This is one of those concepts that I thought I had a handle on until I dug a bit deeper.
I was taught to set the compound (or top slide for those of you across the pond) to 29.5° when cutting a 60° thread. My understanding was that this would allow for you to advance the threading tool at a 29.5° angle into the workpiece so that only the leftmost side of the threading tool would remove chips.
Image from WoodworkForums.com
Recently I learned (like so many other machining operations) that the method I was taught wasn’t the only method out there. It turns out that there are people out there that not only don’t use this method, they’ve never even heard of it! That lead to a forum post that I encouraged Jerry to write about the topic. The purpose was to discuss the method in greater detail and gain some clarity along the way.
Then a forum member mentioned that if the intent was to give relief to the back side of the threading tool, the angle needed to be 30.5° and not 29.5°. This made me curious and I proceeded to draw the concept up in Sketchup to either confirm or deny that 29.5° was the proper angle.
It turns out that it does seem that 30.5° might make more sense (if you’re truly trying to prevent the back side of the threading tool from cutting). Setting the tool at 29.5° actually causes the right side of the tool to remove material (albiet a lot less material than the left). While setting the tool to 30.5° looks as though it would result in the desired relief on the right hand side of the tool.
The image below shows (exaggerated in red) the amount of material that would be removed if you were to set the compound to 29.5° degrees (but it’s actually set to 25° to exaggerate the concept for clarity).
While this image shows how the right hand side of the tool would have clearance if you set the compound to 30.5° (again, exaggerated for clarity by setting the angle to 35°).
So what do you think? How were you taught?
I put together a YouTube video that illustrates the concept and shows what I discovered when I drew things up in Sketchup. If you’re already familiar with the concept, skip ahead to 4:20 in the video, which is where I start talking about my Sketchup findings.
If you have experience with this method and can explain why my Sketchup results were counter intuitive, please feel free to leave a comment and enlighten me (and everyone else) via the forum topic connected to this post.