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Thread: School me in Boring Bar Angles

  1. #1
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    Default School me in Boring Bar Angles

    I am not a machinist, therefore no formal training. Please school me on the appropriate use of my new boring bar. It has a round shank so the angle is up to me. I believe it is called “rake” angle, but I am not sure.
    In the following photo there are three different ways to mount the tool. Which one is best?
    Does it make any difference what the material being machined is?
    Is the point supposed to be on center, above or below center?
    Or is the center of the shank supposed to be what is “on center”?
    I have heard the term "positive" and "negative". Which one is positive, and which one is negative?

    Thanks for taking your valuable time to reply.
    Last edited by Ron of Va; 04-06-2010 at 02:07 PM.

  2. #2
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    A is a negative rake
    B is neutral rake
    C is positive rake

    Regardless of rake, the very tip should always be on-center. Otherwise, your likely to get chatter and your X axis moves to make the bore bigger or smaller won't be 1 for 1.

    For that insert (appears to be a TPMG or similar) you should be using the neutral rake, unless the boring bar itself has flats that would suggest otherwise.

  3. #3
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    Quote Originally Posted by PixMan
    A is a negative rake
    B is neutral rake
    C is positive rake

    Regardless of rake, the very tip should always be on-center. Otherwise, your likely to get chatter and your X axis moves to make the bore bigger or smaller won't be 1 for 1.

    For that insert (appears to be a TPMG or similar) you should be using the neutral rake, unless the boring bar itself has flats that would suggest otherwise.
    Thanks PixMan!

  4. #4
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    I very rarely use anything but 0 rake with a boring bar and the tip is most always at the center of the axis. You can move the tip up and down a few thousandths to try for a better cut and sometimes it works. Above center seems to work better than below center for me but always with a level cutter.

    The best way is to experiment and see what happens. That's the only way to learn for your self what happens and what works or don't work.
    It's only ink and paper

  5. #5
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    It highly depends on rigidity and metal and such.
    Usally, most people try 0 rake to start with, and if its giving bad chatter, bad finish, etc, try adjusting the angle back or forward a few degrees (little less then shown in your pic)

    Note that when using posative rake, too much and you don't have enough 'relief' angle on small holes and the inside of the hole will rub against your tool.

  6. #6
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    Thanks guys, every little bit of information helps.

    It gets confusing. I have two internal threading bars with inserts. Both of them are setup by the factory with a negative rake (flats on the shank). So I falsely assumed that the boring bar should be used with a negative rake, so I thought I would ask. Turns out I am wrong. I am used to it by now. I have had a lot of practice.

  7. #7
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    Ron of Va: nope, rake depends on material, machine, rigidity, yadayadayada.

    As a general rule, posative rake cuts easyer (less hp/rigidity needed) and often produces a better finish, negative rake tends to have cutting edges that last longer but require much more HP (Not strictly true with just tilting a boring bar back, but for tools designed to be negative rake as they don't need a relief angle ground into the cutting edge)

    Some materials hate posative rake, like brass (sucks the tool into the work)

    Other times you just get chatter or bad finish and a degree or two change can fix it.

  8. #8
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    It's been a couple of years since I used this type of boring bar for holes and internal threading but as you say the flats machined by the makers give you a negative rake and that is how I used them, if there wasn't a flat I still set them to achieve the negative rake angle. I have to say using this rake angle gave no particular problems, in my case on industrial machines but it may be a different story on lighter hobby lathes.
    As has been said, experiment to get the best results but keep the tip on centre line

    Tony

  9. #9
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    The rake also affects the tool clearance, and tool clearance ensures there is a minimum diameter you can use as a starting point if doing an inside bore.

    I have a picture of what happens to inserts when there's not enough clearance:

    http://metalworkingathome.com/?page_id=35

  10. #10
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    I've just found a brand new boring bar in my toolbox with a diameter of approx. 8mm. It has a flat machined on it to give about a 10 degree negative rake and this has got me thinking that this negative rake is to give the insert side clearance to allow small holes to be machined without rubbing as the inserts themselves have so little side clearance angle.
    One thing I did find with these carbide inserts is that they didn't like taking very small finishing cuts as they are not really "sharp" even when new.
    Tony

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