Boring Soft Jaws For Large Job
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    Default Boring Soft Jaws For Large Job

    I'm now on an Okuma Vertical Borer VTM 80Yb. I'm machining jobs which weigh around 1 tonne. Just looking for advice on the best way to machine the soft jaws. I've machined my jaws to 20" diameter, 4" deep. I was using a spider to grip on when I machined them, so I could only turn the pressure up to 100 psi. I've got my pressure set at 400 psi and gripped the job. However, the jaws are lifting slightly off the chuck when I clamp the job. This is causing there to be a taper in the job, so I need to eliminate this. When I reduce the pressure, it doesn't lift as much, however I can't afford to be reducing the pressure as the large O.D is 28" and it weighs a tonne... literally! Any help or advice would be welcome guys.

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    Quote Originally Posted by MachinistMike123 View Post
    so I could only turn the pressure up to 100 psi. I've got my pressure set at 400 psi and gripped the job. However, the jaws are lifting slightly off the chuck when I clamp the job.
    That's why they are lifting.
    Either clamp onto something that won't crush at 400psi, or bore your jaws on a taper.

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    The machine is a VTM, why do you need such high clamping pressure?

    and since its such a heavy and large part, your table RPM should be quite low? hence my concern for such high clamping force.

    We have a couple 60+ year old VTLs with 60" tables, and we only use the standard 4 jaws and a big old wrench to clamp everything down.

    Unless you part is very oddly shaped and you are doing some off center drilling, I can't imagine why you need all that clamping force!

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    Jaw boring spiders with 3 adjusting bolts are shit. They can kick the jaws around on loose chucks when you bore the jaws which causes runout issues when you locate on the part. Use something solid when boring the jaws if at all possible.

    Always bore the jaws using same pressure as you will use when chucking on the job--again super important especially on worn out chucks.

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    You don't need 400psi. Like ever.

    R

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    Mtndew: yes I've tried boring the jaws straight and with a taper, same outcome with both 🙈

    Ondori: Yes my RPM is pretty slow. But your correct, it's an odd shape and I am doing off center drilling. I'm gripping on "19.5 diameter, and machining a "30 to "28 and drilling twelve "1.5 inch off centre holes on the edge of the face on the 28" flange.

    Philabuster: yes I agree, it's completely shit and I hate doing it. However, there is no way we could have as many discs as discs as we need in the shop. We run one offs or very small batches of jobs ranging from "2 to "40 in diameter.

    Litlerob: I'm going to have to strongly disagree with you on that one mate. You try running a horizontal chuck on 400RPM with a 1 tonne job in which is 1000mm long and you can only grip on 100mm

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    Quote Originally Posted by MachinistMike123 View Post
    Mtndew: yes I've tried boring the jaws straight and with a taper, same outcome with both ��
    This is why you put taper into the jaws, when they flex, the area that's holding the part makes full contact.
    Play around with amount of taper until you get full contact.
    If you bore them straight and they flex, then the area of contact is only a tiny spot.

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    Quote Originally Posted by MachinistMike123 View Post
    Mtndew: yes I've tried boring the jaws straight and with a taper, same outcome with both 🙈

    Ondori: Yes my RPM is pretty slow. But your correct, it's an odd shape and I am doing off center drilling. I'm gripping on "19.5 diameter, and machining a "30 to "28 and drilling twelve "1.5 inch off centre holes on the edge of the face on the 28" flange.

    Philabuster: yes I agree, it's completely shit and I hate doing it. However, there is no way we could have as many discs as discs as we need in the shop. We run one offs or very small batches of jobs ranging from "2 to "40 in diameter.

    Litlerob: I'm going to have to strongly disagree with you on that one mate. You try running a horizontal chuck on 400RPM with a 1 tonne job in which is 1000mm long and you can only grip on 100mm
    Wait, I thought you said vertical? *Only* grip on 100mm??? That is 4" roughly, that is a whole lot of grip IMO. That is 10% of the total length, I think most of us would rarely get that much to hold onto.

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    He means the face of the Chuck is Horizontal (I'm assuming). But if not, that is a whole different thread topic.

    I machine BIG parts. One of the fundamentals that I learned early on is that if your workholding is correct---Chuck pressure is the last of your concerns. So OP disagree all you want. Get your jaws right. Ditch the Spider. Wind up your RPM to bore.

    OTOH, I think he might mean the serrations are pulling away from the face of the Chuck, not neccessary pulling away from the part. Either way, less Chuck pressure.

    R

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    Mike, are you gripping on a finished surface, or could you get away with using more general-purpose jaws with carbide grip buttons inserted? It might easy your concerns about the clamping capacity and allow a lower pressure, perhaps creating less distortion in the part too.

    What's the condition of the chuck? Are you getting lift of the master jaws, or at the master to soft interface? Any chance of pics?

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    All the weight of the part is toward the Chuck. The Chuck isn't holding anything up. The pressure only needs to be enough to not slip in the jaws, when Rough Turning. Bore jaws within .005" of the part diameter.....it's not rocket surgery.

    R

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    Need a bit more info,

    When you say the jaws are “lifting slightly off the chuck” what do you mean? Do you mean they are angling back past 90deg? Is that because of wear or is the chuck body distorting? Sounds like you need a better chuck for one thing.

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    Quote Originally Posted by MachinistMike123 View Post
    Mtndew: yes I've tried boring the jaws straight and with a taper, same outcome with both 🙈
    As others have said, you've not really explained what exactly that outcome is. In your OP, you said you are getting taper because the jaws are lifting due to the increased jaw pressure??? Taper in the diameters you are turning??? Are you distorting the part???

    Quote Originally Posted by MachinistMike123 View Post
    Ondori: Yes my RPM is pretty slow. But your correct, it's an odd shape and I am doing off center drilling. I'm gripping on "19.5 diameter, and machining a "30 to "28 and drilling twelve "1.5 inch off centre holes on the edge of the face on the 28" flange.
    In that case, I'd be putting support under the lip of the flange to counter the drilling pressure, a jack, or even just a stud bottomed in a t-nut with a union nut on top works great. Can be quickly added after the turning, before the drilling. Using jaw pressure to hold the part flat when applying all that downward force on an unsupported area is not the way to go.


    Quote Originally Posted by MachinistMike123 View Post
    Philabuster: yes I agree, it's completely shit and I hate doing it. However, there is no way we could have as many discs as discs as we need in the shop. We run one offs or very small batches of jobs ranging from "2 to "40 in diameter.
    A spider need not be a separate disk for each size. A disk with three drilled and tapped radial holes can be used to cover a very wide range of sizes, just depends how long the bolts you use are, and, it's infinitely variable in size.

    Quote Originally Posted by MachinistMike123 View Post
    Litlerob: I'm going to have to strongly disagree with you on that one mate. You try running a horizontal chuck on 400RPM with a 1 tonne job in which is 1000mm long and you can only grip on 100mm
    Been there done that, only in sixty year old Bullards where there's nothing between you and the spinning work but your belt buckle.

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    I just got done running a batch of these bearing blocks that were 1900# and 3/4" out of center.
    Doo to the unbalance - I was happy to be able to spin @ 100RPM without getting too much issue.




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    Quote Originally Posted by Ox View Post
    I just got done running a batch of these bearing blocks that were 1900# and 3/4" out of center.
    Doo to the unbalance - I was happy to be able to spin @ 100RPM without getting too much issue.




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    But Ox, I thought 2k lbs was like a jumbo jet or something!!!!

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    Well, mine is a scroll chuck (obviously the pic is of a faceplate with fixtures) and so I don't see the lift like seems to be common on hydraulically actuated chucks for some reason?
    When I put my 8" chuck on my small lathe and watch the jaws flex on a basically brand new chuck - I would prolly get queezy seeing them splay out on a 2000# part too.

    The part in my pic AINT commin' out!

    And if I was him, I'd prefer to run heavy serrated jaws if at all possible.
    Maybe that would require leaving a bit of material on that section and coming back later to kiss it off?
    Depending on how much material needs to come off in his chucking, maybe the extra time to flip it and indicate it could be absorbed, and get a much warmer / fuzzier feeling about the set-up?

    I've seen the inside of some of those inverted Index VTL's and you should see the scars inside the machine from the chuck letting loose of the parts!

    I'd hate to see a 2000# part come off at 400 rpm.
    That'll likely doo more than just leave a mark.



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    I seriously agree with (it’s litlerob1 now???) on the chuck pressure. After “you tubing” one similar it doesn’t look to be a high duty production machine… Onesie-twosie’s or prototype, it looks really cool!

    I’d bet if the OP put paper strips under the part before chucking they’d pull out when gripped at high pressure (that’s not cool!).

    The bit about drilling some 4ish inches outside the chuck jaws shouldn’t matter if the part was really firm on the chuck face (it’s about 3100#’s thrust with a 1&½" twist drill @ .020"IPR). If the part really WAS kicking the other piece part side bottom off the chuck face (over 20” away) → then drill 3/8” before the 1&½" drill.

    Good luck,
    Matt

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    New member asks for help, doesn't like the suggestions so he argues that he is right, and then never comes back.

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    Sorry fellers, had to break off this job just after my last post, so I didn't get a chance to try these ideas. I will be getting back on the job soon, once I am, I will let you know the outcome

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    Quote Originally Posted by litlerob1 View Post
    You don't need 400psi. Like ever.

    R
    For the two hundred and umpteenth time (not looking at you specifically Rob) PSI/KGcm2/Mpa/whatever unit of pressure you like to use is a completely and utterly meaningless way to represent gripping force between different machines.

    Unless you know how the actuator converts it to axial force, and then how the chuck converts it to radial force, and can calculate all that on the fly in your head, it means absolutely jack shit.

    400 PSI might only be half way up the scale on op's machine for all we know.

    Quote Originally Posted by MachinistMike123 View Post
    Litlerob: I'm going to have to strongly disagree with you on that one mate. You try running a horizontal chuck on 400RPM with a 1 tonne job in which is 1000mm long and you can only grip on 100mm
    OP, your terminology is all wrong and it's confusing people.

    In lathe terms horizontal means conventional (workpiece horizontal), your machine is a vertical.

    All that out of the way, rob is completely correct about chuck pressure vs clamping security. If you have bored jaws to fit the part, then using excessive pressure that causes the jaws to tilt provides massively less clamping security than moderate pressure and full contact.

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