Bar push back in Jaws

Captdave · Nov 1, 2014

We are running a bar job of 1.25" 12L14 in soft jaws and the bar is being pushed back in during the OD roughing process. We were using .075 DOC at .015" per rev and getting push back and have since reduced the feed to .010" per rev and its doing better. During the OD rough the Z axis load is only 10-14% so that isn't much.

We have run this job many times and have not had an issue. Took a skin cut on the soft jaws before we began and set chuck pressure to 300PSI would have some up to .050" short. Blued up the jaws and had excellent contact on the length of the jaw and bar so turned the chuck to max pressure of 370 PSI same result. Bars have oil on them, cleaned a few off and had better results but still random push back. Machined off half of the jaws contact area and that helped a bit more.

May try a sharper edge prep insert on Monday.

toolmaker96 · Nov 1, 2014

Any chance you are near the bottom of the chuck stroke?

Captdave · Nov 1, 2014

No, if anything it could be too close to the top limit. Now that you bring that up I think the boy may have bored them too close to the upper limit as the jaws open very little but you would think if that was the culprit it would have shown up during the last run we did.

bigjon61 · Nov 1, 2014

You could add some serrations to the jaws. Take an ID grooving tool and cut a 3-4 grooves. The jaw would look like so: _--_--_--_--_. This should work really well in 12l14. It may mar the clamping surface a bit, but shouldn't be too noticeable.

18my · Nov 1, 2014

Either do as suggested above or reduce the doc and run heavier feed. I find its more to do with doc than anything else.

jims · Nov 1, 2014

slipping in jaws

bigjon61 said:
You could add some serrations to the jaws. Take an ID grooving tool and cut a 3-4 grooves. The jaw would look like so: _--_--_--_--_. This should work really well in 12l14. It may mar the clamping surface a bit, but shouldn't be too noticeable.

I would check that the jaws are bored to size of bar or better yet a few thousands under. When bored a few thousands over jaws only grip in center of part and can slip .
jims

Tonytn36 · Nov 1, 2014

If running cut pieces, cut a step in the jaws with a corner relief so they can't push. Serrations if bar pulling.

Vancbiker · Nov 1, 2014

When was the last time you greased the chuck? Possibly dry inside and binding up instead of gripping.

18my · Nov 1, 2014

That's not a picture of the jaws you're using is it?

Are you using a standard chuck or force compensating?

Kelly@PDM · Nov 1, 2014

In the pic you are not running the cover and the chuck could be full of chips. Also is the bar whipping?

yardbird · Nov 1, 2014

Usually when I have this problem and start to get aggravated I either turn a shoulder/step in the back of the jaws to push against or out comes the back stop. Are you using the tailstock?

Brent

Ox · Nov 1, 2014

If you wunna run bars and 3 jaws - then buy or make a set of jaws that take the W&S Master pads and fetch a set of serrated pads. Heck even serrated pads have their limits. Kant imagine trying to werk within the limitations of a soft 3 jaw. :ack2:

Or gets some hard and sharp upper jaws that will bite right in. Got some like that for Bertha. She doesn't let things push back - even when running a 2-1/2 spade or whatnot ....

-------------------

Think Snow Eh!
Ox

Heinz R. Putz · Nov 1, 2014

Whenever you bore softjaws, always bore them smaller than the OD you want to chuck.
More later.
Heinz at doccnc.com

Heinz R. Putz · Nov 1, 2014

If you bore to the exact diameter, the pressure of chucking will cause a 3 point contact and you are not holding the part well, no matter how high the chucking pressure, as a matter of fact, the higher the pressure, the worse the contact.
Always bore them a little smaller than the part and you create a 6 point contact.
CNC is easy, but you still need some basic machineshop skills, this is one of them.
In my over 30 years of training CNC shops, I used this method and it works good.
Heinz . doccnc.com

Joe788 · Nov 1, 2014

Have you been greasing the chuck regularly?

Captdave · Nov 1, 2014

Vancbiker said:
When was the last time you greased the chuck? Possibly dry inside and binding up instead of gripping.

They all get greased once a week, I know because I do it myself every Monday morning along will all the other PM stuff on all the machines, it's not something I leave for employees to "say" it's been done.

I'll rebore them on Moday a couple thou undersized and throw a couple of grooves in them as well.

We we made a set of soft jaws that hold W&S pads for another Mazak but it's only a 6" chuck, guess its time to make another set for this machine.

yardbird · Nov 1, 2014

I just assumed since you were using soft jaws you were gripping on a 2nd operation and this was pieces not long bar stock. Made my last post sound stupid. lol..

I would look and make sure the chuck and the draw tube aren't coming unscrewed from each other. Be the only other thing i can think of.

Brent

npok · Nov 1, 2014

Are the jaws lifting much? You can bore them with a fast feed so the surface isn't slick.

Or if all else fails: http://www.schunk.com/schunk_files/attachments/j_schunk_sp_KraSta_60_DE_EN.pdf

toolmaker96 · Nov 1, 2014

Why not just run in hard jaws?

yardbird · Nov 1, 2014

toolmaker96 said:
Why not just run in hard jaws?

Yeah that's what threw off in the beginning. Sounds like he doesn't have any for this chuck and has to make them. Kind of weird this has been working up till this point. Makes me wonder if it might be something else.

Brent

Bar push back in Jaws

Titanium

Hot Rolled

Titanium

Hot Rolled

Cast Iron

Hot Rolled

Diamond

Diamond

Cast Iron

Aluminum

Titanium

Diamond

Stainless

Stainless

Titanium

Titanium

Titanium

Aluminum

Hot Rolled

Titanium

Similar threads