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



## skcncx (Aug 26, 2022)

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


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.


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## parshal (Aug 26, 2022)

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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## skcncx (Aug 26, 2022)

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.


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.


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## mmcmdl (Aug 26, 2022)

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.


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 .


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## MrWhoopee (Aug 26, 2022)

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.


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## parshal (Aug 26, 2022)

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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## skcncx (Aug 26, 2022)

parshal said:


> I use a hex er-40 collet block in the dialed in 3-jaw when I need to do repetitive work.


That seems like a nice option, maybe a good way to get into collet chucks and have the ability to dial it in.


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## parshal (Aug 26, 2022)

I have a collet chuck, too, but I rarely, if ever, use it.


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## mksj (Aug 26, 2022)

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


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## mmcmdl (Aug 26, 2022)

Lets not forget about boring soft jaws either .


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## skcncx (Aug 26, 2022)

mksj said:


> 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,


With that the value proposition goes up a lot for me.

I have found myself flipping parts a lot on my Atlas 618 so I would like something on my PM-1236T that is as easy to use as a scroll 3 jaw but still hold good tolerances when I flip parts.

I did lookup a toolmex 6" 3 jaw adjustable, I think they were around $1300... so the PM one for $700 seems like a good deal, and for $50 you can get an extra set of jaws.  PM also states it's precision balanced, I would think that is of benefit, especially for a smaller weight PM-1236T lathe (big for me though)



mksj said:


> My 3J 6" scroll is a PBA Setrite with a single scroll pinion and it repeats very well.


Excuse my ignorance... but single scroll pinion just means there's only one spot on the circumference of the chuck to use the chuck key to clamp it down?

And, am I right, on these adjustable chucks, the way you indicate it in for less TIR, unlike a 4 jaw where you adjust each jaw independently, but rather use some screws on the face of the jaw to move it around?   That's the impression I got by watching a tubalcain video.  So, given the repeatability of an adjustable 3 jaw after you indicate it in, it's set it and forget it.... assuming it doesn't shift out of place.


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## skcncx (Aug 26, 2022)

mmcmdl said:


> Lets not forget about boring soft jaws either .


So, is that like tru-ing up a 3 jaw chuck  just by modifying the jaws themselves while it's turning on the lathe?

I assumed soft jaws were just for softer materials to not mark them and ruin any finish by clamping down on them.


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## mmcmdl (Aug 26, 2022)

skcncx said:


> So, is that like tru-ing up a 3 jaw chuck just by modifying the jaws themselves while it's turning on the lathe?


Soft jaws are either aluminum or steel used for multiple parts requiring close TIR . Bore them to the finish size of your part . They are re-usable also , and can be bored for multiple set-ups . No , they will not mark parts .


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## mikey (Aug 26, 2022)

skcncx said:


> 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.



Unless you're in a production situation, the need to turn the same size stock repeatedly enough to justify a set-tru chuck is not common in a hobby shop. Most work is one off. Flipping a part and getting it concentric again is, however, a common scenario. This can be done with a set-tru or a 4jaw. 

I own two set-tru style chucks - a very fine Yuasa 3 jaw and a mint Pratt Burnerd International 6 jaw. Both are adjustable into the low tenths and neither one is used all that much. 95% of the work I do on a lathe is first operations stuff so a simple 3 jaw is as accurate as I need. If I have to flip a part and tolerances are tight I switch to a 4 jaw, not a set-tru or a collet chuck. 

There is nothing wrong with owning set-tru chucks. Just know that they are not all that necessary in a hobby shop.



skcncx said:


> 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"?


Same means same. If your chuck is adjusted to minimize run out with a specific diameter then if the next part is of a different diameter then run out will increase. This has been measured and proven by many of us. How much pressure you apply to the chuck key, the amount of oil on the stock, and the specific pinion you use to lock the chuck down can also impact on TIR. This really only applies to second op work, most commonly applied to production runs. A collet chuck with a torqued nut may be a better/cheaper option.

I only raise these issues because of the expense of these chucks. They cost a lot of money and I'm not convinced they deliver all that much value in a hobby shop setting. My two chucks alone retail for almost $5,000 and are not used anywhere near enough to justify their cost. Quite frankly, a really good 3 jaw and 4 jaw chuck and a decent collet chuck for threaded or polished work will do most of us just fine.,


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## Jimsehr (Aug 26, 2022)

mikey said:


> Unless you're in a production situation, the need to turn the same size stock repeatedly enough to justify a set-tru chuck is not common in a hobby shop. Most work is one off. Flipping a part and getting it concentric again is, however, a common scenario. This can be done with a set-tru or a 4jaw.
> 
> I own two set-tru style chucks - a very fine Yuasa 3 jaw and a mint Pratt Burnerd International 6 jaw. Both are adjustable into the low tenths and neither one is used all that much. 95% of the work I do on a lathe is first operations stuff so a simple 3 jaw is as accurate as I need. If I have to flip a part and tolerances are tight I switch to a 4 jaw, not a set-tru or a collet chuck.
> 
> ...


A set true chuck needs a little slop to be able to set it true. You can also make a set true chuck by using a larger chuck back plate and counter boring it so that the chuck body fits into the counter bore with a little slop and 4 set screws to adjust it true. That way the chuck body is used as the register.
jimsehr


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## skcncx (Aug 26, 2022)

mikey said:


> I only raise these issues because of the expense of these chucks. They cost a lot of money and I'm not convinced they deliver all that much value in a hobby shop setting. My two chucks alone retail for almost $5,000 and are not used anywhere near enough to justify their cost. Quite frankly, a really good 3 jaw and 4 jaw chuck and a decent collet chuck for threaded or polished work will do most of us just fine.,


I think that's where the value of PMs adjustable 3 jaw chuck made in Taiwan for $700 is a good value.  I was looking at non adjustable scroll jaw chucks from TMX and they are $1000+, I assume that would be a really good 3 jaw.  I guess when I'm ready to upgrade, I'll look around but my guess is that I won't beat PM's adjustable 3 jaw scroll chuck.


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## Ischgl99 (Aug 26, 2022)

The 3 jaw that comes with the PM-1236T might be good enough for what you plan on doing.  When I got mine, it was 0.005-0.007” out, and I was not happy with that.  Disassembling it and cleaning did not help any, but after using it a bit, it is now 0.001-0.003” depending on the diameter and whether using the inner our outer jaw surfaces.  I suspect there was a burr in there somewhere causing the problem that I did not see originally.  Since you are getting it anyways, maybe check and see how good/bad it is and then decide what you need.  I use the 4 jaw frequently since I got pretty good at dialing it in quickly, so I have no plans to get a set through 3 jaw chuck.

I bought the ER-40 and 5c collet chucks from PM, both are set through and I have them dialed in to 0.0001” on the taper.  I prefer using them over the jaw chucks since I can work close to them and not worry about knuckle busters.  I got the inch set of ER collets, I should have gotten the metric set since there are gaps in usable ranges for some of the things I do, and the metric set has more collets.  I got the inch set since I expected to use to mostly for raw stock, and some second operations, but since I do almost exclusively metric parts, I can’t hold some parts in it and have to switch to the 5c chuck.  ER collets need the part to be long enough that it engages the majority of the collet, so you might need a 5c for second operations on short parts, but 5c collets have a very small clamping range that makes ER attractive, and with the PM collet set, your parts can’t be oversize at all, at least with my set.  Which one, if any, are good for you will depend on the work you intend on doing.


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## cathead (Aug 27, 2022)

To me a three jaw chuck is pretty useless so mine sits under the lathe collecting dust.  Once you get used to 
centering a round part with a four jaw chuck, it becomes routine.  Maybe the reason I do this is because I have
two lathes in the shop and one has a home made 5C collet block installed pretty much all the time.  It all depends
on what one machines so if you turn more odd shapes and do one-off repairs like I do, it might be worth some
consideration.  If I did a bunch of pipe work, I might consider using the three jaw but that probably will not happen
any time soon.  Another consideration is that just to change out the four jaw to a three jaw for one operation would
be pointless as it weighs in at about a hundred pounds.


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## erikmannie (Aug 27, 2022)

My 6-jaw chuck is a Set-Tru. At least lately, it has seen the most use because I work on a lot of tubing and pipe.

When I first installed it, I very carefully used the Set-Tru adjustment to get TIR under .001”.

Since then, I use it as one would a 3J or 6J scroll chuck. That is, I have not yet revisited the Set-Tru adjustment screws. It merely functions as a scroll chuck with very low TIR.

I regularly take work out & then put it back in for further machining. To my pleasant surprise, I have had excellent repeatability.

One could imagine that you buy a scroll chuck & test the TIR when you install it. If that TIR was .001”, you would be very happy with that chuck.

Both of my 5C collet chucks are not Set-Tru, & I have to live with whatever TIR I get. Not the best situation for a 5C collet chuck.

If I am going to work on something that (1) will be removed & rechucked, and (2) calls for a very tight (e.g. .001”) tolerance, I use a 4-jaw.


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## skcncx (Aug 29, 2022)

Thanks for every one's input!

For now I'm waiting on upgrading the chuck that comes with the PM-1236T, a 3 jaw economy chuck.  Like @Ischgl99's input, it might be all I need for awhile.  I have so many other things to buy as well.

When I'm ready or need higher precision on a 3 jaw the ultra precision from PM is a great value where price/accuracy meet.  And at that point, it may be just get a 4 jaw for versatility.  Though, I have yet to use a 4 jaw and have one for my atlas 618... for a few reasons, its never been worth it for one or two operations, I'm primarily doing round and hex stock, nothing square/odd shape to date and fear .  I just gotta practice dialing one in.  In the end I'd rather put $$ into a chuck that's likely the primary player most of the time.  Or maybe the way the wind blows when I pick up my lathe in October if I see it on the shelf, could be impulse time.. though, I might reserve that for a small mill.  Aren't we supposed to have both lathe and mill?

Sometimes the only way to know if something is worth it is to buy and try!


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## Jimsehr (Aug 29, 2022)

I mostly use 3 jaw chucks . I like the chucks with top jaws and the ability to mount soft jaws to bore to size if I need them . It is also nice to be able to bore soft jaws to hold a washer like part. Like 1/8 inch thick and have part run true with a built in stop ( back side of bored jaws ) to hold part. I have also turned a regular 3 jaw chuck into a set true chuck. If the back plate register fits into the recess of the chuck you can turn the register down to give the chuck some slop
them put 4 set screws thru back plate to adjust.

Also I have other types of set true chucks that work same way .


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## skcncx (Aug 29, 2022)

Jimsehr said:


> I like the chucks with top jaws and the ability to mount soft jaws to bore to size if I need them . It is also nice to be able to bore soft jaws to hold a washer like part. Like 1/8 inch thick and have part run true with a built in stop ( back side of bored jaws ) to hold part.


I have yet to comprehend the all the great benefits of boring soft jaws.  If they are reversible, stepped jaws ("top jaws"??) that seem common, wouldn't reversing them and holding a washer like part accomplish the same thing?  I ask out of ignorance.  Maybe if the part's diameter is too small, you would have to bore soft jaws in that case.




Jimsehr said:


> I have also turned a regular 3 jaw chuck into a set true chuck. If the back plate register fits into the recess of the chuck you can turn the register down to give the chuck some slop
> them put 4 set screws thru back plate to adjust.


I may give that a try on the economy chuck included with my lathe. Worth a shot before I spend $700. What's the point of set true if it's that easy... maybe the internal mechanism of PM's adjustable 3 jaw is more sophisticated. It seems like it does the same thing though, except your screws to lock it down are on the face, so maybe easier to do....


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## Jimsehr (Aug 29, 2022)

This is an couple examples of set true chucks. Not spindle chucks but same idea.

First pic shows a ring with 4 set screws bolted to back plate with tight fit. Front set of screws used to adjust center part true.

Pic 2 is different tool made to set true center body of tool. These tools are made to inserted into turret lathe or automatic machine. To center cutters true.

I also have more of this type of tool.


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## Larry42 (Aug 29, 2022)

skcncx said:


> Aren't we supposed to have both lathe and mill?


"Depends" on what you intend to do! Some milling can be done on a lathe. BUT having both is a much nicer set of tools. And all the tooling, accessories etc. turns into an expensive rabbit hole.  

One thing I've noticed with my D1-4 spindle is that it makes a difference how I tighten the cams. Not much difference but I can measure it. So I always do the same sequence. 
Both of my collet chucks are "set-tru" types. If the work fits my 5C that's what I'll use. If I'm just turning parts from a piece of round stock any 3 jaw is good enough. If a second operation is needed by reversing the part the level of accuracy required will govern my choice. Putting the 8" 4 jaw on and centering each part can get very good tolerances. But it is time consuming for me. I finally made a second shorty chuck key so I can use it on the back side at the same time as the key on the front side. Speeds up the process. 

I did finally buy a nice Taiwan 8" 3 jaw. Not set true, but it repeats much better than the 6" included in the box, 3 jaw. 
The finest chuck in the would would not make me a master machinist. The only place I find it really important for dead on sizing is for press fits. For press fits both parts need to be the same temperature to measure accurately. Loktite can be your friend.


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## mmcmdl (Aug 29, 2022)

skcncx said:


> I have yet to comprehend the all the great benefits of boring soft jaws. If they are reversible, stepped jaws ("top jaws"??) that seem common, wouldn't reversing them and holding a washer like part accomplish the same thing? I ask out of ignorance. Maybe if the part's diameter is too small, you would have to bore soft jaws in that case.


Can't live without soft jaws .  Maybe a hobbiest can , but in an industrial setting where TIR needs to be spot on , soft jaws are the bees knees . If they are on a set true type , they are even better . Bore them once and you can remove them and re-install them as long as you need them . They are simple to make out of round stock aluminum so no need to buy expensive sets of jaws .


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## skcncx (Aug 29, 2022)

Larry42 said:


> "Depends" on what you intend to do! Some milling can be done on a lathe. BUT having both is a much nicer set of tools. And all the tooling, accessories etc. turns into an expensive rabbit hole.


Ha, I don't have a lot of pre-made plans/intentions, other than I really enjoyed using my Atlas 618 for some tractor parts, shop parts for band saw upgrade, few other projects for friends... but wanted to do some threading and realized, I'm gonna stick with this so might as well upgrade to something with a decent QCGB.... that got the snow ball rolling.  Then, what tooling should I get, and.... should I just get a mill when I go pick up my lathe, save another round trip.  So far, every time I have tried to save I end up spending more .  The PM728VT or the PM833T or TV models are looking good... my responsible side should kick in.... but I have really no justification for any of this.


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## mmcmdl (Aug 29, 2022)

Once I get caught up around the house , I'll have a few pieces of equipment going out depending on what you're looking for . They'll be posted on here .


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## skcncx (Aug 29, 2022)

mmcmdl said:


> Can't live without soft jaws .  Maybe a hobbiest can , but in an industrial setting where TIR needs to be spot on , soft jaws are the bees knees . If they are on a set true type , they are even better . Bore them once and you can remove them and re-install them as long as you need them . They are simple to make out of round stock aluminum so no need to buy expensive sets of jaws .



These things?  You make those out of round stock? 

I think I'm not picturing what you have in mind... no answer needed, I'm just soaking all this in these days.  I'll do some googling.


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## mmcmdl (Aug 29, 2022)

Yes , those things .


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## skcncx (Aug 29, 2022)

mmcmdl said:


> Once I get caught up around the house , I'll have a few pieces of equipment going out depending on what you're looking for . They'll be posted on here .


Sounds good... I guess there is a place to sell... but I have yet to find it on here.  Manual flat bar angle bender, rolling bender possibly, mill stuff interested in.


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## IamNotImportant (Nov 15, 2022)

mmcmdl said:


> Soft jaws are either aluminum or steel used for multiple parts requiring close TIR . Bore them to the finish size of your part . They are re-usable also , and can be bored for multiple set-ups . No , they will not mark parts .


just watched a nice vid about making soft jaws by Haas..


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## Jimsehr (Nov 15, 2022)

skcncx said:


> These things?  You make those out of round stock?
> 
> I think I'm not picturing what you have in mind... no answer needed, I'm just soaking all this in these days.  I'll do some googling.
> 
> View attachment 418639


There are more types of soft jaws. Most CNC machines use power chucks with serrated jaws. Also soft pie jaws can be the only way to hold large rings. And in a production shop it can mean profit or loss with how much time it takes to load parts. I have run tens of thousands of parts with load times of about ten SECONDS per part. And don’t forget many parts are held on the bores with soft collets that are turned to part bore size.


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## davidpbest (Nov 15, 2022)

Soft jaws are not only useful to ensure tolerance from part to part, but but are often absolutely necessary to securely clamp a part that is thin walled.  Example shown below where the soft jaws have been machined to perfectly fit the OD of the thin-walled part that requires additional operations.  This has several benefits:  1 - increases the clamping surface area, 2 - provides a back-stop to register the part depth and flatness in the chuck, and 3 - reduce surface marring where the part is clamped in the chuck since the soft jaws are made from softer material (including plastic at times).







Here's another example using "Pie-shaped" soft jaws that have been machined to fit the part being clamped:


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## skcncx (Nov 15, 2022)

davidpbest said:


> Soft jaws are not only useful to ensure tolerance from part to part, but but are often absolutely necessary to securely clamp a part that is thin walled.  Example shown below where the soft jaws have been machined to perfectly fit the OD of the thin-walled part that requires additional operations.  This has several benefits:  1 - increases the clamping surface area, 2 - provides a back-stop to register the part depth and flatness in the chuck, and 3 - reduce surface marring where the part is clamped in the chuck since the soft jaws are made from softer material (including plastic at times).
> 
> View attachment 426896
> 
> ...


It's been taking me awhile, but I'm now "getting it" as to the beneficial features to having 2 piece jaws, which means you can easily throw on soft jaws for a variety of specific scenarios.  Seems to reason you can easily make your own soft jaws out of aluminum blocks... turn and bore to suite for your needs.

It took me awhile to recognize in pictures of chucks having the screw holes on the face of the jaws and what it meant... you just unscrew and screw new jaws on.  Is there typically a standard hole spacing for a given chuck size?

in that pie-shape soft jaws illustration, is the clamp expanding on the part?  Full faced soft jaws, pretty elaborate.

Kind of a bummer, though not a huge deal, since I am highly considering the bison combo 4 jaw chuck as my 4 jaw in my line up... hard jaws only, weight still has been concerned turning at 1000+rpm.


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## davidpbest (Nov 15, 2022)

skcncx said:


> you just unscrew and screw new jaws on.  Is there typically a standard hole spacing for a given chuck size?



Yes, you are now understanding how they work.  And yes, there are standard hole spacing - metric and imperial.  And it's not only the hole spacing that that matters here - the back of the removable jaw has alignment ways and slots.  The removable jaws have tongue and groove elements that mate with the female version of same on the jaw that remains in the chuck.  You can see these elements in this photo:






Here are the specs for an imperial 8" chuck with 2-piece jaws:





You can buy all kinds and shapes at a cost less than you can buy the material for at this manufacturer:  monsterjaws.com



skcncx said:


> in that pie-shape soft jaws illustration, is the clamp expanding on the part?  Full faced soft jaws, pretty elaborate.



Yes, absolutely.  They have been machined on the lathe to precisely fit the ID of the part and also provide a back-stop registration flat to align the back of the part perpendicular to the spindle axis.


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## Rich V (Nov 16, 2022)

I have an 8 inch Gator 6 jaw set true chuck on my PM 1340GT lathe. Recently I needed to bore a 0.512 hole 2.25 inches deep into D2 tool steel. My solid carbide boring bar will only go ~1.9 inches deep.


First I turned the OD of the bar true then I drilled and bored the hole to 1.9 inches deep. I then parted off the part and flipped it in the chuck. Using the set true feature I indexed the OD with a dial indicator to +/- 0.0001. That took ~90 seconds to dial in. I then bored out the remaining 0.35 inch of ID to 0.512. Finished by polishing the bore with a fine Scotch bright pad. The resulting bore would pass a 0.512 gauge pin freely but a 0.513 pin would not start.

That demonstrates what a set true chuck can do.


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