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Relative newbie question on internal threading?

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  • J Tiers
    replied
    Originally posted by BCRider View Post
    I've yet to NOT get burrs even with freshly sharpened tools. Pretty much anything other than the last couple of one thou whisker cuts or a spring cut raises a burr when I do single point cuts.
    You pretty much HAVE to.....

    Think about it.... no cutter is perfectly sharp, so as it cuts the metal, it has some effect of pushing and lifting the chip as it is cut off. "Displacing metal", which puts pressure on the uncut metal next to the cut. There are constraints on where it can go. The bulk of uncut metal prevents it moving inwards or sideways, the cutter prevents it moving toward the cutter. Only moving toward the OD is left.

    Because nothing is supporting that metal on the OD, it pushes outwards in the only "free" direction there is, forming a burr on the OD.

    With a perfectly sharp tool, an infinitesimal edge of infinite strength, and a material that cuts perfectly freely, there might be no burr. While I would like to work with that stuff, I have never seen any for sale. There is only material that is "sorta like that" and has "less burr". But not zero.

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  • lakeside53
    replied
    Originally posted by BCRider View Post

    I really like the idea of a rubberized abrasive block for this. Lakeside, what sort of Cratex (or other brand) tool are you using for that operation?

    No tool... I just hold a bar (maybe 1/4x1x6) of it on the outside by hand for a few passes. Really works well. for small internal just thread in the round size that fits; for larger, back to a stick of it.

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  • lakeside53
    replied
    Nothing like 1018/A36 to raise burrs when threading at typical HSM speeds.

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  • danlb
    replied
    Originally posted by BCRider View Post
    I've yet to NOT get burrs even with freshly sharpened tools. Pretty much anything other than the last couple of one thou whisker cuts or a spring cut raises a burr when I do single point cuts.
    I'll have to get out the magnifier to see how much burr there is.

    When I first started I'd end up with a sharp crest that needed to be filed off to get the proper form. I've never noticed a burr since I learned to thread properly.

    Dan

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  • BCRider
    replied
    I've yet to NOT get burrs even with freshly sharpened tools. Pretty much anything other than the last couple of one thou whisker cuts or a spring cut raises a burr when I do single point cuts.

    Leave a comment:


  • J Tiers
    replied
    Originally posted by danlb View Post
    This should read "burrs which MAY be on the threads". Burrs are not automatic when cutting threads just as they are not automatic with any other machining process.

    If you use sharp tools, the proper major and minor diameters and follow the recommended infeed steps you are likely to have a burr free thread in many materials.
    Add 5 bucks and you can get a cheap coffee at starbucks with that......

    There is always a burr. ALWAYS. it goes with the geometry of the cutter, cutting forces and the nature of ductile workpieces. Your comment is REALLY about whether or not the burr is large enough to make a difference.

    With the typical materials the new guy works with, and the typical threading cutters (not full profile), there WILL BE a burr, and it will likely be significant. This is fact based from prior threads we have seen.

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  • danlb
    replied
    Originally posted by J Tiers View Post
    He is using full profile cutters, so the burr issue is much reduced.

    For any OTHERS reading, the burrs which WILL BE on the threads unless you deburr, or use a full profile cutter,
    This should read "burrs which MAY be on the threads". Burrs are not automatic when cutting threads just as they are not automatic with any other machining process.

    If you use sharp tools, the proper major and minor diameters and follow the recommended infeed steps you are likely to have a burr free thread in many materials.

    Leave a comment:


  • LKeithR
    replied
    The way you've phrased the question suggests that you're a newbie so the first thing we need to know is how
    good the fit needs to be. As you gain experience you will soon figure out that in many cases "close enough"
    really is good enough. There's a reason why nuts and bolts are made to the tolerances they are. Working to
    tighter tolerances than are needed for a specific job is always a waste of time and money. As an amateur the
    "cost" of your time may not be a big deal but you're still imposing stresses on yourself that may not be necessary.

    The only practical way to measure internal threads is with a GO-NOGO gauge whether purchased or home made.
    If you're working to a standard then the simplest--but most expensive--solution is to purchase a set of gauges for
    the diameter and pitch you're working with. Making your own male gauge is pretty easy because you can measure
    over wires to get the thread to a standard tolerance. You then use your male thread to fit the female.

    As to finishing an internal thread if you're only making one or two pieces the only way to arrive at your final fit is to
    "sneak up" on the dimension. As you gain experience it becomes easier because you learn not only how your lathe
    performs but also how different materials respond to different feeds, speeds and DOC. Making multiple parts is easier
    because by the time you've made a few you can figure out where to set your dials--or the readings on your DRO--to
    hit the numbers every time. Once you reach that point then the only thing you have to worry about is tool wear....

    Leave a comment:


  • J Tiers
    replied
    He is using full profile cutters, so the burr issue is much reduced.

    For any OTHERS reading, the burrs which WILL BE on the threads unless you deburr, or use a full profile cutter, can (will) fool you into thinking you do not have the thread deep enough. Then you cut further, so that when you finally deburr, the fit is like a pencil in an apple barrel.

    Taps are obviously full profile cutters, and work well for finishing to size (or, I suppose, deburring) when it is closer tolerance. Pay attention to the "H" number to get the correct sizing. For large threads on bigger workpieces, that is less practical as each tap can cost $100 or more.

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  • MattiJ
    replied
    End grain on hardwood block works also for deburring. Baltic birch is what we have here, I'd imagine Oak would work same way.
    Add some abrasive if you feel like.

    Leave a comment:


  • BCRider
    replied
    I think "single turn" is more a case of a poor description. Until I have at least a running fit I've never gotten a close but not quite fit to go on even a full turn. Usually it's about a 1/4 turn or slightly more until it wedges tight. And that makes sense if you think about how the thread engages when there's still an interference fit.

    Good point on the deburring step. It's been a long time since my last internal thread. I recall using a triangle scraper as one deburring step and I might have used a patch of fine emery for the fine fitting stages. Emery then a cleanup before the test fit.

    I really like the idea of a rubberized abrasive block for this. Lakeside, what sort of Cratex (or other brand) tool are you using for that operation?

    Ya, know... For a TPI on a larger diameter than usual I wonder if a tap could be used as a deburring tool? Like when I do a 2 1/2" - 8TPI internal thread to fit my lathe spindle could I use a 1"-8TPI tap as a scraper to deburr the inside thread crests? This would be with hand power of course. I can imagine something like that catching and wobbling around in there with me on the other end....
    Last edited by BCRider; 12-10-2017, 02:02 PM.

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  • lakeside53
    replied
    unless you are using full profile inserts, don't try to "test fit" or measure unless you debur. Cratex (rubber/abrasive) works wonders.

    "hardware store bolts", are generally a very sloppy fit but can be all over the map.. Don't get caught by sizing to one then finding another is too tight!
    Last edited by lakeside53; 12-10-2017, 01:52 PM.

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  • danlb
    replied
    The general advice of taking .001 at a time makes good sense, BUT it raises the danger that you might end up with them galling and stuck together forever. Been there, did that.

    If you can thread them together 1 turn and then jam, you have a problem. If I remember correctly, when both threads are the same pitch and cleanly cut, they should continue to thread on if they get by one full turn IF single pointed. The first few tapped threads may be tapered due to the grind of the tap but single point should have threads that are straight and parallel.

    Your use of full profile inserts should guarantee a good thread profile. That assumes that you are using the right insert for the TPI and that you are using a different insert for internal than the external thread. The fishtail gauge is for use with a V profile ground bit, not really meant for full profile inserts. You will have to align the insert by measuring against the insert holder's shaft.

    If I were you, I'd start with the external thread's major diameter exactly spot on and the internal major diameter just .001 less than the calculated dimension.

    Dan

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  • Toolguy
    replied
    I use taps and dies a lot to work to industrial standards. For example, a tap and split (adjustable) die of the same diameter and thread pitch, metric or inch.

    Drill and tap a hole in a piece of steel. You can harden it if you like, or leave it soft. This is the female thread gauge.

    Single point a steel bar a little oversize. Open the thread die a bit oversize and thread it on the bar. Check the fit of the tapped part. It should not quite fit. Close the die a little and rethread. Check the fit. Repeat until you like the fit of the parts threaded together.
    This is the male thread gauge. Leave the thread die at that setting. You can harden this gage or leave it soft.

    Sometimes you won't have a tap or die to use for a reference. In that case, you can single point the male part and use the 3 wire method to measure it. If you have an internal thread, you can use the male part as a plug gauge for the female.

    As for aligning the insert correctly, you can use a square from the chuck face to the insert holder. This will be plenty good for any job. If your plug gauge threads in one turn, you are probably very close to finish size. Advance in .001 increments until you get the desired fit. It's a good practice to make a couple of extra passes at the final setting to smooth out any minor high spots.

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  • BCRider
    replied
    To go from starting but jamming to a good low play easy running fit? It depends on the diameter in a big way. On a little 1/4 to 1/2" thread it may only take another couple of thou. And depending on the bar you're using it may just take another spring cut or two. But on a 2 inch part it may take another 10 thou. After all, what might be considered a good low clearance fit on a 2 inch diameter 8TPI would be disaster on a 6-32 size thread.

    I do know that when I hit that point where the thread starts but jams I only make cuts in 1 or 2 thou steps from that point on and take a lot of spring cuts when it's going on but is still binding. At that point of fitting but with binding it really is only a thou or three away from perfect depending on the size.

    Leave a comment:

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