sfriedberg
Diamond
- Joined
- Oct 14, 2010
- Location
- Oregon, USA
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.
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.