What's the primary benefit of the adjustable 3 jaw scroll chuck?

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skcncx

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mksj said:
I am an advocate of getting a good set-true style 3J scroll, a slightly larger 4J independent for bigger/rougher/odd shaped stock, and a quality collet type chuck for smaller round stock. QMT/PM carries these chucks in mid-priced range versions, and I feel they offer very good performance to price ratio
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That seems cover just about every chuck scenario... but I'm still foggy on the real value of the set-true (or adjustable, etc...) 3 jaw chuck.

Is this a correct statement? The primary benefit is when you plan to chuck the same sized parts and or need to flip a part and keep it concentric. The second you open or close the jaws too much for a larger or smaller part, you lose the benefit of an adjustable (scroll type) 3 jaw chuck where TIR was all but removed... and you are back to the characteristics of a standard, non adjustable 3 jaw chuck.

If I'm tracking correctly, what's the tolerance for "same sized parts" in an adjustable 3 jaw chuck. In order to keep TIR very low between chucking different parts all have to be within a few thousandths? Or is it more like 1/2"?

Now, I guess there's no harm (other than higher cost) in getting an adjustable 3 jaw chuck. I'd assume the 6" 3 jaw adjustable chuck PM sells is of much higher quality than the economy 3 jaw scroll that's coming with my PM-1236T. But when I decide to upgrade/add to my chuck collection (a future purchase), I wonder if spending more on a higher end 3 jaw and not so concerned with higher end 4 jaw (as I have yet to use one, though, one day I will) is good logic.
 
I use PM's 3 jaw adjustable chuck for everything. The only time I pull out the 4 jaw is if I have a part that is not round. It's so much quicker to indicate a rifle barrel with the 3 jaw and, in the end, it's just as 'true' as a 4 jaw when properly indicated. I'd never go back to a regular 3 jaw.

I can't comment on the repeatability if changing parts. If I were doing repetitive parts I'd use a collet.
 
parshal said:
I use PM's 3 jaw adjustable chuck for everything. The only time I pull out the 4 jaw is if I have a part that is not round. It's so much quicker to indicate a rifle barrel with the 3 jaw and, in the end, it's just as 'true' as a 4 jaw when properly indicated. I'd never go back to a regular 3 jaw.

I can't comment on the repeatability if changing parts. If I were doing repetitive parts I'd use a collet.
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Good to know! Sounds like the option to dial it in is key for you.... when needed.

My Atlas 618 4 jaw has never been used. I'm trying not to get ahead of myself with more accessories than I need to start or rather "re"start with my new lathe upgrade and ALL that goes with it. Upgrading the 3 jaw to PM's adjustable might be more bang for my buck prior to a 4 jaw. At least get a lot more use. Having a 4 jaw at some point is just necessary.

I wish their lathes didn't even come with a chuck, let the buyer pick what level they want.... If I did the upgrade, I'll have the included 3 jaw that never sees the light of day... unless I re-purpose for something else.
 
skcncx said:
Upgrading the 3 jaw to PM's adjustable might be more bang for my buck prior to a 4 jaw. At least get a lot more use. Having a 4 jaw at some point is just necessary.
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I would agree . A set true type 3 jaw and a 4 jaw will darn near cover anything . The 3 jaw gets the most use though . You can get into collets of all kinds also , and 4 jaw scroll chucks etc etc etc in the future but a good 4 jaw and adjustable 3 jaw are the basics .
 
Assuming the scroll is not worn or damaged, an adjustable 3-jaw that has been dialed in for a particular diameter will be very close at other diameters. I've only adjusted my Buck once, but then I'm not doing repetitive work with it and rarely require close concentricity with the chucked diameter. My collets get far more use than my 3-jaw, and I have to check my 4-jaw from time to time to make sure it isn't rusting in the cabinet.
 
parshal said:
I use a hex er-40 collet block in the dialed in 3-jaw when I need to do repetitive work.
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That seems like a nice option, maybe a good way to get into collet chucks and have the ability to dial it in.
 
Standard 3J scroll chucks will have different tolerances for TIR depending on the quality and size of the chuck, but for something like a Bison/TMX 6" scroll chuck it is usually within 0.002". But it also depends on the back plate and lathe, one will typically index a chuck on a lathe so it mounts in the same position with the least amount of run out. In addition if they use multiple scroll pinions there can be a variation in TIR based on which pinion is used, and there can be variations across the scroll. A set-tru style one can adjust the TIR down to typically under 0.0004", and it should repeat within this range when you switch out the stock. The tolerance also should repeat within the scroll range, as opposed to just one diameter. So I will often align my chuck with a precision ground 1.25" rod, and will also check it with a 0.5" rod. For the most part it will hold the TIR tolerance.

If you are just turning stock and then cutting it off then there is less of a benefit of a set-true style chuck, but if you remove and reinstall a part for multiple ops or need to do multiple pieces with a high degree of repeatability then it can become a significant factor. My 3J 6" scroll is a PBA Setrite with a single scroll pinion and it repeats very well. The cost difference of getting a set-true style chuck vs. regular of the same quality is usually small, and the chucks tend to be very well made in order to hold their rated TIR spec.

PM/QMT use to carry the Chandox set-true scroll chuck and it had very positive reviews. They now offer a another Taiwanese chuck with same specs., and about 1/2 the price of a similar chuck from Bison/TMX/PBA, so very good value. There are also other factors such as the grind of the jaws and scroll that can effect TIR. Collet chucks work better for smaller diameter stock that cannot be held well in a 3J. Interestingly my 8" 4J combination chuck can hold stock down to 0.2", but I prefer to use collet chucks which are less risky to the hands when working close to the chuck.

6″ Adjustable Ultra Precision 3 Jaw Chuck, D1-4 Mount – Precision Matthews Machinery Co.

www.precisionmatthews.com www.precisionmatthews.com

PBA 6 inch Setrite.jpg
 
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