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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 dp
    So the TNMP insert in your image has 10 degrees rake relative to what?
    The 10° is the chipbreaker angle. That angle is specified w.r.t. a flat top insert. Like all negative rake inserts, the sides are flat.

    So if the tool holder were dipped down 15 degrees and the tip brought to the tangent point again, that would produce a negative rake of 5 degrees and a rather stunning clearance angle.
    Exactly. You have a square-sided negative rake insert, and you have either no chipbreaker, like Evan's insert, or a variety of chipbreakers than range up to 20° (if I remember correctly). So a HSM'er can change the pocket angle to trade-off the rake and clearance angles.

    That image is from the Plastools web page, which makes insert holders for home-shop machinists. So they set the pocket much more positive than you would for an industrial machine.

    The ISO toolholders (i.e., the standard toolholder you buy at MSC or Enco), have specific pocket angles. The MTGNR toolholder I mentioned earlier, which is the toolholder that goes with the TNM* inserts, is something like 5° negative front and side:



    In other words, whether you buy an ANSI/ISO toolholder from Hertel, or Sandvik, or Valenite or Bison/TMX, they all have the same insert pocket angles.

    By the way, the same ANSI designations apply to boring bars. The only difference is the material is pre-pended to the designation. So a TNMG, TNMM, or TNMP insert uses an MTGNR-xxx toolholder, or an Axx-MTGNR boring bar. 'A' designates steel, 'C' is carbide.
    Last edited by lazlo; 07-12-2009, 11:45 PM.

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  • Glenn Wegman
    replied
    Originally posted by lazlo
    and for the first 20 years that carbide inserts were available, most of them were either N or P inserts (TPG's, etc).
    I was going to go there strictly as a guess in regards to the M (molded) G (ground) issue as it seems like a lot of coincidence that from the entire alphabet they chose P,N and M,G.

    But I didn't! (well I guess I did)
    Last edited by Glenn Wegman; 07-12-2009, 11:22 PM.

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  • dp
    replied
    Originally posted by lazlo
    Agreed Negative rake inserts have square sides, period.
    So the TNMP insert in your image has 10 degrees rake relative to what? That "what" is the assumption the plane of the insert will be be in parallel to a ray depending from the center of the chuck when the insert is centered vertically on the work - tangent, as it were. In line with the cross feed, in fact.

    So if the tool holder were dipped down 15 degrees and the tip brought to the tangent point again, that would produce a negative rake of 5 degrees and a rather stunning clearance angle. That may or may not be inside the design scope of the insert - particularly a sintered insert, logic tells me. At some point the shear forces against such configurations have to take a toll. While this is a possible configuration and will provide a chip breaker function, it seems to me there is a better way to achieve this.

    If this is so then all the angles in practice come down to "it depends". In theory it seems easily defined except that I see no specification for tool holder expectations or I'm looking at the forest and seeing nothing but trees.

    I use TNMP's all the time on my little Asian lathe and like the results. I typically cut 1018 and at a wide range of feeds and speeds owing to the curse of change gears and belts and a predisposition to leave the gears where they are for the final op (often threading), and hand feed the roughing passes. And I could get them at Boeing surplus for about $0.25 each.

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  • lazlo
    replied
    Originally posted by BadDog
    But I maintain it's useless except as a mnemonic because there are ground inserts that are not G, and molded that are not M.
    I agree the forward case is correct -- G's are not the only ground inserts, but every "M" insert I've ever bought was molded, and not as sharp as a ground insert.

    I've also found that coated inserts are not as sharp as un-coated -- they grind first then PVD/CVD coat, and the coating rounds off the cutting edges. Which is, I think, why the "Upsharp" or "Honed" inserts are almost always uncoated.

    Man, I wish it would cool down a bit. I got things I want to do in the shop. Suns been down a while, and it's just now dropped to 109...
    We feel your pain down in Austin. For reasons I don't entirely understand, there's been a high-pressure zone that's been sitting on Central Texas, and we've had 105 - 107° for the last 5 weeks straight. That's 15° over our normal June/July temperatures, which is scary, because the temperature here peaks at the end of August...

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  • BadDog
    replied
    Originally posted by lazlo
    Hey, I didn't make this stuff up, I'm just reading the spec. Believe me, I've read far worse ANSI/ISO specs. In fact, based on your profession, I bet you have too Russ
    LOL! That's the truth!

    Ageed in general.

    As I've said, I've read some of the same references stating things just as you describe. But I maintain it's useless except as a mnemonic because there are ground inserts that are not G, and molded that are not M. Someone probably said something to the effect if "In the beginning there were molded and ground inserts, and their tolerances were typically of this value, so we'll use 'M' and 'G' to denote that value for coherency (folks that write specs like words like that ), and add in all the rest to fill out the field..." But regardless of the origins, I don't think you can really say "G is ground and M is molded". <shrug> Doesn't matter, too many "references" out there use this descriptions, and my pedantics aside, really doesn't matter to me either way. At least now folks who didn't know otherwise may register that there are other options beyond M and G, and so not be too surprised if/when they see them. Likewise for "P", though N is pretty clear cut and final...

    Man, I wish it would cool down a bit. I got things I want to do in the shop. Suns been down a while, and it's just now dropped to 109...
    Last edited by BadDog; 07-12-2009, 10:57 PM.

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  • lazlo
    replied
    Originally posted by BadDog
    This is much the same as P in the second position. Folks will say that it means "Positive", but it's really just clearance angle, and there are "positive" inserts that use other designations (and that's as close to that discussion as I care to get! )
    It's actually not a coincidence that negative rake inserts have an "N" designation. "P" was chosen for "Positive Rake" because it's the 11° positive insert, and for the first 20 years that carbide inserts were available, most of them were either N or P inserts (TPG's, etc).

    So if you look at the relief angle table, there's "N", and "P", and the remaining slots are A - G. It's pretty obvious that they reserved N and P for the common Negative and Positive designations:

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  • lazlo
    replied
    Originally posted by BadDog
    If M is Molded and G is Ground, what are A, C, E, F, ... and all the rest? Just arbitrary aberrations with no meaning?

    And if a molded insert process (or selection standard) produced inserts that met the spec for "G", I would expect them to label it "G". That's my story and I'm sticking to it... at least for the moment...
    Hey, I didn't make this stuff up, I'm just reading the spec. Believe me, I've read far worse ANSI/ISO specs. In fact, based on your profession, I bet you have too Russ

    If you look at the tolerances, "M" and "U" have the worst tolerances -- 2 - 5 thou. All the other letters are "G" (ground ) tolerance or better. If you look at the "A" tolerance -- that's 2 tenths. So I bet there's a Godlike Swiss TNAG-xxx insert out there somewhere that's ground to 2 tenths...

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  • BadDog
    replied
    Originally posted by lazlo
    Yeah, I know that's confusing. The tolerance actually indicates whether the insert is ground or molded. With a molded insert, the carbide is sintered in the actual insert mold, so the "M" tolerance is 2 - 5 thou.
    I agree that it works out that way, and I've read it in more than one "reference", but I just don't buy it.

    If M is Molded and G is Ground, what are A, C, E, F, ... and all the rest? Just arbitrary aberrations with no meaning? I can't prove one way or the other, but I expect it is just a convenient mnemonic that became adopted fact by virtue of common application. It shows up in conversational texts, but never on official standard/spec sheets as far as I've seen. Or maybe it's the reverese. Those were common tollerances for the process, so adopted into the standards, and then expanded with others for better definition. But even if so, it still eliminates the original association as far as I'm concerned. For instance, I would expect an "F" to also be Ground, but that does not imiply it's really a G, and so on...

    This is much the same as P in the second position. Folks will say that it means "Positive", but it's really just clearance angle, and there are "positive" inserts that use other designations (and that's as close to that discussion as I care to get! )

    So if a molded insert process (or selection standard) produced inserts that met the spec for "G", I would expect them to label it "G". That's my story and I'm sticking to it... at least for the moment...
    Last edited by BadDog; 07-12-2009, 10:19 PM.

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  • Evan
    replied
    The figure you posted is refering to negative/positive tooling as an insert with a negative side rake. Side rake is altogether different than back rake as it describes the plane of the top of the tool and it's relationship to the long axis of the lathe. If the tool is tilted down on the side toward which it moves as it cuts it has negative side rake regardless of the top rake angle or the cutting edge angle.

    I wrote: "Conversely the ANSI specs for tool holders do not contain rake information. " I said nothing about clearance information which is a different matter. The clearance of an insert does NOT reflect the cutting top rake angle it provides.

    N does not stand for negative, only insert relief angle provided. The tool holder clearance angle provides no information about tool top rake angle since that is dependent on the tool top surface features. Further, that clearance angle only applies to inserts with no clearance.

    It would make no sense to specify both a clearance angle for the insert, which is the case, and another separate clearance angle for the tool holder. One or the other provides the required clearance, not both. In the case of inserts with no clearance then the tool holder provides the clearance. In the case of inserts with clearance then the insert provides the clearance and there is no specification for tool holder angle.

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  • lazlo
    replied
    Originally posted by lane
    If you can turn the insert up side down and use the bottom side it is a negative rake insert. Period .
    Agreed Negative rake inserts have square sides, period.

    Originally posted by lazlo
    Note that these are all negative rake inserts:

    That's a huge advantage for the HSM'er -- because negative rake inserts have square sides, they can be flipped over, and you get twice the number of cutting edges.

    You can't do that with positive rake inserts, because they have the positive rake angle molded or ground into the insert itself.
    Last edited by lazlo; 07-12-2009, 10:15 PM.

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  • lane
    replied
    If you can turn the insert up side down and use the bottom side it is a negative rake insert. Period . Lay a square on top and if the edges are 90؛ it is considered a negative rack insert.Period again.
    Last edited by lane; 07-12-2009, 09:54 PM.

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  • lazlo
    replied
    Originally posted by BadDog
    But a small point, and I'm sure you are aware, but others may not be. That 3rd position is actually "tollerance". "M" is not "Molded", and "G" is not "Ground", though it works out that way often enough that it's a decent mnemonic.

    Reference Chart
    Yeah, I know that's confusing. The tolerance actually indicates whether the insert is ground or molded. With a molded insert, the carbide is sintered in the actual insert mold, so the "M" tolerance is 2 - 5 thou.

    A ground insert is sintered oversize, and ground to final dimension, so the "G" tolerance is 1 thou. In fact, this book is a little dated -- Sandvik spec's a tolerance of 5 tenths for their ground inserts. Also, the text doesn't mention this, but as Glenn was implying, the ground inserts are sharper, and cut nicer, but they're more expensive:

    Last edited by lazlo; 07-12-2009, 09:51 PM.

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  • lazlo
    replied
    Originally posted by Evan
    Then why does the ANSI nomenclature system contain information to identify the rake angle of inserts? Conversely the ANSI specs for tool holders do not contain rake information.
    The ANSI/ISO designations do specify the toolholder rake angles. The TNMG negative rake insert, for example, is intended to be used with the "MTGNR-XXX" toolholder:
    "M" is clamp and pin, "T" for triangular insert, "G" for offset pocket with no additional relief, "N" for negative rake insert, "R" for right-handed tool:

    Last edited by lazlo; 07-12-2009, 09:34 PM.

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  • lazlo
    replied
    Originally posted by dp
    I had thought that the terms rake, clearance, etc., had to do with the angle of the various faces relative to the work.

    by placing it in the proper holder angled down, achieve both negative rake and clearance.
    That's exactly right Dennis. Negative rake inserts don't have negative rake -- they have square sides. They're designed so the toolholder tips the pocket down 5 or 10° to give you the negative rake.

    Positive rake inserts actually have 3 - 30° positive rake, so if you're making your own toolholder, you have much less latitude in changing the rake angles than a negative rake insert.

    If that is so then the concept of rake and clearance in carbide is one of intent of design,

    I thought the angle of the tool holder mattered in establishing rake, etc., and that the machinist would consider the vendor's guidelines as to what the range of operation is. I think, in fact, that is exactly what you have done with your cermet cutters, no?
    Exactly. A flat-topped negative rake insert like the SNGA Evan is using is a worst-case scenario for horsepower and rigidity. But take that same negative rake insert with a positive rake chipbreaker, and you end up with a "Negative/Positive" insert that requires much less power and rigidity:

    Metal cutting theory and practice, David A. Stephenson

    Last edited by lazlo; 07-12-2009, 09:21 PM.

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  • Evan
    replied
    I guess that's a long way of saying I thought the angle of the tool holder mattered in establishing rake, etc., and that the machinist would consider the vendor's guidelines as to what the range of operation is.
    Then why does the ANSI nomenclature system contain information to identify the rake angle of inserts? Conversely the ANSI specs for tool holders do not contain rake information. How you choose to hold it is up to you but when I am buying inserts frequently the only information I have to go on is the ANSI spec in the type number.

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