A traditional carpenter’s holdfast presses a workpiece against a bench using a curved arm and a long shank inserted through a bench hole. This page shows its shape and contact arrangement, then calculates the nominal clearance and geometric tilt available from the entered hole, shank and bore-depth dimensions.
Considering a separate powered clamp
A powered clamp is a separate design from the tap-set holdfast illustrated here. An actuator and its linkage require their own force, travel, release and structural checks; this bore-fit calculator does not size them.
With leverage, force is traded against travel. A bracket change that improves force can also increase required stroke, slow the output motion, or move the actuator into a poor mounting angle.
For a powered linkage, determine the required force at the clamp pad and use the linkage geometry to calculate actuator force throughout travel. These forces are generally different.
- Calculate actuator force at the worst bracket angle, not just at mid-stroke.
- Check stroke, retracted length, and extended length against the mechanism envelope.
- Confirm the structure can accept the higher loads created by a stronger actuator.
For a FIRGELLI actuator application, run the FIRGELLI actuator force calculator, then compare available stroke, force, and mounting envelope in the FIRGELLI linear actuator range. If the goal is to replace an existing unit, use the linear actuator replacement finder before assuming the new actuator is a direct fit.
Carpenter's Bench-clamp Interactive Calculator
Enter the hole diameter, shank diameter and effective straight-bore depth. The right-hand cutaway follows those dimensions. The results describe nominal geometric fit; they do not estimate clamping force or guarantee that a holdfast will lock.
Equation Used
- Hole and shank are circular, straight and rigid in the ideal fit model.
- Effective bore depth is measured between the two constraining bore edges.
- The tilt result is a geometric limit, not a predicted operating angle or proof of locking.
- No impact, spring stiffness, friction, wood crushing or wear model is supplied.
- The whole-tool drawing is illustrative; the separate bore section uses all three entered dimensions at a common scale.
- Highlights identify contact locations without implying motion or measured forces.
Compare the hole and shank diameters with the effective bore depth to explore clearance and opposing-edge contact. The cutaway shows the holdfast, workpiece and bench, while the dimensioned section follows your entries. Clamping force is outside this geometric model.
The curved-arm holdfast
The manufacturer’s photographs show a long round shank, a curved arm and a flattened foot bearing on the workpiece. The shank enters a hole in the bench. The maker describes setting it with a tap on top and releasing it from the side.
The cutaway separates that recognizable tool from a dimensioned bore section. A slightly tilted shank contacts opposite bore edges. The foot, workpiece and supporting bench complete the contact arrangement. Real grip also depends on the spring behavior of the tool and the surfaces in contact.
The animation highlights the bore contacts and pad in sequence. The workpiece remains stationary; impact, deformation and clamping force are not simulated.
Checking nominal fit dimensions
The calculator compares a proposed shank diameter with a measured bench-hole diameter and effective bore depth. It reports nominal clearance and the rigid geometric angle at which opposing-edge contact would occur.
Use the appropriate manufacturer’s dimensions for an actual holdfast. A positive clearance alone does not establish reliable locking. Tool designs, bench materials and contact conditions differ.
Geometry of a tilted shank in a straight bore
Let D be hole diameter, d shank diameter and t effective bore depth, all in mm. The centered radial gap is (D−d)/2. In the plane of tilt, the shank occupies horizontal width d/cos α; its centerline shifts t tan α between the two bore edges.
At opposite-edge contact, t tan α + d sec α = D, or t sin α + d = D cos α. Solving for the nonnegative contact angle gives α = atan(D/t) − asin[d/√(D²+t²)], provided D>d. The result is converted from radians to degrees.
For D=d there is no tilt clearance. For D<d the nominal dimensions interfere, and the calculator returns zero tilt with an interference status.
Default fit example
A 19 mm hole and 17 mm shank give 2 mm diametral clearance and a 1 mm centered radial gap. With a 75 mm effective bore depth, the ideal opposite-edge contact angle is approximately 1.52°.
This angle follows the entered dimensions. It is not a recommended holdfast angle or a prediction of pad force. Increasing bore depth at the same diameters reduces the available tilt; an oversized shank is explicitly shown as interference.
What the calculation cannot infer
Mallet force alone does not define the clamping load after the blow. Contact time, deformation, friction, tool stiffness and bench behaviour also affect it. These effects are outside the dimensional fit calculation.
The rigid geometric contact calculation also omits real edge rounding, roughness and local compression. It helps explain the dimensions, while manufacturer guidance and the actual tool govern fit and use.
Questions about the bench-clamp calculator
Does clearance prove the holdfast will grip?
No. It establishes nominal space for the shank, not the friction or elastic behavior needed to retain a clamping load.
Can this calculation determine clamping force?
No. It reports dimensional fit. The force retained at the pad also depends on tool stiffness, deformation and contact behaviour.
What does effective bore depth mean?
It is the straight constraining portion between the two bore edges. Relief or counterboring can make it different from the overall bench thickness.
Why does the picture remain visible for interference?
The red dimensioned section identifies the overlap. The whole-tool drawing remains as a labelled typical example, not a claim that the selected dimensions assemble.
Primary tool reference
Tools for Working Wood: Holdfasts by Gramercy Tools, manufacturer photographs and description of the curved shank, arm, foot and setting/release action. The separate rigid-fit equation is derived from the displayed bore geometry; it is not a manufacturer force model.
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
