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Thread: Lathe Tooling

  1. #1
    Join Date
    Jun 2004
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    Grafton, N.S.W.
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    Default Lathe Tooling

    G'day All.
    Can anyone explain the difference between positive rake and negative rake carbide replacable tip tools?

    I use 20 and 25mm tip holders and the H&F catalogue lists both.
    Hooroo.
    Regards, Trevor
    Grafton

  2. #2
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    sydney ( st marys )
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    Default

    both tools will perform well,one of the main governing feature has to do with HP and rigidity of your lathe,neg rake tools apply a great load on your machine,pos rake tips are more suited to the smaller type lathes,but seeing you are using 20 and 25 mm centre height tools i dont think HP and rigidity would be a problem,not 100% sure but i think you will find that the neg rake tools will allow you to turn the tip over and use other side of tip where as pos rake tips usually only provide one cutting face.

  3. #3
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    Feb 2006
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    Hobart,Tas
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    Default

    Positive rake carbide tips have angled cutting edges (attempted illustration) \=====/ and sit flat/slightly inclined up in the toolholders toward the workpiece and only have the one angled cutting side which can be used, apart from rotating the tip on the horizontal axis, cut cleaner and easier.

    Negative rake tips have square cutting faces |====| and the cutting tip tends to lean down toward the workpiece, hence the negative apsect, take more power to cut, and the tip can be flipped over for a new cutting face as well as rotating the tip on a horizontal axis. Take more power to cut and not a good idea to use on turning small diameters due to the extra cutting force needed. Good for larger work though.

    and tips cannot be interchanged.

    Hope this makes sense
    24 hours in a day, 24 beers in a carton ...Its no coincidence

  4. #4
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    Default

    G'day All.
    Thanks for the replies.
    It is a little clearer now.
    Hooroo.
    Regards, Trevor
    Grafton

  5. #5
    Join Date
    Dec 2006
    Location
    Hunter Valley
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    Default Inserts

    Selection of inserts is a complicated issue and takes a bit of time to work it out. A Sandvic Coromant Turning
    tools catalogue helps a lot in making the right choice.
    First off if your lathe is a small one say 2 hp or less it will machine better with Positive rake inserts.
    Negative rake inserts are more suitable for industry with their powerful 3 phase lathes etc.
    They get the extra economy from the fact that you can turn the negative insert upside down and use all corners.
    The front rake angle for a negative insert is machined into the seat of the tool holder itself because there is no
    relief angle on the insert itself , hence you can use it upside down.
    Negative rake inserts place a lot more stress on the spindle bearings etc. and suck up HP.
    TCMT is the triangular , positive 7 degree front rake , insert with a centre mounting hole , this suits small
    hobbie lathes that will only chuck small diametre stock.
    TPMR is the triangular, positive 11 degree front rake , insert that is solid and must be top clamped. Its 11 degree
    rake and solid design makes it more suitable for medium lathe say 2 to 3 HP that can take heavier cuts and chuck
    larger diametre stuff.
    This is Sanvic Coromant stuff other companies may make the same size inserts under a different code.
    Also each size and type of insert is available in a range of cutting point radius and different cutting materials
    that make them suitable for different cutting requirements of all the various materials & steels that can be
    turned.
    So you have to decide on positive or Negative rake and insert size and shape/style that suits the tool holder you
    are going to use.
    Then buy the tool holder/s . Take the tool holders with you to the tooling shop and get the right insert to fit.
    Also you have to select corner radius for a roughing, medium or finnish cut . I just use .08 radius for everything.
    Select the insert coating material for a particular metal or whatever. I have used GC 4125 and found it had a wide
    range of use but coatings come and go . However some materials are not available in the smaller inserts. Ask about
    coatings and suitability of and try to get the material that gives you a wide range of application in the insert
    style you need. Make them show you the application range in their catalogue so you can see if it is the best for
    you or what.
    Once you have done it is simple the next time.

  6. #6
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    Default

    G'day Retromill.
    Thanks for the info.
    My lathe is a 3phase 3HP 1060mm between centres.

    She is a solid old bugger. Pacific SSL1060A.
    Hooroo.
    Regards, Trevor
    Grafton

  7. #7
    Join Date
    Dec 2006
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    Default Negative Rake or not

    Thanks Trevor, Glad to help if I can. At 3 HP 3phase unit your lathe could handle negative rake tooling and inserts. However if it is an old lathe then I would test for any movement in the spindle bearings .
    Place a dial gauge on the chuck and wooden block across the bed under the chuck and give it a bit of a leaver with a piece of soft wood.
    Now I can,t tell you how much movement is too much I don't know and I have never been able to find out, but my lathe is fairly new and it only moves the dial gauge .001 with quite a bit of force . So if the spindle bearings are tight you could use negative inserts especially for roughing cuts without placing too much stress on the spindle bearings.
    The thing is that negative inserts are dearer and usually only available in packest of 10 but in the square style inserts with 8 usable corners they can be economical especially if a lot of material has to be removed as roughing cuts.
    However if you use a lot of negative rake cutting and machine a lot of hard heavy stuff dont expect your headstock to chamber any BR / Comp. quallity stuff as it will get a bit loose after a while.
    I think you will know what I mean.

  8. #8
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    I found that the negative rake tools give a better finish with softer materials , some of the special turning steels and some alloys , with copper and al products I used HSS and grind a negative angle to suit .
    Cause that was 10 years ago and I am sure disposable tools have progressed since then
    Ashore




    The trouble with life is there's no background music.

  9. #9
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    Dec 2006
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    Hunter Valley
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    Default

    Dear Ashore, My lathe is too small to use negative rake inserts well but I have heard from other people that soft materials buildup less on a negative rake insert than a positive one . So that may be the reason they produce a better finnish. I used some soft al the other day to make a small job it was supposed to be a Marine grade al , atleast thats what I bought it as because the article has to stand out in the weather.
    Build up on an 11 degree positive rake insert was anoying but the finnish was quite good.
    I am not sure I understand you with grinding a negative angle on HSS tool bits as you would need to be able to angle the tool holder down to get some front clearance . Most newer lathes can't change the down angle of their tooling but It could be done in a single rocker tool post the kind that takes Armstrong type tool holders .
    You have raised a good point maybe I am missing out on some machining flexability by not having a single rocker tool post available .Hmm! I will have to look into this. Regards from Fellow Novacastrian.

  10. #10
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    G'day All.
    I did the dial indicator test as mentioned by Retromill.
    I got 0.001mm using a 3ft length of 3x2 as a lever under the chuck.

    Removes the doubt of worn spindle bearings by doing the test.
    Hooroo.
    Regards, Trevor
    Grafton

  11. #11
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    Default

    Quote Originally Posted by glock40sw View Post
    G'day All.
    I did the dial indicator test as mentioned by Retromill.
    I got 0.001mm using a 3ft length of 3x2 as a lever under the chuck.

    Removes the doubt of worn spindle bearings by doing the test.
    That is real good , .001mm is nothing . The .001 inch (.025mm) that I get on my lathe has a component of flex in it because of the smaller weaker headstock arangement . Your lathe is much stronger and does not flex any so the reading is more accurate. I cant see any impediment to negative tooling for some jobs.

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