skills.


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## RJSakowski (Sep 30, 2019)

chips&more said:


> Hi and welcome!
> I’m not sure about a 5C collet being useful at 1/32” from nominal? Especially going bigger from nominal.
> ER collets, rubber flex and some others can tolerate some variance from nominal.
> The big difference is “holding surface area” of a collet versus a 3 or 6 jaw chuck.
> Everything has a manufactures tolerance (and some are fudged!!!). So, using a collet and or 3-6 jaw chuck will have runout. I would highly recommend getting a collet chuck and or 3-6 jaw chuck that has runout adjustment. Good Luck…Dave.


1/32" is the gripping range of a 5C collet.  The recommended working range is more like half that and I have seen it written that for best concentricity, 1/128" should be used.  To my knowledge, a collet should not be used for work which is larger than the nominal diameter

A 5C collet only grips a full circle at the nominal diameter.  At less than the nominal diameter, the collet grips at three points at the center of the segment while the outside of the collet grips at the edges adjacent to the kerf. Additional the collet segment moves on an arc with the end of the collet making contact first.  As the collet is tightened, the outer surface of the collet comes closer to being in full contact with the collet chuck and the work.  Until full contact is made the work can wobble in the collet.  As the work size gets smaller relative to the nominal diameter, the required force to distort the collet into full contact increases.  This is why the recommended working range is restricted.  Hardinge sells 5C collets in .001" increments.


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## whitmore (Sep 30, 2019)

Julia said:


> ... collet chucks appear to be essentially a many jaw'd chuck, how can they be better at maintaining concentricity than say a 6 jaw chuck?



A collet is ONE part, finished on the exterior, roughed on the internal surfaces, then slit at three (for 5C types) places
and given a little time to 'relax' to shape.
The whole thing is then given a finishing internal grind, as it is spun in an accurate spindle.
As long as you don't mount the collet with metal chips in the holder, it's as concentric as a single
piece of metal that just came from the grinder: six jaws, rubbing on channels, with a history of 
'oops' collisions on the protruding parts, sliding against a spiral scroll... is good, but not perfect.


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## MrWhoopee (Oct 21, 2019)

Never used a collet chuck. I've used lever type drawtube, handwheel drawtube and spindle nose handwheel type frequently. I'm so thankful that my SB Heavy 10L came with a lever type and full set of collets by 16ths. It lives on the lathe and is used much more than either chuck, including a Buck Adjust-tru.


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## savarin (Oct 21, 2019)

I love my ER collet chuck. I do a lot of stainless in various diameters so for me its a godsend.
What I like as well is the rod can pass through so long lengths can be worked on.
I made mine from a chunk of bar, cut the internal thread so it would screw onto the lathe spindle then turned it all to size, bored, cut the internal taper for the collets.
I purchased the collets and collet nut.
I hoped making it on the lathe it will be used on would ensure it was concentric and such was the case.


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## MrWhoopee (Oct 21, 2019)

savarin said:


> I love my ER collet chuck. I do a lot of stainless in various diameters so for me its a godsend.
> What I like as well is the rod can pass through so long lengths can be worked on.
> I made mine from a chunk of bar, cut the internal thread so it would screw onto the lathe spindle then turned it all to size, bored, cut the internal taper for the collets.
> I purchased the collets and collet nut.
> I hoped making it on the lathe it will be used on would ensure it was concentric and such was the case.



Pics please, particularly the tightening system


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## savarin (Oct 21, 2019)

OK I will get some
I did do a build on this site but am unable to find it now.


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## brino (Oct 21, 2019)

MrWhoopee said:


> Pics please, particularly the tightening system





savarin said:


> I did do a build on this site but am unable to find it now.



I found some:
https://www.hobby-machinist.com/threads/adjusting-a-taper-bore.48728/post-411786
https://www.hobby-machinist.com/threads/adjusting-a-taper-bore.48728/post-412148
....but I thought you had posted more.

-brino


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## savarin (Oct 21, 2019)

Thanks Brino I thought there was more as well.
and heres the pin spanner for it








						Potd - Project Of The Day- What Did You Do In Your Shop Today?
					

For the sake of closing this argument and getting back to discussing machining etc. can a ask an administrator to remove the offending two photos from my post #3240.  I can seem to figure out how to edit it myself in this new software.




					www.hobby-machinist.com
				



post 9832
and the wrench for the nut


Dont forget I only made the chuck, I purchased all the collets, nut and special wrench.


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## MrWhoopee (Oct 21, 2019)

brino said:


> I found some:
> https://www.hobby-machinist.com/threads/adjusting-a-taper-bore.48728/post-411786
> https://www.hobby-machinist.com/threads/adjusting-a-taper-bore.48728/post-412148
> ....but I thought you had posted more.
> ...



Ah, thanks. I was, of course, trying to picture a 5c collet.


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## T Bredehoft (Oct 21, 2019)

I use an ER40 chuck for fine work. Before I clamp, I check for concentricity. Subsequent to mounting the chuck I indicate the taper inside, making sure it's as near zero as I can get it (Set True Chuck). after I've tightened the work I check it, usually it within .0005 to .001, but I bump it around to nearest zero I can get it. Then  I re-tighten the chuck and check the part again.

It's tedious, but I get finished parts routinely under .001.  When making tooling, this is ideal.

_Edit; _Correct automatic correction.


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## 682bear (Oct 28, 2019)

To add to everything above...

My South Bend 14 1/2 lathe has 3 chucks, a 9 inch 3 jaw, a 10 inch 3 jaw, and a 12 inch 4 jaw. Because of the flats on the inside of the jaws, the smallest diameter I can hold is about 7/16 inch. I bought a 5C collet chuck for it and a set of collets... this allows me to hold pieces down to 1/16 inch easily.

My 12 inch Hendey lathe has a 10 inch 4 jaw chuck... I have the same problem with it. The jaws, when closed until they are all touching, leaves a hole in the center of about 7/16 inch. The Hendey has a very short bed... I think I may be able to get about 15 inches between centers. The 4 jaw chuck takes about 6 inches away from that. Because of this, I am considering an ER type collet chuck for it, which would only take 3 inches or so of my usable length.

-Bear


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## RJSakowski (Oct 28, 2019)

One of the reasons that I bought a 5C collet chuck was the increased clearance when approaching work from the chuck side.  My ball turner couldn't get close enough with my three jaw to turn a complete ball unless I had an excessive stick-out.


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