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How To Cut Threads On a Lathe?

Jan. 26, 2021

How to cut an internal thread

1.Drilling the Hole for the Threads

(1) Internal taps are designated according to nominal size. For example, an M 6 tap means "metric with a diameter of 6 mm".

The diameter of the twist drill for tappingcan be calculated as follows:

Core hole diameter = Tap diameter - Thread pitch

Example the M6:

Core hole diameter = 6 mm - 1.0 mm = 5.0 mm

(2) Clamp the workpiece in place with a vise or C-clamp so it doesn’t move

(3) Use a center punch to position the place where you plan to drill

(4) Drilling the hole with bench drilling machine or handy drilling machine while using the cutting oil

Machine Tap

Machine Tap

2. Cutting internal threads(by hand)

(1) Put the tap into a tap wrench (T-handle or tool holder with ratchet).

(2) Use cutting oil into the hole to lubricate it

(3) Fit the end of the tap into the hole to make it straight

(4) Rotate the tap wrench to thread your hole with the tap

When using tap sets you must follow the tap sequence (taper, plug, and bottom). You can recognize the order by the rings on the shank:

taper - one ring

plug - two rings

bottom - no ring

cutting edges that go from rough to fine

When you have cut the thread, unscrew the tap in the opposite direction

The thread cutting is finished

Cutting internal threads(by machine)

Tighten the machine tap in the machine and cut the thread.

Insert the Machine Tap slowly and straight into the core hole at the appropriate speed while using the cutting oil.

Friction generated between the chips and the tap cutting edges and thus the inflict torque is reduced which ensures better chip removal.

After a few turns, the machine tap pulls itself into the workpiece and cuts the thread.

When the machine tap is through, stop it and switch to the opposite direction and restart the machine. The machine tap will pull itself out of the hole

The thread cutting is finished

How to cut an external thread

1.Select the correct diameter of round rod and die. 

2. File a chamfer at the head of the round rod. This will help start the thread cutting,the chamfer can be created more accurately on a centre lathe

3. Fix the die in the die stock, tightening the centre adjusting screw and widening the die slightly, as well as holding it firmly in the die stock. 

4. Place the die on the round rod, keeping it straight with the rod. Add a little pressure and turn the die stock in a clockwise direction. 

The first couple of ‘turns’ of the die are critical. Make the die stock parallel to the vice

Continue to rotate the die stock in a clockwise direction. Once the thread has been started, for every clockwise rotation, rotate the stock in an anticlockwise direction, for half a turn. This will cut off the chips and remove them out of the die.

Thread Cutting can continue until the needed length is been achieved.

Remove the die stock from the thread by rotating it in an anticlockwise direction itll it is aparted of the rod entirely.

The thread cutting is finished

If you want to know more thread cutting tools, please click here