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IndustryArena Forum > General Manufacturing Processes > Turning > Help with 1/4 npt G71 on Okuma Lathe
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  1. #1

    Help with 1/4 npt G71 on Okuma Lathe

    Hello, first post here. I feel like I'm losing it a bit here. My G code (according to my eyes) is correct I think. I keep having troubles with this internal toolpath.

    It seems like no matter what I do, I keep breaking inserts/destroying tools. Any other thread (npt or otherwise) I have zero problems with.

    OK, so here's the setup.

    Machine- Okuma Lb3000 EXII

    Tap drill- ISCAR SumoCham 5x drill w/ .437 insert

    Threading tool- Carmex 08IR with 18 npt laydown threading insert. Recommended sfm 295 min 450 max in SS

    Material- 303ss

    My process-

    Drill, put on 30* cham (as per our company req.), THEN cut the taper for the NPT, after that I thread with a G71, chase/top threads with boring bar, rerun threading cycle.


    G71 X.540 Z-.4375 H.088 B60 A-1.79 D.023 U.0005 Q1 M32 M73 F.0556

    RPM's- 2000 as per my math for correct sfm on the insert. (3.82x295)/.540. Initially, had the RPM's @ 500 but changed to be in line with the inserts "requirements".

    I don't have any one to bounce ideas off at work and am completely at a loss for what I have wrong. If you have any advice, I'd truly appreciate it.

  2. #2
    Join Date
    Jun 2015

    Re: Help with 1/4 npt G71 on Okuma Lathe

    hy are you braking only threading inserts, or also others ?

    when i need to debug threading operations, i stop and inspect thread and tool after each pass, sometimes also recovering the chip from the machine, and having them in order on the table ( color, shape, may tell a few things ); during testing, cutting specs are low, moderated, and after i fix, or at least improve, then i raise them

    between others, is important that there are no chips on tool, or inside part, thus operation must run clean; importance of this is higher as threads get lower in size

    for such testing, i use a threading code that is performing a tool retract & M0 between passes

    possible causes :
    ... insert interference ( check attached image )
    ... toolholder interference ( use a marker, and inspect the toolholder as you go, so to identify marks that should not be there, caused by a wrong tool mounting, or chips squished between part and toolholder )
    ... low quality inserts, that look nice, but simply don't cut ( i take the tool to a clasical lathe, and check it with a veteran )
    ... improper cooling ( internal coolant is better; if also outside available, try to use 2 nozzles, one hitting insert top, the other hitting insert side, thus using internal coolant toghether with outside coolant may be better than only internal )
    ... entangled chips ( increase clearance so there to be enough room for the chips to fall, lower doc, etc )
    ... blind hole ( causing chips to gather/squish at the end; try to attack the hole in 2 or 3 steps, like threading 60% depth, then 90%, then the rest 10% )
    ... tool overhang ( lower specs as much as possible )

    sometimes chips will entangle, one way or another, so i look/listen carefully, and input an M0 not after each pass, but after a few, like after 5, etc, and M0s may be more frequent near the final passes ( for example, tool may behave ok with an entangled chip at the 1st few passes, while an entangled chip right before the last pass, may damage the thread ); for small threads, this requires a bit of silence, because inside a noisy shop is hard to hear a small vibration

    refresh operations after threading :
    ... recut the front chamfer towards the root
    ... be sure that you use as many spring passes as needed, and listen the tool during spring passes; sound should be clean, cutting chips should not exist, or be minimal

    for small threads, the machining tolerance of the bore, before threading, matters, because a bore size that is minimal, will put too much force on the insert, especially at the 1st few passes, so, consider :
    ... having the bore near the maximum tolerance, and/or
    ... before threading, thus before the 1st pass, cut a 0 pass ( or more if needed ), designed to deal with any bore variations, so to minimize starting stress during threading operation; in others words, a 0 pass ensures that 1st pass always has constant doc

    for small threads, a partial insert may behave better than a full profile insert, simply because there is more clearance for the chip

    how you are using an okuma machine, inspect load diagram for each pass, because it can show anomalies that you may not be aware of, or hard to detect; it's sensibility should help, hoping that the little thread can be felt by the machine

    by the way, there is an okuma forum here : www.cnczone.com/forums/okuma/


  3. #3

    Re: Help with 1/4 npt G71 on Okuma Lathe

    Thanks for replying.

    I've tried most of that. I'm more so concerned about my code and approach. I'm going to keep those in mind. If I have a moment I'll run it again tomorrow. I think I'm going to change my initial DOC.

    The tool holder I'm using has great geometry imo and it's an insert that's specifically for npt18. Frankly I'm at a bit of a loss and I feel like it's something stupid I'm missing.

    Sent from my SM-G960U using Tapatalk

  4. #4
    Join Date
    Jun 2015

    Re: Help with 1/4 npt G71 on Okuma Lathe

    yes, sometimes may be just a little thing

    i forgot to tell you, on small id threads, thus when there are small inner clearances, insert may recut the thread on it's way out; this is hapening because, after final z is reached, x travel is short, and z out motion begins too fast; is a cinematic thing; when it hapens, it may not recut the entire thread, but only the final few pitches; this can be prevented by using a lower rpm and lower rapids; also there are other methods / kindly

  5. #5

    Re: Help with 1/4 npt G71 on Okuma Lathe

    Thanks, I'm going to give it a go again this morning. I appreciate your input. Thank you

    Sent from my SM-G960U using Tapatalk

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