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OPERATING NOTES
INTRODUCTION
Coventry type dieheads are the most commonly used of the thread cutting dieheads in U.K. and many other parts of the world. They are robust and reliable, simple to use and easy to set and can be used on manual or automatic machinery. Changeover of the thread being cut is quick, and standard thread chasers are fairly inexpensive.
Several types of Coventry type dieheads have been produced over the years, many for different applications. We can advise you which type you need for your particular application.
CHASERS
Chasers for Coventry type dieheads 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 dieheads 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.
BASIC OPERATION
a) Closing the diehead
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 diehead 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 diehead
Coventry type dieheads 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 diehead.
DX dieheads 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 diehead 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 diehead by hand, this can be done 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 diehead 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 dieheads the gap between the shank and backplate 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.
| PROBLEM |
POSSIBLE CAUSE |
REMEDY |
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| Head does not open |
Broken opening spring |
Replace |
| Chasers faulty |
Replace |
| Etching proud on chasers |
Stone off |
| Chasers incorrectly ground |
Regrind |
| Chasers too sharp, clinging |
Stone face |
| Diehead clogged with chips |
Clean off |
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Head will not close,
or opens early |
Tooth or detent pin worn |
Replace |
| Indexing throwing head open |
Close on last index |
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| Variation in thread diameter |
Tooth or detent pin worn |
Replace |
| Chasers incorrectly ground |
Regrind |
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| Tapered threads |
Front plate loose |
Tighten |
| Tooth or detent pin worn |
Replace |
| Chasers incorrectly ground |
Regrind |
| Wear in diehead body |
Send to us! |
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| No thread produced |
Chasers incorrectly fitted |
Check/refit |
| Chasers faulty/mixed set |
Replace |
| Thread pulled off when |
Use DX or |
| diehead opens |
XT2 diehead |
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| Poor thread quality |
Chasers wrong grades |
Replace |
| Chasers incorrectly ground |
Regrind |
| Misalignment head/machine |
Check |
| Wear in diehead |
Recondition |
| Wear in diehead |
Recondition |
| Material too tough |
Use ALCO thread |
These are a few of the problems that may be encountered.
If you come across a problem not listed here change the diehead.
If this cures the problem remember that we have a
Diehead reconditioning service.
We can bring your diehead any type up to
A good as new standard for a fraction of the cost of new.
Our fitters are fully trained and
we can usually turn around in 24 48 hours.
This applies to all types of heads, Coventry
Landis, Alco, Namco etc.
All heads are costed before work starts.
We hold very large stocks of chasers for Coventry type dieheads.
We stock the entire range of chasers for standard listed threads both right and left hand, and for various materials.
Around two thirds of our stocks are for non standard thread forms, or non standard pitch/diameter relationship. These include: UNS, UNEF, SPECIAL WHIT, NPS, API, ACME, TRAPEZOIDAL, WOODSCREW FORM, CYCLE, METRIC FINE, UNJ, etc, etc.
 Current stock lists for standard chasers can be found in our Cutting Tools catalogue. Please call us if you need a copy. We cannot list non standards as the range is infinite. PLEASE ENQUIRE!! We can normally satisfy most requirements from stock, or can offer a suitable alternative.
Our sales staff are able to assist you with any technical problems, and help you select the correct chasers for the job.
RECOMMENDED GRADES OF CHASERS FOR VARIOUS MATERIALS
| MATERIAL |
GRADE |
| Mild steel, wrought iron, free cutting aluminium |
S |
| Cast iron, phosphor bronze, gunmetal. |
M |
| Brass |
B |
| High tensile steels, free cutting stainless steel |
AM5, S5, Tin coated |
NOTE: Sometimes the grade required is not supplied as standard, and so may be expensive. We would be pleased to advise you of alternatives that may be available.
We also stock parts for other types of Coventry diehead including:
DS, DRX, XT, XT2, CH, FT
Lists and prices available on request
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