Hiscox’s double-acting screwdriver has an inner spindle with a left-hand thread and an outer hollow spindle with a right-hand thread. Locking nuts can secure either stage. The animation lets you choose which stage works while the other is locked, then shows how axial travel produces opposite bit rotation for the two thread hands.
Double-acting Screwdriver Interactive Calculator
Select the working inner or outer spindle. Adjust its pitch diameter, lead angle and axial stroke to inspect bit rotation and compare the ideal push-force/torque relationship.
Equation Used
- One thread stage works and the other is locked.
- Lead angle is measured from the circumferential plane.
- The selected thread stays engaged on the animated return.
- Force/torque results are ideal and lossless.
Selected stage stays engaged during the illustrated push and rewind. No automatic ratchet, spring-return behavior, efficiency or tool-strength rating is assumed.
Nested opposite-hand threaded spindles
The inner blue spindle carries the bit and has a left-hand working thread. The hollow gold outer spindle has a right-hand thread. A collar at each engagement represents its nut; locking one stage leaves the other available for axial-to-rotary motion. Hiscox identifies these two thread hands and locking nuts in figure 1392.
With the outer stage locked, the handle and outer sleeve translate together. Their nut advances along the inner left-hand spindle, rotating the bit. With the inner stage locked to the sleeve, the inner spindle and sleeve rotate together while the handle nut travels along the outer right-hand thread.
The stage choice is manual and stays selected throughout playback. The return shown is a kinematic rewind of that same engaged thread, so rotation reverses on the return. Automatic pawls, freewheeling spring return and automatic stage switching are not modeled.
The larger end view follows the same bit angle as the side view. Thread pitch spacing follows the entered lead; handle, inactive spindle and locking-nut proportions remain illustrative.
What this historical tool illustrates
This device illustrates selectable opposite-hand screw motion and nested spindles. It should not be conflated with every later spiral ratchet screwdriver or with the adjacent reciprocating drill in Hiscox’s collection.
The calculator compares thread lead, turns per axial stroke and ideal force-to-torque conversion. It does not identify an interchangeable bit, a model-specific stroke or a practical torque rating. Use the dimensions, friction characteristics and engagement details of the actual tool when assessing practical performance.
Lead angle sets axial travel per revolution
Unroll one turn of the pitch cylinder. Its circumferential length is πd and its axial rise is the thread lead ℓ. If α is measured from the plane perpendicular to the axis, tan α=ℓ/(πd), hence ℓ=πd tan α.
A stroke S produces a magnitude of S/â„“ revolutions. The selected thread hand determines its sign: the inner left-hand stage is negative and the outer right-hand stage positive under the displayed end-view convention. Increasing lead angle increases axial lead and therefore reduces turns per stroke, with diameter and stroke held fixed.
For an ideal lossless conversion, F·S=τ·2π|n|. Thus torque magnitude is τ=Fℓ/(2π), using lead in metres, and work per push is F·S with stroke in metres. These are ideal energy relationships; friction, nut contact, locking strength and user technique are excluded.
At fixed diameter and stroke, a larger lead gives fewer revolutions. At the same axial force, it also gives greater ideal torque, preserving the work balance between push and rotation.
Example with a 12 mm pitch diameter
At a 45° lead angle and 12 mm pitch diameter, lead is π×12×tan45°=37.70 mm/rev. A 100 mm axial stroke therefore gives 2.65 bit turns in magnitude. Selecting the inner left-hand stage gives −2.65 turns; selecting the outer right-hand stage gives +2.65 turns.
With an ideal 60 N axial force, the corresponding torque magnitude is 0.360 N·m and work per push is 6.00 J. Multiplying torque by the absolute angular travel, 2π×2.65 rad, recovers that same work.
At 30° instead, lead falls to 21.77 mm/rev and the same stroke gives about 4.59 turns. Ideal torque falls to 0.208 N·m at the same force. This is the geometric speed/torque trade, not an empirical claim about a particular vintage tool.
What remains schematic
The selected thread uses the entered pitch diameter, lead angle and stroke. The inactive stage is shown at a compatible illustrative diameter, and its exact historic dimensions are not supplied by the engraving. The body is fitted to the complete entered stroke so nuts stay on their working lengths.
Playback prescribes a smooth push and return for inspection. It does not solve acceleration, input speed, freewheeling, spring forces or dynamic engagement. The selected stage remains engaged for the displayed rewind. The end view makes the accumulated rotation visible even when several turns occur during one push.
The torque result is an ideal lossless value. Real helical drives can lose considerable input to friction, and shallow leads can resist back-driving. No efficiency, self-locking threshold, allowable push force or nut strength is inferred from this diagram.
Double-acting screwdriver questions
Is this a Yankee ratchet screwdriver?
The drawing follows Hiscox’s nested opposite-hand spindles and locking nuts. It does not claim the pawl and spring-return arrangement of a particular later Yankee model.
What does the spindle selector change?
It selects the working left-hand inner stage or right-hand outer stage. The other stage is locked, and the bit’s rotation sign changes.
Does playback change the lock setting automatically?
No. Your selection stays fixed. The return phase simply reverses the selected screw motion.
Why do steeper lead angles produce fewer turns?
The angle here is measured from the circumferential plane. A steeper helix advances farther axially per revolution, so a fixed stroke contains fewer revolutions.
Is calculated torque the usable tool torque?
No. It is the lossless work-equivalent torque. Friction and the real tool’s strength and engagement must be evaluated separately.
Does a longer push increase ideal torque?
At fixed force and thread geometry, a longer push increases turns and work but leaves the ideal torque unchanged.
Primary construction reference
Hiscox, Mechanical Movements, Powers, Devices and Appliances, printed page 336, figure 1392. The caption identifies a left-hand inner spindle, a right-hand hollow outer spindle, and locking nuts. Figure 1394 on the same page is a separate reciprocating drill and is not used to infer an automatic return for this screwdriver.
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