How to cut a screw?

 
Cutting of female screw.
There are four conditions for successful screw cutting.
1. Sharp and quality screwing tap. If you found the rusty one in a box with nails, - throw it away. If the cutting edge is blunt, there will be no good cutting; such screwing tap will brake in the first hole. It will be hard for you to get it out from the hole; it’s easier to buy a good one. Chinese instruments are made of low-quality steel are useful only when you work with plastic.
2. Greasing. To make screwing tap work long you should always grease it. Few oil or kerosene drops would improve screw quality and would ease your work. It's crucially important for closed holes. And aluminum won’t stick to cutting edges. And grease chills down the cutting edges of a screwing tap. Don't forget to grease your screwing tap.
3. Carefully choose the hole diameter. It’s very important. It the hole will be big, the screw would dangle, if it will be small, the screwing tap would brake from overload. I won’t write calculating formula here, I’ll just list the most common diameters. For metric screw thread M2 screw 1.6 mm diameter; M2.5 - d2.2mm; M3 - d2,5; M4 – d3.3 mm; M5 – 4.2mm; M6 – 5mm; M8 – 6.7 mm and M10 – 8.3mm.
4. Skills. You should have skills in everything and it comes with practice.  Practice, brake screwing taps and at last you'll do it. At first cut the screw with the first screwing tap, then with the second. Clean the hole and the screwing tap from chip scrap with old toothbrush. If the hole is deep, you'll need to unscrew the screwing tap periodically to clean it. After each turn you should do half-turn in the opposite direction to shear chip scrap.
 

 
  It’s easier to cut a screw in a thru hole, there will be no problems. But it’s hard to cut in “detail’s body" as locksmiths always say. There is a risk to break a screwing tap, because of filling the hole with chip scrap. There are several ways to put out the fragment of a screwing tap if such situation has happened with you and you can’t screw another hole. The best variant is when a screwing tap part sticks out from the hole. In such case you can sharpen it with polishing stone, to make it look like scapula, and try to screw it out with pliers.
But it’s much harder to put out a screwing tap if it broke in the deep of a hole. In such case you’ll need patience, azotic acid and wooden stick. Put one drop of acid to a broken part. Repeat this action until a broken part won't have cutting edges and then you can take it with pincers or beat it out from the other side. You can protect the detail from the acid with lamp wax. You can take out the broken drill in the same way.
  Don’t forget about security measures while working with acid.