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Thread: How to - cutting lathe soft jaws?

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    Default How to - cutting lathe soft jaws?

    Starting a new project this week... bought a couple of sets of soft (steel) jaws for the chuck to do it. I know you need to have clamp pressure on the jaws before you machine. This is my first real project on the CNC lathe... so, a few questions:

    First, how do you guys do this? One machinist I know suggested putting a small disc in the back of the jaws up against the chuck face to keep them open, then undercutting later and removing the small ledge that would be left. Wouldn't this potentially cause a problem with the jaws not clamping perfectly square? And also it seems you lose some effectiveness of the jaws this way. I have seen clamping plates that are inserted on the face of the jaws using one of the jaw screws through the plate, through the jaw and into the jaw's bottom piece. Couldn't I just take a piece of 7075 maybe 3/8" and drill holes and bolt that to the front of the jaws in the same manner, then bore right through that and the chuck jaws? Or is there a better way?

    Second, I am machining them to hold a 2.5" bar of stock. I'm guessing I would want max clamp pressure so I could measure how much the jaws move on un-clamping, and machine to 2.5" minus this distance (plus a little more to give me some room to insert my stock), so that when I clamp I get max pressure. Am I thinking right on this one?

    Finally, given they are square ended, if I start out boring, thats gonna be one hell of an interrupted cut. Would I be better off starting with a drill (I do have a 1.5" insert drill) or an endmill? Or should I not worry about it and go for the boring bar right off? Machine ridigity and HP should not be an issue - this is a giant 60hp CNC lathe.

    Any other advice or tips so I don't mess up a $100 set of jaws

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    Last edited by SRT Mike; 09-11-2007 at 01:52 AM.


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    I have seen other opinions but I think the best approach is to load the jaws in a manner as close as possible the the load they will experience when they are in use and machine them in that state. This means that they should be clamped on something toward the front of the jaws not towards the rear.

    I simply made some sturdy rings and dowel pins that would fit in the counterbore for the mounting bolts. Put the dowel pins in the counterbore holes and then clamp them down on the ring when the chuck closes. Adjust the size of the ring so that the sliding jaws are something like 0.03" from being fully closed then bore the soft jaws to the size of you stock or a thou or two under. Now when you clamp on the stock you have the best contact possible and any bellmouthing of the jaws has been compensated for.

    It is one hell of an interrupted cut but if you try drilling you stand a good chance of getting fragments of drill sprayed around inside the machine because it is still an interrupted cut which drill do not like.

    When doing this I normally put it on a G71 with a small D value and wander off and have a cup of coffee.

    Don't worry too much about messing them up, you can also bolt them back on a couple of serrations closer in and start again.

    An open mind is a virtue...so long as all the common sense has not leaked out.


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    Quote Originally Posted by Geof View Post
    I simply made some sturdy rings and dowel pins that would fit in the counterbore for the mounting bolts. Put the dowel pins in the counterbore holes and then clamp them down on the ring when the chuck closes. Adjust the size of the ring so that the sliding jaws are something like 0.03" from being fully closed then bore the soft jaws to the size of you stock or a thou or two under. Now when you clamp on the stock you have the best contact possible and any bellmouthing of the jaws has been compensated for.
    Thanks for your help Geof - I am not sure I understand the above 100%? I thought that I would want to clamp the ring down before I engage the chuck clamp, no? That way, I would have full clamping pressure going, but the way I read the above, you are saying I would put the dowel pins in the jaw bores, and have it so the ring stops the jaw movement .03" from where they would be if I let the jaws close freely without anything in them or bolted to them. But if they are only engaging the stock in the final .03" of travel, wouldn't that produce quite low clamp pressure? Wouldnt I want to end up such that when the jaws are open, my stock juuuuust fits in the jaws, and when I clamp it, I get the full clamping pressure? Maybe I just read it wrong.

    I was thinking of mounting the jaws far enough out so that I have the minimum amount to bore out. Then mount the ring as you mentioned, then clamp the chuck (and the jaws will barely move because the pins and ring are holding them in place). Then bore to 2.525 (in the example that my stock is 2.5") so when I unclamp, the jaws only move back a hair and lets say its 2.550", meaning most of the jaw movement when I hit the clamp button will go towards clamping pressure and not radial movement of the jaws.

    Then again this is my first time doing it so I may have no clue what I'm talking about!

    Last edited by SRT Mike; 09-11-2007 at 02:07 AM.


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    Let me say it in different words.

    You want the bore of the jaws to be a perfect (as close as possible) match to the stock you are holding when they are at clamping pressure. This means they have to be clamped during the boring in a manner as close as possible to the way they will be clamped during use.

    The clamping pressure does not vary throughout the stroke of the jaws so it does not matter whether you bore them when they are near the top of their stroke or near the bottom.

    The reason I suggest closing them down so there is only a small bit of travel remaining is that when they are open you have plenty of clearance to put you stock in.

    Chuck jaws always bellmouth when closing: It is impossible to avoid because the closing force is behind the jaw slots and the part being clamped is in front; on a good chuck it is not much but it is there. This means the grip of a chuck is not even along the length of the jaw and tends to be tighter toward the chuck body.

    The way to compensate for this is to clamp the jaws in a way that maximizes the bell mouthing while they are being bored. This is why the dowels are put in the counterbores and closed down on a ring; the dowels are acting like jaw extensions so the jaws are clamping at a distance further from the chuck body than they will clamp in use.

    Then when the jaws are clamped on the stock they make the best contact along their entire length.

    You are correct about positioning the jaws to require the minimum amount of boring. This means you can rebore several times before you start to run out of material.

    I have found with the soft jaw blanks they are often so big they stick out beyond the chuck and greatly increase the inertia of the chuck causing the acceleration times to be longer. With 60 horse they may not be a worry for you but on my small machines I often machine the OD and will releive the front face back to get rid of excess weight. Now that is an interrupted cut!

    An open mind is a virtue...so long as all the common sense has not leaked out.


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    Gotcha - thanks Geof. Where I was running astray was in thinking the clamping pressure was different at various parts of the stroke. Thats what a few machinists I talked to have told me, but I wasnt too sure about it then and definitely dont buy it now.

    The idea was almost like the hydraulic chuck was volume based -so that when you pushed the clamp button, it would force a given quantity of hydraulic fluid into the chuck closer - and that fluid could be used up moving the jaws or applying pressure to the stock to clamp. But I thought that more likely, the guys who design the chucks have it just keep pumping fluid until it reaches a given pressure, whether that pressure was reached by clamping on a piece of stock, or whether it was reached by hitting the end of travel and being unable to move anymore.

    I guess I'll chalk that up to a machinists myth

    I hear you on the inertia of the jaws. Its a 12" chuck that weighs a ton to start with, so the jaws may not be the end of the world.

    Is there any easy way to tell what are hard jaws and what are soft jaws? Its not always apparrent by looking. I have probably 50 sets that came with the machine. some that look like they were machined (very nicely machined) and I have some that are stepped to hold multiple diameters. I bet I could re-machine some of those as extras, but I am not sure if the were ever sent out to be hardened or if people even bother to harden them. Could I tap them with a hammer or use a scribe to mark them to determine for sure they are soft enough to be machined?



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    Quote Originally Posted by SRT Mike View Post
    ...the clamping pressure was different at various parts of the stroke. Thats what a few machinists I talked to have told me, but I wasnt too sure about it then and definitely dont buy it now.

    ...I guess I'll chalk that up to a machinists myth

    .... Could I tap them with a hammer or use a scribe to mark them to determine for sure they are soft enough to be machined?
    To answer the hardness question...hit them with a file, if it just glides over the surface they are hard, too hard to machine but if you can remove metal with a file they will be machineable.

    Regarding the myth all myths have a basis in fact. I think this one probably comes from the early days of CNC when "air chucks" were common. A lot of times the air opened the chucks and they were closed by big springs, actually a stack of Belleville washers normally. On these the clamping pressure does vary with stroke and that is how it is adjusted by moving some big adjustment nuts at the back end of the drawtube. On a hydraulic chuck the pressure is adjusted.

    Adjusting the pressure brings up a point, you may need to back it off a bit when you clamp on your ring with the dowels; they are not going to be as strong as the jaws clamping normally and you don't want to rip them out of the counterbore.

    An open mind is a virtue...so long as all the common sense has not leaked out.


  7. #7

    Default holding bar stock

    Do you have any other options? Maybe a set of hardened serrated jaws? You don't have to be right on the 2.5 dia to hold it with serrated. Good investment if you don't have any available. It'll will cover a good range of diameter's. The holding is not too critical if you are holding rough barstock. Very little run out will transfer to the cut if any, most of the time anyway. What are you making?



  8. #8

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    while I 'll agree that clamping at the outer edge of the jaws would be the best situation for a chuck that has a lot of slop, I go the route of a ring inside the chuck behind the jaws .
    The most important thing is to bore the jaws at the diameter of the workpiece . Actually if you think about it , boring slightly smaller , would cause the jaws to contactat their outer edges, causing a 6 point contact.
    I laugh when I see these sets of boring rings or plates all stacked up in a neat pile . I made up a set of boring "spiders". a steel doughnot with 3 holes tapped equally spaced on the circumference . just adjust the setscrews in or out and clamp the thing inside the jaws . It doesn't even have to run true.
    All you are doing is holding tension so you can bore . If its a manual chuck , use a paint stick to mark the chuck hole . Yea , I know its supposed to be a 3 jaw universal, but there are different stresses generated on each hole .
    All of my engine lathes have 4 jaw chucks . 3 (hard) jaws are for close enough work only . 3 jaw production is always a soft jaw application .
    When I do large , thin wall pipe , I custom make jaw extensions so I can get a large surface area , with minimal clamping pressure .



  9. #9

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    Hi, I have a question. When you turn a set of soft jaws in a 3 jaw chuck, can you take the jaws off and then put them back on later for the same diameter part later? Don't know much about all this stuff.



  10. #10

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    yes. mark your soft jaws to what position they are in, if your head has numbered slots. that way you put them back in to the right place.



  11. #11

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    Thanks Shane. So if we run say about 100 different diameter parts (Different part numbers), from 5 -15 parts per batch run, is it best to make the jaws for each batch run then mark them correctly to fit back on the correct chuck position and machine and part number of part being run later. Or is it better to make new jaws every time for each part? I would assume it best to keep track of things and mark the jaws, etc but I need your expert oppinion.



  12. #12

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    I would first ask what is the runout tolerance of these parts ?
    As I stated above , boring to a set diameter slightly smaller than the part will result in each jaw contacting the work piece in 2 spots .
    This will give you a set of jaws that will be reasonably close over a range of diameters.
    But the most accurate way , is to preload the jaws , and take a minimum clean up cut before each part run .
    I run a series of cast parts where runout to .010" is acceptable , so I just bolt up the jaws to their numbered locations and go.

    I was selling an old lathe recently , and one buyer said " If it don't have a 3 jaw , I 'm not interested ".
    I replied " do you want to do work that is right on , or just close enough " .
    Eventually ,an Amish guy bought it , and insisted he only wants a 4 jaw .
    ( pretty fitting actually , since the lathe was a 1900 vintage )



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