A holding fixture for a work piece 1 in thick at clamp locations is being designed.

Question:

A holding fixture for a work piece 1 ½ in thick at clamp locations is being designed. The detail of one of the clamps is shown in the figure. A spring is required to drive the clamp upward while removing or inserting a work piece. A clamping force of 10 lbf is satisfactory. The base plate is 5/8 in thick. The clamp screw has a 7/16 in-20 UNF thread. It is useful to have the free length L0 short enough so that the clamp screw can compress the spring upon fixture reassembly during inspection and service, say L0 ≤ 1.5 + 3/8 in. The spring cannot close solid at a length greater than 1 ¼ in. The safety factor when compressed solid should be ns ≥ 1.2, and at service load n1 ≥ 1.5. Design a suitable helical coil compression spring for this fixture.