Another single point threading question, sorry guys.

While I think I sometimes get off in the weeds on a topic, learning how the details work can be useful for me. I rarely cut threads to spec, but knowing how means that I better understand things and have an easier time making normal threads.

It's a bit like accuracy. As I do things for hobby reasons, time isn't a big issue. So sometimes I try to aim for tighter tolerance than I need to improve my ability to hit them, and measure them. It's a way to practice with less chance of blowing the part. Sometimes I miss, but am still within the tolerance I actually need, but I'm improving. It's been a bit of a trip from thinking 1/8" was "small". :)
 
Old thread, but I wanted to add an inexpensive, easily tested, practice I've found. In the US, 3/4" PVC schedule 40 can be used to practice 3/4" 14 tpi Straight Pipe for Mechanical Joints, Loose-Fitting Mechanical Joints for Hose Couplings (NPSH), the fitting found on common propane/MAPP gas canisters. The PVC OD is ~ 1.055, the ID is ~.81" The minor diameter is.925", the pitch diameter is min .9822, max .9956. "Opisies"/practice runs are easily parted off with either HHS or an "E" type insert tool. The thread is easily tested with the propane nozzle. For what it's worth . . .

Ron
 
Probably bad practice but I start single point at 90 degrees, finish with die then use a fine wire wheel in an electric drill to "smooth" the threads first one way then in reverse.
I find this gives me almost polished threads.
I started doing this when I cut a heap in hot rolled bar that left the crests like saw blades.
 
darkzero said:
Check to see if your threading tool is ground to a 60° taper. If it is, it shouldn't reduce the major diameter when you get to the proper pitch diameter. It would only reduce the major diameter if taken well past the spec pitch diameter as it will be cutting down the crest. When properly done, the crest should not be touched by the tool so the major diameter that you initially machined would stay the same.

Also, if your are using the modified flank method for infeeding, check that your compound is set to just under 30°, 29.5° is most commonly used. Note the diagram below. If you have a smaller import lathe, most have a limited scale protractor that only goes up to 45° or 50° so if you set the mark on the compound to 29° on the protractor it's wrong & can cause the same issue you are having, it needs to be set at 60.5°. If you did have it set this way it would probably be more obvious as the threads would look funny.

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I understand the 29.5 degrees but the tool position confuses me. Should it not be perpendicular to the workpiece??
 
fixit said:
I understand the 29.5 degrees but the tool position confuses me. Should it not be perpendicular to the workpiece??
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Yes, the tool needs to be set with a thread gauge (a tool with a 60º cut in it that is held against your work and then you align your tool tip into the cut). Set the 29.5º angle of the compound first, then align your tool to your work. Then proceed to thread.
 
I understand that cut threads are never allowed on aircraft (as the sharp edge can act as a stress riser). Rather, all threads on an aircraft must be rolled, and, as such, never made in a home shop. I could be misinformed here.
 
For a drawbar, I wouldn't use cut threads. Because it will be cycled many times, use a rolled thread. I make all my drawbars with a grade 8 bolt attached to the drawbar via a socket and welded or brazed.
 
MrWhoopee said:
This is one of the most important things you can learn, both in machining and life in general. If that little voice says something is not right, LISTEN TO IT! That little voice knows whereof it speaks. Virtually every time I have ignored it I have come to regret it. Take the time to double check, call the client, question that irritable, self-righteous engineer. Whatever you do, DO NOT IGNORE THAT VOICE!
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That is not until you've proven it wrong beyond the shadow of a doubt, twice!
 
I never use dies. Just single point every thing. That said, are you using a split die? Cranking down on it could make it cut undersize.
 
I just used a Carbide insert threading tool yesterday to cut a 7/8-20tpi and now I'm absolutely hooked it's the only thing I will use anymore....
 
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