Thread cutting 

Every well equipped DIY workshop should have a tap and die set for the most commonly used bolt sizes. It's what you need for repairing a damaged thread, or cutting a new, larger thread if the old one is beyond repair. You can also create an internal or external thread on parts that you make yourself. A good tap and die set contains three-stage taps for each size plus various dies so that you can cut both external and internal threads. Simple single-stage taps are suitable for rethreading or for soft material.

The key requirement for achieving a durable new thread is to drill a pilot hole with the correct diameter.

Before you start drilling, you must first mark the centre of the hole with a centre punch. To do this, place the part you are going to drill on a firm, flat surface.

Now you can drill the hole. Take care to align the drill perfectly straight. If you drill at an angle, the part may be ruined. So, to be safe, it's advisable to secure the part in a vice or bench drill, making sure it's exactly horizontal. When drilling a relatively large new hole, start with a smaller drill or a step drill. Clamp the drill bit in the drill and lubricate it with cutting oil (or spray oil) to prevent it from overheating or seizing. Start drilling at slow speed. If necessary, tidy up the hole with a countersink, but do not countersink too deep, especially on thin metal.

Select the correct size of tap for the pilot hole. Three-stage taps are marked with rings on the shank. Stage 1 (one ring - taper tap). Stage 2 (two rings) and bottoming tap (no ring).

Screw the tap into a suitable tool holder, lubricate it with cutting oil (or spray oil) and insert it vertically into the drill hole.

Then cut the thread slowly. In soft material (e.g. aluminium). you can cut continuously. But in hard metal (steel, stainless steel), you should slowly screw the tap in by one full turn of the thread, then break the chip (swarf) by screwing the tap out again a quarter turn. Continue like this until the tap has advanced as far as possible into the pilot hole.

If you're drilling a blind hole (i.e. a hole that does not go all the way through the material), you may not need to cut to the end. Always work slowly, smoothly and gradually, making sure you keep the tap vertical in the pilot hole.

When the first step is complete, unscrew the taper tap, wipe away the metal chips and blow the thread clean with compressed air. Lubricate the second tap (two rings) and screw it in the same way. Clean the drill hole again and finish the thread with the lubricated bottoming tap (no rings). 

Now you can try the bolt in the threaded hole. If it doesn't turn smoothly and easily, do a bit more work with the bottoming tap.

In our DIY tip Undoing Screws, we explain how to repair a thread using a “V-Coil” thread repair kit.

Cutting a new external thread demands a degree of skill and practice. The key requirement is a perfectly cylindrical bolt with a suitable diameter. For example, the bolt for an M3 thread must be 3 mm in diameter, M4 = 4 mm, M5 = 5 mm ...

You need to give the end of the bolt a slight taper, e.g. using a file. Without a chamfer, it's not possible to position the die properly. It would immediately skew and chew up the bolt.

Oil the die, insert it in a tool holder and position the tool holder with its arms at 90° to the bolt. Now screw the die onto the bolt with slow, steady movements.

Soft metal (aluminium) can be cut in one go, but with hard metal (steel, stainless steel), you need to break the chip (swarf) after one full turn of the thread by reversing the die by a quarter turn.

When the die reaches the base of the bolt, apply some spray oil and run the die up and down the bolt a few times to smooth the thread.

A flattened, damaged external thread can be made fully functional again by running a lubricated die up and down it a couple of times. Here too, it's essential to make sure the die is positioned perfectly straight on the thread.

A flattened, damaged external thread can be made fully functional again by running a lubricated die up and down it a couple of times. Here too, it's essential to make sure the die is positioned perfectly straight on the thread.