Single point cutting at 60 degree compound.

Any sudden section change is a stress riser. The thread root itself is a prime offender. The key is gradual section transitions. Surely a threading gully that is a sharp 90 degree trench is bad. But a gully with a gradual change can actually be a stress reliever. Common practice with highly stressed engine bolts is to have the shank at the threaded end be smaller than the thread minimum diameter, then gradually taper back up to full diameter.

I dunno. Maybe I've read the Caroll Smith books too many times. I'm always watching stress patterns. If I decide I want a gully in a place that matters, I use a rounded form tool to cut it.
 
The difference between a relief groove and a thread run out is very small. Yes no groove is stronger. The only time it comes into play is when the threaded joint will be used right at the failure points. In many if not most cases for hobby use a thread is used because of its dimensions and not its ultimate strength. We will often use a 1/4 inch fastener because it is a convenient size when a much smaller fastener would still have plenty of strength. Hobbyists love overkill
 
Sometimes hobbyists are making plant stands. Sometimes they are making race car parts. Stress concentrations are of most concern with cyclic loads.
 
I have nothing against stress analysis in parts, my son earns a good living as a stress expert in the aviation industry.
But when I am at my lathe making things for my own pleasure, stress reduction in the operator comes first.
 
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I agree that for highly stressed components, a rounded or tapered relief profile is superior to a simple squared thread relief. This is standard practice for power transmission components and high tension fasteners. Such relief profiles require a deeper cut at the bottom of the radius than a typical thread relief so you have to consider that reduction in cross section in the design.

I suppose the lesson in this digression is to be aware of stress risers in stressed components as BtoVin83 pointed out and profile it properly if the situation calls for it or not use it altogether. With that said, a rounded thread relief will be wider and deeper than a typical squared edge thread relief and, as hobby guys in a non-critical situation in most cases, we just may not have room for that width and depth. I've decided that I won't worry about it and continue to cut thread relief grooves with a parting tool as I've done in the past unless I have a situation in which the part is highly stressed - then I'll worry about it.

Good discussion.
 
I remember watching Mr. Pete explain thread cutting. If my memory serves me correctly when cutting threads with the compound at 30 or 29.5 degrees. When you advance the compound by say .010 on the dial the actual depth is increased by only .05. This on direct read dials.
Could someone confirm this. I can't find the particular video.
 
With the compound at 30, to keep it simple, advancing the compound by X will move the leading edge X into the work. Your DoC will be X. Transferring this movement to relative to the work, the tool will advance cos(30) into the work and move sin(30) towards the chuck. Sin(30) is 0.500. Cos(30) is 0.866-something, if I remember.
 
Watch the series of 4.
As stated above there are some very good instructional videos on Youtube.
You are on your way my friend.
Have fun and be careful. A lathe can be deadly.
 
I didn't mean to stress everyone out just pointing out how I was taught. Old habits die hard and as pointed out the best was to reduce stress is go out and take something big and make it smaller
 
I understand the how's , I just didn't understand the compound vs cross feed. Now I do . Thanks every one. Lathe got shipped today, finally!
 
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