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How much difference can tooling make on a small, low power lathe?

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  • lazlo
    replied
    Originally posted by nheng
    In Stephenson and Agapiou's "Metal Cutting Theory ..." book here:

    http://books.google.com/books?id=19A...esult&resnum=4

    They refer to negative/positive inserts as negative side rake, positive back rake. Robert's illustration is also correct, showing a negative back rake insert that has become positive due to the chipbreaker.
    ...and as I explained 102 posts back, flat-topped negative rake inserts with purely negative shear angles are not well-suited to a Home Shop Machinist -- they require maximal power and rigidity.

    Get the same negative rake insert Evan used, but with a positive chipbreaker -- the Negative/Positive insert in Figure C in the page I posted from Moltrecht -- it will require much less horsepower and rigidity than the exact same negative insert with a flat top, in the same toolholder.

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  • Evan
    replied
    The ANSI specs refer to the side rake as the negative rake as refered to in the definition being used to identify a negative/positive insert. The confusion arises because a negative rake insert also has no clearance and so must be inclined toward the work (tool holder inclination) to provide clearance.

    In the tool holder I made to use the insert in my original post I provided about 7 degrees of forward inclination and the same -7 degrees side rake. Because the top rake is negative and the side rake is negative it is a negative/negative insert.

    It isn't a matter of different manufacturers using a different definition since if they ignore the side rake then they only have one rake to talk about, the cutting edge. Inclination provides clearance, the shape of the cutting edge determines the top rake angle. Those are two different things. To refer to the forward inclination as the negative in negative/positive is a paradox and an oxymoron. It must be one or the other. This is the point I have been making all along. The insert has either a positive edge, a neutral edge or a negative edge. By convention a neutral edge is considered negative, a bit like a temperature of -0.

    What makes an insert with a positive rake edge a negative insert isn't the fact it is inclined to provide clearance. It's the fact that the top plane of the insert is inclined toward the chuck.

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  • nheng
    replied
    Although I skipped 102 replies, I can see Robert's and Evan's points.

    In Stephenson and Agapiou's "Metal Cutting Theory ..." book here:

    http://books.google.com/books?id=19A...esult&resnum=4

    They refer to negative/positive inserts as negative side rake, positive back rake. Robert's illustration is also correct, showing a negative back rake insert that has become positive due to the chipbreaker.

    So, it seems to be a simple matter of the same name (sort of) having been used for two different things in the same industry. Not a big surprise. Guess you gotta read the fine print before plunking down $10 - $20 per insert

    I've had major headaches in electronics dealing with printed circuit board (PCB) manufacturers. Here's an industry that is quite mature yet 3 major suppliers across the country do not agree on the definitions of a number of common fab options

    Den

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  • Evan
    replied
    That's right (post 103) and it has nothing to do with the top rake (aka back rake) or the inclination of the tool holder toward the work. My illustration shows negative side rake in which the cutting edge on the SIDE is negative in respect of the work. That is the "negative" in "negative/ positive", not the fact that the insert may be tilted by the tool holder toward the work, which is how you have been explaining it all along. You can have a tool holder that has positive rake with respect to the work and negative side rake at the same time.

    Tool holder inclination provides clearance while in the case of inserts with a chipbreaker feature that feature may provide a positive cutting edge. I grind HSS tools with negative side rake and positive back rake for boring. They take a heavy cut on the way in which deflects the tool but on the way out it is cutting positive/positive and takes a fine cut which compensates the spring.

    Look at Kyocera tool holders. They don't call the tilt of the tool holder "rake". It's the inclination angle. They list side rake seperately which is the negative in negative/positive. Your diagram above in post 104 is incorrect.

    http://global.kyocera.com/prdct/tool...alog_cp231.pdf
    Last edited by Evan; 07-18-2009, 11:04 PM.

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  • lazlo
    replied
    Originally posted by Evan
    You still don't have it right Robert.
    An insert cannot have a negative and positive back rake at the same time regardless of the chipbreaker style or how the tool is tilted toward the work.
    Sigh. One last time. From Moltrecht:

    "The relief angle on the negative rake insert, shown at B, is provided by the inclination at which the insert is held on the tool holder. Held in this position, the flanks of the insert can be perpendicular with both faces and all of the edges on both faces can be used as cutting edges. Therefore, negtive rake inserts have twice as many cutting edges as comparable positive rake inserts. Another advantage of negative rake inserts is that they are stronger than positive rake inserts and more able to withstand shock loads, such as encountered when taking interrupted cuts. The advantage of positive rake inserts is that when cutting, the cutting force is significantly less. When the cutting force must be kept low as possible, as when cutting thin material sections, a positive rake insert should be used.

    Positive-Negative rake inserts, view C, are held on negative rake tool holder but have an effective positive rake angle, provided by the groove on the face of the insert."




    And open any tool catalog, and it's exactly as Moltrecht and the other machinist texts I've posted describe:

    Last edited by lazlo; 07-18-2009, 11:08 PM.

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  • lazlo
    replied
    Originally posted by Evan
    Evan, Side Rake Angle is the angle of the top of the tool, with respect to the side. In other words, it's the angle the top of the tool (the positive chipbreaker) is sloping backwards:

    Last edited by lazlo; 07-18-2009, 10:57 PM.

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  • Glenn Wegman
    replied
    5.5" bushings turned from 36RC 4140 turned at 900 fpm using Cermets. (Turned in pairs, back to back on a manual lathe)



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  • Evan
    replied
    You still don't have it right Robert. An insert cannot have a negative and positive back rake at the same time regardless of the chipbreaker style or how the tool is tilted toward the work. That isn't what negative/positive means. Your diagram explains that.

    In order for an insert to be considered "negative/positive" it must be cutting with negative SIDE RAKE. Negative rake on the tool holder toward the work is irrelevant if the cutting edge is still positive. Kyocera lists side rake separately from back rake (top rake) or insert holder inclination (not negative rake).

    See below.

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  • noah katz
    replied
    "For our HLV clone, I use very positive (20-35 degrees of positive rake) stainless steel finishing inserts from Sandvik that have a long (~.4") cutting edge and are capable of deep cuts. They work brilliantly on just about everything."

    Doesn't that make the insert fragile? Do you mean everything as long as it's finishing cuts?

    It's great to be able to economize by being able to flip the insert over, but what about the cost of toolholders for these?

    Can they be used in a standard toolholder for BX toolpost with no rake?

    Would a standard zero front rake toolholder work as long as the cutting edge is at centerline height or slightly below?
    Last edited by noah katz; 07-18-2009, 03:24 PM.

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  • lazlo
    replied
    Originally posted by Glenn Wegman
    I was just browsing the Korloy catalog and throughout the catalog inserts are classified by two catagories, positive and negative, regardless of chip breaker configuration.
    All tool catalogs, and any modern machine shop text, including the half dozen or so I posted (and confirmed by Lane) say the same thing: xNxx inserts, with square sides, are negative regardless of the chipbreaker. So the Koroloy catalog, like the Sandvik, Kennametal, and Valenite catalogs, indentify CNxx, TNxx, SNxx, et al as negative inserts.

    A hybrid insert like the TNMP or CNMP is referred to as a "Negative rake insert with a positive chipbreaker" or a "Negative/Positive insert."

    So as this picture from Metal cutting theory and practice shows, the toolholder and insert orientation with respect to the workpiece is negative, regardless of the chipbreaker. If you use a flat-top negative insert ( a "Negative/Negative" insert), it's purely negative. But replace that insert in the same toolholder, with the same pocket angles, with one with a positive chipbreaker (a "Negative/Positive" insert), and you have a negative insert angle with a positive shear angle -- the best of both worlds:

    Last edited by lazlo; 07-14-2009, 12:13 PM.

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  • Glenn Wegman
    replied
    Originally posted by Evan
    The issue is one about insert nomenclature. It seems that the ANSI nomenclature for insert designation is meaningless. It is being argued that the rake of the cutting edge relative to the work does not determine the insert type as per ANSI nomenclature. In actual fact it does although apparently you may need to buy what is being called a negative insert in order to get a positive one. Proper interpretation of the ANSI labeling system does reveal the type of insert which is of course the objective of a standard.

    The argument posed by Robert results in the rather awkward circumstance of specifiying a negative rake insert to obtain one with a positive rake cutting edge. He has also made some statements that are flat out incorrect, such as that the second letter of the insert identification system indicates the rake angle of the cutter when in fact it only specifies the clearance angle below the cutting edge.
    I was just browsing the Korloy catalog and throughout the catalog inserts are classified by two catagories, positive and negative, regardless of chip breaker configuration.

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  • J Tiers
    replied
    Hss seems to prefer no more than a brownish chip, not into the purple area. Actually, if you look up the temper colors, you can estimate the chip temp, but that may not reflect the cutting edge temperature.

    While grades of HSS claim to work red hot, it seems generally to be a bad idea to push that.

    As mentioned, carbide will take/make much hotter chips.

    Curiously, the last time I had occasion to compare them, I was in fact getting blue chips with HSS, but too much edge wear on a cutter with considerable side rake. When the chips were colder, I was getting cobwebs waiting for each pass.

    When I switched to carbide, at first via a TNMG insert, and then later via an older USA-made brazed cutter with neutral rake, I got much colder chips at a similar DOC and speed/feed to that which had produced the blue chips with HSS.

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  • toastydeath
    replied
    Originally posted by Alistair Hosie
    As usual a lovely job well done Evan well done but are the chips meant to be so blue I thought that meant you were cutting too hot and needed to cut back a little ?Alistair
    Carbide doesn't care about blue/purple (and even sometimes cherry red) chips; the only way to gauge the "right" temperature is to check the edge wear periodically. If it's wearing faster per minute than you want it to, then you need to back the speed down.

    You can use the chip color as a rough guide on HSS because the temperature range where HSS really starts to break down fast is in the range where chip color has the most change. It's just a convenient coincidence.

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  • Alistair Hosie
    replied
    As usual a lovely job well done Evan well done but are the chips meant to be so blue I thought that meant you were cutting too hot and needed to cut back a little ?Alistair

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  • Peter.
    replied
    Lazlo in the diagram above, looking at the holder in plan, which way does the pocket negative rake angle go? I need to make a holder for my TNMP inserts which arrived today. My first guess is that the pocket rake would be (for instance) 5 degrees directly 'south' on the diagram to provide clearance along the cutting edge, but wouldn't this leave the forwardmost tip of the insert cutting tangentially and with no clearance at all, so I guess I need an amount of negative rake to the 'west' on the diagram too or it will chip-off the cutting edge on the other side of the insert?

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