Rough threads - what went wrong?

Probably the steel is of a poor grade and has impurities, plus your cutting tool might be partially to blame
When using mystery steel you never quite know what results you're gonna get
 
Looks like the typical mild steel cut with carbide too slow. Try High speed steel. With mystery steel markba633csi has the best answer
 
If you are using a partial form tool i.e. with no provision to form the thread crest then file the sharp crests off the thread before testing in the female thread. You can have a correct thread with the sharp crests preventing it engaging, if you keep taking cuts without removing the crests the thread will be loose & only engaging on the sharp crests.

There are no references to minimum cutting speeds, so choose the speed you are comfortable with and use cutting oil instead of soluble oil (coolant).
 
I agree that HSS is best for manual machines, and I use it for all my threading, and it is quite forgiving as to chipping which is all too easy with carbide, I use TapMagic for most all threading, whether by single pointing or Geometric die head.
 
The fact that carbide tools can cut with high speed does not mean using them at low speed will create poor results. If you are doing machining for a living, the only poor result is that you will make less money. If you can maintain a steady cutting speed and depth of cut, sure you can use carbide tools at low speed. I use carbide tool for threading and I turn the chuck with my hand. Slow enough ?

mild steel is well known to be difficult to cut cleanly. To get better results, you will need very sharp tools. Unfortunately carbide tools are usually made to be not that sharp in order to lengthen the tool life. Thats why HSS is better because you can make it as sharp as you like. Being less brittle, HSS can also stand rough cutting ( erractic speed, interrupted cut, etc ) better.

so the solution I would propose is using sharper carbide tools or grind your own HSS tool.
 
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Your threads have a sharp point. There should be a flat equal to 1/8 the pitch. Take a file to the OD until you see flats at the crest. Your threads will look a lot better. I am also a believer in sharp HSS tools. I use dark thread cutting oil of the type plumbers use for pipe threading. It is designed to work with gummy steel.
 
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