Rookie Lathe Soft Jaw Questions

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rmac
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Re: Rookie Lathe Soft Jaw Questions

Post by rmac » Fri Jan 07, 2022 1:43 am

Well, well. After reading the comments here, the thread that Harold referred to, and some other stuff that IT linked to, I guess I need to see what I'm missing by not using soft jaws. My chuck uses the American standard tongue and groove system that SteveM mentioned. And sure enough, people do sell soft jaws that would fit. But where's the fun in that? As it turns out, I have a big chunk of thick aluminum plate that's just begging to be rent asunder. So here's the plan:

jaws.png

This thing hangs out beyond the master jaws by about 3/8" on both ends, so maybe I can nibble away at the pointed end for small diameter work, and the other end for larger diameters. The 1-1/4" width matches the thickness of my stock. The height seems "about right", and everything else kind of has to be the way it is to mate with the master jaws. This is all still subject to change if anybody sees something I've missed.

Getting a good fit with the master jaws will be a challenge for me and my despicable mill/drill.

-- Russell Mac

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Re: Rookie Lathe Soft Jaw Questions

Post by Harold_V » Fri Jan 07, 2022 2:28 am

My only comment is that the more material you can leave on the ends, the longer life the jaws will enjoy. There's a balance between scroll engagement with the master jaws and how much material would be acceptable. Also, depending on the nature of your lathe, the extended jaws may interfere with chuck rotation.

If you make them a little too long and they're troublesome, you can always remove a little. If, on the other hand, you make them short, when they're useful life is gone, you have to make new ones. Welding on them isn't a good idea unless you use the proper alloy (6061) and have the capability to restore their aged condition by heat treat after welding. Left soft they won't be very useful. That's the one advantage of making them from steel. They can easily be rebuilt or modified for special applications.

Many of my soft jaw sets are made of aluminum.

H
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RSG
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Re: Rookie Lathe Soft Jaw Questions

Post by RSG » Fri Jan 07, 2022 8:34 am

As Harold mentions leaving them longer on one end is useful. You can pop them off and reface as needed. I like to scroll the chuck jaws out as far as I dare for mine so there's lots of meat in the beginning. Here's a few pics of the standard size in my shop, some in use and the spares on the shelf along with the spacers and rings.

Image

Image

Image
Vision is not seeing things as they are, but as they will be.

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rmac
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Re: Rookie Lathe Soft Jaw Questions

Post by rmac » Fri Jan 07, 2022 9:00 am

Harold_V wrote: There's a balance between scroll engagement with the master jaws and how much material would be acceptable. Also, depending on the nature of your lathe, the extended jaws may interfere with chuck rotation.
Thanks, Harold. I actually thought about both of those things, but perhaps not very clearly. I installed the master jaws with just one or two teeth engaging the scroll, and then sized the soft jaws to just barely avoid interference. But it's doubtful that I'll often (or ever) need the jaws extended that far, and maybe a good idea anyway to engage more of the scroll. Is there a rule of thumb on this? How far out is too far out? I'll consider longer jaws. I haven't cut any material yet, and it's easy to change the CAD drawings.
Harold_V wrote: Welding on them isn't a good idea unless you use the proper alloy (6061) and have the capability to restore their aged condition by heat treat after welding. Left soft they won't be very useful. That's the one advantage of making them from steel. They can easily be rebuilt or modified for special applications.
Unfortunately, I'm not set up for welding. Maybe someday. Sigh.

-- Russell Mac
Last edited by rmac on Wed Jan 12, 2022 12:05 am, edited 1 time in total.

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rmac
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Re: Rookie Lathe Soft Jaw Questions

Post by rmac » Fri Jan 07, 2022 9:16 am

RSG wrote: As Harold mentions leaving them longer on one end is useful.
Is there an advantage to putting the extra material on just one end instead of dividing it equally between the two ends?
RSG wrote: Here's a few pics of the standard size in my shop, some in use and the spares on the shelf along with the spacers and rings.
The northeast jaw in the first picture doesn't show any crossways feature to transmit the clamping force between the master jaw and the soft jaw. I guess you're relying on the two screws for that? Maybe because you're always taking relatively light cuts?

-- Russell Mac

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Re: Rookie Lathe Soft Jaw Questions

Post by RSG » Fri Jan 07, 2022 2:44 pm

rmac wrote:
Fri Jan 07, 2022 9:16 am
Is there an advantage to putting the extra material on just one end instead of dividing it equally between the two ends?
If you look at the hole spacing you'll see it is set as far back as it can so work can only be done on one side.
rmac wrote:
Fri Jan 07, 2022 9:16 am
The northeast jaw in the first picture doesn't show any crossways feature to transmit the clamping force between the master jaw and the soft jaw. I guess you're relying on the two screws for that? Maybe because you're always taking relatively light cuts?

-- Russell Mac
I've never bothered to get that fancy with them by adding that cross feature because when setting them up I pre-load the jaws before final tightening of the screws. Also, I only really use them for aluminium work but that doesn't stop me from taking heavy cuts. You'll be surprised what these jaws can handle.
Vision is not seeing things as they are, but as they will be.

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Re: Rookie Lathe Soft Jaw Questions

Post by Harold_V » Fri Jan 07, 2022 5:01 pm

rmac wrote:
Fri Jan 07, 2022 9:00 am
Is there a rule of thumb on this? How far out is too far out?
There may be, but I am not aware of one. In my case, I've run my soft jaws with just one tooth engaged with the scroll, but in such a case I'm very careful to not take any serious cuts. I do NOT recommend that this becomes routine, but when I don't have a set of jaws that will accommodate a given operation and there's just one piece to be machined, I'll take the risk. I like to have all the teeth engaged, but it's not always possible. It goes without saying, one can't predict the size of work that will be demanded tomorrow.

If you make your jaws deep enough, the face can be altered. If, for example, you must hold a large disc, instead of expanding the jaws, they are moved towards center, then the face of the jaws is machined to create the needed cavity. That's the beauty of soft jaws. They are sacrificial---they can be altered in any way you wish--the choice is yours. If you start with the largest possible jaws, they'll enjoy a longer life, but you have to balance function with longevity. If you make them outlandishly oversized, they'll last an eternity, because they won't fit most of your needs. That's why one has multiple sets. They are chosen by their machined condition so the next application demands little stock removal. Multiple sets prolongs the life of all of the sets for that reason. In RSG's case, where he makes similar parts, his jaws will last a very long time, assuming he removes only a few thou for each setup. That's why I like a spider---you can skim a few thou from jaws to get them to run true each time. I like the solid discs, but you'd need a huge array of sizes to duplicate the capability of a spider. Note, too, that a spider can be inserted when a disc cannot. That has saved me on numerous occasions.

Note that in spite of the fact that my jaws fit tightly, removing them and returning them generally results in minor run-out. They really do need to be machined after they are installed unless you aren't concerned about a couple thou concentricity. And, re-machining the cavity ensures that parts can be held without a hint of damage. Soft jaws don't harm the parts, not even when the parts have an exceptional finish, assuming you keep 'em clean.

H
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pete
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Re: Rookie Lathe Soft Jaw Questions

Post by pete » Fri Jan 07, 2022 7:11 pm

Fwiw, one of my older UK published books mentions at least 3 scroll teeth engaged as a minimum. I guess it depends on how heavy handed you are, if you over tighten everything in sight, that less than 3 tooth rule may not end well. My first lathe was a tiny Shereline, I broke a jaw tooth because I over tightened with the jaws extended too far even though they still cleared the lathe bed. It was a good lesson about using proper feel and judgement. For round parts, if you bore the jaws slightly undersized as others have mentioned, you'll then have 6 points of contact on the part surface. So that alone helps to increase your grip with the same torque on the chuck key as the standard jaws. There's less part marking as well compared to hard jaws. And you can then use less torque with the non recommended number of teeth and/or lighter feeds and depths of cut. Plus they can really help with thin walled or soft material parts.

I wish I had chucks with that soft jaw capability for the multiple reasons others have already mentioned. And most of this wouldn't apply to a real money making shop. But they only add another and admittedly usually faster, easier and most times better method of part holding. For multiple parts they also have a huge advantage comparable to collets that may be incapable of even holding parts of that size or shape. But I also try to remember that at one time 3 jaw scroll and prior to that the independent 4 jaw chucks weren't even invented yet. Yet they still managed to get anything that showed up accomplished using nothing more than a face plate, bolts, hold downs or turning between centers. My guess is it didn't take too long for bolted on adjustable jaws to be added to those face plates and you then have a rudimentary 4 jaw. The trade off without those faster and easier methods is time. Depending on what the part features allow, once you get towards the upper limits of your lathes swing, or it's an odd shape with enough off set, a face plate might still be the only method that can be used and that's why there still made. My collets are pretty accurate, yet if the concentricity requirement is high enough and the part size allows it I'll usually at least finish turn between centers. It's still the most accurate method if the part has to be moved or checked against a mating part at some point. The downside is there's much less holding torque to resist high feed rates and depths of cut compared to multiple jaws. So it's now not used much in commercial shops other than on grinders.

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Re: Rookie Lathe Soft Jaw Questions

Post by Harold_V » Sat Jan 08, 2022 2:49 am

pete wrote:
Fri Jan 07, 2022 7:11 pm
if you bore the jaws slightly undersized as others have mentioned, you'll then have 6 points of contact on the part surface. So that alone helps to increase your grip with the same torque on the chuck key as the standard jaws.
Hmmm. I wonder who those people might be? I wouldn't recommend sizing the jaws anywhere beyond a snug slip fit. To do so is to encourage run-out. If you happen to have a chuck with little error in the scroll, that may not be much of an issue, but I can assure you that my chuck introduces error when the jaws aren't a proper fit with the part.

All of this relates to what one expects from the setup. If you don't care about a thou or two runout, no big deal. If you're trying to hold less than a thou, size of the parts as well as the size of the bored chuck jaws is ultra critical.

I have little understanding about the issue of holding parts with undersized jaws, so there's six point contact. I've used soft jaws for heavy roughing cuts in production runs and have never had a problem with parts slipping or shifting. The properly nested part performs perfectly well. I wouldn't endorse that idea under any circumstance. (YMMV)

H
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Re: Rookie Lathe Soft Jaw Questions

Post by pete » Sat Jan 08, 2022 6:31 am

I had to go back and re-read SteveM's post on the first page. Apparently I misinterpreted what he said when he mentioned to adjust the preload size just slightly smaller than the finish size you need. What his method does and what he meant was it uses up less of the soft jaws instead of the 6 point contact I first assumed he meant. My own reading comprehension was what made that mistake. Either way, how do you go about machining and then measuring the recess your boring to exactly match the part. So lets look at it logically, yes there's a certain amount of deflection that would happen with soft jaws to help match the part radius. And without a way to exactly measure what the bore recess is, your either going to be at a slightly under or over radius unless you get really lucky every time you bore the jaws. Even that snug slip fit is an over size condition. No it's not much, it's still present though. But the same amount undersized somehow can't or won't work as well? If anything that gives you more contact once the chuck is fully tightened and would allow the jaw tips to conform to the exact part size and shape a bit better, plus there's then more surface contact that may or may not be needed. Two points on each jaw even helps centralize and probably maintain the jaw position better within there jaw slots and closer to how the jaws were first machined. But that extra contact is there anyway and at zero additional cost or effort. More contact results in a more rigidly held part, maybe not that important with shorter parts. But I'll take more rigidity every time over a yes it will work. And the larger the recess the farther apart those two points on each jaw are = more stability and rigidity.

Ok it's a long ways from part holding with soft jaws. But even with high precision machined bench rest rifle actions the general accepted practice when custom fitting the over sized scope ring bases to the action is to bore the radius in them very slightly less than what the action has. The logic behind that is because of how tough it is to match that radius exactly. Hand scraping to fit might be the only real viable method to get an almost exact bearing between the parts. But I've never seen that done so far. But machined over size by even a few 10ths gives you a single line point contact at the mounting screws location = less stable and rigid. Under size by a few 10ths gives you two line points separated by the width of the scope base = not the ideal 3 point contact, but still a much more secure and stable two point line contact. With a soft jaw chuck, then the same effect would be multiplied by 3.

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Re: Rookie Lathe Soft Jaw Questions

Post by LIALLEGHENY » Sat Jan 08, 2022 10:42 am

How do you go about boring a set of jaws accurately every time? Use an inside mic, such as the B&S Intirimik, where you get 3 points of contact , one on each jaw. Telescoping gauges will work as well if your part is small enough that the jaws encapsulate it, or you can get a good 2 points of contact, and you have the feel for using them. For larger OD parts, I do as Harold states, a good snug slip fit. I'm also in agreement with Harold on the 6 point contact argument, I don't recommend it, besides the fact that the points of contact can actually mark or mar the part you are trying to hold, which doesn't work if your OD is already finished. In the world of CNC turning, soft jaws are predominately used, it's not uncommon to be holding onto only an 1/8" of a part.....something your not going to do with hard jaws. One thing not mentioned thus far, one of the benefits of using soft jaws is the ability to lessen the effects of chatter/ harmonics when turning or boring your work. The better the fit, the more rigid you can hold the part = less chatter/ harmonics.

Nyle

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GlennW
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Re: Rookie Lathe Soft Jaw Questions

Post by GlennW » Sat Jan 08, 2022 11:33 am

If you are working with thin wall tubing you definitely don't want six point contact. You want the diameters to match to keep from distorting the tubing.

When that is the case, I bore a piece of stock first to get the diameter set and then bore the soft jaws.
Glenn

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