1. The pivot is a bearing, not a bolt
A phalanx pivot looks like a screw because it has a head and a thread, but its job is to locate a rotating member. That makes it a bearing shaft with a threaded retention feature, and it should be toleranced like a shaft: ground diameter, controlled straightness, fine surface finish.
A turned shoulder screw with a good diameter but 0.03 mm of bend will bind in a 4 mm bore. Straightness, not diameter, is usually the cause of a stiff finger joint.
2. Design the joint to be opened, then decide how to lock it
During development a finger joint may be disassembled fifty times. A joint locked with encapsulated adhesive cannot support that. Use a nylon patch for prototyping, accept a lower break-loose torque, and switch to encapsulated adhesive only at design freeze.
Document the change. Teams that forget this step often ship prototypes with production locking compound and then cannot service them.
3. Mass at the fingertip costs more than mass at the palm
Every gram in the distal phalanx must be accelerated by every upstream actuator. A titanium screw that saves 0.3 g at the fingertip is worth more than a 3 g saving in the forearm, because the distal mass participates in the highest-acceleration part of the motion.
This is also why small screws are often specified in titanium even when steel would be stronger — the design is optimising dynamic response, not static strength.
4. Test to the cycle count, not to a feeling
A million cycles is a week of continuous operation in a test rig and a year in the field. Specify the number, the direction of loading and the temperature, then measure backlash and break-loose torque at intervals rather than only at the end.
A joint that passes at 10⁶ cycles but loses 60% of its preload in the first 10⁵ is not a passing joint — it is a joint that has already failed and is waiting to be discovered.
5. Magnetic fields change the material choice
Joint motors produce fields that will attract a loose ferrous screw. Beyond the obvious risk of a screw migrating into a magnet gap, ferromagnetic fasteners can distort the field and add cogging torque.
SUS316 and titanium are effectively non-magnetic. If the design uses steel screws for strength, consider a captive or encapsulated design so that a loose screw is not possible.
6. Put the tolerance where the function is
A finger assembly may have eight dimensions in the stack, but only two determine tip position. Spending precision on all eight multiplies cost without improving accuracy; spending it on the two that matter is cheaper and more effective.
In practice this usually means a ground pin and a precision bore, with everything else at commercial tolerance. The fastener is often the cheapest place in the stack to buy accuracy — if it is specified correctly.