Screw head formation

Abstract

The screw has two or three non-circular super-imposed recesses. The depth of each recess is less than 1.2 mm and there is a less than 1° of draw in each recess. The screw head recess is formed using a punch (10) in a cold-heading process. The punch is formed in a profilating CNC machine, cutting the profiles (12, 14, 16) of the punch to a tolerance of better than ±0.025 mm. A method of manufacture of a cold-forming punch for such a screw comprises providing a profilating head as one of the tools on a turret mounted adjacent the chuck in the CNC machine. The profilating head has several cutting elements, each being adjustable with respect to one another: axially, radially and angularly.

Description

BACKGROUND OF THE INVENTION [0001] The present invention relates to a form of screw head for a screw fastener, and to a tool to drive such screw. In particular, the invention relates to a method of manufacture of a punch for cold forming such screw head. [0002] GB1150382 appears to be the first patent to recognise the benefits of a multi-tiered screw head design that has stepped tiers between a deep central recess and a wider top recess, the recess having sides parallel a central axis of the screw. Such recesses are effective at reducing “cam out”. Furthermore, the recess can be deeper in the head of the screw without weakening the connection between the shaft of the screw and its head. This is because, at its deepest, where the countersink is narrowest or the transition between head and screw shank occurs, the recess is smallest. [0003] GB2285940 elaborated on this principle and pointed out that the same screwdriver could be employed to drive large screws (having several tiers of r...

AI Technical Summary

Benefits of technology

[0039] By virtue of the three degrees of freedom of each cutting element it takes considerable skill and patience to adjust the cutting elements to reach the desired precision of position. The desired precision is in the order of ±2.5 microns in each direction. However, once the system is set up, it is simply a matter of turning the CNC machine on and letting it run. Once set-up, operation and production of punches can be automatic.

[0040] Profilating is a known art. However employing it to the precision anticipated by the present invention has not been contemplated hitherto. Precision of the order c...

Problems solved by technology

However, such a deep recess can neither be formed without substantial draw (to permit release of the punch after forming of the recess), nor can it be formed in a single strike.

But the multiple strike using progressively larger punches does result in greatly increased costs.

In this market, cost is vitally important.

However, this is not only because of the cost associated with the other polygon recesses.

Despite the apparent advantages of the multi-tiered screw-head design, however, they have not yet succeeded in penetrating the market.

The reason for this is simply that hitherto it...

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