The Coventry Diehead was the main method of producing threads in volume on capstan and turret lathes, before the advent of CNC machines.
Alfred Herbert Ltd owned the business, and held a major position world-wide in the business of dieheads and chasers, supplying probably 50% of the
world market at one point. Others such as Landis and Concentric also produced dieheads, but Herbert's position as the supplier of both turret/capstan
lathes and the dieheads gave them a huge advantage.
They are still in use today, although the thread-cutting performance of machining centres has ousted them from all but the smaller machine shops. Spares are
still available, as are new chasers, both from surplus dealers and from sources of new product.
A box of goodies! Dragged out from under a bench where it had lain for years, the box
has a pair of dieheads, four chaser grinding jigs and a box of tool retainers for the turret.
the smaller diehead has its front plate missing
Cleaned up ready to go! The 1" diehead after an hours cleanup and lube.
Cleaned up sharpening jig for chasers that fit the 1-1/4" diehead. Note that this jig
right-handed thread chasers, while some do both right-hand and left hand.
Close up shot of the sharpening jig, showing the aperture for holding four chaser bits.
are to rest the bottom of the chasers onto, so their height is uniform. A thumb-screw on the right holds the
sharpening is in progress. The slope of the aperture is deliberate and not a camera fault, see the next picture
for a shot
of this part with chasers fitted.
The jig aperture with four chasers fitted. The rollers are omitted on left-hand sharpening jigs,
the angled base
of each chaser sitting directly on the base of the aperture. Note the numbering
of the chasers, they must be installed in the
diehead in sequence.
Close up of the holding arrangement for individual chasers on the jig. The left-hand screw
tightens the clamp,
while the screw in front sets the position of the chaser for depth.
The next page has a picture of a single chaser mounted in this fixture.
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