Boring Bar Push Off?
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  1. #1
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    Default Boring Bar Push Off?

    I am pretty new to machining, and I attempted to bore a hole the other day. I needed it to be 0.702" in diameter, and I used a 5/8 drill before boring. The drill hole was pretty true after measurement. The depth of bore and length of boring tool was about 1.5 inches off the tool post.

    I noticed after awhile that it pretty much stopped cutting further into the hole, and basically tapered inside. I think the boring bar itself left a lot to be desired. Seemed to bend fairly easily just when touching off on the inside wall. Is this normal? Basically when I was done with it, only the first 0.200" of depth measured correctly, and the rest of the hole was around 0.68 or less. I kept trying to make cuts and it simply walked right up that taper. Tried increasing and decreasing speed, and same thing.

    The tool looked cheap to me. Could it be it's not sharp enough? I ordered a new boring bar. After this happened I read about it and saw that "push-off" is an issue with boring? Is this common? Does this mean that I should take spring cuts after every single pass to avoid this in the future? I spent 5 hours on this part, only to screw up the most critical cut in the bore. I can't afford to do this many more times.

    All the help I can get would be great. Thank you

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    What were you trying to bore in?......the same thing would happen to me if I was boring a piston pin that was file hard......and Ive seen many newbies try to do just that.......the other factor is have you eliminated the free play in the lathe.....to bore a hole,you must wind back a little towards you ,and not then advance the tool ,for as soon as the load is applied,the slides simply move over to eliminate as much of the backlash as the force will do.

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    What kind of boring tool are you using? Is it "as purchased" or did you touch up the edges before using it?

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    When I use a Spaghetti noodle to Bore holes in Adamantium running 10k SFM, and .5 chipload---I have problems also.....

    No information to help the OP.

    R

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    There's almost always some push-off but what you're describing is way beyond that. You don't any details but since you acknowledge your boring bar is cheap, that's never a good sign.

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    I knew lathes that would do that because the bed was so worn under the chuck.....the cure was to lock up the slides and bore with the compound slide.I did a lot of bearing bores with those machines ,to excellent fits.

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    Quote Originally Posted by john.k View Post
    I knew lathes that would do that because the bed was so worn under the chuck.....the cure was to lock up the slides and bore with the compound slide.I did a lot of bearing bores with those machines ,to excellent fits.
    I test ran a small lathe that would drag the boring bar really bad when withdrawing from a bore. The saddle was cracked at the back of the front V guideway.

    The 'price' of the boring bar has little to do with the performance. I've made boring bars out of 'free' steel. The tool tip has to be ground with the proper clearance angles and set to the work so that it does not drag. This typically means at or slightly above centerline height. Use a sharp edge tool for the finish cut: standard carbide inserts may have too much of a hone on the edge which will increase bar deflection slightly. If it doesn't feel sharp, then it ain't sharp. I sharpen my own finishing inserts for critical bores. if you can't grind carbide, then you'll need to use HSS which you can work with a little more easily.

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    i always wondered why a boring bar would deflect more the further it gets into the bore?
    the pressure in the beginning would be the same as the pressure well into the bore i would have thought.

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    Quote Originally Posted by john.k View Post
    What were you trying to bore in?......the same thing would happen to me if I was boring a piston pin that was file hard......and Ive seen many newbies try to do just that.......the other factor is have you eliminated the free play in the lathe.....to bore a hole,you must wind back a little towards you ,and not then advance the tool ,for as soon as the load is applied,the slides simply move over to eliminate as much of the backlash as the force will do.
    7075 Aluminum

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    Quote Originally Posted by litlerob1 View Post
    When I use a Spaghetti noodle to Bore holes in Adamantium running 10k SFM, and .5 chipload---I have problems also.....

    No information to help the OP.

    R
    Funny. It was 900 rpm and I took 0.010 to 0.030 DOP

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    Quote Originally Posted by janvanruth View Post
    i always wondered why a boring bar would deflect more the further it gets into the bore?
    the pressure in the beginning would be the same as the pressure well into the bore i would have thought.
    Not sure if this has anything to do with it, but solid mechanically speaking, the deflection of the work itself would be less the closer you get to the chuck.

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    Quote Originally Posted by GregSY View Post
    There's almost always some push-off but what you're describing is way beyond that. You don't any details but since you acknowledge your boring bar is cheap, that's never a good sign.
    Actually, I forgot to mention that the x changed on the DRO as it fed towards the chuck. The dude running the shop tightened the way screw(or something) and it stopped moving.

    So I wonder if that was the initial issue, and then it just walked after that.

    I don't even know how to avoid it again.

    So my question is, is it necessary to take spring cuts when boring? There's an instructional video on YouTube and this guy claims you should, at least when getting close to your size

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    Mentioned at first .....failure to take up backlash.......remember ,the backlash for boring is opposite to OD turning.......if the DRO reading changes ,then the topslide is moving .........your problem solved.

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    Quote Originally Posted by janvanruth View Post
    i always wondered why a boring bar would deflect more the further it gets into the bore?
    the pressure in the beginning would be the same as the pressure well into the bore i would have thought.
    If the boring bar is set a little bit too low (heel rubbing) or there is not enough clearance angle the error tends to cumulate further you go with the cut. Started from chuck side the taper would be in opposite direction.

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    Pix of bar, and general set-up pulleaze.

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    Quote Originally Posted by john.k View Post
    What were you trying to bore in?......the same thing would happen to me if I was boring a piston pin that was file hard......and Ive seen many newbies try to do just that.......the other factor is have you eliminated the free play in the lathe.....to bore a hole,you must wind back a little towards you ,and not then advance the tool ,for as soon as the load is applied,the slides simply move over to eliminate as much of the backlash as the force will do.
    I just wanna get this straight. Are you saying to move back towards me then advance in Z? The wording was a little hard to understand when you say "and not then advance.."

    Wouldn't I want to advance away from me and then advance in Z?

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    Quote Originally Posted by digger doug View Post
    Pix of bar, and general set-up pulleaze.
    I'm away from the setup and have a new bar coming in. I'll take a pic next go-round just in case I fail again. I never know how much info you all need given that I have no idea whether these problems are simple or not. For those of you on these forums that request diagrams and every detail possible, I think most people don't have that much time on their hands. I'm just in search of possible problems when push off occurs. But I think what most of you have told me gives me enough ideas as to what it could be. I think the tool is crap and there was too much play in the x direction.

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    Quote Originally Posted by Mot3s View Post
    I just wanna get this straight. Are you saying to move back towards me then advance in Z? The wording was a little hard to understand when you say "and not then advance.."

    Wouldn't I want to advance away from me and then advance in Z?
    I believe he was saying the tool is always advanced “toward” the new value. So that means when setting you touch off on the 0.625” bore and “move plus” in the same axis direction to enlarge the bore.

    Also +1 to all that mentioned the tool needs proper relief or clearance to work right.

    Good luck,
    Matt

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    Quote Originally Posted by Matt_Maguire View Post
    I believe he was saying the tool is always advanced “toward” the new value. So that means when setting you touch off on the 0.625” bore and “move plus” in the same axis direction to enlarge the bore.

    Also +1 to all that mentioned the tool needs proper relief or clearance to work right.

    Good luck,
    Matt
    You sure about that? Haha now I'm confused.

    I'm pretty certain I naturally creeped X toward me when setting the new DOP, and did not feed away before taking the cut, when I'm thinking now I should have? Unless you all are saying the opposite.

    The DRO dropped in diameter as I advanced in Z, which means the cross slide shifted away from me. In that case, wouldn't I want to feed toward me and then away from me to my new DOP to eliminate the play in the cross slide and the potential for it to move away from me?

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    What type/brand Lathe are we talking about here?

    If there is something wrong with the actual cutting action, then logic dictates that it will not get better as you keep going (with exceptions). So if the Bore diameter is getting smaller, then you have to take the opposing action to counteract it.

    R


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