Strap
In CNC machining and millwork, a strap, or strap clamp, is a flat clamp bar spanning over a workpiece, tightened via a stud, nut, and T-nut seated in the machine table T-slot. It holds the part firmly against the table against cutting forces. It is part of a stack-up: T-nut, stud, strap, step block, and nut.
On the shop floor, strap clamps come into play when a part is too large, thin, or irregular for a vise. A typical setup uses a T-nut in the table slot, a threaded stud, a strap, and a step block; the step block is set slightly higher than the workpiece so the strap presses down with a defined force vector. This matters in CNC because the programmed toolpath assumes the part will not lift or shift. A poorly adjusted clamp can create dimensional error, chatter, broken tools, or even a fixture collision. In millwork and cabinet shops, the same principle appears on CNC router tables when parts are held down for trimming, boring, or edge work. Thin material may need adjustable strap systems that avoid flexing or crushing the surface.
Why use a strap clamp instead of a vise?
Strap clamps handle parts that exceed vise capacity, have awkward geometry, or require top-down access across a broad area of the table.
What establishes clamp force in a strap setup?
The stud, nut torque, step-block geometry, and the clamp's bridge height determine how much downward load reaches the part.
What protects a finished surface from clamp marks?
Shim stock or a sacrificial pad placed under the strap contact area reduces marking and surface indentation.