Improvements in Mechanism for Screw Tapping.
Year: 1892

page_text: Date of Application, 16th Dec., 1892-Accepted, 28th Jan., 1893
COMPLETE SPECIFICATION.
Improvements in Mechanism for Screw Tapping.
I, ARCHIBALD COCHRANE, of East Hurlet House, Hurlet, in the County of
Renfrew, Engineer do hereby declare the nature of this invention and in what
manner the same is to be performed, to be particularly described and ascertained
in and by the following statement:-
My invention relates to an improved mechanism for tapping screw threads, for
use in a drilling machine provided with reversing gear, and affixed to a twist
drill in said machine.
The object of the mechanism is to avoid undue straining or breaking of a tap
when it arrives at the bottom of a hole, or where resistance is excessive; by
allowing tap to disengage whilst the driving socket continues to rotate freely.
The manner in which this is effected is as follows.
A socket or tap holder, as shewn on accompanying sheet of drawings Figure 1,
is bored on end A. to fit the standard tapping size of twist drill for the tap to be
used, the other end being made to receive tap. On the plain part of tap is formed
a longitudinal slot B. with edges bevelled to an angle of about 30 degrees. This
slot is made shallower at inner end as shewn. C. is a pawl or feather which is
made with bevelled edges to fit sides of slot accurately. On the side of socket is
fixed a layer of springs to keep pawl in gear at a pressure suited to the size of tap
being used. D. is a screw pin, with point rounded to fit groove of twist drill, to
act as the positive driver.
Two slots may be formed in tap and an extra set of springs, with pawl,
attached if required; but one set as shewn is thought to be sufficient for general
purposes.
Figure 2 shews socket arranged with male end for direct attachment to drilling
machine spindle if so required.
Fig. 3 represents the cross section at E. E.
The action of the tapper is as follows:-
After a hole has been drilled with a standard tapping size of twist drill, the drill
is withdrawn from the hole and tapping socket slipped with a screwing motion, on
to the point of twist drill. The tap is now "fed" into the hole to be tapped, by the
gear of the drilling machine. When the strain on the tap becomes excessive, the pawl
is forced out of gear by the bevelled edges of tap, thus allowing socket to rotate freely
without injury to the tap. The drilling machine is reversed after the hole has
been tapped and the tap unscrewed. The slot B. being formed shallower at inner
end, the pressure of the springs through the pawl on the tapered surface or incline,
tends to keep tap in its normal position in socket.
Having now particularly described and ascertained the nature of my said inven-
tion and in what manner the same is to be performed, I declare that what I
claim is:
1. The combination of mechanism substantially as hereinbefore described, and
for the purpose specified.
2. The combination of such mechanism with a twist drill.
3. The method of providing an escapement in a tapping apparatus, by means
of a longitudinal slot and feather with springs, as described herein.
4. The application of laminated springs to keep driving feather in gear as herein
described.
Dated this the 15th day of December 1892.