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My views on soft jaws

Posted: Sat Jan 25, 2003 4:10 pm
by Harold_V
Running soft jaws is a great way to hold almost anything that you have need to chuck, be it a straight bar or a complex piece. The advantage is that you can chuck items time and again with precision with no effort aside from properly setting up the soft jaws. When properly prepared, soft jaws will generally repeat to within .0005" consistently. A further benefit of soft jaws is that you can hold delicate items without fear of marking, crushing or distortion because the jaws can be made to almost totally enclose your item. Using your imagination, soft jaws are nearly a perfect solution to almost any chucking problem. Even strange configurations can be held if you're willing to do your jaw machining on a mill. They are truly one of the best methods of holding parts.

How I came to run soft jaws routinely came about because of a negative experience with my 3 jaw shortly after I bought the lathe. I had the chuck on edge and turned to pick up my air hose to blow off the D1-4 mount before installing it in the lathe. The chuck rolled and hit the floor, landing on one of the jaws, shearing off the teeth. The scroll was apparently not hurt, for it is still in use to this day.

A new set of jaws was requested from the dealership. They were ordered, though never received. The dealer had a set of jaws on hand that had come from another chuck, which he offered to me while I waited (in vain) for the new jaws to come in. The dealer finally told me to keep the jaws I had borrowed. These used jaws had a considerable amount of runout, so I started using a set of soft jaws for gripping straight material.

My chuck came with three sets of jaws, the third set being the two piece type jaws that were already fitted with soft jaws. To this day, I generally have soft jaws in my chuck, though there are times when I do use the ill fitting jaws, along with the reverse gripping jaws that also came with the chuck. What I learned from that experience is that, in general, soft jaws are superior in almost every way to hardened jaws. They grip material very well, even for roughing operations, with the advantage of running far more precisely than the hardened counterparts.

If you're interested in trying soft jaws, your chuck must be fitted with the two piece jaws that permit reversing the top jaw. Instead of using the top jaws, a set of soft jaws is purchased or machined from steel or aluminum. I prefer steel, but aluminum has its place. Once the new soft jaws are installed in the chuck, the jaws are then bored to accommodate the form of the part to be held. This brief outline will be discussed in detail below so you can follow, step by step, the procedure to try them yourself.

A safety note: The larger the soft jaws are, the better the chance of getting clipped when running them. They stick out considerably farther than hard jaws, so you must exercise good work practice. They can be dangerous, but running machinery is dangerous business. Be ever aware of the spinning jaws, soft or otherwise.

To make soft jaws, consider these things:

The jaws you make should be a snug slip fit on top of the master jaws so they locate the same way each time they are installed, and can't shift under load. You should duplicate, in every detail, the mounting face of the soft jaws as compared to a hard jaw.

The larger you make the soft jaws, both in length and height, the longer life they will have. The wider you make them, the more area you will have gripping your part, something that can be important for thin pieces.

Wide jaws limit how far you can close your chuck. There is no good reason why you can't taper the jaws so they can close down farther. Just make sure you remove an identical amount from each jaw so they remain the same relative weight from jaw to jaw so the chuck runs in balance. I recommend wider jaws over narrow ones. They will serve you better in most instances. If you feel you will hold a lot of small parts, make more than one set while you're making them, and make one set as narrow as possible. Even tapering the portion where they come together so you can grip small diameters. Anything goes with these things as long as you keep them in balance.

Be certain that the ends of the jaws extend beyond the base jaws both inside and outside. There will be occasions when you bore through the jaws, so you must clear the base jaw to do so. You will have to do that in order to hold long items that will pass through the chuck body.

When you counterbore for the socket head cap screws, go deep enough that you can use the original screws. If you use longer screws, the life of your jaws will be restricted to the depth of the counterbore. Once you get to the screw, you can no longer machine the jaws. The counterbore should be no larger diameter than necessary.

Your jaws should be numbered so they can be placed on the same base jaw each time. Make your permanent mark by stamping, and do it below the area of the screws so you can't machine the number off inadvertently.

You MUST always use the same wrench hole when tightening your chuck. If you do not have one that is marked, mark one in a permanent way and use only this hole each time. Any other hole will yield runout. The scroll is loose enough in the chuck that it will load differently with each hole, giving more or less runout from hole to hole.

I have attached a photo of my chuck with one set of soft jaws partially installed, with various views of these jaws. These were made about 35 years ago for a specific purpose, but I have used them countless times since for gripping similar items.

Keep in mind that soft jaws can be reversed on the top jaw, so you can use both ends of the jaws for different configurations. Thus, each set is actually two sets of jaws. Those in the pic were used that way, plus the projecting jaw was also used to hold a part internally, so this set of jaws was used in three different ways.

When jaws have been machined until there is no more life in them, it is customary to weld on new stock and start over. They don't have to be pretty, and any precision that is demanded comes from the machining, not the welding.

I'll discuss boring the jaws in this post, and I'll discuss the making of the little adjustable spider in the following post directly under this one, along with a picture of it for clarification.

To begin boring soft jaws, once installed, set the jaws such that you can bore them by removing the absolute minimum of material to create the pocket that will hold your part. That way you will get the best mileage from your jaws. At that point, the jaws should be blocked by closing down on the spider, which has been pre-set to support the jaws at that particular location. The length of each of the stops in the spider should be identical, which can be measured from the straight face of the spider body by using a scale.

When you install the spider, it should be as close to the outside edge of the jaws as possible, just beyond the deepest point you will machine. If you are boring jaws straight through, you will locate the spider on the base jaw, just slightly beyond the end of the soft jaw so you can bore through but not hit the spider. Remember that the bulk of error in your chuck is because of slide distortion, so you want to load the jaws in the slide such that they can't move, and the loading will be identical to the loading in application. That's how you generate the high precision.

It is VERY important that the spider be at right angles, not cocked and running wonky. If you close your chuck on a piece that is not dead perpendicular, what it does is load the jaws differently from one another, yielding jaws that won't run on center. You'll be hard pressed to get your jaws to run under a thou that way. Be certain to also have the spider centered on the jaws so they aren't loaded to the side, which also yields jaws that don't run well. Tighten your chuck as tight as you feel you'll tighten it when you are actually running your part. Try to duplicate, as near as possible, the identical conditions that you will use when you are machining. That yields the highest degree of precision.

Once you have your spider properly located, bore your jaws to the exact size of your part. The part should start, but not necessarily go in when you try fitting for size. Approach size carefully, for an oversized bore will yield run-out. If you go oversize, you can reset the spider and close the jaws, then re-bore , so all is not lost.

Soft jaws run true because you machine the pocket to exact size of the part being held. If you advance the scroll beyond or retract it to a position before the one that it occupied when boring the soft jaws, there is no guarantee that the jaws will run concentric, and there will no longer be a match of holding surfaces, jaw to part. The soft jaws will grip only at their outer edges if the item is larger, and only in the center if the item is smaller. It takes very little to effect the fit, and also the runout

If you intend to use the bottom of the bore to register your part, bore your jaws to the desired depth, then face the back of the bore so you have a perfect right angle. It is always a good idea to slightly undercut the inside corner so it does not interfere with the part touching the shoulder. I generally use a relatively sharp pointed bar and just go slightly deeper in both depth and diameter at the corner, yielding some clearance so a sharp corner will clear. That seems to be adequate.

If you intend to hold a long item that won't bottom out in the jaws, bore them clear through, and to the diameter of the item to be held.

If you intend to make several parts, hold the chucking diameter as close as possible from part to part. Even a thou can change the way your chuck runs, so that is important.

Once bored to size, remove the spider, clean out the chuck as required, and carefully deburr the jaws so you can't roll over the burrs and trap them between the parts and the jaws. I make it a practice to draw my file away from the jaws so I don't roll any burrs inside, where they will mark your parts and create runout if you happen to miss them. It's a good policy to go over the edges with some abrasive cloth if you have parts that are finished and you don't want to mar them in any way.

Once you've cleaned up your jaws, put in a part and check it for runout with a DTI. If you aren't happy with how the part runs, repeat the operation. There are times when you may have to bore two or three times to get the runout down, a result of the spider not being located perfectly. Each re-bore need be only a thou or two in depth, just enough to clean up again and hit size. That will be determined by how you set the spider.

There is a follow up photo in the next post, showing a set of soft jaws made from aluminum. I generally make them without the step, but the piece of stock I used was tapered, so I cut the step to eliminate the taper. They normally would be rectangular in shape. That permits the most life and options of how you will machine the jaws. The step in this set of jaws will limit me in how I can use that end of the jaws, but the price was right!


A pic of a set of soft jaws

Posted: Sat Jan 25, 2003 4:14 pm
by Harold_V
Remember to make yours rectangular, without the step.



Posted: Sat Jan 25, 2003 4:22 pm
by Harold_V
In order for soft jaws to be used properly they must be bored to an exact fit to the portion of the part being held. Building this simple tool, which I call a spider, makes setting the jaws at the proper attitude very easy, and you will use is as long as you bore soft jaws. The attached pic is of one of these simple devices, along with several sets of extension fingers, which are nothing more than various lengths of socket head cap screws.

The spider is nothing more than a fairly large hex nut that has holes drilled and tapped in it at 120° intervals ( to match the jaws). What this little tool will do for you is prevent the hunt for a proper sized piece to chuck each time you bore soft jaws. That can be a valuable asset in that you normally need take out of the existing jaw only enough material to make it run true and to fit the contour of the part you desire to hold. The size of the piece chucked to bore the jaws becomes somewhat critical unless you don't mind wasting material from the jaws. This will make all kinds of sense to you once you use soft jaws.

The size of screw and base nut you use will be dictated by the size of your chuck. It is difficult for anyone to tell you what size is good without actually seeing the particular setup, so you will have to use some judgment in selecting for your machine.

For a guideline, I have an 8" 3 jaw and use a nut that is 1½" across the flats (a 1" nut). On the flats I have drilled and tapped for (3) 5/16"-18 socket head cap screws. With this setup you can put in longer or shorter screws and adjust them to fit the exact size that you need when boring your soft jaws. To set the screws to the same length as one another, I measure from the flat of the screw to the end of the head, using my 6" scale. You can hold them within a few thou that way, which appears to be close enough for good running jaws. Drastically different lengths shifts the centerline of the heads such that they will bear on the edges of the jaws, causing them to load sideways to some degree, yielding poor running jaws.

Do keep a couple things in mind as you make your spider. In use, it may well be located on the master jaw deep within the chuck if you are boring through the entire soft jaw, or it may be located inside the soft jaw itself when boring a pocket that will accommodate an object that is to be held square to the shoulder left by the bore. You will want to bore right up to the nut in some cases and will need the clearance afforded by tapping your holes as near the end of your nut as is reasonably possible.

The spacing of your tapped holes should be of concern. They should be installed in such a way that the three holes aim for exact center and are 120° apart. Reason? The loading pressure will not be uniform otherwise. If your chuck has loose slides for the jaws, the jaws will not close in line as they should. That manifests itself as runout to some degree when you install the part to be machined.

Last thing of concern is the screws you will use in your spider. Socket head cap screws are an excellent choice. They take a minimum of room and are easily adjusted by finger with their knurled head. Face the head of each one if it is not square, and cut a small chamfer on it so it doesn't have sharp corners. It helps to have the head dead square with the axis of the screw to minimize runout of the final bore.

Enjoy running soft jaws~


Re: A pic of a set of soft jaws

Posted: Sat Jan 25, 2003 4:25 pm
by Hanz
Nice post Harold. I have said before that I use my soft (aluminum) jaws all the time. I am not sure what a 'spider' is, what I do is stick an appropriate diameter piece in the inner jaw to tighten down on, and what I usually find convenient is an impact socket of the right size.

Althought you already said this in many words, one reason I use them is that with the bored jaws fitting the piece precisely, you don't have to tighten the chuck as much as you would normally just clamping by three points. It becomes almost collet like. I just bored out some very thin walled camshaft bearings, with the jaws just ever so snug, say with two fingers.

Re: The "Spider"

Posted: Sat Jan 25, 2003 4:36 pm
by Hanz
OK, now I see the spider. Harold, is there another type of spider? There was a post in which someone was asking about mounting one near the handwheel, I pictured that it was to stop a long rod from whipping around.

Re: The "Spider"

Posted: Sat Jan 25, 2003 4:52 pm
by JeffinWI
Hanz, what I know as a "spider" has a similar design to the "3-legged soft jaw boring spider" but with four or eight screws and a design that allows it to be attached to some type of tailstock center...or a complete unit, with Morse taper that would fit in the tailstock. Used inside of a long tube type part, to allow cutting a steady rest spot on the O.D. Conversely a "cathead" is an appropriately size piece of stock with a c'bore in one end and a center on the other end. usually four screws positioned radially and entering the c'bore. For long shaft type parts held in chuck and needing a steady rest spot cut on the other end. Cathead goes on outboard end of shaft/roll/etc., and tailstock center into center hole of cathead. Can then be adjusted to true up part, and hold it while cutting steady rest spot. Handy and easily made "tools" to ahve around a lathe.

Re: The "Spider"

Posted: Sat Jan 25, 2003 5:03 pm
by Harold_V
Hi Hanz,
I think I'm guilty of tossing around the term "spider" quite loosely. I get the idea that anything that looks even close to this type device gets tossed in the same basket. You know how us brain dead machinists are! [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]

I didn't apply for a 'copyright' on the name, so I guess we'll let the other guys use the term, too. [img]/ubb/images/graemlins/blush.gif"%20alt="[/img]

Actually, I have no idea what a spider really is aside from that eight legged thing most of us don't like. I, too, have seen the term as applied to the device you speak of. I recall a conversation some time ago with a fellow that works in a shop that does large shaft work, in which he talked of a spider for holding a shaft in place at the tailstock end.

Maybe we need to have a contest, sort of a name the "xxxx" kind of thing! [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]

I hope you can see the value of using this device. It comes in real handy when you have a complex profile in a set of jaws and you want to just clean it up when you use it again. It allows for a minimum amount of stock removal because it is adjustable. Like you, I've used any conceivable thing in the shop when boring jaws, and finally got tired of looking for just the right piece. I came up with this simple device and have never looked back. They work very well.

Thanks for the compliments on the post. I'm hopeful that it will be the push some of these guys need to explore soft jaws. They really make a difference in a shop.


Re: The "Spider"

Posted: Sat Jan 25, 2003 5:15 pm
by Hanz
Harold, I reread your spider instructions a few times, but can't understand how you set the screws evenly? Maybe I was reading too fast though...

Re: The

Posted: Sat Jan 25, 2003 5:24 pm
by Harold_V

It's more likely that I didn't say it well. :(

What I do is measure from the flat of the nut to the top of the socket head screw, using my 6" rule, setting the screws to the same length. I've found you can hold them identical in length within a few thou very easily, which doesn't seem to effect how the jaws run. By having them drastically out of length with one another shifts the centerline, which effects how well the finished jaw runs because it moves the heads to the edges of the jaws, cocking them sideways slightly in the slots. Hope that makes it a little more clear. :wink:

I have edited the original post to include this procedure and clarify a couple other things I said that were somewhat redundant. Sorry about my poor writing, folks.

Re: The "Spider"

Posted: Sat Jan 25, 2003 6:55 pm
by Michael_Az
Great post Harold! Expecially about the spider. I will save this post. Would you believe I need the soft jaws right now? Turned down a part today and cut it off. Has to go back with no run out so I'm going to have to make a locking mandrel to do it. Is this fifty year old technology? Now, lets see if I can find a chuck with removable jaws.

Excellent post Harold!

Posted: Sat Jan 25, 2003 7:40 pm
by Matt_Isserstedt
Now I've gotta get busy hunting up some flat stock....

Wow, that was Great!

Posted: Sat Jan 25, 2003 7:59 pm
by MarkLong
I never undestood why you would use soft jaws when you have an adjust-tru 3 jaw chuck or collets available. If you need to run a part on center, you can use the adjust-tru option to get it on center-but the chuck jaws may mark the part. And it may not be as repeatable as collets-esp on a semi worn chuck.

Collets generally run real close to center and are pretty repeatable and don't mark. But my 5C's only go to 1-1/16 inch. And they can't grip on the outside of the jaw (ID of part) !

Soft jaws give you the best features of both with the added benefit of sz being only limited by the swing of your lathe, the sz of your chuck and soft jaws.

Outstanding post, clearly written and good pics help to explain. I am saving this post into my lathe folder on my computer! Thanks!

But problem for me is...

My chuck only has one set of jaws and they are one piece. It is a 6" Buck-Forkhardt Adjust tru 3 jaw chuck. Would 2 pc jaws be available for it?? It is probably 10 yrs old but unused when I got it at the university. They were throwing it away because it was plain back and had no way to fit to spindle. Grad students didn't know about making a back plate!