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Tapping a thread in steel is a useful workshop skill

This post is for people with little experience of working in metal. Tapping a thread is a useful workshop skill. It’s easy to cut threads in steel with a tap. You can use a drill and tap to fit a cutting bit to a tool shaft, assemble steel parts, or to make up frames for tool stands etc.

Drill the hole

Drill the hole for the thread using a tapping drill. This is just a normal twist bit the correct size for the particular thread. When drilling holes in steel, you need a bench drill with the speed set slow. This makes a hole that is square to the surface of the metal. When drilling metal like this it is essential to use a vice or clamp. The drill can catch when it cuts through, spinning the metal round and doing your hand no good at all.

The size tolerance is small, and accuracy is important. If the hole is bigger than recommended, the thread will be weaker, though the tapping will be easier. If it is too small, you will find the tap makes the hole bigger without getting a grip in the metal and cutting a thread.

The correct size of hole depends on factors including thread form and thread size. There are a lot of thread forms, such as Whitworth, BSF and BA, for which drill size tables are available online. I include here tables for the smaller metric threads and Whitworth threads that are commonly used in older machinery.

Taps

Taps come in all sizes and threads. You can identify the right tap to match an existing male thread such as a bolt by placing them side by side with the thread peaks lined up. Hold them up to the light, and you can see any mismatch further along the tap. If the diameter matches too, you probably have the right one.

Each thread type and size has three corresponding taps, though you don’t need all three for through holes. They differ only at the point. The first cut tap has a long taper to help start the thread, the second cut has less taper and the bottoming tap has none, because it works when the other taps have already made the start.

As well as the taps, you need a tap wrench.

Cut the thread

When tapping a thread, it’s necessary to align the tap accurately with the hole so that they are coaxial. A crooked tap will not produce a good thread. In shallow holes it is less critical, but the deeper you go the more severe any misalignment becomes, and the tap will break. The first couple of threads set the alignment. Don’t try to pull the tap straight once the thread has gripped the tap, it’s too late then. You can sometimes correct it by drilling the hole bigger at its opening and re-starting the thread further in.

A good way to start the tap squarely is to grip it in the chuck of the bench drill used to make the hole. That keeps it on track, but you have to turn it by hand, using the drill lever to keep gentle pressure on the tap so it enters the hole and starts self-feeding. Once the first cut tap is securely held by the thread it has cut, you can switch to the tap wrench. Just wind the tap in as far as it will go, taking care not to bend it. Small taps break easily. Then, if it is a blind hole, change to the second cut, which will go further in, then to the bottoming tap, which will complete the thread to the bottom of the hole. Before bottoming, clear out the swarf.

Another method for aligning the tap is to drill a clearance hole in a bit of scrap metal or wood and clamp it above the hole for the tap. The tap will slide through the clearance hole, which will hold it square. The clearance hole must of course be drilled square.

Lubricate the tap

Lubricate the tap to give a better finish to the thread. There are special compounds, but oil will do. The swarf cut by the tap has to be broken up as you go. To do this, advance a little then turn backwards a bit, going two steps forward and one back. If you don’t, the tap can get locked.

Blunt taps are hard to turn and may seize. You can sharpen them with a narrow, round-edge grinding wheel, but it is easier to replace them. Watch for this if buying second-hand taps, they were probably disposed of because they are blunt.

Metric tapping drill sizes

Tap                        Metric drill               Imperial drill

3 mm × 0.5           2.5 mm –

4 mm × 0.7           3.3 mm –

5 mm × 0.8           4.2 mm –

6 mm × 1.0            5.0 mm –

7 mm × 1.0            6.0 mm                      15/64

8 mm × 1.25           6.8 mm                      17/64

8 mm × 1.0             7.0 mm –

10 mm × 1.5            8.5 mm –

10 mm × 1.25          8.8 mm                      11/32

10 mm × 1.0            9.0 mm –

12 mm × 1.75          10.3 mm –

12 mm × 1.5             10.5 mm                    27/64

14 mm × 2.0             12.0 mm –

14 mm × 1.5              12.5 mm                    1/2

16 mm × 2.0              14.0 mm                   35/64

16 mm × 1.5                14.5 mm –

Whitworth tapping drill sizes

Size (in)                  Tapping drill size

1/16                          Number Drill 56 (1.2 mm)

3/32                          Number Drill 49 (1.85 mm)

1/8                            Number Drill 39 (2.55 mm)

5/32                          Number Drill 30 (3.2 mm)

3/16                           Number Drill 26 (3.7 mm)

7/32                           Number Drill 16 (4.5 mm)

1/4                             Number Drill 9 (5.1 mm)

5/16                           Letter Drill F (6.5 mm)

3/8                             5/16 in (7.94 mm)

7/16                           Letter Drill U (9.3 mm)

1/2                             Letter Drill Z (10.5 mm)

9/16                           12.1 mm (0.4764 in)

5/8                             13.5 mm (0.5315 in)

 

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