Clamping Force Is About Friction, Not Just Squeeze
A workholding clamp doesn't stop a part from moving by brute strength alone, it relies on friction between the part and the fixture surface, generated by the clamping force pressing the two together. The required clamping force depends directly on how slippery that contact is: a lower friction coefficient means you need proportionally more clamp force to hold the same cutting load.
Why the Safety Factor Matters Here
Cutting forces aren't perfectly steady, interrupted cuts, chatter, and tool wear all create spikes above the nominal force, so a safety factor of 2 or more is standard practice rather than cutting it close to the calculated minimum. Lighter, more interrupted cuts or harder, more brittle workpiece materials are good reasons to push the safety factor higher.
Splitting Force Across Multiple Clamps
Spreading the total required clamping force across more clamping points reduces the load each individual clamp needs to apply, which is often the easier fix than searching for one enormous clamp, and also helps even out the holding pressure across a larger or more flexible part.