Cramp Drill (form 2)

The Cramp Drill (form 2) in Action

The cramp drill (form 2) is a screw-feed hand drill. You clamp the frame to the work — either across a bench edge or onto the part itself — and the cramp screw at the back of the frame pushes against a fixed surface to load the bit into the cut. Turn the crank handle, and bevel gears step the rotation down to the spindle that holds the drill bit. Because the body is locked relative to the workpiece, every turn of the cramp screw advances the bit by exactly the screw pitch — typically 1.5 to 2 mm per revolution on a Victorian-era machinist's drill. That's the whole point. The operator is no longer fighting to keep the tool square while pushing on a breast plate.

Geometry matters. The spindle axis must sit perpendicular to the cramp-screw axis within about 0.5°, otherwise the bit walks sideways as feed builds up and you end up with an oversized, tapered hole. The bevel gears are usually 2:1 or 3:1 reduction, so a comfortable 60 RPM at the crank gives 20 to 30 RPM at the bit — exactly where a carbon-steel twist drill wants to run in mild steel. Push the crank faster and the bit overheats; the cutting edge anneals and the hole stops progressing. Slow the crank and load the screw harder, and you'll snap a small drill bit because the feed-per-rev exceeds what the flutes can clear.

Failure modes are predictable. Worn bevel teeth give a rattling backlash that telegraphs into the bit and chips the cutting edge. A bent cramp screw deflects under load and pulls the hole off-axis. The most common builder mistake is undersizing the cramp itself — if the frame springs even 0.2 mm under feed pressure, the drill chatters and the hole goes oval.

Key Components

• Crank Handle: The hand input. Typical throw is 100 to 140 mm radius, giving comfortable continuous cranking at 40 to 80 RPM. The handle pin should run on a bronze bushing — a plain steel-on-steel pin wears oval inside 50 hours of use and introduces a wobble that you feel as fatigue in the wrist.
• Bevel Gear Pair: Converts horizontal crank rotation to vertical (or in-line) spindle rotation. Reduction ratios sit between 2:1 and 3:1. Tooth contact must cover at least 60% of the face width — anything less and the gears wear unevenly and start to skip under heavy feed.
• Spindle and Chuck: Holds the drill bit. On period cramp drills the chuck is a square taper or a 3-jaw screw chuck rated for bits up to 13 mm. Spindle runout above 0.05 mm at the chuck nose causes the hole to oversize by twice that figure once the bit starts cutting.
• Cramp Screw (Feed Screw): The defining feature. A coarse Acme or square-thread screw, pitch 1.5 to 2.5 mm, that pushes the back of the frame against a fixed reaction surface — bench, post, or the workpiece itself. Each full turn of the cramp head advances the bit by one pitch. Backlash in this thread directly becomes lost feed, so the nut runs as a long bronze block, 25 to 40 mm of engagement minimum.
• Cramp Frame: The structural loop carrying load between the spindle and the cramp screw. Cast iron or wrought iron in original tools, with a section thick enough to keep deflection under 0.1 mm at full feed load (around 200 N). A flexy frame is the single biggest cause of out-of-round holes.
• Reaction Pad / Centre: The hardened pad on the cramp screw tip that bears against the reaction surface. A loose or rotating pad lets the screw walk sideways under load. Most surviving examples use a swivel cup that self-aligns to the back surface within ±3°.

Real-World Applications of the Cramp Drill (form 2)

The cramp drill (form 2) earned its place in fitter's shops, gunsmith benches and clockmaker stations from roughly 1850 through the early 20th century, anywhere accurate holes were needed in metal but a powered machine wasn't available or practical. The screw feed mechanism gave the user controlled axial pressure, which is why the tool is still chosen today by restorers and bladesmiths working on irreplaceable parts where a hand-held electric drill would skid and mark the surface. Use cases tend to fall in two camps — locations a drill press cannot reach, and materials where over-speeding ruins the cut.

• Antique Firearm Restoration: Drilling sight-mount holes on original Winchester 1873 receivers where clamping in a mill is impossible without damaging case-hardened surfaces
• Clockmaking and Horology: Boring pivot holes in brass clock plates on a workbench, where a Pulteney-style cramp drill running at 25 RPM gives the operator the feel needed for a 1.5 mm hole
• Timber-Frame Restoration: Drilling drift-pin holes in standing oak posts on heritage barns — Trillium Dell Timberworks and similar restoration crews use cramp-style hand drills where bringing power to the post is impractical
• Locksmithing and Safe Work: Field drilling of safe lock cases where a cordless drill walks on hardened plate but a screw-fed cramp drill bites in cleanly
• Shipwright and Wooden Boat Repair: Boring through-hull and keel-bolt holes on traditional wooden vessels at yards like the Northwest School of Wooden Boatbuilding, where the tool clamps directly to the timber
• Bladesmithing and Knife Making: Drilling pin holes in heat-treated knife tangs, where controlled feed prevents work-hardening of the cut
• Heritage Toolmaking: Boring tap-drill holes in cast iron tool bodies during museum-grade reproductions of 19th-century machinist tools

The Formula Behind the Cramp Drill (form 2)

The number that matters on a cramp drill is feed rate — how fast the bit advances into the work per minute. Get it right and the bit cuts cleanly with chips evacuating up the flutes. Push the low end of the operating range and you'll burnish the hole instead of cutting it; push the high end and you'll snap the bit or stall the crank. The sweet spot for a 6 mm twist drill in mild steel sits at roughly 0.05 mm of feed per spindle revolution — and the formula below tells you what crank speed delivers that, given your gear ratio and screw pitch.

Variables

Worked Example: Cramp Drill (form 2) in a brass watchmaker's plate

You are drilling a 2 mm pivot hole in a 3 mm thick yellow brass clock plate clamped in a Pulteney-pattern cramp drill. The bevel gears are 2.5:1 reduction, the cramp screw has a 2 mm pitch, and you intend to crank at a steady 60 RPM. You want to know the feed rate and whether the resulting feed-per-rev is correct for a HSS twist drill in brass.

Given

• N_crank = 60 RPM
• R_gear = 2.5 dimensionless
• p_screw = 2.0 mm/rev
• n_screw = 1.2 RPM (one-fiftieth of crank, by feel)

Solution

Step 1 — at the nominal 60 RPM crank speed, find the spindle speed:

Step 2 — compute the nominal feed rate, with the cramp screw advancing at roughly 1.2 RPM (the operator turning the cramp head about once every 50 seconds):

Step 3 — convert to feed-per-spindle-rev to check the cut:

For a 2 mm HSS bit in brass, 0.05 to 0.10 mm/rev is the textbook range, so 0.10 mm/rev sits right at the upper edge — chips will be substantial and the hole will cut quickly. At the low end of practical operation — crank at 30 RPM, screw at 0.6 RPM — feed rate drops to f_rate,low = 1.2 mm/min and feed-per-rev stays near 0.10 mm/rev because both numbers scale together. You'll feel the cut as slow but controlled, ideal for the last 0.5 mm before breakthrough where you want to avoid tearing the back face. At the high end — crank at 90 RPM, screw at 1.8 RPM — f_rate,high climbs to 3.6 mm/min. The bit progresses fast but the small 2 mm flutes start to clog with brass swarf, and you'll hear the cut change pitch as the drill begins to rub instead of cut.

Result

Nominal feed rate is 2. 4 mm/min at a feed-per-rev of 0.10 mm/rev — about 75 seconds to drill through the 3 mm plate. That's a steady, productive cut where chips peel out the flutes in continuous yellow curls. The low-end 30 RPM crank gives 1.2 mm/min and a much gentler cut suitable for breakthrough or for thin watch plates; the high-end 90 RPM crank doubles output to 3.6 mm/min but pushes the flutes past their swarf-clearance limit. If your measured feed rate falls 30% below predicted, the most likely causes are: (1) cramp screw backlash above 0.3 mm, where the screw rotates without advancing because the bronze nut is worn, (2) the cramp frame springing under load — check for visible flex by sighting along the cramp axis while feeding, or (3) a dull bit refusing to bite, which manifests as the crank getting easier to turn rather than harder.

Choosing the Cramp Drill (form 2): Pros and Cons

The cramp drill (form 2) competes with the breast drill and the bench-mounted hand drill press. Each has a window where it wins, and the right choice comes down to portability, feed control, and the size of hole you're cutting.

Frequently Asked Questions About Cramp Drill (form 2)