Thread cutting tool did a lot of thread forming
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  1. #1
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    Default Thread cutting tool did a lot of thread forming

    I usually cut threads with die heads or taps, and do relatively little conventional thread cutting on the manual lathe. Over the holiday I needed to cut some simple 1/2" long 3/8-16NC threads on some 1/2" 1018 rod, and had a very strange experience that I probably should have had during an apprenticeship.

    I cut the relief groove and major diameter of the threaded section, and proceeded to cut threads with a Vardex AG60 laydown tool with a pronounced chipbreaker mold (SCB). It is significant that the AG60 is a "partial profile" cutter, not a "topping" cutter intended for a specific pitch. My preferred method is to feed in directly with the cross-slide, not mess about with 29.5 degree angles or whatnot with the compound slide. Spindle at 35 RPM, which is about as fast as I can get the half-nuts disengaged within the relief groove. After some heavier first passes, I settled down to feeding 5 thou on diameter where the tool was taking a bright continuous shaving on the cut.

    The strangeness began when the threads started looking complete after far too few passes, and with the minor diameter still far too large. To skip over all the head scratching, I eventually discovered that the tool was not only cutting, but also forming. The thread crests were extruded as much as 20 or 25 thou above the level of the major diameter cut.

    I had a couple of these to do. The first one, I just kept doing passes until a thread micrometer gave me my target number (there was adequate relief in the female anvil of the micrometer), then filed the crests down to the desired major diameter at 400 RPM. I didn't like this, as the thread crests turned into foil burrs and got messy during the cutting. So on the next few, I stopped every time the crests got sharp and filed them down until I could see appropriate flats before continuing with more passes. I liked this better.

    I didn't have time to play around experimenting, but there were several things going on. Foremost was the use of a generic threading insert. Using a pitch-specific tool would have cut the crests as they got extruded upward. 1018 can be a bit gummy. In this situation, I got bright, clean-cut threads rather than tearing, but the material probably tended to form more than a harder or crisper-cutting steel would. I don't know if the straight-in feed magnified the forming effect compared to an angular feed.

  2. #2
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    Check your tool height. If the tool is above center, there can be inadequate relief below the cutting edge and it will rub. You could have been in a situation where the depth of cut resulted in both cutting and rubbing to form the threads. The effect of being too high on the tool would be larger at smaller diameters, perhaps explaining the change you saw as you cut the threads. I've had a similar problem with a regular tool when I was careless and had a chip under the QCTP holder height adjuster when I swapped tools.

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  4. #3
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    I’m thinking the tool height is off also. NOW, that 3 & 1/2 feet per minute surface speed is prolly lower than just doing it with a hand held die stock… LOL! So 30ish SFPM would get you 300RPM and 19ish inch per minute saddle travel speed.

    Usual allowance for thread relief is 2-3 leads to allow a proper radius in the groove corners. My guess is you could do this at a 12-19IPM saddle travel speeds after half a dozen dry runs over the top of the part (2-300rpm). Maybe blue sky after your have a revelation on how well you can track things.

    I’d regularly do bearing retainer threads at 20+IPM & the real trick is to relax & get the start engagement smooth & firm. I do use the compound & the cross slide handle is 10-12o’clock & marked. Then I start a few inches back drop the half nut in & when done the left hand goes down (cross slide back) and the right hand goes up (half nuts open). Basically tilt head left too, so three motions happen at the same time…

    I worked on this after watching someone going more than double the above travel speed on 1&1/4" x 12tpi cylinder rods in the 70's. The little boy in me just couldn’t let it go after seeing it done...

    Good luck,
    Matt


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