rmac wrote: ↑Wed May 19, 2021 10:05 pm
When you're threading up to a shoulder, what do you look at to know when to stop the cut? Do you watch the tool approach the shoulder? Or do you set up a carriage indicator like I mentioned above? Or something else?
I tend to use a combination of the shank of the tool, and the tip. The shank and tip when threading to a shoulder, but the tip, alone, when threading to a specific stopping point. Doesn't make much difference if I'm threading with or without an undercut. It's more about getting a rhythm --- a sort if "feel" when to open the half nuts and retract the cross slide. That's the method I used for this part, a replacement auger for Susan's wheat grinder. The original, made of cast iron, broke. It's a three pitch lead, created by starting the cut in a groove, but ending by pulling out at the appropriate time. The amount of tool contact was quite large, so it would have been very unforgiving of an overrun.
If you are curious, the part shown was made from 17-4PH stainless. The color is the result of heat treat to condition H900.
As far as a stop goes, it generally isn't of much value, as the relative position of the tool tip is ever changing due to the compound feed. If you're threading internally to a shoulder, it can get you in trouble quickly, especially with a coarse thread.
For internal threading, the one thing that remains constant is the relative position of the tool tip (and end) as it relates to the part. When I'm faced with that situation, what I do is determine where I must disengage the half nuts, then I use a pencil to make a line on the top of the threading tool. When the line is even with the end of the part, I open the half nuts. Works a charm.
I strongly encourage you (everyone, really) to pursue threading as if it's always up to a shoulder. By doing so, you gain the necessary skill required to generate threads to a specific stopping point. If you rely on an undercut, which may or may not always be possible, you don't get the necessary practice it takes to become proficient.
A comment about a thread relief. It stands to reason that the home shop type is free to cut one. The choice is usually his/hers alone. However, where I was trained, that was not the case. And, when there was one permitted, it had rigid specifications so the undercut didn't lend to the problems that come from sharp corners. Undercuts, like all other features, were provided by engineering and held to tight tolerances as a result. I make mention so you'll understand my comments. I had no choice but to learn to thread to shoulders, generally without a thread relief.
H