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  1. #1
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    Default Tool Post Thread grinder

    So I set up my TP grinder on the lathe, dressed a 120G P hardness wheel and tried out a 1/4 x 20 thread. Poor results were had. The edge of the wheel lost its form really quick. I expected helix angle issues, but the blunting of the point on the wheel made that for naught.

    So what kind of wheel is needed, I was using a 4" D wheel. Assuming a finer grit wheel to hold the edge, what hardness.

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    What does the root of the thread look like? Do you have the grinding wheel axes tilted to the helix angle? I think you are either having the point of the wheel scraping the flanks making the V profile more of a spade profile, or you are needlessly grinding the root deeper.

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    First pass at .010 infeed left a fine line as expected, second pass at .015 deeper and wider, two more .015 passes, and the vee groove was rounded like a .062 radius tool would leave. As stated, the grinder was not tilted. I then stopped the carriage and just feed the wheel straight in, the profile matched the thread, crap.

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    Are you talking inches or mm.......your depth of cut is from 10x to 100x too great......with the setup you have ,tenths( of a thousandth of an inch )is a viable cut ......and trying to grind from solid on a toolpost grinder is folly.

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    Quote Originally Posted by john.k View Post
    ......and trying to grind from solid on a toolpost grinder is folly.
    Certainly not the shortest way around the barn.

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    Hi Toms Wheels:
    John.k has a point about the amount of material you're hoping to remove with this setup, both with each cut and the total material removal when you are grinding from the solid.
    However, thread grinding from the solid is a perfectly viable process, just not with a wimpy setup and most importantly not with a weenie little wheel.

    When you look at a dedicated thread grinder you will see that they run large diameter wheels, in big rigid spindles, tilted at the helix angle of the thread, and deficiencies in these three factors are why the process is not working for you.
    A big wheel means that every grain at the periphery of the wheel shares the work with many others, which obviously is not true with a small wheel.

    This primarily is why the corner of your wheel is degrading so fast.
    That and you can't conveniently re-dress the wheel on the fly and you (presumably) don't have flood coolant.
    Also you do not have the benefit of the research thread grinder makers have undertaken to find the best combination of bond, grain size and wheel hardness to give the best balance of free, cool cutting and wheel shape retention.
    All of these things matter immensely as you're finding out.

    So given the limitations of your gear, you need to circumvent the worst of it.
    If you are willing to accept a hard wheel you can preserve the shape better.
    To get a hard wheel to cut without burning you need better cooling and you can't load it up by hogging with it.

    So roughing the thread with a single point threading tool, dressing the wheel often (rig something up so you don't have to tear down your setup for each dress) find a way to flood cool it without trashing the lathe: all will help you.
    Grab an "L" hardness wheel that's as coarse as you can get away with, and maybe snip out bits of the periphery to break up the cut (it'll run cooler that way).
    That will also help you.
    Tilt the toolpost grinder to the helix angle as Alex recommends in post #2.

    These are all tricks you can try to help you overcome the fact that you're not running a 36" diameter wheel on a 50 horsepower wheelhead using a 15000 lb machine with a built in dresser.

    If you do all those things you can grind decent threads...they won't be even remotely as good as what a thread grinder can make, but they will be good enough for many things.

    My personal experience was with threaded mold cores...I've ground dozens in my career as a moldmaker and they were done with a surface grinder running a 7 1/4" wheel.
    They had to have good fidelity of thread form and a great finish but did not need to be particularly accurate by the criteria that normally apply to ground threads.
    I and about a bazillion other toolbreakers were able to routinely achieve threads that were plenty good enough for those kinds of applications, but they weren't micrometer spindle or leadscrew grade threads by any stretch.

    So it's by no means impossible so long as you are realistic in your expectations.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

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    John K. Not sure why you think it is folly to use a TP grinder in this application. This was an exercise to see what needs to be for a project in the planning. Tilting the grinder to the helix is possible, but adjusting the angle of the dress on the trailing side of the wheel would be easier.

    In the past I have used a Ruby wheel dressed to 45 degrees on Vee grooves in Hardened D2, and did not recall excessive wear. That was a 7" wheel not a 4".

    Before my Cylindrical Grinder, there was a bunch of grinding done with TP grinders, just never threads.

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    Old (1954) Norton book says "S" hardness for from-the-solid 13 to 24 pitch

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    Dumb questions - can you rough-in the thread with a carbide insert? And is the purpose due to hard stock, or trying for accuracy? If your lathe and carriage/compound are worn, I'd question the final precision of the part. Not to mention leadscrew wear unknowns...

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    IMO, not tilting the wheel is a big problem. Not even sure what kind of weird geometry you'll get, not matter how you dress it. A lathe makes a really crappy grinder compared to a grinder. That said, it will be interesting to see how good you can do when you fix the various problems.

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    Hi again Toms Wheels:
    In post # 7 you wrote "but adjusting the angle of the dress on the trailing side of the wheel would be easier"

    By leaving the wheelhead spindle co-planar to the workpiece spindle and trying to compensate by dressing the wheel you ignore the fact that you are side loading the tip of the profile, and increasing the likelihood of popping abrasive grains from the corner of the wheel.
    My expectation is that you will accelerate the wear on the wheel with this strategy and that appears to be happening based on your report.
    It's corroborated by this comment from you:
    "In the past I have used a Ruby wheel dressed to 45 degrees on Vee grooves in Hardened D2, and did not recall excessive wear"

    I would really re-think whether it's worth it to resist tilting the wheel.
    If you make a little dressing template you can mount to the TP grinder spindle and incrementally move toward its axis, you can dress the wheel accurately as often as you need to and it's a simple dress that will make an accurate profile completely painlessly with nothing more complicated than a single point diamond in a little rectangular steel block and your template against which the block can run when you push it by hand.

    This is how we always did it when grinding the mold cores I spoke of in post #6 and it worked very well.
    Using an "L" hardness wheel we were able to grind threads accurately to form with none of the trouble you described in your first post...it's been decades but I recall getting more than one core completely finished with a single wheel dress using a Norton or Rappold 80L aluminum oxide wheel (with flood cooling) and the requirements for threadform were typically quite fussy as I recall (in the tenths range for sure).
    We pulled off the wheelguard and rigged a smaller one so we could access the dressing template that was mounted horizontally.
    The Harig whirlyjig was mounted on a small ball slide we could twist on the mag chuck to the helix angle of the thread and we had a dummy threaded plug mounted in the vee block of the Harig with a follower nut to force the workpiece to advance with the slide as the Harig spindle was turned.

    So this was a truly ghetto kind of setup but it ran like a bunny and made great parts in spite of all its shortcomings.
    I see no reason why you could not duplicate my success using your own ghetto setup.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

    Oh yeah: while I'm at it, I assume you're not running the workpiece too fast???
    I don't think you want to be above maybe 150 RPM on the lathe spindle and slower is better.
    Anything much faster than that and you'll strip the grains right off the wheel as soon as it touches.
    You also want to make sure the workpiece doesn't bounce or vibrate...a little sprung follower will often work wonders.
    I'f you've been cylindrical grinding for a while you will know all this and I apologize for stating the obvious; but we have no idea how much of this sort of work you've done before.

    MC
    Last edited by implmex; 10-16-2019 at 07:04 PM.

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