Newbie. Need to make soft chuck jaws for Mori lathe

[bob1112]

We are new in the CNC lathe stuff. How does one usually make soft jaws or any formed jaw for a Mori with a standard three jaw chuck? Do I just bolt in some material, clamp, and bore a hole or what? Thanks.

[Eurisko]

If the jaws don't have to be bored all the way through, grip on the OD of some round stock placed near the chuck face. The diameter should allow the jaws to travel about .030-.050 before gripping. Now bore the jaws.

If the jaws have to be bored through, you'll have to grip on the ID of a ring. You can place pins in the bolt holes of the jaws to grip the ring.

The important thing is to watch how much travel the jaws have. Stay near the top of the stroke.

[bob1112]

Is there a "special" ring for this or would about any thick wall tubing do?

[Geof]

Originally Posted by viper6383 Is there a "special" ring for this or would about any thick wall tubing do?

I have seen some places advertising these rings but we make our own.

Actually the way we do it is make the pins that fit in the counterbores for the bolt.

Then figure out what diameter disc we need to go inside these pins and hold the jaws maybe 50 thou smaller than their open position.

Turn a piece of round bar to this size and cut off a disc about 1/2" long.

Close the jaws with the disc inside the pins in the counterbore.

Bore the size you need all the way through the disc and jaws. You need to make sure there is enough meat on the jaws to do this.

One thing to watch is that the disc is large enough that it does not crush as the hole is bored through it.

[bob1112]

I am still trying to caprende what you just wrote. I am not familiar with the pins you speak of but I think what you are talking about is to cut a disk of material maybe slightly smaller than the stock diameter and pinch it in the jaws and bore it to size? I guess I am wondering of you can just cut pipe into rings to get this type of thing done? I guess I need to scratch my head a bit on this issue.

I know we have some slug parts to run asap but will soon run a bar puller and pull through the spindle so I need to know how to do it both ways.

[Geof]

Originally Posted by viper6383 ..I am not familiar with the pins you speak of but I think what you are talking about is to cut a disk of material maybe slightly smaller than the stock diameter and pinch it in the jaws and bore it to size? ...

The thing about a picture being worth a thousand words applies here, but I don't have any pictures.

The pins I am referring to are just little lengths of round bar that fit in the counterbore of the bolt holes the adjustable jaws are fastened on with.

Small lengths of bar in these counterbores are like little jaw extenders.

The procedure for boring the soft jaws is to open the chuck.

Adjust the position of the soft jaws so the ID of the three jaws is smaller than the size you want to bore.

Put the pins in the counterbores.

Now you want the chuck to close by about one third of its total travel, so you turn a disc of steel so that when the pins close on the OD of this disc the chuck has closed by about one third. The pins are what is gripping the disc.

The disc is much bigger than the size you will be boring the jaws.

Now you drill through the disc and bore the soft jaws.

Because you are gripping the disc with the pins sticking out of the jaws the jaws are forced to take up all the slack and go sort of bell mouthed, like chuck jaws always do a little bit.

You bore them when they are held like this so that when the chuck is opened the jaws spring back ever so slightly and they are smaller at the front.

But when you put your stock in and close them they go slightly bell mouthed and grip the stock over the whole length.

The reason cutting a disc is better than cutting a ring off a piece of pipe is that the wall thickness of the pipe is probably not strong enough and it will crush.

[SRT Mike]

Geof helped me with this same question not too long ago - and having gone through this maybe I can help.


Look at this picture:

http://images.andale.com/f2/130/104/...oring_ring.jpg

The thing with boring jaws is that if the force on the jaws when machining isn't similar to the force on them when clamping, then they won't be correctly aligned. So if you don't have them clamped when you bore, and let's say you turn a 1" bore - well then when you clamp a 1" bar in there and turn on the spindle it will be wobbling. I found out the hard way

But if you are turning a bore that goes right through - how do you do that? You can't grab something in the jaws or else how can you turn it? The answer is you need the jaws to be held with something larger (larger ID) than the bore you are turning. That way the jaws are still clamped on this clamping jig, but now you can bore your hole without boring out the piece that the jaws are clamped on.

So how do you do that? Some guys put a disc way at the back of the jaws, bore the hole almost through, then remove the piece and do a back cut to remove the extra material in back. I prefer to fixture the jaws using the mounting holes as Geof said. You can make pins that go in the holes that affix the jaws, then you just need a disc with matching holes (actually slots to allow a little movement) such that when you clamp the jaws they clamp on the large disc which is braced with the pins in the fixture holes, allowing you to bore through the center and bore the jaws right through with no problems.

Personally if you can swing it I would go to ebay and search on "boring ring" and buy the one like in the picture. It's a fair bit of work IMO to make a new ring every time you make custom jaws, and if you do it a lot (like I do) it becomes a huge chore. The boring ring lets you do it quick and easy!

[bob1112]

I think I am feeling you. Thanks for the pic and explanation. As I understand, the ring has studs that go into the mount holes and just give some resistance for the chuck to clamp down on? The center of the jaws remain undisturbed for boring? If clamping the OD of a part or bar, the jaws should be cut while clamped down on something, not out?

[Geof]

Clamped down, correct. Clamped in the direction they will be used.

That ring thing SRT Mike linked to is very spiffy; much nicer than mine, and adjustable with the slots.

[SRT Mike]

Originally Posted by viper6383 I think I am feeling you. Thanks for the pic and explanation. As I understand, the ring has studs that go into the mount holes and just give some resistance for the chuck to clamp down on? The center of the jaws remain undisturbed for boring? If clamping the OD of a part or bar, the jaws should be cut while clamped down on something, not out?

Yep, you got it. Like it was said before, the jaws tend to deform slightly when they clamp since they are being clamped from the rear of the jaws. So if you turned a smooth bore while they were unclamped, then measured it, you would find that your bore had now becmoe a cone - slightly smaller diameter at the back. But if you bore them while clamped, this slight rotation that the jaws experience when clamped will already be accounted for, and while your bore will not be perfectly true while unclamped, when clamped it will be perfectly true.

On that pic, yes the pins you see go into the screw holes on the chuck jaws, then you can rotate the ring to where it's going to prevent the chuck closing, then you close the chuck and let it clamp down on the rings (through the pins), while still leaving the center open to bore your ID.

The extra fun comes when you want to bore jaws for ID holding For that I turned down the jaws oversize, then machined a ring to go around the jaws at the very base, then with the uncut portion of the jaws extending up through my ring I cut the profile I needed with the jaws clamped for ID holding. Then close the jaws, take off the ring and I was good to go.

When I first read how to do this, I thought "come on - do you REALLY need to go to that extent to have the jaws clamped? Does it make THAT much of a difference???". The answer is that it does... I have 2 sets of jaws for holding 2.5" stock. The ones I bored correctly I put the stock in and when I turn on the spindle it doesn't even look like it's moving - it runs nice and true. The other set that I bored without using a ring are crap... put the stock in and close it and you see the stock wobbling because it's not being gripped perfectly squarely.

Good luck!

P.S. You can get the rings from MTM Tool Supply or US Shop Tools - for a 12" chuck I think they are around $350, less for smaller chucks. You can also get them on eBay for less, I think.

[g-codeguy]

Or if you don't want to spend that kind of money, you can do what I do for jaws that have to be bored thru. Make a ring that is 1/4 inch thick. Make the I.D. bigger than the largest thru size you will ever need by at least 2 inches. Make the O.D. large enough that the ring won't be deformed by the chuck pressure. One half inch wide should be more than enough. I believe the one I have is around 3/8 inch wide.

Make a little program with a face groove tool that will remove the width of the ring plus extra for closing. Example: Ring O.D. is 6.0 and I.D. is 5.25. You want the jaws to close .02 from the open position. The program would be made to face groove from 6.02 to 5.24. I.D. isn't important as long as the ring will fit in the groove. I use a .125 wide top notch groove tool, and make the face groove .17 deep because that is what I originally had to work with.

With the jaws in the open position, run the face groove program. Insert the ring. Usually I run a big enough endmill thru so that I can use a decent size boring bar to avoid chatter. Bore to size using the pressure you plan on holding with. (Or a little less if planning on maximum pressure.)

Ring I use was actually something left over from a previous job. Cost nothing to buy or make.