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Coventry Diechasers 01.jpg Coventry Diechasers 02.jpg
Coventry Diechasers 03.jpg Ridgid Logo.gif


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Coventry dieheads are still a great way to produce threads on a lathe, even against faster CNC lathes, some small diameter and longer threads are ideally suited to thread chasing.

Coventry dieheads and diechaser  sets are avail for a huge range of thread forms still. Coventry dieheads can be used on capstan and turret lathes, on CNC or automatic screwing machines, and on multi-spindle automatics or drilling machines. The Coventry diehead range includes stationary, rotating and taper threading heads.

We can even cater for stainless steel leads, brass leads or general purpose steel leads in the die chaser sets. The sets come in a set of 4 pieces.

Please try to specify the size of Coventry diehead you are using when ordering.



Mild steel, wrought iron, free cutting aluminum S
Cast iron, phosphor bronze, gun metal. M
Brass B
High tensile steels, free cutting stainless steel AM5, S5, Tin coated
(i.e. ½” UNC , S rake , die chaser set to suit a 1” diehead)  



Genuine “Coventry” dieheads and Coventry chasers have become a by-word for external threading. Originally developed by Alfred Herbert and sold world-wide, the Coventry diehead is probably used in more countries than any other system.Coventry Diechasers 05.jpg


Tangential chaser sections to suit “Landis” dieheads and similar diehead systems can also be purchased from Miller’s Tooling Pty Ltd.






Coventry Diechasers 06.jpg“Ridgid” die chasers machines & die section chasers are available from Miller’s Tooling as well.Ridgid Logo.gif






Chasers for Coventry type die heads are supplied in sets of four for all heads except the 3.1/2”FT, which has a set of 6. They are numbered and are fitted into the die heads in numerical order in a clockwise direction facing the front of the head. This applies to both right and left threads, but for the latter the spindle is reversed.

Chasers are marked with the thread form, tpi or pitch, and diameter, LH if left hand, the grade, which applies to the material that they are ground for, and the gauge number which is required when regrinding.



a) Closing the Die head.

The head should be mounted with the closing handle or auto die closer (ADC) in a convenient position, preferably near the top. The handle or ADC is then pushed away until the head locks closed. If operating manually, and the thread is to be cut in one pass, the detent handle should be left in the finish (-) position.

b) Adjusting the thread diameter.

Set the indicator line on the graduated scale opposite to the zero line by use of the adjusting screw. When doing this the pressure of the opening springs should be relieved, by pushing against the handle or ADC. This makes it easier to turn the adjusting screws, and makes the adjustment more sensitive. The nominal diameter should be obtained but further slight adjustment may be needed, so it is best to make a trial run.

c) Changing the chasers.

Close the die head to take the pressure off the chasers and remove the front plate. The new chasers must be fitted clockwise in numerical order, starting in any position.

d) Opening the Die head.

Coventry type die heads open automatically. Types CH, CHS and DS open when the travel of the head along the work piece is halted by a suitable stop. Initially only the back half of the head stops, the chasers, in contact with the rotating work piece cause the front of the head to be pulled a little further forward until the detent pin disengages, opening the die head. DX die heads can also be arranged to open in this way, but they possess an external plunger, which can trip the head open as it meets a stop suitably positioned. This is to overcome stripping of the thread if a short fine thread is being cut in soft material. Another die head that operates in

this way is the XT2, but this type cannot be made to ‘pull-off’. This can be an advantage on fast indexing autos where another type of head may be thrown open on indexing. If the need arises to open a die head by hand this can be achieved by holding the shank securely, and pulling on the front of the head. For CH, CHS, DS, and DX types, or by pushing in the plunger on DX and XT2 types.

Operating Speed. This depends on many factors such as the material, the thread form and the finish required


Spindle speed = 12S Pi.D


Where D = thread diameter, Pi = 3.142, S = feed speed (Ft/min)

S varies with the material being threaded, a rough guide is:-


stainless steel and other tough materials 5 – 8 ft/min

mild steel 10 –20 ft/min

cast iron 8 –12 ft/min

brass and copper turning speed.


If in doubt a rough rule of thumb is that threading speed should be around half turning speed.


Cutting the thread.

The material should be turned to around 0.001” above the finished thread major diameter. A good flow of cutting fluid, ideally through the shank is recommended to improve the finish of the thread and the life of the dies and prevent the die head clogging with chips. The head should be offered up to the work piece with a steady gentle pressure, but without forcing, until the dies bite. The pressure should be maintained as the head travels along. For CHS die heads the gap between the shank and back plate should be maintained. When the required length of thread has been obtained the travel of the head must be arrested by a suitably set stop, or by the feed handle being firmly held. The dies will carry on cutting for about 2 – 3 mm until the detent pin disengages.

S-type die chasers, B-type die chasers, AM5 die chasers, AM die chasers, S5 die chasers, AS die chasers, M5 die chasers ,S-20 die chasers, B-33  die chasers and AM5 20 die chasers are most popular die chasers.